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Herschel Spectroscopic Observations of Little Things Dwarf Galaxies
Cigan, Phil; Young, Lisa; Cormier, Diane; Lebouteiller, Vianney; Madden, Suzanne; Hunter, Deidre; Brinks, Elias; Elmegreen, Bruce; Schruba, Andreas; Heesen, Volker
Astronomical Journal 151(14), 2016
We present far-infrared (FIR) spectral line observations of five galaxies from the Little Things sample: DDO 69, DDO 70, DDO 75, DDO 155, and WLM. While most studies of dwarfs focus on bright systems or starbursts due to observational constraints, our data extend the observed parameter space into the regime of low surface brightness dwarf galaxies with low metallicities and moderate star formation rates. Our targets were observed with Herschel at the [C ii] 158 mum, [O i] 63 mum, [O iii] 88 mum, and [N ii] 122 mum emission lines using the PACS Spectrometer. These high-resolution maps allow us for the first time to study the FIR properties of these systems on the scales of larger star-forming complexes. The spatial resolution in our maps, in combination with star formation tracers, allows us to identify separate photodissociation regions (PDRs) in some of the regions we observed. Our systems have widespread [C ii] emission that is bright relative to continuum, averaging near 0.5% of the total infrared (TIR) budget---higher than in solar-metallicity galaxies of other types. [N ii] is weak, suggesting that the [C ii] emission in our galaxies comes mostly from PDRs instead of the diffuse ionized interstellar medium (ISM). These systems exhibit efficient cooling at low dust temperatures, as shown by ([O i]+[C ii])/TIR in relation to 60 mum/100 mum, and low [O i]/[C ii] ratios which indicate that [C ii] is the dominant coolant of the ISM. We observe [O iii]/[C ii] ratios in our galaxies that are lower than those published for other dwarfs, but similar to levels noted in spirals.
A Warp in Progress: H I and Radio Continuum Observations of the Spiral NGC 3145
M. Kaufman, E. Brinks, C. Struck, B.~G. Elmegreen, D.~M. Elmegreen
Astronomical Journal150, 65, 2015
VLA H i observations and lambda6 cm radio continuum observations are presented of the barred-spiral galaxy NGC 3145. In optical images NGC 3145 has stellar arms that appear to cross, forming ``X''-features. Our radio continuum observations rule out shock fronts at three of the four ``X''-features, and our H i data provide evidence of gas motions perpendicular to the disk of NGC 3145. In large portions of NGC 3145, particularly in the middle-to-outer disk, the H i line profiles are skewed. Relative to the disk, the gas in the skewed wing of the line profiles has z-motions away from us on the approaching side of the galaxy and z-motions of about the same magnitude (~40 km s-1) toward us on the receding side. These warping motions imply that there has been a perturbation with a sizeable component perpendicular to the disk over large spatial scales. Two features in NGC 3145 have velocities indicating that they are out-of-plane tidal arms. One is an apparent branch of a main spiral arm on the northeastern side of NGC 3145; the velocity of the branch is ~150 km s-1 greater than the spiral arm where they appear to intersect in projection. The other is the arm on the southwestern side that forms three of the ``X''-features. It differs in velocity by ~56 km s-1 from that of the disk at the same projected location. H i observations are presented also of the two small companions NGC 3143 and PGC 029578. Based on its properties (enhanced SFR, H i emission 50% more extended on its northeastern side, etc.), NGC 3143 is the more likely of the two companions to have interacted with NGC 3145 recently. A simple analytic model demonstrates that an encounter between NGC 3143 and NGC 3145 is a plausible explanation for the observed warping motions in NGC 3145.
Hierarchical star formation across the ring galaxy NGC 6503
D.~A. Gouliermis, D. Thilker, B.~G. Elmegreen, D.~M. Elmegreen, D. Calzetti, J.~C. Lee, A. Adamo, A. Aloisi, M. Cignoni, D.~O. Cook, D.~A. Dale, J.~S. Gallagher, K. Grasha, E.~K. Grebel, A.~H. Davo, D.~A. Hunter, K.~E. Johnson, H. Kim, P. Nair, A. Not
Monthly Notices of the Royal Astronomical Society452, 3508-3528, 2015
We present a detailed clustering analysis of the young stellar population across the star-forming ring galaxy NGC 6503, based on the deep Hubble Space Telescope photometry obtained with the Legacy ExtraGalactic UV Survey. We apply a contour-based map analysis technique and identify in the stellar surface density map 244 distinct star-forming structures at various levels of significance. These stellar complexes are found to be organized in a hierarchical fashion with 95 per cent being members of three dominant super-structures located along the star-forming ring. The size distribution of the identified structures and the correlation between their radii and numbers of stellar members show power-law behaviours, as expected from scale-free processes. The self-similar distribution of young stars is further quantified from their autocorrelation function, with a fractal dimension of ~1.7 for length-scales between ~20 pc and 2.5 kpc. The young stellar radial distribution sets the extent of the star-forming ring at radial distances between 1 and 2.5 kpc. About 60 per cent of the young stars belong to the detected stellar structures, while the remaining stars are distributed among the complexes, still inside the ring of the galaxy. The analysis of the time-dependent clustering of young populations shows a significant change from a more clustered to a more distributed behaviour in a time-scale of ~60 Myr. The observed hierarchy in stellar clustering is consistent with star formation being regulated by turbulence across the ring. The rotational velocity difference between the edges of the ring suggests shear as the driving mechanism for this process. Our findings reveal the interesting case of an inner ring forming stars in a hierarchical fashion.
Wiggle Instability of Galactic Spiral Shocks: Effects of Magnetic Fields
Y. Kim, W.-T. Kim, B.~G. Elmegreen
Astrophysical Journal809, 33, 2015
It has been suggested that the wiggle instability (WI) of spiral shocks in a galactic disk is responsible for the formation of gaseous feathers observed in grand-design spiral galaxies. We perform both a linear stability analysis and numerical simulations to investigate the effect of magnetic fields on the WI. The disk is assumed to be infinitesimally thin, isothermal, and non-self-gravitating. We control the strengths of magnetic fields and spiral-arm forcing using the dimensionless parameters beta and F, respectively. By solving the perturbation equations as a boundary-eigenvalue problem, we obtain dispersion relations of the WI for various values of beta =1 to infinity and F=5% and 10%. We find that the WI arising from the accumulation of potential vorticity at disturbed shocks is suppressed, albeit not completely, by magnetic fields. The stabilizing effect of magnetic fields is not from the perturbed fields but from the unperturbed fields that reduce the density compression factor in the background shocks. When F=5% and beta <+10 or F=10% and beta ~5-10, the most unstable mode has a wavelength of ~0.1-0.2 times the arm-to-arm separation, which appears consistent with a mean spacing of observed feathers.
