Kyle R. Stewart

MERGERS AND BULGE FORMATION IN ΛCDM: WHICH MERGERS MATTER?

We use a suite of semi-empirical models to predict the galaxy-galaxy merger rate and relative contributions to bulge growth as a function of mass (both halo and stellar), redshift, and mass ratio. The models use empirical constraints on the halo occupation distribution, evolved forward in time, to robustly identify where and when galaxy mergers occur. Together with the results of high-resolution merger simulations, this allows us to quantify the relative contributions of mergers with different properties (e.g., mass ratios, gas fractions, redshifts) to the bulge population. We compare with observational constraints, and find good agreement. We also provide useful fitting functions and make public a code to reproduce the predicted merger rates and contributions to bulge mass growth. We identify several robust conclusions. (1) Major mergers dominate the formation and assembly of ~L * bulges and the total spheroid mass density, but minor mergers contribute a non-negligible ~30%. (2) This is mass dependent: bulge formation and assembly is dominated by more minor mergers in lower-mass systems. In higher-mass systems, most bulges originally form in major mergers near ~L *, but assemble in increasingly minor mergers. (3) The minor/major contribution is also morphology dependent: higher B/T systems preferentially form in more major mergers, with B/T roughly tracing the mass ratio of the largest recent merger; lower B/T systems preferentially form in situ from minor mergers. (4) Low-mass galaxies, being gas-rich, require more mergers to reach the same B/T as high-mass systems. Gas-richness dramatically suppresses the absolute efficiency of bulge formation, but does not strongly influence the relative contribution of major versus minor mergers. (5) Absolute merger rates at fixed mass ratio increase with galaxy mass. (6) Predicted merger rates agree well with those observed in pair and morphology-selected samples, but there is evidence that some morphology-selected samples include contamination from minor mergers. (7) Predicted rates also agree with the integrated growth in bulge mass density with cosmic time, but with a factor ~2 uncertainty in both—up to half the bulge mass density could come from non-merger processes. We systematically vary the model assumptions, totaling ~103 model permutations, and quantify the resulting uncertainties. Our conclusions regarding the importance of different mergers for bulge formation are very robust to these changes. The absolute predicted merger rates are systematically uncertain at the factor ~2 level; uncertainties grow at the lowest masses and high redshifts.

Merger Histories of Galaxy Halos and Implications for Disk Survival

We study the merger histories of galaxy dark matter halos using a high-resolution ΛCDM N-body simulation. Our merger trees follow ~17,000 halos with masses M0 = 1011–1013 h−1 M☉ at z = 0 and track accretion events involving objects as small as m 1010 h−1 M☉. We find that mass assembly is remarkably self-similar in m/M0 and dominated by mergers that are ~10% of the final halo mass. While very large mergers, m 0.4M0, are quite rare, sizeable accretion events, m ~ 0.1M0, are common. Over the last ~10 Gyr, an overwhelming majority (~95%) of Milky Way-sized halos with M0 = 1012 h−1 M☉ have accreted at least one object with greater total mass than the Milky Way disk (m > 5 × 1010 h−1 M☉), and approximately 70% have accreted an object with more than twice that mass (m > 1011 h−1 M☉). Our results raise serious concerns about the survival of thin-disk-dominated galaxies within the current paradigm for galaxy formation in a ΛCDM universe. In order to achieve a ~70% disk-dominated fraction in Milky Way-sized ΛCDM halos, mergers involving m 2 × 1011 h−1 M☉ objects must not destroy disks. Considering that most thick disks and bulges contain old stellar populations, the situation is even more restrictive: these mergers must not heat disks or drive gas into their centers to create young bulges.

