Ariyeh H. Maller

Multiphase galaxy formation: high-velocity clouds and the missing baryon problem

The standard treatment of cooling in Cold Dark Matter halos assumes that all of the gas within a “cooling radius” cools and contracts monolithically to fuel galaxy formation. Here we take into account the expectation that the hot gas in galactic halos is thermally unstable and prone to fragmentation during cooling and show that the implications are more far-reaching than previously expected: allowing multi-phase cooling fundamentally alters expectations about gas infall in galactic halos and naturally gives rise to a characteristic upper-limit on the masses of galaxies, as observed. Specifically, we argue that cooling should proceed via the formation of high-density, � 10 4 K clouds, pressure-confined within a hot gas background. The background medium that emerges has a low density, and can survive as a hydrostatically stable corona with a long cooling time. The fraction of halo baryons contained in the residual hot core component grows with halo mass because the cooling density increases with gas temperature, and this leads to an upper-mass limit in quiescent, non-merged galaxies of � 10 11 M⊙. In this scenario, galaxy formation is fueled by the infall of pressure-supported clouds. For Milky-Way-size systems, clouds of mass � 5 × 10 6 M⊙ that formed or merged within the last several Gyrs should still exist as a residual population in the halo, with a total mass in clouds of � 2 × 10 10 M⊙. The baryonic mass of the Milky Way galaxy is explained naturally in this model, and is a factor of two smaller than would result in the standard treatment without feedback. We expect clouds in galactic halos to be � 1kpc in size and to extend � 150kpc from galactic centers. The predicted properties of Milky Way clouds match well the observed radial velocity distribution, angular sizes, column densities, and velocity widths of High Velocity Clouds around our Galaxy. The clouds we predict are also of the type needed to explain high-ion absorption systems at z < 1, and the predicted covering factor around external galaxies is consistent with observations.

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.

Galaxy Merger Statistics and Inferred Bulge-to-Disk Ratios in Cosmological SPH Simulations

We construct merger trees for galaxies identified in a cosmological hydrodynamic simulation and use them to characterize predicted merger rates as a function of redshift, galaxy mass, and merger mass ratio. At z = 0.3, we find a mean rate of 0.054 mergers per galaxy per Gyr above a 1 : 2 mass ratio threshold for massive galaxies (baryonic mass above 6.4 × 1010 M☉), but only 0.018 Gyr-1 for lower mass galaxies. The mass ratio distribution is ∝R for the massive galaxy sample, so high-mass mergers dominate the total merger growth rate. The predicted rates increase rapidly with increasing redshift, and they agree reasonably well with observational estimates. A substantial fraction of galaxies do not experience any resolved mergers during the course of the simulation, and even for the high-mass sample, only 50% of galaxies experience a greater than 1 : 4 merger since z = 1. Typical galaxies thus have fairly quiescent merger histories. We assign bulge-to-disk ratios to simulated galaxies by assuming that mergers above a mass ratio threshold Rmajor convert stellar disks into spheroids. With Rmajor values of 1 : 4, we obtain a fairly good match to the observed dependence of the early-type fraction on galaxy mass. However, the predicted fraction of truly bulge-dominated systems (fbulge > 0.8) is small, and producing a substantial population of bulge-dominated galaxies may require a mechanism that shuts off gas accretion at late times and/or additional processes (besides major mergers) for producing 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.

Towards a resolution of the galactic spin crisis: mergers, feedback and spin segregation

We model in simple terms the angular momentum problems of galaxy formation in cold dark matter cosmologies, and identify the key elements of a scenario that may solve them. The buildup of angular momentum is modelled viadynamical friction and tidal stripping in a sequence of mergers. We demonstrate how overcooling in incoming haloes leads to a transfer of angular momentum from the baryons to the dark matter, in conflict with observations. By incorporating a simple recipe of supernova feedback, we are able to solve the problems of angular momentum in disc formation. Gas removal from the numerous small incoming haloes, which merge to become the low specific angular momentum (low-j) component of the product, eliminates the low-j baryons. Heating and puffing-up of the gas in larger incoming haloes, combined with efficient tidal stripping, reduces the angular momentum loss of baryons due to dynamical friction. Dependence of the feedback effects on the progenitor halo mass implies that the spin of baryons is typically higher for lower-mass haloes. The observed low baryonic fraction in dwarf galaxies is used to calibrate the characteristic velocity associated with supernova feedback, yielding V f b ∼100 km s - 1 , within the range of theoretical expectations. We then find that the model naturally produces the observed distribution of the spin parameter among dwarf and bright disc galaxies, as well as the j profile inside these galaxies. This suggests that the model indeed captures the main features of a full scenario for resolving the spin crisis.