Halpha kinematics of S^4G spiral galaxies - II. Data description and non-circular motions
S. Erroz-Ferrer, J.~H. Knapen, R. Leaman, M. Cisternas, J. Font, J.~E. Beckman, K. Sheth, J.~C. Munoz-Mateos, S. Diaz-Garcia, A. Bosma, E. Athanassoula, B.~G. Elmegreen, L.~C. Ho, T. Kim, E. Laurikainen, I. Martinez-Valpuesta, S.~E. Meidt,
Monthly Notices of the Royal Astronomical Society451, 1004-1024, 2015
We present a kinematical study of 29 spiral galaxies included in the Spitzer Survey of Stellar Structure in Galaxies, using Halpha Fabry-Perot (FP) data obtained with the Galaxy Halpha Fabry-Perot System instrument at the William Herschel Telescope in La Palma, complemented with images in the R band and in Halpha. The primary goal is to study the evolution and properties of the main structural components of galaxies through the kinematical analysis of the FP data, complemented with studies of morphology, star formation and mass distribution. In this paper we describe how the FP data have been obtained, processed and analysed. We present the resulting moment maps, rotation curves, velocity model maps and residual maps. Images are available in FITS format through the NASA/IPAC Extragalactic Database and the Centre de Donnes Stellaires. With these data products we study the non-circular motions, in particular those found along the bars and spiral arms. The data indicate that the amplitude of the non-circular motions created by the bar does not correlate with the bar strength indicators. The amplitude of those non-circular motions in the spiral arms does not correlate with either arm class or star formation rate along the spiral arms. This implies that the presence and the magnitude of the streaming motions in the arms is a local phenomenon.
The Odd Offset between the Galactic Disk and Its Bar in NGC 3906
B. de Swardt, K. Sheth, T. Kim, S. Pardy, E. D'Onghia, E. Wilcots, J. Hinz, J.-C. Munoz-Mateos, M.~W. Regan, E. Athanassoula, A. Bosma, R.~J. Buta, M. Cisternas, S. Comeron, D.~A. Gadotti, A. Gil de Paz, T.~H. Jarrett, B.~G. Elmegreen, S.
Astrophysical Journal808, 90, 2015
We use mid-infrared 3.6 and 4.5 mum imaging of NGC 3906 from the Spitzer Survey of Stellar Structure in Galaxies (S4G) to understand the nature of an unusual offset between its stellar bar and the photometric center of an otherwise regular, circular outer stellar disk. We measure an offset of ~910 pc between the center of the stellar bar and photometric center of the stellar disk; the bar center coincides with the kinematic center of the disk determined from previous HI observations. Although the undisturbed shape of the disk suggests that NGC 3906 has not undergone a significant merger event in its recent history, the most plausible explanation for the observed offset is an interaction. Given the relatively isolated nature of NGC 3906 this interaction could be with dark matter substructure in the galaxy's halo or from a recent interaction with a fast moving neighbor that remains to be identified. Simulations aimed at reproducing the observed offset between the stellar bar/kinematic center of the system and the photometric center of the disk are necessary to confirm this hypothesis and constrain the interaction history of the galaxy.
A Star Formation Law for Dwarf Irregular Galaxies
B.~G. Elmegreen, D.~A. Hunter
Astrophysical Journal805, 145, 2015
The radial profiles of gas, stars, and far-ultraviolet radiation in 20 dwarf Irregular galaxies are converted to stability parameters and scale heights for a test of the importance of two-dimensional (2D) instabilities in promoting star formation. A detailed model of this instability involving gaseous and stellar fluids with self-consistent thicknesses and energy dissipation on a perturbation crossing time gives the unstable growth rates. We find that all locations are effectively stable to 2D perturbations, mostly because the disks are thick. We then consider the average volume densities in the midplanes, evaluated from the observed H i surface densities and calculated scale heights. The radial profiles of the star-formation rates are equal to about 1% of the H i surface densities divided by the free fall times at the average midplane densities. This 1% resembles the efficiency per unit free fall time commonly found in other cases. There is a further variation of this efficiency with radius in all of our galaxies, following the exponential disk with a scale length equal to about twice the stellar mass scale length. This additional variation is modeled by the molecular fraction in a diffuse medium using radiative transfer solutions for galaxies with the observed dimensions and properties of our sample. We conclude that star formation is activated by a combination of three-dimensional gaseous gravitational processes and molecule formation. Implications for outer disk structure and formation are discussed.
High-resolution Mass Models of Dwarf Galaxies from LITTLE THINGS
S.-H. Oh, D.~A. Hunter, E. Brinks, B.~G. Elmegreen, A. Schruba, F. Walter, M.~P. Rupen, L.~M. Young, C.~E. Simpson, M.~C. Johnson, K.~A. Herrmann, D. Ficut-Vicas, P. Cigan, V. Heesen, T. Ashley, H.-X. Zhang
Astronomical Journal149, 180, 2015
We present high-resolution rotation curves and mass models of 26 dwarf galaxies from ``Local Irregulars That Trace Luminosity Extremes, The H i Nearby Galaxy Survey'' (LITTLE THINGS). LITTLE THINGS is a high-resolution (~6" angular; <2.6 km s-1 velocity resolution) Very Large Array H i survey for nearby dwarf galaxies in the local volume within 11 Mpc. The high-resolution H i observations enable us to derive reliable rotation curves of the sample galaxies in a homogeneous and consistent manner. The rotation curves are then combined with Spitzer archival 3.6 mum and ancillary optical U, B, and V images to construct mass models of the galaxies. This high quality multi-wavelength data set significantly reduces observational uncertainties and thus allows us to examine the mass distribution in the galaxies in detail. We decompose the rotation curves in terms of the dynamical contributions by baryons and dark matter (DM) halos, and compare the latter with those of dwarf galaxies from THINGS as well as LambdaCDM Smoothed Particle Hydrodynamic (SPH) simulations in which the effect of baryonic feedback processes is included. Being generally consistent with THINGS and simulated dwarf galaxies, most of the LITTLE THINGS sample galaxies show a linear increase of the rotation curve in their inner regions, which gives shallower logarithmic inner slopes alpha of their DM density profiles. The mean value of the slopes of the 26 LITTLE THINGS dwarf galaxies is alpha =-0.32+/- 0.24 which is in accordance with the previous results found for low surface brightness galaxies (alpha =-0.2+/- 0.2) as well as the seven THINGS dwarf galaxies (alpha =-0.29+/- 0.07). However, this significantly deviates from the cusp-like DM distribution predicted by DM-only LambdaCDM simulations. Instead our results are more in line with the shallower slopes found in the LambdaCDM SPH simulations of dwarf galaxies in which the effect of baryonic feedback processes is included. In addition, we discuss the central DM distribution of DDO 210 whose stellar mass is relatively low in our sample to examine the scenario of inefficient supernova feedback in low mass dwarf galaxies predicted from recent LambdaCDM SPH simulations of dwarf galaxies where central cusps still remain.
