We explore the dependence of galaxy stellar population properties that are derived from broad-band spectral energy distribution fitting - such as age, stellar mass, dust reddening, etc. - on a ...variety of parameters, such as star formation histories, age grid, metallicity, initial mass function (IMF), dust reddening and reddening law, filter setup and wavelength coverage. Mock galaxies are used as test particles. We confirm our earlier results based on real z= 2 galaxies, that usually adopted τ-models lead to overestimate the star formation rate and to underestimate the stellar mass. Here, we show that - for star-forming galaxies - galaxy ages, masses and reddening can be well determined simultaneously only when the correct star formation history is identified. This is the case for inverted-τ models at high-z, for which we find that the mass recovery (at fixed IMF) is as good as ∼0.04 dex. However, since the right star formation history is usually unknown, we quantify the offsets generated by adopting standard fitting setups. Stellar masses are generally underestimated, which results from underestimating the age. For mixed fitting setups with a variety of star formation histories the median mass recovery at z∼ 2-3 is as decent as ∼0.1 dex (at fixed IMF), albeit with large scatter. The situation worsens towards lower redshifts, because of the variety of possible star formation histories and ages. At z∼ 0.5 the stellar mass can be underestimated by as much as ∼0.6 dex (at fixed IMF). A practical trick to improve upon this figure is to exclude reddening from the fitting parameters, as this helps to avoid unrealistically young and dusty solutions. Stellar masses are underestimated by a smaller amount (∼0.3 dex at z∼ 0.5). Reddening and the star formation rate should then be determined via a separate fitting. As expected, the recovery of properties is better for passive galaxies, for which e.g. the mass can be fully recovered (within ∼0.01 dex at fixed IMF) when using a fitting setup including metallicity effects. In both cases of star-forming as well as passive galaxies, the recovery of physical parameters is dependent on the spectral range involved in the fitting. We find that a coverage from the rest-frame ultraviolet to the rest-frame near-infrared appears to be optimal. We also quantify the effect of narrowing the wavelength coverage or adding and removing filter bands, which can be useful for planning observational surveys. Finally, we provide scaling relations that allow the transformation of stellar masses obtained using different template fitting setups and stellar population models.
This paper describes a new publicly available codebase for modeling galaxy formation in a cosmological context, the "Semi-Analytic Galaxy Evolution" model, or sage for short. super(5) sage is a ...significant update to the 2006 model of Croton et al. and has been rebuilt to be modular and customizable. The model will run on any N-body simulation whose trees are organized in a supported format and contain a minimum set of basic halo properties. In this work, we present the baryonic prescriptions implemented in sage to describe the formation and evolution of galaxies, and their calibration for three N-body simulations: Millennium, Bolshoi, and GiggleZ. Updated physics include the following: gas accretion, ejection due to feedback, and reincorporation via the galactic fountain; a new gas cooling-radio mode active galactic nucleus (AGN) heating cycle; AGN feedback in the quasar mode; a new treatment of gas in satellite galaxies; and galaxy mergers, disruption, and the build-up of intra-cluster stars. Throughout, we show the results of a common default parameterization on each simulation, with a focus on the local galaxy population.
