The Impact of Cooling and Feedback on Disc Galaxies

We present detailed, analytical models for the formation of disc galaxies to investigate the impact that cooling and feedback have on their structural properties. In particular, we investigate which observables extracted directly from the models are best suited as virial mass estimators, and to what extent they allow the recovery of the model input parameters regarding the feedback and cooling efficiencies. Contrary to naive expectations, the luminosities and circular velocities of disc galaxies are extremely poor indicators of total virial mass. Instead, we show that the product of disc scale length and rotation velocity squared yields a much more robust estimate. We show that feedback can cause a narrow correlation between galaxy mass fraction and halo spin parameter, similar to that found recently by van den Bosch, Burkert and Swaters from an analysis of dwarf galaxy rotation curves. Finally we investigate the impact that cooling and feedback have on the colors, metallicities, star formation histories and Tully-Fisher relation of disc galaxies.Comment: 20 pages, 12 figures. To be published in MNRA

The structural and dynamical properties of compact elliptical galaxies

Dedicated photometric and spectroscopic surveys have provided unambiguous evidence for a strong stellar mass-size evolution of galaxies within the last 10 Gyr. The likely progenitors of today's most massive galaxies are remarkably small, disky, passive and have already assembled much of their stellar mass at redshift z=2. An in-depth analysis of these objects, however, is currently not feasible due to the lack of high-quality, spatially-resolved photometric and spectroscopic data. In this paper, we present a sample of nearby compact elliptical galaxies (CEGs), which bear resemblance to the massive and quiescent galaxy population at earlier times. Hubble Space Telescope (HST) and wide-field integral field unit (IFU) data have been obtained, and are used to constrain orbit-based dynamical models and stellar population synthesis (SPS) fits, to unravel their structural and dynamical properties. We first show that our galaxies are outliers in the present-day stellar mass-size relation. They are, however, consistent with the mass-size relation of compact, massive and quiescent galaxies at redshift z=2. The compact sizes of our nearby galaxies imply high central stellar mass surface densities, which are also in agreement with the massive galaxy population at higher redshift, hinting at strong dissipational processes during their formation. Corroborating evidence for a largely passive evolution within the last 10 Gyr is provided by their orbital distribution as well as their stellar populations, which are difficult to reconcile with a very active (major) merging history. This all supports that we can use nearby CEGs as local analogues of the high-redshift, massive and quiescent galaxy population, thus providing additional constraints for models of galaxy formation and evolution.Comment: 33 pages, 27 figures and 20 tables (with most of the tables provided as online-only supporting information). Accepted for publication in MNRA

Preheating by Previrialization and its Impact on Galaxy Formation

We use recent observations of the HI-mass function to constrain galaxy formation. The data conflicts with the standard model where most of the gas in a low-mass dark matter halo is assumed to settle into a disk of cold gas that is depleted by star formation and supernova-driven outflows until the disk becomes gravitationally stable. A consistent model can be found if low-mass haloes are embedded in a preheated medium, with a specific gas entropy ~ 10Kev cm^2. Such a model simultaneously matches the faint-end slope of the galaxy luminosity function. We propose a preheating model where the medium around low-mass haloes is preheated by gravitational pancaking. Since gravitational tidal fields suppress the formation of low-mass haloes while promoting that of pancakes, the formation of massive pancakes precedes that of the low-mass haloes within them. We demonstrate that the progenitors of present-day dark matter haloes with M<10^{12}h^{-1}\msun were embedded in pancakes of masses ~5x10^{12}h^{-1}\msun at z~2. The formation of such pancakes heats the gas to a temperature of 5x10^5K and compresses it to an overdensity of ~10. Such gas has a cooling time that exceeds the age of the Universe at z~2, and has a specific entropy of ~15Kev cm^2, almost exactly the amount required to explain the stellar and HI mass functions. (Abridged)Comment: 13 pages, 3 figures. Accepted for publication in MNRA

Satellite Kinematics I: A New Method to Constrain the Halo Mass-Luminosity Relation of Central Galaxies

Satellite kinematics can be used to probe the masses of dark matter haloes of central galaxies. In order to measure the kinematics with sufficient signal-to-noise, one uses the satellite galaxies of a large number of central galaxies stacked according to similar properties (e.g., luminosity). However, in general the relation between the luminosity of a central galaxy and the mass of its host halo will have non-zero scatter. Consequently, this stacking results in combining the kinematics of satellite galaxies in haloes of different masses, which complicates the interpretation of the data. In this paper we present an analytical framework to model satellite kinematics, properly accounting for this scatter and for various selection effects. We show that in the presence of scatter in the halo mass-luminosity relation, the commonly used velocity dispersion of satellite galaxies can not be used to infer a unique halo mass-luminosity relation. In particular, we demonstrate that there is a degeneracy between the mean and the scatter of the halo mass-luminosity relation. We present a new technique that can break this degeneracy, and which involves measuring the velocity dispersions using two different weighting schemes: host-weighting (each central galaxy gets the same weight) and satellite-weighting (each central galaxy gets a weight proportional to its number of satellites). The ratio between the velocity dispersions obtained using these two weighting schemes is a strong function of the scatter in the halo mass-luminosity relation, and can thus be used to infer a unique relation between light and mass from the kinematics of satellite galaxies.Comment: 8 pages, 3 figures, MNRAS submitte