GAS-RICH MERGERS IN LCDM: DISK SURVIVABILITY AND THE BARYONIC ASSEMBLY OF GALAXIES

We use N-body simulations and observationally normalized relations between dark matter halo mass, stellar mass, and cold gas mass to derive robust expectations about the baryonic content of major mergers out to redshift z ~ 2. First, we find that the majority of major mergers (m/M>0.3) experienced by the Milky Way size dark matter halos should have been gas-rich, and that gas-rich mergers are increasingly common at high redshifts. Though the frequency of major mergers into galaxy halos in our simulations greatly exceeds the observed early-type galaxy fraction, the frequency of gas-poor major mergers is consistent with the observed fraction of bulge-dominated galaxies across the halo mass range M DM ~ 1011-1013 M ☉. These results lend support to the conjecture that mergers with high-baryonic gas fractions play an important role in building and/or preserving disk galaxies in the universe. Second, we find that there is a transition mass below which a galaxys past major mergers were primarily gas-rich and above which they were gas-poor. The associated stellar mass scale corresponds closely to that marking the observed bimodal division between blue, star-forming, disk-dominated systems and red, bulge-dominated systems with old populations. Finally, we find that the overall fraction of a galaxys cold baryons deposited directly via major mergers is significant. Approximately ~20%-30% of the cold baryonic material in M star ~ 1010.5 M ☉ (M DM ~ 1012 M ☉) galaxies is accreted as cold gas or stars via major mergers since z = 2, with most of this accretion in the form of cold gas. For more massive galaxies with M star ~ 1011 M ☉ (M DM ~ 1013 M ☉), the fraction of baryons amassed in mergers since z = 2 is even higher, ~40%, but most of these accreted baryons are delivered directly in the form of stars. This baryonic mass deposition is almost unavoidable, and provides a limit on the fraction of a galaxys cold baryons that can originate in cold flows or from hot halo cooling.

ORBITING CIRCUMGALACTIC GAS AS A SIGNATURE OF COSMOLOGICAL ACCRETION

We use cosmological smoothed particle hydrodynamic simulations to study the kinematic signatures of cool gas accretion onto a pair of well-resolved galaxy halos. We find that cold-flow streams and gas-rich mergers produce a circumgalactic component of cool gas that generally orbits with high angular momentum about the galaxy halo before falling in to build the disk. This signature of cosmological accretion should be observable using background-object absorption-line studies as features that are offset from the galaxys systemic velocity by ~100 km s-1. In most cases, the accreted gas co-rotates with the central disk in the form of a warped, extended cold flow disk, such that the observed velocity offset will be in the same direction as galaxy rotation, appearing in sight lines that avoid the galactic poles. This prediction provides a means to observationally distinguish accreted gas from outflow gas: the accreted gas will show large one-sided velocity offsets in absorption-line studies while radial/bi-conical outflows will not (except possibly in special polar projections). Such a signature of rotation has already been seen in studies of intermediate-redshift galaxy-absorber pairs, and we suggest that these observations may be among the first to provide indirect observational evidence for cold accretion onto galactic halos. This cold-mode halo gas typically has ~3-5 times more specific angular momentum than the dark matter. The associated cold-mode disk configurations are likely related to extended H I/extended UV disks that are seen around galaxies in the local universe. The fraction of galaxies with extended cold flow disks and associated offset absorption-line gas should decrease around bright galaxies at low redshift as cold-mode accretion dies out.

Galaxy Mergers and Dark Matter Halo Mergers in LCDM: Mass, Redshift, and Mass-Ratio Dependence