Fitting synthetic spectral energy distributions (SEDs) to the multiband photometry of galaxies to derive their star formation rates (SFRs), stellar masses, ages, etc. requires making a priori ...assumptions about their star formation histories (SFHs). A widely adopted parametrization of the SFH, the so-called τ models where SFR ∝ e−t/τ is shown to lead to unrealistically low ages when applied to a sample of actively star-forming galaxies at z∼ 2, a problem shared by other SFHs when the age is left as a free parameter in the fitting procedure. This happens because the SED of such galaxies, at all wavelengths, is dominated by their youngest stellar populations, which outshine the older ones. Thus, the SED of such galaxies conveys little information on the beginning of star formation (SF), i.e. on the age of their oldest stellar populations. To cope with this problem, besides τ models (hereafter called direct-τ models), we explore a variety of SFHs, such as constant SFR and inverted-τ models (with SFR ∝ e+t/τ), along with various priors on age, including assuming that SF started at high redshift in all the galaxies in the test sample. We find that inverted-τ models with such latter assumption give SFRs and extinctions in excellent agreement with the values derived using only the UV part of the SED, which is the one most sensitive to ongoing SF and reddening. These models are also shown to accurately recover the SFRs and masses of mock galaxies at z∼ 2 constructed from semi-analytic models, which we use as a further test. All other explored SFH templates do not fulfil these two tests as well as inverted-τ models do. In particular, direct-τ models with unconstrained age in the fitting procedure overestimate SFRs and underestimate stellar mass, and would exacerbate an apparent mismatch between the cosmic evolution of the volume densities of SFR and stellar mass. We conclude that for high-redshift star-forming galaxies an exponentially increasing SFR with a high formation redshift is preferable to other forms of the SFH so far adopted in the literature.
We build a theoretical model to study the origin of the globular cluster metallicity bimodality in the hierarchical galaxy assembly scenario. The model is based on empirical relations such as the ...galaxy mass-metallicity relation O/H-M sub(star) as a function of redshift, and on the observed galaxy stellar mass function up to redshift z ~ 4. We derive a new galaxy Fe/H-M sub(star) relation as a function of redshift, and by assuming that globular clusters share the metallicity of their original parent galaxy at the time of their formation, we populate the merger tree with globular clusters. The main results and predictions of the model are the following. (1) The hierarchical clustering scenario naturally predicts a metallicity bimodality in the galaxy globular cluster population, where the metal-rich subpopulation is composed of globular clusters formed in the galaxy main progenitor around redshift z ~ 2, and the metal-poor subpopulation is composed of clusters accreted from satellites, and formed at redshifts z ~ 3-4.
We investigate the evolution of brightest cluster galaxies (BCGs) from redshift z ~ 1.6 to z = 0. We compare the model predictions of the K-band luminosity evolution and the J - K, V - I, and I - K ...color evolution with a series of data sets, including those of Collins et al. who argued that semi-analytic models based on the Millennium simulation cannot reproduce the red colors and high luminosity of BCGs at z > 1. We show instead that the model is well in range of the observed luminosity and correctly reproduces the color evolution of BCGs in the whole redshift range up to z ~ 1.6. We argue that the success of the semi-analytic model is in large part due to the implementation of a more sophisticated spectro-photometric model. In a ACDM universe, we define such evolution as "passive in the hierarchical sense."
Abstract
We present the ∼800 star formation rate maps for the Sydney-AAO Multi-object Integral field spectrograph (SAMI) Galaxy Survey based on H α emission maps, corrected for dust attenuation via ...the Balmer decrement, that are included in the SAMI Public Data Release 1. We mask out spaxels contaminated by non-stellar emission using the O iii/H β, N ii/H α, S ii/H α, and O i/H α line ratios. Using these maps, we examine the global and resolved star-forming main sequences of SAMI galaxies as a function of morphology, environmental density, and stellar mass. Galaxies further below the star-forming main sequence are more likely to have flatter star formation profiles. Early-type galaxies split into two populations with similar stellar masses and central stellar mass surface densities. The main-sequence population has centrally concentrated star formation similar to late-type galaxies, while galaxies >3σ below the main sequence show significantly reduced star formation most strikingly in the nuclear regions. The split populations support a two-step quenching mechanism, wherein halo mass first cuts off the gas supply and remaining gas continues to form stars until the local stellar mass surface density can stabilize the reduced remaining fuel against further star formation. Across all morphologies, galaxies in denser environments show a decreased specific star formation rate from the outside in, supporting an environmental cause for quenching, such as ram-pressure stripping or galaxy interactions.