The Impact of Feedback on Disk Galaxy Scaling Relations

We use a disk galaxy evolution model to investigate the impact of mass outflows (a.k.a. feedback) on disk galaxy scaling relations. Our model follows the accretion, cooling, star formation and ejection of baryonic mass inside growing dark matter haloes, with cosmologically motivated specific angular momentum distributions. Models without feedback produce disks that are too small and rotate too fast. Feedback reduces the baryonic masses of galaxies, resulting in larger disks with lower rotation velocities. Models with feedback can reproduce the zero points of the scaling relations between rotation velocity, stellar mass and disk size, but only in the absence of adiabatic contraction. Our feedback mechanism is maximally efficient in expelling mass, but our successful models require 25% of the SN energy, or 100% of the SN momentum, to drive the outflows. It remains to be seen whether such high efficiencies are realistic or not. Our energy and momentum driven wind models result in different slopes of various scaling relations, such as size - stellar mass, stellar mass - halo mass, and metallicity - stellar mass. Observations favor the energy driven wind at stellar masses below Mstar = 10^{10.5} Msun, but the momentum driven wind model at high masses. The ratio between the specific angular momentum of the baryons to that of the halo, (j_gal/m_gal), is not unity in our models. Yet this is the standard assumption in models of disk galaxy formation. Feedback preferentially ejects low angular momentum material because star formation is more efficient at smaller galactic radii. This results in (j_gal/m_gal) increasing with decreasing halo mass. This effect helps to resolve the discrepancy between the high spin parameters observed for dwarf galaxies with the low spin parameters predicted from LCDM. [Abridged]Comment: 27 pages, 16 figures, accepted to MNRAS, two new figure

The Substructure Hierarchy in Dark Matter Haloes

We present a new algorithm for identifying the substructure within simulated dark matter haloes. The method is an extension of that proposed by Tormen et al. (2004) and Giocoli et al. (2008a), which identifies a subhalo as a group of self-bound particles that prior to being accreted by the main progenitor of the host halo belonged to one and the same progenitor halo (hereafter satellite). However, this definition does not account for the fact that these satellite haloes themselves may also have substructure, which thus gives rise to sub-subhaloes, etc. Our new algorithm identifies substructures at all levels of this hierarchy, and we use it to determine the mass function of all substructure (counting sub-haloes, sub-subhaloes, etc.). On average, haloes which formed more recently tend to have a larger mass fraction in substructure and to be less concentrated than average haloes of the same mass. We provide quantitative fits to these correlations. Even though our algorithm is very different from that of Gao et al. (2004), we too find that the subhalo mass function per unit mass at redshift z = 0 is universal. This universality extends to any redshift only if one accounts for the fact that host haloes of a given mass are less concentrated at higher redshifts, and concentration and substructure abundance are anti-correlated. This universality allows a simple parametrization of the subhalo mass function integrated over all host halo masses, at any given time. We provide analytic fits to this function which should be useful in halo model analyses which equate galaxies with halo substructure when interpreting clustering in large sky surveys. Finally, we discuss systematic differences in the subhalo mass function that arise from different definitions of (host) halo mass.Comment: 18 pages, 24 figures, accepted for publication on MNRA

Unitarity Bounds and the Cuspy Halo Problem

Conventional Cold Dark Matter cosmological models predict small scale structures, such as cuspy halos, which are in apparent conflict with observations. Several alternative scenarios based on modifying fundamental properties of the dark matter have been proposed. We show that general principles of quantum mechanics, in particular unitarity, imply interesting constraints on two proposals: collisional dark matter proposed by Spergel & Steinhardt, and strongly annihilating dark matter proposed by Kaplinghat, Knox & Turner. Efficient scattering required in both implies m < 12 GeV and m < 25 GeV respectively. The same arguments show that the strong annihilation in the second scenario implies the presence of significant elastic scattering, particularly for large enough masses. Recently, a variant of the collisional scenario has been advocated to satisfy simultaneously constraints from dwarf galaxies to clusters, with a cross section that scales inversely with velocity. We show that this scenario likely involves super-elastic processes, and the associated kinetic energy change must be taken into account when making predictions. Exceptions and implications for experimental searches are discussed.Comment: 4 pages, references adde

