Aaron A. Dutton

Concentration, spin and shape of dark matter haloes as a function of the cosmological model: WMAP1, WMAP3 and WMAP5 results

We investigate the effects of changes in the cosmological parameters between the Wilkinson Microwave Anisotropy Probe (WMAP) 1st, 3rd and 5th year results on the structure of dark matter haloes. We use a set of simulations that cover five decades in halo mass ranging from the scales of dwarf galaxies (V c ≈ 30 km s -1 ) to clusters of galaxies (V c ≈ 1000 km s -1 ). We find that the concentration mass relation is a power law in all three cosmologies. However, the slope is shallower and the zero-point is lower moving from WMAP1 to WMAP5 to WMAP3. For haloes of mass log M 200 /[h -1 M ⊙ ] = 10, 12 and 14 the differences in the concentration parameter between WMAP1 and WMAP3 are a factor of 1.55,1.41 and 1.29, respectively. As we show, this brings the central densities of dark matter haloes in good agreement with the central densities of dwarf and low surface brightness galaxies inferred from their rotation curves, for both the WMAP3 and WMAP5 cosmologies. We also show that none of the existing toy models for the concentration-mass relation can reproduce our simulation results over the entire range of masses probed. In particular, the model of Bullock et al. fails at the higher mass end (M ≥ 10 13 h-1 M ⊙ ), while the NFW model of Navarro, Frenk and White fails dramatically at the low-mass end (M ≤ 10 12 h -1 M ⊙ ). We present a new model, based on a simple modification of that of Bullock et al., which reproduces the concentration-mass relations in our simulations over the entire range of masses probed (10 10 ≤ M ≤ 10 15 h -1 M ⊙ ). Haloes in the WMAP3 cosmology (at a fixed mass) are more flatted compared to the WMAP1 cosmology, with a medium to long axis ration reduced by ≈ 0 per cent. Finally, we show that the distribution of halo spin parameters is the same for all three cosmologies.

UBIQUITOUS OUTFLOWS IN DEEP2 SPECTRA OF STAR-FORMING GALAXIES AT z = 1.4

Galactic winds are a prime suspect for the metal enrichment of the intergalactic medium (IGM) and may have a strong influence on the chemical evolution of galaxies and the nature of QSO absorption-line systems. We use a sample of 1406 galaxy spectra at z ~ 1.4 from the DEEP2 redshift survey to show that blueshifted Mg IYI ?? 2796, 2803 absorption is ubiquitous in star-forming galaxies at this epoch. This is the first detection of frequent outflowing galactic winds at z ~ 1. The presence and depth of absorption are independent of active galactic nuclei spectral signatures or galaxy morphology; major mergers are not a prerequisite for driving a galactic wind from massive galaxies. Outflows are found in co-added spectra of galaxies spanning a range of 30 times in stellar mass and 10 times in star formation rate (SFR), calibrated from K-band and from the Multiband Imaging Photometer for Spitzer IR fluxes. The outflows have column densities of order NH ~ 1020 cm-2 and characteristic velocities of ~?300-500?km?s?1, with absorption seen out to 1000?km?s?1 in the most massive, highest SFR galaxies. The velocities suggest that the outflowing gas can escape into the IGM and that massive galaxies can produce cosmologically and chemically significant outflows. Both the Mg II equivalent width and the outflow velocity are larger for galaxies of higher stellar mass and SFR, with V wind ~ SFR0.3, similar to the scaling in low redshift IR-luminous galaxies. The high frequency of outflows in the star-forming galaxy population at z ~ 1 indicates that galactic winds occur in the progenitors of massive spirals as well as those of ellipticals. The increase of outflow velocity with mass and SFR constrains theoretical models of galaxy evolution that include feedback from galactic winds, and may favor momentum-driven models for the wind physics.