A high-resolution LCDM N-body simulation is employed to present merger rate predictions for dark matter halos and to investigate how common merger-related observables—such as close pair counts, starburst counts, and the morphologically disturbed fraction—are likely scale with luminosity, stellar mass, and redshift from z = 0 to z = 4. We provide a simple ‘universal’ fitting formula that describes our derived merger rates for dark matter halos a function of halo mass, merger mass ratio, and redshift. Using number density-matching as a means of assigning galaxy properties to halos, we show, for example, that the instantaneous merger rate of r > 0.3 mass ratio events into typical L & f L∗ galaxies follows the simple relation dN/dt ≃ 0.03(1 + f)Gyr −1 (1 + z) 2.1 . Despite the rapid increase in merger rate with redshift, only a small fraction of > 0.4L∗ high-redshift galaxies (∼ 3% at z = 2) should have experienced a major merger (m/M > 0.3) in the very recent past (t < 100 Myr). This suggests that short-lived, merger-induced bursts of star formation should not contribute significantly to the global star formation rate at early times, in agreement with observational indications. In contrast, a fairly high fraction (∼ 20%) of those z = 2 galaxies should have experienced a morphologically transformative merger within a virial dynamical time. We compare our results to observational merger rate estimates from both morphological indicators and pair-fraction based determinations between z = 0 − 2 and show that they agree with our predictions to within observational uncertainties. We find that a majority of bright z = 3 Lyman Break Galaxy halos should have undergone a major merger in the last 500 Myr and conclude that mergers almost certainly play an important role in delivering baryons and influencing the kinematic properties of the highest redshift galaxies. Subject headings: cosmology: theory — dark matter — galaxies: formation — galaxies: halos — methods: N-body simulations

OBSERVING THE END OF COLD FLOW ACCRETION USING HALO ABSORPTION SYSTEMS

The radio continuum emission from the Galaxy has a rich mix of thermal and non-thermal emission. This very richness makes their interpretation challenging since the low radio opacity means that a radio image represents the sum of all emission regions along the line-of-sight. These challenges make the existing narrow-band radio surveys of the Galactic plane difficult to interpret: e.g. a small region of emission might be a supernova remnant (SNR) or an HII region, or a complex combination of both. Instantaneous wide bandwidth radio observations in combination with the capability for high resolution spectral index mapping, can be directly used to disentangle these effects. Here we demonstrate simultaneous continuum and spectral index imaging capability at the full continuum sensitivity and resolution using newly developed wide-band wide-field imaging algorithms. Observations were done in the L- and C-Band with a total bandwidth of 1 and 2 GHz respectively. We present preliminary results in the form of a full-field continuum image covering the wide-band sensitivity pattern of the EVLA centered on a large but poorly studied SNR (G55.7+3.4) and relatively narrower field continuum and spectral index maps of three fields containing SNR and diffused thermal emission. We demonstrate that spatially resolved spectral index maps differentiates regions with emission of different physical origin (spectral index variation across composite SNRs and separation of thermal and non-thermal emission), superimposed along the line of sight. The wide-field image centered on the SNR G55.7+3.4 also demonstrates the excellent wide-field wide-band imaging capability of the EVLA.We use cosmological smoothed particle hydrodynamic simulations to study the cool, accreted gas in two Milky Way size galaxies through cosmic time to z = 0. We find that gas from mergers and cold flow accretion results in significant amounts of cool gas in galaxy halos. This cool circum-galactic component drops precipitously once the galaxies cross the critical mass to form stable shocks, M vir = M sh ~ 1012 M ?. Before reaching M sh, the galaxies experience cold mode accretion (T ?cm?2. These values are considerably lower than observed covering fractions, suggesting that outflowing gas (not included here) is important in simulating galaxies with realistic gaseous halos. Within ~500?Myr of crossing the M sh threshold, each galaxy transitions to hot mode gas accretion, and fc drops to ~5%. The sharp transition in covering fraction is primarily a function of halo mass, not redshift. This signature should be detectable in absorption system studies that target galaxies of varying host mass, and may provide a direct observational tracer of the transition from cold flow accretion to hot mode accretion in galaxies.

