Short-lived star-forming giant clumps in cosmological simulations of z~2 disks

Many observed massive star-forming z\approx2 galaxies are large disks that exhibit irregular morphologies, with \sim1kpc, \sim10^(8-10)Msun clumps. We present the largest sample to date of high-resolution cosmological SPH simulations that zoom-in on the formation of individual M*\sim10^(10.5)Msun galaxies in \sim10^(12)Msun halos at z\approx2. Our code includes strong stellar feedback parameterized as momentum-driven galactic winds. This model reproduces many characteristic features of this observed class of galaxies, such as their clumpy morphologies, smooth and monotonic velocity gradients, high gas fractions (f_g\sim50%) and high specific star-formation rates (\gtrsim1Gyr^(-1)). In accord with recent models, giant clumps (Mclump\sim(5x10^8-10^9)Msun) form in-situ via gravitational instabilities. However, the galactic winds are critical for their subsequent evolution. The giant clumps we obtain are short-lived and are disrupted by wind-driven mass loss. They do not virialise or migrate to the galaxy centers as suggested in recent work neglecting strong winds. By phenomenologically implementing the winds that are observed from high-redshift galaxies and in particular from individual clumps, our simulations reproduce well new observational constraints on clump kinematics and clump ages. In particular, the observation that older clumps appear closer to their galaxy centers is reproduced in our simulations, as a result of inside-out formation of the disks rather than inward clump migration.Comment: 11 pages, 6 figures, 1 table. Accepted for publication in the Astrophysical Journa

Star-Forming Galaxies at Cosmic Noon

Ever deeper and wider lookback surveys have led to a fairly robust outline of the cosmic star formation history, which culminated around z~2 -- a period often nicknamed "cosmic noon." Our knowledge about star-forming galaxies at these epochs has dramatically advanced from increasingly complete population censuses and detailed views of individual galaxies. We highlight some of the key observational insights that influenced our current understanding of galaxy evolution in the equilibrium growth picture: ∙ scaling relations between galaxy properties are fairly well established among massive galaxies at least out to z~2, pointing to regulating mechanisms already acting on galaxy growth; ∙ resolved views reveal that gravitational instabilities and efficient secular processes within the gas- and baryon-rich galaxies at z~2 play an important role in the early build-up of galactic structure; ∙ ever more sensitive observations of kinematics at z~2 are probing the baryon and dark matter budget on galactic scales and the links between star-forming galaxies and their likely descendants; ∙ towards higher masses, massive bulges, dense cores, and powerful AGN and AGN-driven outflows are more prevalent and likely play a role in quenching star formation. We outline emerging questions and exciting prospects for the next decade with upcoming instrumentation, including the James Webb Space Telescope and the next generation of Extremely Large Telescopes

Resolved views on early galaxy evolution

Resolved observations of star-forming galaxies at cosmic noon with the Hubble Space Telescope and large ground-based facilities provide a view on the spatial distribution of stars, gas and dust, and probe gaseous motions revealing the central gravitational potential and local feedback processes at play. In this paper, we review recent insights gained from such observations, with an emphasis on results obtained through optical/near-infrared imaging and imaging spectroscopy. Their context and implications are documented more fully in a forthcoming review article by Förster Schreiber & Wuyts (in prep)

Measurement of the Mass and Stellar Population Distribution in M82 with the LBT

We present a K-band spectroscopic study of the stellar and gas kinematics, mass distribution, and stellar populations of the archetypical starburst galaxy M82. Our results are based on a single spectrum at a position angle of 67.5 degrees through the K-band nucleus. We used the CO stellar absorption band head at 2.29 {\mu}m (CO_2.29) to measure the rotation curve out to nearly 4 kpc radius on both the eastern and western sides of the galaxy. Our data show that the rotation curve is flat from 1 - 4 kpc. This stands in sharp contrast to some previous studies, which have interpreted H I and CO emission-line position-velocity diagrams as evidence for a declining rotation curve. The kinematics of the Br\gamma, H_2, and He I emission lines are consistent with, although characterized by slightly higher velocities than, the stellar kinematics. We derived M82's mass distribution from our stellar kinematic measurements and estimate its total dynamical mass is ~10^10 Msun. We measured the equivalent width of CO_2.29 (W_2.29) as a function of distance from the center of the galaxy to investigate the spatial extent of the red supergiant (RSG) population. The variation in W_2.29 with radius clearly shows that RSGs dominate the light inside 500 pc radius. M82's superwind is likely launched from this region, where we estimate the enclosed mass is <= 2 x 10^9 Msun.Comment: 6 pages, 6 figures. Submitted to ApJ. For a brief video explaining the key result of this paper, see http://www.youtube.com/user/OSUAstronom

Star formation in the CDFS: observations confront simulations

We investigate the star formation history of the universe using FIREWORKS, a multiwavelength survey of the CDFS. We study the evolution of the specific star formation rate (sSFR) with redshift in different mass bins from z = 0 to z ~ 3. We find that the sSFR increases with redshift for all masses. The logarithmic increase of the sSFR with redshift is nearly independent of mass, but this cannot yet be verified at the lowest-mass bins at z > 0.8, due to incompleteness. We convert the sSFRs to a dimensionless growth rate to facilitate a comparison with a semi-analytic galaxy formation model that was implemented on the Millennium Simulation. The model predicts that the growth rates and sSFRs increase similarly with redshift for all masses, consistent with the observations. However, we find that for all masses, the inferred observed growth rates increase more rapidly with redshift than the model predictions. We discuss several possible causes for this discrepancy, ranging from field-to-field variance, conversions to SFR, and shape of the IMF. We find that none of these can solve the discrepancy completely. We conclude that the models need to be adapted to produce the steep increase in growth rate between redshift z=0 and z=1.Comment: 7 pages, 5 figure