THE INTRINSIC PROPERTIES OF SDSS GALAXIES

The observed properties of galaxies vary with inclination; for most applications we would rather have properties that are independent of inclination, intrinsic properties. One way to determine inclination corrections is to consider a large sample of galaxies, study how the observed properties of these galaxies depend on inclination and then remove this dependence to recover the intrinsic properties. We perform such an analysis for galaxies selected from the Sloan Digital Sky Survey which have been matched to galaxies from the Two-Micron All Sky Survey. We determine inclination corrections for these galaxies as a function of galaxy luminosity and Sersic index. In the g-band these corrections reach as as high as 1.2 mag and have a median value of 0.3 mag for all galaxies in our sample. We find that the corrections show little dependence on galaxy luminosity, except in the u band, but are strongly dependent on galaxy Sersic index. We find that the ratio of red-to-blue galaxies changes from 1:1 to 1:2 when going from observed to intrinsic colors for galaxies in the range 22.75 < MK < 17.75. We also discuss how survey completeness and photometric redshifts should be determined when taking into account that observed and intrinsic properties differ. Finally, we examine whether previous determinations of stellar mass give an intrinsic quantity or one that depends on galaxy inclination. Subject headings: galaxies: clusters: general—galaxies: statistics—methods:statistical:surveys

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.

Redistributing hot gas around galaxies: do cool clouds signal a solution to the overcooling problem?

We present a pair of high-resolution smoothed particle hydrodynamics simulations that explore the evolution and cooling behaviour of hot gas around Milky Way size galaxies. The simulations contain the same total baryonic mass and are identical other than their initial gas density distributions. The first is initialized with a low-entropy hot gas halo that traces the cuspy profile of the dark matter, and the second is initialized with a high-entropy hot halo with a cored density profile as might be expected in models with pre-heating feedback. Galaxy formation proceeds in dramatically different fashion depending on the initial setup. While the low-entropy halo cools rapidly, primarily from the central region, the high-entropy halo is quasi-stable for ∼4 Gyr and eventually cools via the fragmentation and infall of clouds from ∼100 kpc distances. The low-entropy halo’s X-ray surface brightness is ∼100 times brighter than current limits and the resultant disc galaxy contains more than half of the system’s baryons. The high-entropy halo has an X-ray brightness that is in line with observations, an extended distribution of pressure-confined clouds reminiscent of observed populations and a final disc galaxy that has half the mass and ∼50 per cent more specific angular momentum than the disc formed in the low-entropy simulation. The final high-entropy system retains the majority of its baryons in a low-density hot halo. The hot halo harbours a trace population of cool, mostly ionized, pressure-confined clouds that contain ∼10 per cent of the halo’s baryons after 10 Gyr of cooling. The covering fraction for H I and Mg II absorption clouds in the high-entropy halo is ∼0.4 and ∼0.6, respectively, although most of the mass that fuels disc growth is ionized, and hence would be under counted in H I surveys.

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.

How to zoom: bias, contamination and Lagrange volumes in multimass cosmological simulations

We perform a suite of multimass cosmological zoom simulations of individual dark matter halos and explore how to best select Lagrangian regions for resimulation without contaminating the halo of interest with low-resolution particles. Such contamination can lead to significant errors in the gas distribution of hydrodynamical simulations, as we show. For a fixed Lagrange volume, we find that the chance of contamination increases systematically with the level of zoom. In order to avoid contamination, the Lagrangian volume selected for resimulation must increase monotonically with the resolution difference between parent box and the zoom region. We provide a simple formula for selecting Lagrangian regions (in units of the halo virial volume) as a function of the level of zoom required. We also explore the degree to which a halos Lagrangian volume correlates with other halo properties (concentration, spin, formation time, shape, etc.) and find no significant correlation. There is a mild correlation between Lagrange volume and environment, such that halos living in the most clustered regions have larger Lagrangian volumes. Nevertheless, selecting halos to be isolated is not the best way to ensure inexpensive zoom simulations. We explain how one can safely choose halos with the smallest Lagrangian volumes, which are the least expensive to resimulate, without biasing ones sample.