Concentration, spin and shape of dark matter haloes: scatter and the dependence on mass and environment

We use a series of cosmological N-body simulations for a flat A cold dark matter (ACDM) cosmology to investigate the structural properties of dark matter haloes, at redshift zero, in the mass range 3 x 10 9 h -1 ≤ M vir ≤ 3 x 10 13 h -1 M ⊙ . These properties include the concentration parameter, c, the spin parameter, λ, and the mean axis ratio, q. For the concentration-mass relation we find c oc M -0.11 vir agreement with the model proposed by Bullock et al., but inconsistent with the alternative model of Eke et al. The normalization of the concentration-mass relation, however, is 15 per cent lower than suggested by Bullock et al. The results for X and q are in good agreement with previous studies, when extrapolated to the lower halo masses probed here, while c and λ are anticorrelated, in that high-spin haloes have, on average, lower concentrations. In an attempt to remove unrelaxed haloes from the sample, we compute for each halo the offset parameter, x off , defined as the distance between the most bound particle and the centre of mass, in units of the virial radius. Removing haloes with large x off increases the mean concentration by ∼ 10 per cent, lowers the mean spin parameter by ∼ 15 per cent, and removes the most prolate haloes. In addition, it largely removes the anticorrelation between c and λ, though not entirely. We also investigate the relation between halo properties and their large-scale environment density. For low-mass haloes we find that more concentrated haloes live in denser environments than their less concentrated counterparts of the same mass, consistent with recent correlation function analyses. Note, however, that the trend is weak compared to the scatter. For the halo spin parameters we find no environment dependence, while there is a weak indication that the most spherical haloes reside in slightly denser environments. Finally, using a simple model for disc galaxy formation we show that haloes that host low surface brightness galaxies are expected to be hosted by a biased subset of haloes. Not only do these haloes have spin parameters that are larger than average, they also have concentration parameters that are ∼15 per cent lower than the average at a given halo mass. We discuss the implications of all these findings for the claimed disagreement between halo concentrations inferred from low surface brightness rotation curves, and those expected for a ACDM cosmology.

A Revised Model for the Formation of Disk Galaxies: Low Spin and Dark Halo Expansion

We use observed rotation velocity-luminosity (VL) and size-luminosity (RL) relations to single out a specific scenario for disk galaxy formation in the ΛCDM cosmology. Our model involves four independent lognormal random variables: dark halo concentration c, disk spin λgal, disk mass fraction mgal, and stellar mass-to-light ratio I. A simultaneous match of the VL and RL zero points with adiabatic contraction requires low-c halos, but this model has V2.2 ~ 1.8Vvir (where V2.2 and Vvir are the circular velocity at 2.2 disk scale lengths and the virial radius, respectively), which will be unable to match the luminosity function (LF). Similarly models without adiabatic contraction but standard c also predict high values of V2.2/Vvir. Models in which disk formation induces an expansion rather than the commonly assumed contraction of the dark matter halos have V2.2 ~ 1.2Vvir, which allows a simultaneous fit of the LF. This may result from nonspherical, clumpy gas accretion, where dynamical friction transfers energy from the gas to the dark matter. This model requires low λgal and mgal values, contrary to naive expectations. However, the low λgal is consistent with the notion that disk galaxies predominantly survive in halos with a quiet merger history, while a low mgal is also indicated by galaxy-galaxy lensing. The smaller than expected scatter in the RL relation and the lack of correlation between the residuals of the VL and RL relations, respectively, imply that the scatter in λgal and in c needs to be smaller than predicted for ΛCDM halos, again consistent with the idea that disk galaxies preferentially reside in halos with a quiet merger history.

