Stellar Populations in Bulges

We present line strengths in the bulges and inner disks of 38 galaxies in the local universe, including several galaxies whose bulges were previously identified as being disk-like in their colors or kinematics, to see if their spectral properties reveal evidence for secular evolution. We find that red bulges of all Hubble types are similar to luminous ellipticals in their central stellar populations. They have large luminosity-weighted ages, metallicities, and alpha/Fe ratios. Blue bulges can be separated into a metal-poor class that is restricted to late-types with small velocity dispersion and a young, metal-rich class that includes all Hubble types and velocity dispersions. Luminosity-weighted metallicities and alpha/Fe ratios are sensitive to central velocity dispersion and maximum disk rotational velocity. Red bulges and ellipticals follow the same scaling relations. We see differences in some scaling relations between blue and red bulges and between bulges of barred and unbarred galaxies. Most bulges have decreasing metallicity with increasing radius; galaxies with larger central metallicities have steeper gradients. Where positive age gradients (with the central regions being younger) are present, they are invariably in barred galaxies. The metallicities of bulges are correlated with those of their disks. While this and the differences between barred and unbarred galaxies suggest that secular evolution cannot be ignored, our results are generally consistent with the hypothesis that mergers have been the dominant mechanism responsible for bulge formation.Comment: 30 pages, 21 figures; submitted to MNRA

Structure Formation with Cold + Hot Dark Matter

We report results from high-resolution particle-mesh (PM) N-body simulations of structure formation in an $\Omega=1$ cosmological model with a mixture of Cold plus Hot Dark Matter (C+HDM) having $\Omega_{\rm cold}=0.6$, $\Omega_\nu=0.3$, and $\Omega_{\rm baryon}=0.1$. We present analytic fits to the C+HDM power spectra for both cold and hot ($\nu$) components, which provide initial conditions for our nonlinear simulations. In order to sample the neutrino velocities adequately, these simulations included six times as many neutrino particles as cold particles. Our simulation boxes were 14, 50, and 200~Mpc cubes (with $H_0=50$ km s$^{-1}$ Mpc$^{-1}$); we also did comparison simulations for Cold Dark Matter (CDM) in a 50~Mpc box. C+HDM with linear bias factor $b=1.5$ is consistent both with the COBE data and with the galaxy correlations we calculate. We find the number of halos as a function of mass and redshift in our simulations; our results for both CDM and C+HDM are well fit by a Press-Schechter model. The number density of galaxy-mass halos is smaller than for CDM, especially at redshift $z>2$, but the numbers of cluster-mass halos are comparable. We also find that on galaxy scales the neutrino velocities and flatter power spectrum in C+HDM result in galaxy pairwise velocities that are in good agreement with the data, and about 30\% smaller than in CDM with the same biasing factor. On scales of several tens of Mpc, the C+HDM streaming velocities are considerably larger than CDM. Thus C+HDM looks promising as a model of structure formation.Comment: 33pp., 16+ figures not included (available by mail), SCIPP-92/5

Linearizing the Observed Power Spectrum

Reconstruction of the linear power spectrum from observational data provides a way to compare cosmological models to a large amount of data, as Peacock & Dodds (1994, 1996) have shown. By applying the appropriate corrections to the observational power spectrum it is possible to recover the underlying linear power spectrum for any cosmological model. Using extensive N-body simulations we demonstrate that the method is applicable to a wide range of cosmological models. However, we find that the recovery of the linear power spectrum from observations following PD94 is misleading because the corrections are model- dependent. When we apply the proper corrections for a given model to the observational power spectrum, we find that no model in our test group recovers the linear power spectrum well for the bias suggested by PD94 between Abell, Radio, Optical, and IRAS catalogs 4.5:1.9:1.3:1, with b_IRAS=1. When we allow b_IRAS to vary we find that: (i)CHDM models give very good fits to observations if optically-selected galaxies are slightly biased b_Opt=1.1 (ii) Most LCDM models give worse but acceptable fits if blue galaxies are considerably antibiased: 0.6<b_Opt<0.9 and fail if optical galaxies are biased. (iii)There is a universal shape of the recovered linear power spectrum of all LCDM models over their entire range of explored wavenumbers,0.01<k<0.6h\Mpc. Recovered spectra of CDM and CHDM models are nearly the same as that of LCDM in the region 0.01<k<0.2h/Mpc but diverge from this spectrum at higher k.Comment: submitted to the Mon.Not.R.Astron.Soc., LaTeX (uses mn.sty, graphics.sty, endfloat.sty, trig.sty), 15 pages, 10 figures, also available at http://astro.nmsu.edu/~akravtso/GROUP/group_publications.html or at ftp://charon.nmsu.edu/pub/aklypin/LINOB