Globular Cluster Populations: First Results from S^4G Early-type Galaxies
D. Zaritsky, M. Aravena, E. Athanassoula, A. Bosma, S. Comeron, B.~G. Elmegreen, S. Erroz-Ferrer, D.~A. Gadotti, J.~L. Hinz, L.~C. Ho, B. Holwerda, J.~H. Knapen, J. Laine, E. Laurikainen, J.~C. Munoz-Mateos, H. Salo, K. Sheth
Astrophysical Journal799, 159, 2015
Using 3.6 mum images of 97 early-type galaxies, we develop and verify methodology to measure globular cluster populations from the S4G survey images. We find that (1) the ratio, T N, of the number of clusters, N CL, to parent galaxy stellar mass, M *, rises weakly with M * for early-type galaxies with M * > 1010 M &sun; when we calculate galaxy masses using a universal stellar initial mass function (IMF) but that the dependence of T N on M * is removed entirely once we correct for the recently uncovered systematic variation of IMF with M *; and (2) for M * < 1010 M &sun;, there is no trend between N CL and M *, the scatter in T N is significantly larger (approaching two orders of magnitude), and there is evidence to support a previous, independent suggestion of two families of galaxies. The behavior of N CL in the lower-mass systems is more difficult to measure because these systems are inherently cluster-poor, but our results may add to previous evidence that large variations in cluster formation and destruction efficiencies are to be found among low-mass galaxies. The average fraction of stellar mass in clusters is ~0.0014 for M * > 1010 M &sun; and can be as large as ~0.02 for less massive galaxies. These are the first results from the S4G sample of galaxies and will be enhanced by the sample of early-type galaxies now being added to S4G and complemented by the study of later-type galaxies within S4G.
Legacy Extragalactic UV Survey (LEGUS) With the Hubble Space Telescope. I. Survey Description
D. Calzetti, J.~C. Lee, E. Sabbi, A. Adamo, L.~J. Smith, J.~E. Andrews, L. Ubeda, S.~N. Bright, D. Thilker, A. Aloisi, T.~M. Brown, R. Chandar, C. Christian, M. Cignoni, G.~C. Clayton, R. da Silva, S.~E. de Mink, C. Dobbs, B.~G. Elmegreen, D.~M. Elmegreen
Astronomical Journal149, 51, 2015
The Legacy ExtraGalactic UV Survey (LEGUS) is a Cycle 21 Treasury program on the Hubble Space Telescope aimed at the investigation of star formation and its relation with galactic environment in nearby galaxies, from the scales of individual stars to those of ~kiloparsec-size clustered structures. Five-band imaging from the near-ultraviolet to the I band with the Wide-Field Camera 3 (WFC3), plus parallel optical imaging with the Advanced Camera for Surveys (ACS), is being collected for selected pointings of 50 galaxies within the local 12 Mpc. The filters used for the observations with the WFC3 are F275W(lambda2704 A), F336W(lambda3355 A), F438W(lambda4325 A), F555W(lambda5308 A), and F814W(lambda8024 A) the parallel observations with the ACS use the filters F435W(lambda4328 A), F606W(lambda5921 A), and F814W(lambda8057 A). The multiband images are yielding accurate recent (<=50 Myr) star formation histories from resolved massive stars and the extinction-corrected ages and masses of star clusters and associations. The extensive inventories of massive stars and clustered systems will be used to investigate the spatial and temporal evolution of star formation within galaxies. This will, in turn, inform theories of galaxy evolution and improve the understanding of the physical underpinning of the gas-star formation relation and the nature of star formation at high redshift. This paper describes the survey, its goals and observational strategy, and the initial scientific results. Because LEGUS will provide a reference survey and a foundation for future observations with the James Webb Space Telescope and with ALMA, a large number of data products are planned for delivery to the community.
The interplay between a galactic bar and a supermassive black hole: nuclear fuelling in a subparsec resolution galaxy simulation
E. Emsellem, F. Renaud, F. Bournaud, B. Elmegreen, F. Combes, J.~M. Gabor
Monthly Notices of the Royal Astronomical Society446, 2468-2482, 2015
We study the connection between the large-scale dynamics and the gas fuelling towards a central black hole via the analysis of a Milky Way-like simulation at subparsec resolution. This allows us to follow a set of processes at various scales (e.g. the triggering of inward gas motion towards inner resonances via the large-scale bar, the connection to the central black hole via minispirals) in a self-consistent manner. This simulation provides further insights on the role of shear for the inhibition of star formation within the bar in regions with significant amount of gas. We also witness the decoupling of the central gas and nuclear cluster from the large-scale disc, via interactions with the black hole. This break of symmetry in the mass distribution triggers the formation of gas clumps organized in a time-varying 250 pc ring-like structure, the black hole being offset by about 70 pc from its centre. Some clumps form stars, while most get disrupted or merge. Supernovae feedback further create bubbles and filaments, some of the gas being expelled to 100 pc or higher above the galaxy plane. This helps remove angular momentum from the gas, which gets closer to the central dark mass. Part of the gas raining down is being accreted, forming a 10 pc polar disc-like structure around the black hole, leading to an episode of star formation. This gives rise to multiple stellar populations with significantly different angular momentum vectors, and may lead to a natural intermittence in the fuelling of the black hole.
The Mass Profile and Shape of Bars in the Spitzer Survey of Stellar Structure in Galaxies (S^4G): Search for an Age Indicator for Bars
T. Kim, K. Sheth, D.~A. Gadotti, M.~G. Lee, D. Zaritsky, B.~G. Elmegreen, E. Athanassoula, A. Bosma, B. Holwerda, L.~C. Ho, S. Comeron, J.~H. Knapen, J.~L. Hinz, J.-C. Munoz-Mateos, S. Erroz-Ferrer, R.~J. Buta, M. Kim, E. Laurikainen, H. Salo, B.~
Astrophysical Journal799, 99, 2015
We have measured the radial light profiles and global shapes of bars using two-dimensional 3.6 mum image decompositions for 144 face-on barred galaxies from the Spitzer Survey of Stellar Structure in Galaxies. The bar surface brightness profile is correlated with the stellar mass and bulge-to-total (B/T) ratio of their host galaxies. Bars in massive and bulge-dominated galaxies (B/T > 0.2) show a flat profile, while bars in less massive, disk-dominated galaxies (B/T ~ 0) show an exponential, disk-like profile with a wider spread in the radial profile than in the bulge-dominated galaxies. The global two-dimensional shapes of bars, however, are rectangular/boxy, independent of the bulge or disk properties. We speculate that because bars are formed out of disks, bars initially have an exponential (disk-like) profile that evolves over time, trapping more disk stars to boxy bar orbits. This leads bars to become stronger and have flatter profiles. The narrow spread of bar radial profiles in more massive disks suggests that these bars formed earlier (z > 1), while the disk-like profiles and a larger spread in the radial profile in less massive systems imply a later and more gradual evolution, consistent with the cosmological evolution of bars inferred from observational studies. Therefore, we expect that the flatness of the bar profile can be used as a dynamical age indicator of the bar to measure the time elapsed since the bar formation. We argue that cosmic gas accretion is required to explain our results on bar profile and the presence of gas within the bar region.