Stealth Galaxies in the Halo of the Milky Way

We predict that there is a population of low-luminosity dwarf galaxies orbiting within the halo of the Milky Way that have surface brightnesses low enough to have escaped detection in star-count surveys. The overall count of stealth galaxies is sensitive to the presence (or lack) of a low-mass threshold in galaxy formation. These systems have luminosities and stellar velocity dispersions that are similar to those of known ultrafaint dwarf galaxies but they have more extended stellar distributions (half light radii greater than about 100 pc) because they inhabit dark subhalos that are slightly less massive than their higher surface brightness counterparts. As a result, the typical peak surface brightness is fainter than 30 mag per square arcsec. One implication is that the inferred common mass scale for Milky Way dwarfs may be an artifact of selection bias. If there is no sharp threshold in galaxy formation at low halo mass, then ultrafaint galaxies like Segue 1 represent the high-mass, early forming tail of a much larger population of objects that could number in the hundreds and have typical peak circular velocities of about 8 kms −1 and masses within 300 pc of about 5 million solar masses. Alternatively, if we impose a low-mass threshold in galaxy formation in order to explain the unexpectedly high densities of the ultrafaint dwarfs, then we expect only a handful of stealth galaxies in the halo of the Milky Way. A complete census of these objects will require deeper sky surveys, 30m-class follow-up telescopes, and more refined methods to identify extended, self-bound groupings of stars in the halo. Subject headings: cosmology: theory — dark matter — galaxies: formation — galaxies: halos — methods: N-body simulations

Mergers and bulge formation in lambda CDM: which mergers matter?

We use a suite of semi-empirical models to predict the galaxy-galaxy merger rate and relative contributions to bulge growth as a function of mass (both halo and stellar), redshift, and mass ratio. The models use empirical constraints on the halo occupation distribution, evolved forward in time, to robustly identify where and when galaxy mergers occur. Together with the results of high-resolution merger simulations, this allows us to quantify the relative contributions of mergers with different properties (e.g., mass ratios, gas fractions, redshifts) to the bulge population. We compare with observational constraints, and find good agreement. We also provide useful fitting functions and make public a code to reproduce the predicted merger rates and contributions to bulge mass growth. We identify several robust conclusions. (1) Major mergers dominate the formation and assembly of ~L * bulges and the total spheroid mass density, but minor mergers contribute a non-negligible ~30%. (2) This is mass dependent: bulge formation and assembly is dominated by more minor mergers in lower-mass systems. In higher-mass systems, most bulges originally form in major mergers near ~L *, but assemble in increasingly minor mergers. (3) The minor/major contribution is also morphology dependent: higher B/T systems preferentially form in more major mergers, with B/T roughly tracing the mass ratio of the largest recent merger; lower B/T systems preferentially form in situ from minor mergers. (4) Low-mass galaxies, being gas-rich, require more mergers to reach the same B/T as high-mass systems. Gas-richness dramatically suppresses the absolute efficiency of bulge formation, but does not strongly influence the relative contribution of major versus minor mergers. (5) Absolute merger rates at fixed mass ratio increase with galaxy mass. (6) Predicted merger rates agree well with those observed in pair and morphology-selected samples, but there is evidence that some morphology-selected samples include contamination from minor mergers. (7) Predicted rates also agree with the integrated growth in bulge mass density with cosmic time, but with a factor ~2 uncertainty in both—up to half the bulge mass density could come from non-merger processes. We systematically vary the model assumptions, totaling ~103 model permutations, and quantify the resulting uncertainties. Our conclusions regarding the importance of different mergers for bulge formation are very robust to these changes. The absolute predicted merger rates are systematically uncertain at the factor ~2 level; uncertainties grow at the lowest masses and high redshifts.

High Angular Momentum Halo Gas: A Feedback and Code-independent Prediction of LCDM

We investigate angular momentum acquisition in Milky Way-sized galaxies by comparing five high resolution zoom-in simulations, each implementing identical cosmological initial conditions, but utilizing different hydrodynamic codes: Enzo, Art, Ramses, Arepo, and Gizmo-PSPH. Each code implements a distinct set of feedback and star formation prescriptions. We find that while many galaxy and halo properties vary between the different codes (and feedback prescriptions), there is qualitative agreement on the process of angular momentum acquisition in the galaxys halo. In all simulations, cold filamentary gas accretion to the halo results in ~4 times more specific angular momentum in cold halo gas (

Mergers and Disk Survival in ΛCDM

\lambda_{cold} \gtrsim 0.1