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Register a new userDisk Galaxy Formation in a LambdaCDM Universe
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B. E. Robertson
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We describe hydrodynamical simulations of galaxy formation in a Lambda cold dark matter (CDM) cosmology performed using a subresolution model for star formation and feedback in a multiphase interstellar medium (ISM). In particular, we demonstrate the formation of a well-resolved disk galaxy. The surface brightness profile of the galaxy is exponential, with a B-band central surface brightness of 21.0 mag arcsec^-2 and a scale-length of R_d = 2.0 h^-1 kpc. We find no evidence for a significant bulge component. The simulated galaxy falls within the I-band Tully-Fisher relation, with an absolute magnitude of I = -21.2 and a peak stellar rotation velocity of V_rot=121.3 km s^-1. While the total specific angular momentum of the stars in the galaxy agrees with observations, the angular momentum in the inner regions appears to be low by a factor of ~2. The star formation rate of the galaxy peaks at ~7 M_sun yr^-1 between redshifts z=2-4, with the mean stellar age decreasing from ~10 Gyrs in the outer regions of the disk to ~7.5 Gyrs in the center, indicating that the disk did not simply form inside-out. The stars exhibit a metallicity gradient from 0.7 Z_sun at the edge of the disk to 1.3 Z_sun in the center. Using a suite of idealized galaxy formation simulations with different models for the ISM, we show that the effective pressure support provided by star formation and feedback in our multiphase model is instrumental in allowing the formation of large, stable disk galaxies. If ISM gas is instead modeled with an isothermal equation of state, or if star formation is suppressed entirely, growing gaseous disks quickly violate the Toomre stability criterion and undergo catastrophic fragmentation.
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We explore galaxy properties and their link with environment and clustering using a population of ~1000 galaxies formed in a high resolution hydrodynamic simulation of the Lambda CDM cosmology. At the redshift we concentrate on, z=1, the spatial resolution is 1.4 proper kpc/h and Milky-way sized disk galaxies contain ~10^5 particles within their virial radii. We include supermassive black hole accretion and feedback as well as a multiphase model for star formation. We find that a number of familiar qualitative relationships hold approximately between galaxy properties, for example, galaxies lie between two broad extremes of type, where ``late types tend to be smaller in size, have lower circular velocities, younger stars, higher star formation rates, larger disk to bulge ratios and lower Sersic indices than ``early types. As in previous studies the stellar component of disk galaxies is not as rotationally supported as in observations. Bulges contain too much of the stellar mass, although disks do have scale lengths compatible with observations. The addition of black hole physics to the simulations does not appear to have an impact on the angular momentum results, nor do we find that it is affected in an identical simulation with significantly lower mass resolution. Despite this, we can profitably use the rank order of either disk to total ratio, Sersic index, or galaxy age to separate galaxies into morphological classes and examine the density-morphology relation and morphology dependence of clustering. We find that while at redshift z=0, the well known preponderance of early types in dense environments is seen, at z=1 the density-morphology relation becomes flatter and late type galaxies are even seen to have a higher clustering amplitude than early types (abridged).
We combine a semi-analytical model of galaxy formation with a very large simulation which follows the growth of large scale structure in a LambdaCDM universe to predict the clustering of Ly-alpha emitters. We find that the clustering strength of Ly-alpha emitters has only a weak dependence on Ly-alpha luminosity but a strong dependence on redshift. With increasing redshift, Ly-alpha emitters trace progressively rarer, higher density regions of the universe. Due to the large volume of the simulation, over 100 times bigger than any previously used for this application, we can construct mock catalogues of Ly-alpha emitters and study the sample variance of current and forthcoming surveys. We find that the number and clustering of Ly-alpha emitters in our mock catalogues are in agreement with measurements from current surveys, but that there is a considerable scatter in these quantities. We argue that a proposed survey of emitters at z=8.8 should be extended significantly in solid angle to allow a robust measurement of Ly-alpha emitter clustering.