Environmental regulation of cloud and star formation in galactic bars
F. Renaud, F. Bournaud, E. Emsellem, O. Agertz, E. Athanassoula, F. Combes, B. Elmegreen, K. Kraljic, F. Motte, R. Teyssier
Monthly Notices of the Royal Astronomical Society454, 3299-3310, 2015
The strong time-dependence of the dynamics of galactic bars yields a complex and rapidly evolving distribution of dense gas and star forming regions. Although bars mainly host regions void of any star formation activity, their extremities can gather the physical conditions for the formation of molecular complexes and mini-starbursts. Using a sub-parsec resolution hydrodynamical simulation of a Milky Way-like galaxy, we probe these conditions to explore how and where bar (hydro-)dynamics favours the formation or destruction of molecular clouds and stars. The interplay between the kpc-scale dynamics (gas flows, shear) and the parsec-scale (turbulence) is key to this problem. We find a strong dichotomy between the leading and trailing sides of the bar, in term of cloud fragmentation and in the age distribution of the young stars. After orbiting along the bar edge, these young structures slow down at the extremities of the bar, where orbital crowding increases the probability of cloud-cloud collision. We find that such events increase the Mach number of the cloud, leading to an enhanced star formation efficiency and finally the formation of massive stellar associations, in a fashion similar to galaxy-galaxy interactions. We highlight the role of bar dynamics in decoupling young stars from the clouds in which they form, and discuss the implications on the injection of feedback into the interstellar medium (ISM), in particular in the context of galaxy formation.
The Spatial Distribution of the Young Stellar Clusters in the Star-forming Galaxy NGC 628
K. Grasha, D. Calzetti, A. Adamo, H. Kim, B.~G. Elmegreen, D.~A. Gouliermis, A. Aloisi, S.~N. Bright, C. Christian, M. Cignoni, D.~A. Dale, C. Dobbs, D.~M. Elmegreen, M. Fumagalli, J.~S. Gallagher III, E.~K. Grebel, K.~E. Johnson, J.~C. Lee, M. Messa, L.~
Astrophysical Journal815, 93, 2015
We present a study of the spatial distribution of the stellar cluster populations in the star-forming galaxy NGC 628. Using Hubble Space Telescope broadband WFC3/UVIS UV and optical images from the Treasury Program LEGUS (Legacy ExtraGalactic UV Survey), we have identified 1392 potential young (<= 100 Myr) stellar clusters within the galaxy using a combination of visual inspection and automatic selection. We investigate the clustering of these young stellar clusters and quantify the strength and change of clustering strength with scale using the two-point correlation function. We also investigate how image boundary conditions and dust lanes affect the observed clustering. The distribution of the clusters is well fit by a broken power law with negative exponent alpha. We recover a weighted mean index of alpha ~ -0.8 for all spatial scales below the break at 3."3 (158 pc at a distance of 9.9 Mpc) and an index of alpha ~ -0.18 above 158 pc for the accumulation of all cluster types. The strength of the clustering increases with decreasing age and clusters older than 40 Myr lose their clustered structure very rapidly and tend to be randomly distributed in this galaxy, whereas the mass of the star cluster has little effect on the clustering strength. This is consistent with results from other studies that the morphological hierarchy in stellar clustering resembles the same hierarchy as the turbulent interstellar medium.
On the Star Formation Law for Spiral and Irregular Galaxies
Astrophysical Journal Letters814, L30, 2015
A dynamical model for star formation on a galactic scale is proposed in which the interstellar medium is constantly condensing to star-forming clouds on the dynamical time of the average midplane density, and the clouds are constantly being disrupted on the dynamical timescale appropriate for their higher density. In this model, the areal star formation rate scales with the 1.5 power of the total gas column density throughout the main regions of spiral galaxies, and with a steeper power, 2, in the far outer regions and in dwarf irregular galaxies because of the flaring disks. At the same time, there is a molecular star formation law that is linear in the main and outer parts of disks and in dIrrs because the duration of individual structures in the molecular phase is also the dynamical timescale, canceling the additional 0.5 power of surface density. The total gas consumption time scales directly with the midplane dynamical time, quenching star formation in the inner regions if there is no accretion, and sustaining star formation for ~100 Gyr or more in the outer regions with no qualitative change in gas stability or molecular cloud properties. The ULIRG track follows from high densities in galaxy collisions.
The Brightest Young Star Clusters in NGC 5253.
D. Calzetti, K.~E. Johnson, A. Adamo, J.~S. Gallagher III, J.~E. Andrews, L.~J. Smith, G.~C. Clayton, J.~C. Lee, E. Sabbi, L. Ubeda, H. Kim, J.~E. Ryon, D. Thilker, S.~N. Bright, E. Zackrisson, R.~C. Kennicutt, S.~E. de Mink, B.~C. Whitmore, A. Aloisi, R.
Astrophysical J.811, 75, 2015
The nearby dwarf starburst galaxy NGC 5253 hosts a number of young, massive star clusters, the two youngest of which are centrally concentrated and surrounded by thermal radio emission (the ``radio nebula''). To investigate the role of these clusters in the starburst energetics, we combine new and archival Hubble Space Telescope images of NGC 5253 with wavelength coverage from 1500 Angstrom to 1.9 microns in 13 filters. These include Halpha, Pbeta, and Palpha, and the imaging from the Hubble Treasury Program LEGUS (Legacy Extragalactic UV Survey). The extraordinarily well-sampled spectral energy distributions enable modeling with unprecedented accuracy the ages, masses, and extinctions of the nine optically brightest clusters (MV < -8.8) and the two young radio nebula clusters. The clusters have ages ~1-15 Myr and masses ~1x10^4-2.5x10^5 M&sun;. The clusters' spatial location and ages indicate that star formation has become more concentrated toward the radio nebula over the last ~15 Myr. The most massive cluster is in the radio nebula; with a mass ~2.5x10^5 M&sun; and an age ~1 Myr, it is 2-4 times less massive and younger than previously estimated. It is within a dust cloud with AV ~50 mag, and shows a clear near-IR excess, likely from hot dust. The second radio nebula cluster is also ~1 Myr old, confirming the extreme youth of the starburst region. These two clusters account for about half of the ionizing photon rate in the radio nebula, and will eventually supply about 2/3 of the mechanical energy in present-day shocks. Additional sources are required to supply the remaining ionizing radiation, and may include very massive stars.