Mid-IR FORCAST/SOFIA Observations of M82

We present 75"x75" size maps of M82 at 6.4 micron, 6.6 micron, 7.7 micron, 31.5 micron, and 37.1 micron with a resolution of ~4" that we have obtained with the mid-IR camera FORCAST on SOFIA. We find strong emission from the inner 60" (~1kpc) along the major axis, with the main peak 5" west-southwest of the nucleus and a secondary peak 4" east-northeast of the nucleus. The detailed morphology of the emission differs among the bands, which is likely due to different dust components dominating the continuum emission at short mid-IR wavelengths and long mid-IR wavelengths. We include Spitzer-IRS and Herschel/PACS 70 micron data to fit spectral energy distribution templates at both emission peaks. The best fitting templates have extinctions of A_V = 18 and A_V = 9 toward the main and secondary emission peak and we estimated a color temperature of 68 K at both peaks from the 31 micron and 37 micron measurement. At the emission peaks the estimated dust masses are on the order of 10^{4} M_sun.Comment: ApJL in press (ApJ, 749, L19), article also available at at http://stacks.iop.org/2041-8205/749/L1

Spatially Resolved Star Formation History Along the Disk of M82 Using Multi-Band Photometric Data

We present the results on the star formation history and extinction in the disk of M82 over spatial scales of 10" (~180 pc). Multi-band photometric data covering from the far ultraviolet to the near infrared bands were fitted to a grid of synthetic spectral energy distributions. We obtained distribution functions of age and extinction for each of the 117 apertures analyzed, taking into account observational errors through Monte-Carlo simulations. These distribution functions were fitted with gaussian functions to obtain the mean ages and extinctions along with errors on them. The analyzed zones include the high surface brightness complexes defined by O'Connell & Mangano (1978). We found that these complexes share the same star formation history and extinction as the field stellar populations in the disk. There is an indication that the stellar populations are marginally older at the outer disk (450 Myr at ~3 kpc) as compared to the inner disk (100 Myr at 0.5 kpc). For the nuclear regions (radius less than 500 pc), we obtained an age of less than 10 Myr. The results obtained in this work are consistent with the idea that the 0.5-3 kpc part of the disk of M82 formed around 90% of the stellar mass in a star-forming episode that started around 450 Myr ago lasting for about 350 Myr. We found that field stars are the major contributors to the flux over the spatial scales analyzed in this study, with stellar cluster contribution being 7% in the nucleus and 0.7% in the disk.Comment: 19 pages, 14 figures. Accepted for publication in The Astrophysical Journa

The Gemini spectral library of near-IR late type stellar templates and its application for velocity dispersion measurements

We present a spectroscopic library of late spectral type stellar templates in the near-IR range 2.15-2.42microns, at R=5300-5900 resolution, oriented to support stellar kinematics studies in external galaxies, such as the direct determination of the masses of supermassive black-holes in nearby active (or non-active) galaxies. The combination of high spectral resolution and state-of-the-art instrumentation available in 8-m class telescopes has made the analysis of circumnuclear stellar kinematics using the near-IR CO band heads one of the most used techniques for such studies, and this library aims to provide the supporting datasets required by the higher spectral resolution and larger spectral coverage currently achieved with modern near-IR spectrographs. Examples of the application for kinematical analysis are given for data obtained with two Gemini instruments, but the templates can be easily adjusted for use with other near-IR spectrographs at similar or lower resolution. The example datasets are also used to revisit the "template mismatch" effect and the dependence of the velocity dispersion values obtained from the fitting process with the characteristics of the stellar templates. The library is available in electronic form from the Gemini web pages (link above).Comment: To appear in the ApJ Supplement Series, December 2009. AASTex, 25 pages, 17 figures. The library spectra are available in standard FITS format from the Gemini Observatory webpage at http://www.gemini.edu/sciops/instruments/nearir-resources/?q=node/1016

Spatially resolved Halpha maps and sizes of 57 strongly star-forming galaxies at z~1 from 3D-HST: evidence for rapid inside-out assembly of disk galaxies

We investigate the build-up of galaxies at z~1 using maps of Halpha and stellar continuum emission for a sample of 57 galaxies with rest-frame Halpha equivalent widths >100 Angstroms in the 3D-HST grism survey. We find that the Halpha emission broadly follows the rest-frame R-band light but that it is typically somewhat more extended and clumpy. We quantify the spatial distribution with the half-light radius. The median Halpha effective radius r_e(Halpha) is 4.2+-0.1 kpc but the sizes span a large range, from compact objects with r_e(Halpha) ~ 1.0 kpc to extended disks with r_e(Halpha) ~ 15 kpc. Comparing Halpha sizes to continuum sizes, we find =1.3+-0.1 for the full sample. That is, star formation, as traced by Halpha, typically occurs out to larger radii than the rest-frame R-band stellar continuum; galaxies are growing their radii and building up from the inside out. This effect appears to be somewhat more pronounced for the largest galaxies. Using the measured Halpha sizes, we derive star formation rate surface densities. We find that they range from ~0.05 Msun yr^{-1} kpc^{-2} for the largest galaxies to ~5 Msun yr^{-1} kpc^{-2} for the smallest galaxies, implying a large range in physical conditions in rapidly star-forming z~1 galaxies. Finally, we infer that all galaxies in the sample have very high gas mass fractions and stellar mass doubling times < 500 Myr. Although other explanations are also possible, a straightforward interpretation is that we are simultaneously witnessing the rapid formation of compact bulges and large disks at z~1.Comment: ApJ Letters, in press. The public 3D-HST website is http://3dhst.research.yale.edu/Home.htm