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 σ_(ln)(VL) ~ 0.13, and show a small dependence on color and morphological type in the sense that redder, earlier 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 size-luminosity (RL) relations is a strong function of morphology and varies from 0.25 to 0.5, with a mean of 0.32 for all Hubble types. At most luminosities, early-type disk galaxies have shorter scale lengths than later type ones. The average dispersion σ_(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. The VL and RL residuals are largely uncorrelated with each other with a correlation coefficient r = -0.16 and Δ log V|L/Δ log R|L = -0.07 ± 0.01; the RV - RL residuals show a weak positive correlation with r = 0.53. These correlations suggest that scatter in luminosity is not a significant source of the scatter in the VL and RL relations. We discuss in two Appendices various pitfalls of standard analytical derivations of galaxy scaling relations, including the Tully-Fisher relation with different slopes. Our galaxy database is available at http://www.astro.queensu.ca/~courteau/data/VRL2007.dat.

On the origin of the galaxy star-formation-rate sequence: evolution and scatter

We use a semi-analytic model for disk galaxies to explore the origin of the time evolution and small scatter of the galaxy SFR sequence - the tight correlation between star formation rate (SFR) and stellar mass (M star ). The steep decline of SFR from z ~ 2 to the present, at fixed M star ,is a consequence of the following. First, disk galaxies are in a steady state with the SFR following the net (i.e. inflow minus outflow) gas accretion rate. The evolution of the SFR sequence is determined by evolution in the cosmological specific accretion rates, α (1 + z) 2.25 , but is found to be independent of feedback. Although feedback determines the outflow rates, it shifts galaxies along the SFR sequence, leaving its zero-point invariant. Second, the conversion of accretion rate to SFR is materialized through gas density, not gas mass. Although the model SFR is an increasing function of both gas mass fraction and gas density, only the gas densities are predicted to evolve significantly with redshift. Third, star formation is fueled by molecular gas. Since the molecular gas fraction increases monotonically with increasing gas density, the model predicts strong evolution in the molecular gas fractions, increasing by an order of magnitude from z = 0 to z ~ 2. On the other hand, the model predicts that the effective surface density of atomic gas is ~10 M ⊙ pc -2 , independent of redshift, stellar mass or feedback. Our model suggests that the scatter in the SFR sequence reflects variations in the gas accretion history, and thus is insensitive to stellar mass, redshift or feedback. The large scatter in halo spin contributes negligibly, because it scatters galaxies along the SFR sequence. An observational consequence of this is that the scatter in the SFR sequence is independent of the size (both stellar and gaseous) of galaxy disks.

The kinematic connection between galaxies and dark matter haloes

Using estimates of dark halo masses from satellite kinematics, weak gravitational lensing and halo abundance matching, combined with the Tully-Fisher (TF) and Faber-Jackson relations, we derive the mean relation between the optical, V opt, and virial, V 200, circular velocities of early- and late-type galaxies at redshift z � 0. For late-type galaxies, VoptV200 over the velocity range V opt = 90-260 km s −1 , and is consistent with V opt = V max,h (the maximum circular velocity of NFW dark matter haloes in the concordancecold dark matter (� CDM) cosmology). However, for early-type galaxies VoptV200, with the exception of early-type galaxies with Vopt � 350 km s −1 . This is inconsistent with early-type galaxies being, in general, globally isothermal. For low-mass (Vopt 250 km s −1 ) early-types V opt > V max,h, indicating that baryons have modified the potential well, while high-mass (Vopt 400 km s −1 ) early-types have V opt < V max,h. Folding in measurements of the black hole mass-velocity dispersion relation, our results imply that the supermassive black hole-halo mass relation has a logarithmic slope which varies from � 1. 4a t halo masses of� 10 12 h −1 Mto � 0.65 at halo masses of 10 13.5 h −1 M� . The values of V opt/V 200 we infer for the Milky Way (MW) and M31 are lower than the values currently favoured by direct observations and dynamical models. This offset is due to the fact that the MW and M31 have higher V opt and lower V 200 compared to typical late-type galaxies of the same stellar masses. We show that current high-resolution cosmological hydrodynamical simulations are unable to form galaxies which simultaneously reproduce both the V opt/V 200 ratio and the V opt-Mstar (Tully-Fisher/Faber-Jackson) relation.