Dense cloud cores revealed by CO in the low metallicity dwarf galaxy WLM
M. Rubio, B.~G. Elmegreen, D.~A. Hunter, E. Brinks, J.~R. Cortes, P. Cigan
Nature525, 218-221, 2015
Understanding stellar birth requires observations of the clouds in which they form. These clouds are dense and self-gravitating, and in all existing observations they are molecular, with H2 the dominant species and carbon monoxide (CO) the best available tracer. When the abundances of carbon and oxygen are low compared with that of hydrogen, and the opacity from dust is also low, as in primeval galaxies and local dwarf irregular galaxies, CO forms slowly and is easily destroyed, so it is difficult for it to accumulate inside dense clouds. Here we report interferometric observations of CO clouds in the local group dwarf irregular galaxy Wolf-Lundmark-Melotte (WLM), which has a metallicity that is 13 per cent of the solar value and 50 per cent lower than the previous CO detection threshold. The clouds are tiny compared to the surrounding atomic and H2 envelopes, but they have typical densities and column densities for CO clouds in the Milky Way. The normal CO density explains why star clusters forming in dwarf irregulars have similar densities to star clusters in giant spiral galaxies. The low cloud masses suggest that these clusters will also be low mass, unless some galaxy-scale compression occurs, such as an impact from a cosmic cloud or other galaxy. If the massive metal-poor globular clusters in the halo of the Milky Way formed in dwarf galaxies, as is commonly believed, then they were probably triggered by such an impact.
Localized Starbursts in Dwarf Galaxies Produced by the Impact of Low-metallicity Cosmic Gas Clouds
J. Sanchez Almeida, B.~G. Elmegreen, C. Munoz-Tunon, D.~M. Elmegreen, E. Perez-Montero, R. Amorin}, M.~E. Filho, Y. Ascasibar, P. Papaderos, J.~M. Vilchez
Astrophysical Journal Letters810, L15, 2015
Models of galaxy formation predict that gas accretion from the cosmic web is a primary driver of star formation over cosmic history. Except in very dense environments where galaxy mergers are also important, model galaxies feed from cold streams of gas from the web that penetrate their dark matter halos. Although these predictions are unambiguous, the observational support has been indirect so far. Here, we report spectroscopic evidence for this process in extremely metal-poor galaxies (XMPs) of the local universe, taking the form of localized starbursts associated with gas having low metallicity. Detailed abundance analyses based on Gran Telescopio Canarias optical spectra of 10 XMPs show that the galaxy hosts have metallicities around 60% solar, on average, while the large star-forming regions that dominate their integrated light have low metallicities of some 6% solar. Because gas mixes azimuthally in a rotation timescale (a few hundred Myr), the observed metallicity inhomogeneities are only possible if the metal-poor gas fell onto the disk recently. We analyze several possibilities for the origin of the metal-poor gas, favoring the metal-poor gas infall predicted by numerical models. If this interpretation is correct, XMPs trace the cosmic web gas in their surroundings, making them probes to examine its properties.
Shrinking Galaxy Disks with Fountain-driven Accretion from the Halo
B.~G. Elmegreen, C. Struck, D.~A. Hunter
Astrophysical Journal796, 110, 2014
Star formation in most galaxies requires cosmic gas accretion because the gas consumption time is short compared to the Hubble time. This accretion presumably comes from a combination of infalling satellite debris, cold flows, and condensation of hot halo gas at the cool disk interface, perhaps aided by a galactic fountain. In general, the accretion will have a different specific angular momentum than the part of the disk that receives it, even if the gas comes from the nearby halo. The gas disk then expands or shrinks over time. Here we show that condensation of halo gas at a rate proportional to the star formation rate in the fountain model will preserve an initial shape, such as an exponential, with a shrinking scale length, leaving behind a stellar disk with a slightly steeper profile of younger stars near the center. This process is slow for most galaxies, producing imperceptible radial speeds, and it may be dominated by other torques, but it could be important for blue compact dwarfs, which tend to have large, irregular gas reservoirs and steep blue profiles in their inner stellar disks.
A New Method to Estimate Local Pitch Angles in Spiral Galaxies: Application to Spiral Arms and Feathers in M81 and M51
I. Puerari, B.~G. Elmegreen, D.~L. Block
Astronomical Journal148, 133, 2014
We examine 8 mu m IRAC images of the grand design two-arm spiral galaxies M81 and M51 using a new method whereby pitch angles are locally determined as a function of scale and position, in contrast to traditional Fourier transform spectral analyses which fit to average pitch angles for whole galaxies. The new analysis is based on a correlation between pieces of a galaxy in circular windows of (ln R,theta ) space and logarithmic spirals with various pitch angles. The diameter of the windows is varied to study different scales. The result is a best-fit pitch angle to the spiral structure as a function of position and scale, or a distribution function of pitch angles as a function of scale for a given galactic region or area. We apply the method to determine the distribution of pitch angles in the arm and interarm regions of these two galaxies. In the arms, the method reproduces the known pitch angles for the main spirals on a large scale, but also shows higher pitch angles on smaller scales resulting from dust feathers. For the interarms, there is a broad distribution of pitch angles representing the continuation and evolution of the spiral arm feathers as the flow moves into the interarm regions. Our method shows a multiplicity of spiral structures on different scales, as expected from gas flow processes in a gravitating, turbulent and shearing interstellar medium. We also present results for M81 using classical 1D and 2D Fourier transforms, together with a new correlation method, which shows good agreement with conventional 2D Fourier transforms.
The H I Chronicles of LITTLE THINGS BCDs II: The Origin of IC 10's H I Structure
T. Ashley, B.~G. Elmegreen, M. Johnson, D.~L. Nidever, C.~E. Simpson, N.~R. Pokhrel
Astronomical Journal148, 130, 2014
In this paper we analyze Very Large Array (VLA) telescope and Green Bank Telescope (GBT) atomic hydrogen (H I) data for the LITTLE THINGS (Local Irregulars That Trace Luminosity Extremes, The H I Nearby Galaxy Survey; https://science.nrao.edu/science/surveys/littlethings) blue compact dwarf galaxy IC 10. The VLA data allow us to study the detailed H I kinematics and morphology of IC 10 at high resolution while the GBT data allow us to search the surrounding area at high sensitivity for tenuous H I. IC 10's H I appears highly disturbed in both the VLA and GBT H I maps with a kinematically distinct northern H I extension, a kinematically distinct southern plume, and several spurs in the VLA data that do not follow the general kinematics of the main disk. We discuss three possible origins of its H I structure and kinematics in detail: a current interaction with a nearby companion, an advanced merger, and accretion of intergalactic medium. We find that IC 10 is most likely an advanced merger or a galaxy undergoing accretion.