Abstract
The fraction of galaxies supported by internal rotation compared to galaxies stabilized by internal pressure provides a strong constraint on galaxy formation models. In integral field ...spectroscopy surveys, this fraction is biased because survey instruments typically only trace the inner parts of the most massive galaxies. We present aperture corrections for the two most widely used stellar kinematic quantities V/σ and λR (spin parameter proxy). Our demonstration involves integral field data from the SAMI (Sydney-AAO Multi-object Integral-field spectrograph) Galaxy Survey and the ATLAS3D survey. We find a tight relation for both V/σ and λR when measured in different apertures that can be used as a linear transformation as a function of radius, i.e. a first-order aperture correction. In degraded seeing, however, the aperture corrections are more significant as the steeper inner profile is more strongly affected by the point spread function than the outskirts. We find that V/σ and λR radial growth curves are well approximated by second-order polynomials. By only fitting the inner profile (0.5Re), we successfully recover the profile out to one Re if a constraint between the linear and quadratic parameter in the fit is applied. However, the aperture corrections for V/σ and λR derived by extrapolating the profiles perform as well as applying a first-order correction. With our aperture-corrected λR measurements, we find that the fraction of slow rotating galaxies increases with stellar mass. For galaxies with log M*/M⊙ > 11, the fraction of slow rotators is 35.9 ± 4.3 per cent, but is underestimated if galaxies without coverage beyond one Re are not included in the sample (24.2 ± 5.3 per cent). With measurements out to the largest aperture radius, the slow rotator fraction is similar as compared to using aperture-corrected values (38.3 ± 4.4 per cent). Thus, aperture effects can significantly bias stellar kinematic integral field spectrograph studies, but this bias can now be removed with the method outlined here.
We examine the kinematic morphology of early-type galaxies (ETGs) in eight galaxy clusters in the Sydney-AAO Multi-object Integral-field spectrograph Galaxy Survey. The clusters cover a mass range of ...and we measure spatially resolved stellar kinematics for 315 member galaxies with stellar masses within 1 R200 of the cluster centers. We calculate the spin parameter, λR, and use this to classify the kinematic morphology of the galaxies as fast or slow rotators (SRs). The total fraction of SRs in the ETG population is FSR = 0.14 0.02 and does not depend on host cluster mass. Across the eight clusters, the fraction of SRs increases with increasing local overdensity. We also find that the slow-rotator fraction increases at small clustercentric radii (Rcl < 0.3 R200), and note that there is also an increase in the slow-rotator fraction at Rcl ∼ 0.6 R200. The SRs at these larger radii reside in the cluster substructure. We find that the strongest increase in the slow-rotator fraction occurs with increasing stellar mass. After accounting for the strong correlation with stellar mass, we find no significant relationship between spin parameter and local overdensity in the cluster environment. We conclude that the primary driver for the kinematic morphology-density relationship in galaxy clusters is the changing distribution of galaxy stellar mass with the local environment. The presence of SRs in the substructure suggests that the cluster kinematic morphology-density relationship is a result of mass segregation of slow-rotating galaxies forming in groups that later merge with clusters and sink to the cluster center via dynamical friction.
We introduce the Theoretical Astrophysical Observatory (TAO), an online virtual laboratory that houses mock observations of galaxy survey data. Such mocks have become an integral part of the modern ...analysis pipeline. However, building them requires expert knowledge of galaxy modeling and simulation techniques, significant investment in software development, and access to high performance computing. These requirements make it difficult for a small research team or individual to quickly build a mock catalog suited to their needs. To address this TAO offers access to multiple cosmological simulations and semi-analytic galaxy formation models from an intuitive and clean web interface. Results can be funnelled through science modules and sent to a dedicated supercomputer for further processing and manipulation. These modules include the ability to (1) construct custom observer light cones from the simulation data cubes; (2) generate the stellar emission from star formation histories, apply dust extinction, and compute absolute and/or apparent magnitudes; and (3) produce mock images of the sky. All of TAO's features can be accessed without any programming requirements. The modular nature of TAO opens it up for further expansion in the future.
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