Weak Lensing by Galaxies in Groups and Clusters: I.--Theoretical Expectations

Galaxy-galaxy lensing is rapidly becoming one of the most promising means to accurately measure the average relation between galaxy properties and halo mass. In order to obtain a signal of sufficient signal-to-noise, one needs to stack many lens galaxies according to their property of interest, such as luminosity or stellar mass. Since such a stack consists of both central and satellite galaxies, which contribute very different lensing signals, the resulting shear measurements can be difficult to interpret. In the past, galaxy-galaxy lensing studies have either completely ignored this problem, have applied rough isolation criteria in an attempt to preferentially select `central' galaxies, or have tried to model the contribution of satellites explicitely. However, if one is able to {\it a priori} split the galaxy population in central and satellite galaxies, one can measure their lensing signals separately. This not only allows a much cleaner measurement of the relation between halo mass and their galaxy populations, but also allows a direct measurement of the sub-halo masses around satellite galaxies. In this paper, we use a realistic mock galaxy redshift survey to show that galaxy groups, properly selected from large galaxy surveys, can be used to accurately split the galaxy population in centrals and satellites. Stacking the resulting centrals according to their group mass, estimated from the total group luminosity, allows a remarkably accurate recovery of the masses and density profiles of their host haloes. In addition, stacking the corresponding satellite galaxies according to their projected distance from the group center yields a lensing signal that can be used to accurate measure the masses of both sub-haloes and host haloes. (Abridged)Comment: 16 pages, 10 figures, Accepted for publication in MNRA

Scaling Relations of Spiral Galaxies

We construct a large data set of global structural parameters for 1300 field and cluster spiral galaxies and explore the joint distribution of luminosity L, optical rotation velocity V, and disk size R at I- and 2MASS K-bands. The I- and K-band velocity-luminosity (VL) relations have log-slopes of 0.29 and 0.27, respectively with sigma_ln(VL)~0.13, and show a small dependence on color and morphological type in the sense that redder, early-type disk galaxies rotate faster than bluer, later-type disk galaxies for most luminosities. The VL relation at I- and K-bands is independent of surface brightness, size and light concentration. The log-slope of the I- and K-band RL relations is a strong function of morphology and varies from 0.25 to 0.5. The average dispersion sigma_ln(RL) decreases from 0.33 at I-band to 0.29 at K, likely due to the 2MASS selection bias against lower surface brightness galaxies. Measurement uncertainties are sigma_ln(V)~0.09, sigma_ln(L)~0.14 and somewhat larger and harder to estimate for ln(R). The color dependence of the VL relation is consistent with expectations from stellar population synthesis models. The VL and RL residuals are largely uncorrelated with each other; the RV-RL residuals show only a weak positive correlation. These correlations suggest that scatter in luminosity is not a significant source of the scatter in the VL and RL relations. The observed scaling relations can be understood in the context of a model of disk galaxies embedded in dark matter halos that invokes low mean spin parameters and dark halo expansion, as we describe in our companion paper (Dutton et al. 2007). We discuss in two appendices various pitfalls of standard analytical derivations of galaxy scaling relations, including the Tully-Fisher relation with different slopes. (Abridged).Comment: Accepted for publication at ApJ. The full document, with high-resolution B&W and colour figures, is available at http://www.astro.queensu.ca/~courteau/papers/VRL2007ApJ.pdf . Our data base for 1303 spiral galaxies is also available at http://www.astro.queensu.ca/~courteau/data/VRL2007.da

Semi-Analytical Models for the Formation of Disk Galaxies: I. Constraints from the Tully-Fisher Relation

We present new semi-analytical models for the formation of disk galaxies with the purpose of investigating the origin of the near-infrared Tully-Fisher (TF) relation. The models assume that disks are formed by cooling of the baryons inside dark halos with realistic density profiles, and that the baryons conserve their specific angular momentum. Only gas with densities above the critical density given by Toomre's stability criterion is considered eligible for star formation, and a simple recipe for supernovae feedback is included. We emphasize the importance of extracting the proper luminosity and velocity measures from the models, something that has often been ignored in the past. The observed K-band TF relation has a slope that is steeper than simple predictions based on dynamical arguments suggest. Taking the stability related star formation threshold densities into account steepens the TF relation, decreases its scatter, and yields gas mass fractions that are in excellent agreement with observations. In order for the TF slope to be as steep as observed, further physics are required. We argue that the characteristics of the observed near-infrared TF relation do not reflect systematic variations in stellar populations, or cosmological initial conditions, but are governed by feedback. Finally we show that our models provide a natural explanation for the small amount of scatter that makes the TF relation useful as a cosmological distance indicator.Comment: 20 pages, 10 figures. Accepted for publication in Ap