Spitzer/Infrared Array Camera near-infrared features in the outer parts of S^4G galaxies
S. Laine, J.~H. Knapen, J.-C. Munoz-Mateos, T. Kim, S. Comeron, M. Martig, B.~W. Holwerda, E. Athanassoula, A. Bosma, P.~H. Johansson, S. Erroz-Ferrer, D.~A. Gadotti, A.~G. de Paz, J. Hinz, J. Laine, E. Laurikainen, K. Menendez-Delmestre, T. M
Monthly Notices of the Royal Astronomical Society444, 3015-3039, 2014
We present a catalogue and images of visually detected features, such as asymmetries, extensions, warps, shells, tidal tails, polar rings, and obvious signs of mergers or interactions, in the faint outer regions (at and outside of R25) of nearby galaxies. This catalogue can be used in future quantitative studies that examine galaxy evolution due to internal and external factors. We are able to reliably detect outer region features down to a brightness level of 0.03 MJy sr-1 pixel-1 at 3.6 mum in the Spitzer Survey of Stellar Structure in Galaxies (S4G). We also tabulate companion galaxies. We find asymmetries in the outer isophotes in 22 pm 1 per cent of the sample. The asymmetry fraction does not correlate with galaxy classification as an interacting galaxy or merger remnant, or with the presence of companions. We also compare the detected features to similar features in galaxies taken from cosmological zoom re-simulations. The simulated images have a higher fraction (33 per cent) of outer disc asymmetries, which may be due to selection effects and an uncertain star formation threshold in the models. The asymmetries may have either an internal (e.g. lopsidedness due to dark halo asymmetry) or external origin.
Evidence for the concurrent growth of thick discs and central mass concentrations from S^4G imaging
S. Comeron, B.~G. Elmegreen, H. Salo, E. Laurikainen, B.~W. Holwerda, J.~H. Knapen
Astronomy and Astrophysics571, A58, 2014
We have produced 3.6 mum + 4.5 mum vertically integrated radial luminosity profiles of 69 edge-on galaxies from the Spitzer Survey of Stellar Structure in Galaxies (S4G). We decomposed the luminosity profiles into a disc and a central mass concentration (CMC). These fits, combined with thin/thick disc decompositions from our previous studies, allow us to estimate the masses of the CMCs, the thick discs, and the thin discs (MCMC, MT, and MT). We obtained atomic disc masses (Mg) from the literature. We then consider the CMC and the thick disc to be dynamically hot components and the thin disc and the gas disc to be dynamically cold components. We find that the ratio between the mass of the hot components and that of the cold components, (MCMC + MT)/(Mt + Mg), does not depend on the total galaxy mass as described by circular velocities (vc). We also find that the higher the vc, the more concentrated the hot component of a galaxy. We suggest that our results are compatible with having CMCs and thick discs built in a short and early high star forming intensity phase. These components were born thick because of the large scale height of the turbulent gas disc in which they originated. Our results indicate that the ratio between the star forming rate in the former phase and that of the formation of the thin disc is of the order of 10, but the value depends on the duration of the high star forming intensity phase.
Astrophysics: How tiny galaxies form stars
Nature514, 310-311, 2014
Observations of two faint galaxies with a low abundance of elements heavier than helium show that the galaxies have an efficiency of star formation less than one-tenth of that of the Milky Way and similar galaxies. See Letter p.335
Big Fish in Small Ponds: Massive Stars in the Low-mass Clusters of M83
J.~E. Andrews, D. Calzetti, R. Chandar, B.~G. Elmegreen, R.~C. Kennicutt, H. Kim, M.~R. Krumholz, J.~C. Lee, S. McElwee, R.~W. O'Connell, B. Whitmore
Astrophysical Journal793, 4, 2014
We have used multi-wavelength Hubble Space Telescope WFC3 data of the starbursting spiral galaxy M83 in order to measure variations in the upper end of the stellar initial mass function (uIMF) using the production rate of ionizing photons in unresolved clusters with ages <= 8 Myr. As in earlier papers on M51 and NGC 4214, the uIMF in M83 is consistent with a universal IMF, and stochastic sampling of the stellar populations in the <=10^3 Msun clusters are responsible for any deviations in this universality. The ensemble cluster population, as well as individual clusters, also imply that the most massive star in a cluster does not depend on the cluster mass. In fact, we have found that these small clusters seem to have an over-abundance of ionizing photons when compared to an expected universal or truncated IMF. This also suggests that the presence of massive stars in these clusters does not affect the star formation in a destructive way.
Star formation sustained by gas accretion
J. Sanchez Almeida, B.~G. Elmegreen, C. Munoz-Tunon, D.~M. Elmegreen
Astronomy and Astrophysics Reviews22, 71, 2014
Numerical simulations predict that metal-poor gas accretion from the cosmic web fuels the formation of disk galaxies. This paper discusses how cosmic gas accretion controls star formation, and summarizes the physical properties expected for the cosmic gas accreted by galaxies. The paper also collects observational evidence for gas accretion sustaining star formation. It reviews evidence inferred from neutral and ionized hydrogen, as well as from stars. A number of properties characterizing large samples of star-forming galaxies can be explained by metal-poor gas accretion, in particular, the relationship among stellar mass, metallicity, and star-formation rate (the so-called fundamental metallicity relationship). They are put forward and analyzed. Theory predicts gas accretion to be particularly important at high redshift, so indications based on distant objects are reviewed, including the global star-formation history of the universe, and the gas around galaxies as inferred from absorption features in the spectra of background sources.
What controls star formation in the central 500 pc of the Galaxy?