The Properties of Galaxies in Voids

We present a comparison of the properties of galaxies in the most underdense regions of the Universe, where the galaxy number density is less than 10% of the mean density, with galaxies from more typical regions. We have compiled a sample of galaxies in 46 large nearby voids that were identified using the Sloan Digital Sky Survey DR4, which provides the largest coverage of the sky. We study the u-r color distribution, morphology, specific star formation rate, and radial number density profiles for a total of 495 galaxies fainter than M_r=-20.4 +5logh located inside the voids and compare these properties with a control sample of field galaxies. We show that there is an excess of blue galaxies inside the voids. However, inspecting the properties of blue and red galaxies separately, we find that galaxy properties such as color distribution, bulge-to-total ratios, and concentrations are remarkably similar between the void and overall sample. The void galaxies also show the same specific star formation rate at fixed color as the control galaxies. We compare our results with the predictions of cosmological simulations of galaxy formation using the Millennium Run semi-analytic galaxy catalog. We show that the properties of the simulated galaxies in large voids are in reasonably good agreement with those found in similar environments in the real Universe. To summarize, in spite of the fact that galaxies in voids live in the least dense large-scale environment, this environment makes very little impact on properties of galaxies.Comment: 11 pages, 15 figures, Submitted to MNRA

Metallicity Distribution Functions of Four Local Group dwarf galaxies

We present stellar metallicities in Leo I, Leo II, IC 1613, and Phoenix dwarf galaxies derived from medium (F390M) and broad (F555W, F814W) band photometry using the Wide Field Camera 3 (WFC3) instrument aboard the Hubble Space Telescope. We measured metallicity distribution functions (MDFs) in two ways, 1) matching stars to isochrones in color-color diagrams, and 2) solving for the best linear combination of synthetic populations to match the observed color-color diagram. The synthetic technique reduces the effect of photometric scatter, and produces MDFs 30-50 % narrower than the MDFs produced from individually matched stars. We fit the synthetic and individual MDFs to analytical chemical evolution models (CEM) to quantify the enrichment and the effect of gas flows within the galaxies. Additionally, we measure stellar metallicity gradients in Leo I and II. For IC 1613 and Phoenix our data do not have the radial extent to confirm a metallicity gradient for either galaxy. We find the MDF of Leo I (dwarf spheroidal) to be very peaked with a steep metal rich cutoff and an extended metal poor tail, while Leo II (dwarf spheroidal), Phoenix (dwarf transition) and IC 1613 (dwarf irregular) have wider, less peaked MDFs than Leo I. A simple CEM is not the best fit for any of our galaxies, therefore we also fit the `Best Accretion Model' of Lynden-Bell 1975. For Leo II, IC 1613 and Phoenix we find similar accretion parameters for the CEM, even though they all have different effective yields, masses, star formation histories and morphologies. We suggest that the dynamical history of a galaxy is reflected in the MDF, where broad MDFs are seen in galaxies that have chemically evolved in relative isolation and narrowly peaked MDFs are seen in galaxies that have experienced more complicated dynamical interactions concurrent with their chemical evolution.Comment: 15 pages, 8 figures, accepted in A

The Bulge-Halo Connection in Galaxies: A Physical Interpretation of the Vcirc-sigma_0 Relation

We explore the dependence of the ratio of a galaxy's circular velocity, Vcirc, to its central velocity dispersion, sigma_0, on morphology, or equivalently total light concentration. Such a dependence is expected if light traces the mass. Over the full range of galaxy types, masses and brightnesses, and assuming that the gas velocity traces the circular velocity, we find that galaxies obey the relation log(Vcirc/sigma_0)= 0.63-0.11*C28 where C28=5log(r80/r20) and the radii are measured at 80 percent and 20 percent of the total light. Massive galaxies scatter about the Vcirc = sqrt(2)*sigma_0 line for isothermal stellar systems. Disk galaxies follow the simple relation Vcirc/sigma_0=2(1-B/T), where B/T is the bulge-to-total light ratio. For pure disks, C28~2.8, B/T -> 0, and Vcirc~=2*sigma_0. Self-consistent equilibrium galaxy models from Widrow & Dubinski (2005) constrained to match the size-luminosity and velocity-luminosity relations of disk galaxies fail to match the observed Vcirc/sigma_0 distribution. Furthermore, the matching of dynamical models for Vcirc(r)/sigma(r) with observations of dwarf and elliptical galaxies suffers from limited radial coverage and relatively large error bars; for dwarf systems, however, kinematical measurements at the galaxy center and optical edge suggest Vcirc(Rmax) > 2*sigma_0 (in contrast with past assumptions that Vcirc = sqrt(2)*sigma_0 for dwarfs.) The Vcirc-sigma_0-C28 relation has direct implications for galaxy formation and dynamical models, galaxy scaling relations, the mass function of galaxies, and the links between respective formation and evolution processes for a galaxy's central massive object, bulge, and dark matter halo.Comment: Accepted for publication in ApJL. Current version matches ApJL page requiremen