At redshift z = 2, when the Universe was just three billion years old, half of the most massive galaxies were extremely compact and had already exhausted their fuel for star formation(1-4). It is believed that they were formed in intense nuclear starbursts and that they ultimately grew into the most massive local elliptical galaxies seen today, through mergers with minor companions(5,6), but validating this picture requires higher-resolution observations of their centres than is currently possible. Magnification from gravitational lensing offers an opportunity to resolve the inner regions of galaxies(7). Here we report an analysis of the stellar populations and kinematics of a lensed z = 2.1478 compact galaxy, which surprisingly turns out to be a fast-spinning, rotationally supported disk galaxy. Its stars must have formed in a disk, rather than in a merger-driven nuclear starburst(8). The galaxy was probably fed by streams of cold gas, which were able to penetrate the hot halo gas until they were cut off by shock heating from the dark matter halo(9). This result confirms previous indirect indications(10-13) that the first galaxies to cease star formation must have gone through major changes not just in their structure, but also in their kinematics, to evolve into present-day elliptical galaxies.
We report new observations of the galaxy UGC8802 obtained through GASS, the GALEX Arecibo SDSS Survey, which show this galaxy to be in a remarkable evolutionary state. UGC8802 (GASS35981) is a disk galaxy with stellar mass M*=2x10^10 Msolar which appears to contain an additional 2.1x10^10 Msolar of HI gas. New millimeter observations with the IRAM 30m telescope indicate a molecular gas mass only a tenth this large. Using deep long-slit spectroscopy, we examine the spatially resolved star formation rate and metallicity profiles of GASS35981 for clues to its history. We find that the star formation surface density in this galaxy is low (Sigma_SFR=0.003 Msolar/yr/kpc^2) and that the star formation is spread remarkably evenly across the galaxy. The low molecular gas masses measured in our three IRAM pointings are largely consistent with the total star formation measured within the same apertures. Our MMT long-slit spectrum reveals a sharp drop in metallicity in the outer disk of GASS35981. The ratio of current star formation rate to existing stellar mass surface density in the outer disk is extremely high, implying that all the stars must have formed within the past ~1Gyr. At current star formation rates, however, GASS35981 will not consume its HI reservoir for another 5-7 Gyr. Despite its exceptionally large gas fraction for a galaxy this massive, GASS35981 has a regular rotation curve and exhibits no sign of a recent interaction or merger. We speculate that GASS35981 may have acquired its gas directly from the inter-galactic medium, and that it and other similar galaxies identified in the GASS survey may provide rare local glimpses of gas accretion processes that were more common during the prime epoch of disk galaxy formation at z~1.
[Abridged] Typical disc galaxies forming in a LambdaCDM cosmology encounter a violent environment, where they often experience mergers with massive satellites. The fact that disc galaxies are ubiquitous in the local Universe suggests that a quiescent history is not necessary for their formation. Modern cosmological simulations can now obtain relatively realistic populations of disc galaxies, but it still remains to be clarified how discs manage to survive massive mergers. Here we use a suite of high-resolution hydrodynamical simulations set in a LambdaCDM cosmology to elucidate the fate of discs encountering massive mergers. We extract a sample of approximately 100 disc galaxies and follow the changes in their post-merger morphologies, as tracked by their disc-to-total ratios (D/T). We also examine the relations between their present-day morphology, assembly history and gas fractions. We find that approximately half of present-day disc galaxies underwent at least one merger with a satellite of total mass exceeding the host systems stellar mass, a third had mergers with satellites of mass exceeding 3 times the hosts stellar mass, and approximately one-sixth had mergers with satellites of mass exceeding 10 times of the hosts stellar mass. These mergers lead to a sharp, but often temporary, decrease in the D/T of the hosts, implying that discs are usually disrupted but then quickly re-grow. To do so, high cold gas fractions are required post-merger, as well as a relatively quiescent recent history (over a few Gyrs before z=0). Our results show that discs can form via diverse merger pathways and that quiescent histories are not the dominant mode of disc formation.
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