J.~M.~D. Kruijssen, S.~N. Longmore, B.~G. Elmegreen, N. Murray, J. Bally, L. Testi, R.~C. Kennicutt
Monthly Notices of the Royal Astronomical Society440, 3370-3391, 2014
The star formation rate (SFR) in the Central Molecular Zone (CMZ, i.e. the central 500 pc) of the Milky Way is lower by a factor of >=10 than expected for the substantial amount of dense gas it contains, which challenges current star formation theories. In this paper, we quantify which physical mechanisms could be responsible. On scales larger than the disc scaleheight, the low SFR is found to be consistent with episodic star formation due to secular instabilities or possibly variations of the gas inflow along the Galactic bar. The CMZ is marginally Toomre-stable when including gas and stars, but highly Toomre-stable when only accounting for the gas, indicating a low condensation rate of self-gravitating clouds. On small scales, we find that the SFR in the CMZ may be caused by an elevated critical density for star formation due to the high turbulent pressure. The existence of a universal density threshold for star formation is ruled out. The H I-H2 phase transition of hydrogen, the tidal field, a possible underproduction of massive stars due to a bottom-heavy initial mass function, magnetic fields, and cosmic ray or radiation pressure feedback also cannot individually explain the low SFR. We propose a self-consistent cycle of star formation in the CMZ, in which the effects of several different processes combine to inhibit star formation. The rate-limiting factor is the slow evolution of the gas towards collapse - once star formation is initiated it proceeds at a normal rate. The ubiquity of star formation inhibitors suggests that a lowered central SFR should be a common phenomenon in other galaxies. We discuss the implications for galactic-scale star formation and supermassive black hole growth, and relate our results to the star formation conditions in other extreme environments.
Hierarchical Star Formation in Nearby LEGUS Galaxies
D.~M. Elmegreen, B.~G. Elmegreen, A. Adamo, A. Aloisi, J. Andrews, F. Annibali, S.~N. Bright, D. Calzetti, M. Cignoni, A.~S. Evans, J.~S. Gallagher III, D.~A. Gouliermis, E.~K. Grebel, D.~A. Hunter, K. Johnson, H. Kim, J. Lee, E. Sabbi, L.~J. Smith, D. Thilker, M. Tosi, and L. Ubeda
Astrophysical Journal787, L15, 2014
Hierarchical structure in ultraviolet images of 12 late-type LEGUS galaxies is studied by determining the numbers and fluxes of nested regions as a function of size from ~1 to ~200 pc, and the number as a function of flux. Two starburst dwarfs, NGC 1705 and NGC 5253, have steeper number-size and flux-size distributions than the others, indicating high fractions of the projected areas filled with star formation. Nine subregions in seven galaxies have similarly steep number-size slopes, even when the whole galaxies have shallower slopes. The results suggest that hierarchically structured star-forming regions several hundred parsecs or larger represent common unit structures. Small galaxies dominated by only a few of these units tend to be starbursts. The self-similarity of young stellar structures down to parsec scales suggests that star clusters form in the densest parts of a turbulent medium that also forms loose stellar groupings on larger scales. The presence of super star clusters in two of our starburst dwarfs would follow from the observed structure if cloud and stellar subregions more readily coalesce when self-gravity in the unit cell contributes more to the total gravitational potential.
The Role of Turbulence in Star Formation Laws and Thresholds
K. Kraljic, F. Renaud, F. Bournaud, F. Combes, B. Elmegreen, E. Emsellem, R. Teyssier
Astrophysical Journal784, 112, 2014
The Schmidt-Kennicutt relation links the surface densities of gas to the star formation rate in galaxies. The physical origin of this relation, and in particular its break, i.e., the transition between an inefficient regime at low gas surface densities and a main regime at higher densities, remains debated. Here, we study the physical origin of the star formation relations and breaks in several low-redshift galaxies, from dwarf irregulars to massive spirals. We use numerical simulations representative of the Milky Way and the Large and Small Magellanic Clouds with parsec up to subparsec resolution, and which reproduce the observed star formation relations and the relative variations of the star formation thresholds. We analyze the role of interstellar turbulence, gas cooling, and geometry in drawing these relations at 100 pc scale. We suggest in particular that the existence of a break in the Schmidt-Kennicutt relation could be linked to the transition from subsonic to supersonic turbulence and is independent of self-shielding effects. With this transition being connected to the gas thermal properties and thus to the metallicity, the break is shifted toward high surface densities in metal-poor galaxies, as observed in dwarf galaxies. Our results suggest that together with the collapse of clouds under self-gravity, turbulence (injected at galactic scale) can induce the compression of gas and regulate star formation.
Metallicity Inhomogeneities in Local Star-forming Galaxies as a Sign of Recent Metal-poor Gas Accretion
J. S'anchez Almeida, A.~B. Morales-Luis, C. Munoz-Tunon, D.~M. Elmegreen, B.~G. Elmegreen, J. Mendez-Abreu
Astrophysical Journal783, 45, 2014
We measure the oxygen metallicity of the ionized gas along the major axis of seven dwarf star-forming galaxies. Two of them, SDSSJ1647+21 and SDSSJ2238+14, show sime0.5 dex metallicity decrements in inner regions with enhanced star formation activity. This behavior is similar to the metallicity drop observed in a number of local tadpole galaxies by Sanchez Almeida et al., and was interpreted as showing early stages of assembling in disk galaxies, with the star formation sustained by external metal-poor gas accretion. The agreement with tadpoles has several implications. (1) It proves that galaxies other than the local tadpoles present the same unusual metallicity pattern. (2) Our metallicity inhomogeneities were inferred using the direct method, thus discarding systematic errors usually attributed to other methods. (3) Taken together with the tadpole data, our findings suggest a threshold around one-tenth the solar value for the metallicity drops to show up. Although galaxies with clear metallicity drops are rare, the physical mechanism responsible for them may sustain a significant part of the star formation activity in the local universe. We argue that the star formation dependence of the mass-metallicity relationship, as well as other general properties followed by most local disk galaxies, is naturally interpreted as side effects of pristine gas infall. Alternatives to the metal-poor gas accretion are examined as well.
A Density Dependence for Protostellar Luminosity in Class I Sources: Collaborative Accretion
B.~G. Elmegreen, R. Hurst, X. Koenig
Astrophysical Journal782, L1, 2014
Class I protostars in three high-mass star-forming regions are found to have correlations among the local projected density of other Class I protostars, the summed flux from these other protostars, and the protostellar luminosity in the WISE 22 mum band. Brighter Class I sources form in higher-density and higher-flux regions, while low luminosity sources form anywhere. These correlations depend slightly on the number of neighbors considered (from 2 to 20) and could include a size-of-sample effect from the initial mass function (i.e., larger numbers include rarer and more massive stars). Luminosities seem to vary by neighborhood with nearby protostars having values proportional to each other and higher density regions having higher values. If Class I luminosity is partially related to the accretion rate, then this luminosity correlation is consistent with the competitive accretion model, although it is more collaborative than competitive. The correlation is also consistent with primordial mass segregation and could explain why the stellar initial mass function resembles the dense core mass function even when cores form multiple stars.