The impact of feedback on disc galaxy scaling relations

We use a disc galaxy evolution model to investigate the impact of mass outflows (a.k.a. feedback) on disc galaxy scaling relations, mass fractions and spin parameters. Our model follows the accretion, cooling, star formation and ejection of baryonic mass inside growing dark matter haloes, with cosmologically motivated angular momentum distributions, and dark matter halo structure. Models without feedback produce discs that are too small, too gas-poor and which rotate too fast. Feedback reduces the galaxy formation efficiency ∈ GF (defined as the fraction of the universally available baryons that end up as stars and cold gas in a given galaxy), resulting in larger discs with higher gas fractions and lower rotation velocities. Models with feedback can reproduce the zero-points of the scaling relations among rotation velocity, stellar mass and disc size, but only in the absence of adiabatic contraction. Our feedback mechanism is maximally efficient in expelling mass, but our successful models require 25 per cent of the supernova (SN) energy or 100 per cent of the SN momentum to drive an outflow. 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. Energy-driven winds result in steeper slopes to the galaxy-mass―halo-mass and stellar-mass-halo-mass relations, a shallower slope to the galaxy-size-stellar-mass relation at z = 0 and a steeper slope to the cold gas metallicity-stellar-mass relation at z ≃ 2. Observations favour the energy-driven wind at stellar masses below M star ≤ 10 10.5 M ⊙ , 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 with inflow and outflow. Yet this is the standard assumption in models of disc formation. Above a halo mass of M vir ≃ 10 12 M ⊙ , cooling becomes increasingly inefficient, which results in (j gal /m gal ) decreasing with increasing halo mass. Below a halo mass of M vir ≃ 10 12 M ⊙ , feedback becomes increasingly efficient. Feedback preferentially ejects low-angular-momentum material because star formation is more efficient at smaller galactic radii and at higher redshifts. 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 A cold dark matter.

The DEEP3 Galaxy Redshift Survey: the impact of environment on the size evolution of massive early-type galaxies at intermediate redshift

Using data drawn from the DEEP2 and DEEP3 Galaxy Redshift Surveys, we investigate the relationship between the environment and the structure of galaxies residing on the red sequence at intermediate redshift. Within the massive (10 < log_(10)(M_(★)/h^(−2) M_⊙) < 11) early-type population at 0.4 < z < 1.2, we find a significant correlation between local galaxy overdensity (or environment) and galaxy size, such that early-type systems in higher density regions tend to have larger effective radii (by ∼0.5 h^(−1) kpc or 25 per cent larger) than their counterparts of equal stellar mass and Sersic index in lower density environments. This observed size–density relation is consistent with a model of galaxy formation in which the evolution of early-type systems at z < 2 is accelerated in high-density environments such as groups and clusters and in which dry, minor mergers (versus mechanisms such as quasar feedback) play a central role in the structural evolution of the massive, early-type galaxy population.

The Redshift Evolution of LCDM Halo Parameters: Concentration, Spin, and Shape

We present a detailed study of the redshift evolution of dark matter halo structural parameters in a LambdaCDM cosmology. We study the mass and redshift dependence of the concentration, shape and spin parameter in Nbody simulations spanning masses from 10^{10} Msun/h to 10^{15} Msun/h and redshifts from 0 to 2. We present a series of fitting formulas that accurately describe the time evolution of the concentration-mass relation since z=2. Using arguments based on the spherical collapse model we study the behaviour of the scale length of the density profile during the assembly history of haloes, obtaining physical insights on the origin of the observed time evolution of the concentration mass relation. We also investigate the evolution with redshift of dark matter halo shape and its dependence on mass. Within the studied redshift range the relation between halo shape and mass can be well fitted by a power law. Finally we show that although for z=0 the spin parameter is practically mass independent, at increasing redshift it shows a increasing correlation with mass.