Unveiling the Structure of Barred Galaxies at 3.6 micron with the Spitzer Survey of Stellar Structure in Galaxies (S^4G). I. Disk Breaks
T. Kim, D.~A. Gadotti, K. Sheth, E. Athanassoula, A. Bosma, M.~G. Lee, B.~F. Madore, B. Elmegreen, J.~H. Knapen, D. Zaritsky, L.~C. Ho, S. Comeron, B. Holwerda, J.~L. Hinz, J.-C. Munoz-Mateos, M. Cisternas, S. Erroz-Ferrer, R. Buta, E. Laurikainen
Astrophysical Journal782, 64, 2014
We have performed two-dimensional multicomponent decomposition of 144 local barred spiral galaxies using 3.6 mum images from the Spitzer Survey of Stellar Structure in Galaxies. Our model fit includes up to four components (bulge, disk, bar, and a point source) and, most importantly, takes into account disk breaks. We find that ignoring the disk break and using a single disk scale length in the model fit for Type II (down-bending) disk galaxies can lead to differences of 40% in the disk scale length, 10% in bulge-to-total luminosity ratio (B/T), and 25% in bar-to-total luminosity ratios. We find that for galaxies with B/T >= 0.1, the break radius to bar radius, r br/R bar, varies between 1 and 3, but as a function of B/T the ratio remains roughly constant. This suggests that in bulge-dominated galaxies the disk break is likely related to the outer Lindblad resonance of the bar and thus moves outward as the bar grows. For galaxies with small bulges, B/T < 0.1, r br/R bar spans a wide range from 1 to 6. This suggests that the mechanism that produces the break in these galaxies may be different from that in galaxies with more massive bulges. Consistent with previous studies, we conclude that disk breaks in galaxies with small bulges may originate from bar resonances that may be also coupled with the spiral arms, or be related to star formation thresholds.
The Onset of Spiral Structure in the Universe
D.~M. Elmegreen, B.~G. Elmegreen
Astrophysical Journal781, 11, 2014
The onset of spiral structure in galaxies appears to occur between redshifts 1.4 and 1.8 when disks have developed a cool stellar component, rotation dominates over turbulent motions in the gas, and massive clumps become less frequent. During the transition from clumpy to spiral disks, two unusual types of spirals are found in the Hubble Ultra Deep Field that are massive, clumpy, and irregular like their predecessor clumpy disks, yet spiral-like or sheared like their descendants. One type is "woolly" with massive clumpy arms all over the disk and is brighter than other disk galaxies at the same redshift, while another type has irregular multiple arms with high pitch angles, star formation knots, and no inner symmetry like today's multiple-arm galaxies. The common types of spirals seen locally are also present in a redshift range around z ~ 1, namely grand design with two symmetric arms, multiple arm with symmetry in the inner parts and several long, thin arms in the outer parts, and flocculent, with short, irregular, and patchy arms that are mostly from star formation. Normal multiple-arm galaxies are found only closer than z ~ 0.6 in the Ultra Deep Field. Grand design galaxies extend furthest to z ~ 1.8, presumably because interactions can drive a two-arm spiral in a disk that would otherwise have a more irregular structure. The difference between these types is understandable in terms of the usual stability parameters for gas and stars, and the ratio of the velocity dispersion to rotation speed.
The Long Lives of Giant Clumps and the Birth of Outflows in Gas-rich Galaxies at High Redshift
F. Bournaud, V. Perret, F. Renaud, A. Dekel, B.~G. Elmegreen, D.~M. Elmegreen, R. Teyssier, P. Amram, E. Daddi, P.-A. Duc, D. Elbaz, B. Epinat, J.~M. Gabor, S. Juneau, K. Kraljic, E. Le Floch'
Astrophysical Journal780, 57, 2014
Star-forming disk galaxies at high redshift are often subject to violent disk instability, characterized by giant clumps whose fate is yet to be understood. The main question is whether the clumps disrupt within their dynamical timescale (<=50 Myr), like the molecular clouds in today's galaxies, or whether they survive stellar feedback for more than a disk orbital time (~300 Myr) in which case they can migrate inward and help building the central bulge. We present 3.5-7 pc resolution adaptive mesh refinement simulations of high-redshift disks including photoionization, radiation pressure, and supernovae feedback. Our modeling of radiation pressure determines the mass loading and initial velocity of winds from basic physical principles. We find that the giant clumps produce steady outflow rates comparable to and sometimes somewhat larger than their star formation rate, with velocities largely sufficient to escape the galaxy. The clumps also lose mass, especially old stars, by tidal stripping, and the stellar populations contained in the clumps hence remain relatively young (<=200 Myr), as observed. The clumps survive gaseous outflows and stellar loss, because they are wandering in gas-rich turbulent disks from which they can reaccrete gas at high rates compensating for outflows and tidal stripping, overall keeping realistic and self-regulated gaseous and stellar masses. The outflow and accretion rates have specific timescales of a few 108 yr, as opposed to rapid and repeated dispersion and reformation of clumps. Our simulations produce gaseous outflows with velocities, densities, and mass loading consistent with observations, and at the same time suggest that the giant clumps survive for hundreds of Myr and complete their migration to the center of high-redshift galaxies. These long-lived clumps are gas-dominated and contain a moderate mass fraction of stars; they drive inside-out disk evolution, thickening, spheroid growth, and fueling of the central black hole.
Embedded Star Formation in S^4G Galaxy Dust Lanes
D.~M. Elmegreen, B.~G. Elmegreen, S. Erroz-Ferrer, J.~H. Knapen, Y. Teich, M. Popinchalk, E. Athanassoula, A. Bosma, S. Comeron, Y.~N. Efremov, D.~A. Gadotti, A. Gil de Paz, J.~L. Hinz, L.~C. Ho, B. Holwerda, T. Kim, J. Laine, E. Laurikainen, Ka. Menendez-Delmestre, T. Mizusawa, J.-C. Munoz-Mateos, M.W. Regan, H. Salo, M. Seibert, and K. Sheth
Astrophysical Journal780, 32, 2014
Star-forming regions that are visible at 3.6 mum and Halpha but not in the u, g, r, i, z bands of the Sloan Digital Sky Survey are measured in five nearby spiral galaxies to find extinctions averaging ~3.8 mag and stellar masses averaging ~5 x 10^4 Msun. These regions are apparently young star complexes embedded in dark filamentary shock fronts connected with spiral arms. The associated cloud masses are ~107 M &sun;. The conditions required to make such complexes are explored, including gravitational instabilities in spiral-shocked gas and compression of incident clouds. We find that instabilities are too slow for a complete collapse of the observed spiral filaments, but they could lead to star formation in the denser parts. Compression of incident clouds can produce a faster collapse but has difficulty explaining the semi-regular spacing of some regions along the arms. If gravitational instabilities are involved, then the condensations have the local Jeans mass. Also in this case, the near-simultaneous appearance of equally spaced complexes suggests that the dust lanes, and perhaps the arms too, are relatively young.