Chiara Tonini

Star formation rates and masses of z∼ 2 galaxies from multicolour photometry

Fitting synthetic spectral energy distributions (SED) to the multi-band photometry of galaxies to derive their star formation rates (SFR), stellar masses, ages, etc. requires making a priori assumptions about their star formation histories (SFH). A widely adopted parameterization of the SFH, the so-called �-models where SFR / e t/� 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, i.e., on the age of their oldest stellar populations. To cope with this problem, besides �-models (hereafter called direct-� models), we explore a variety of SFHs, such as constant SFR and inverted-� models (with SFR / e +t/� ), along with various priors on age, including assuming that star formation started at high redshift in all the galaxies in the test sample. We find that inverted-� 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 star formation 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 test as well as inverted-� models do. In particular, direct-� models with unconstrained age in the fitting procedure overstimate 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.

The SAMI Galaxy Survey: instrument specification and target selection

The SAMI Galaxy Survey will observe 3400 galaxies with the Sydney-AAO Multi- object Integral-field spectrograph (SAMI) on the Anglo-Australian Telescope (AAT) in a 3-year survey which began in 2013. We present the throughput of the SAMI system, the science basis and specifications for the target selection, the survey observation plan and the combined properties of the selected galaxies. The survey includes four volume-limited galaxy samples based on cuts in a proxy for stellar mass, along with low-stellar-mass dwarf galaxies all selected from the Galaxy And Mass Assembly (GAMA) survey. The GAMA regions were selected because of the vast array of ancillary data available, including ultraviolet through to radio bands. These fields are on the celestial equator at 9, 12, and 14.5 hours, and cover a total of 144 square degrees (in GAMA-I). Higher density environments are also included with the addition of eight clusters. The clusters have spectroscopy from 2dFGRS and SDSS and photometry in regions covered by the Sloan Digital Sky Survey (SDSS) and/or VLT Survey Telescope/ATLAS. The aim is to cover a broad range in stellar mass and environment, and therefore the primary survey targets cover redshifts 0.004 < z < 0.095, magnitudes rpet < 19.4, stellar masses 107– 1012M⊙, and environments from isolated field galaxies through groups to clusters of _ 1015M⊙.

Recovering galaxy stellar population properties from broad-band spectral energy distribution fitting

We explore the dependence of galaxy stellar population properties that are derived from broad-band spectral energy distribution (SED) 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 ages. 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 UV to the rest-frame near-IR 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.

The SAMI Galaxy Survey: Early Data Release

We present the Early Data Release of the Sydney–AAO Multi-object Integral field spectrograph (SAMI) Galaxy Survey. The SAMI Galaxy Survey is an ongoing integral field spectroscopic survey of _3400 low-redshift (z < 0:12) galaxies, covering galaxies in the field and in groups within the Galaxy And Mass Assembly (GAMA) survey regions, and a sample of galaxies in clusters. In the Early Data Release, we publicly release the fully calibrated datacubes for a representative selection of 107 galaxies drawn from the GAMA regions, along with information about these galaxies from the GAMA catalogues. All datacubes for the Early Data Release galaxies can be downloaded individually or as a set from the SAMI Galaxy Survey website. In this paper we also assess the quality of the pipeline used to reduce the SAMI data, giving metrics that quantify its performance at all stages in processing the raw data into calibrated datacubes. The pipeline gives excellent results throughout, with typical sky subtraction residuals in the continuum of 0.9–1.2 per cent, a relative flux calibration uncertainty of 4.1 per cent (systematic) plus 4.3 per cent (statistical), and atmospheric dispersion removed with an accuracy of 0:0009, less than a fifth of a spaxel.

THE METALLICITY BIMODALITY OF GLOBULAR CLUSTER SYSTEMS: A TEST OF GALAXY ASSEMBLY AND OF THE EVOLUTION OF THE GALAXY MASS-METALLICITY RELATION

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 star as a function of redshift, and on the observed galaxy stellar mass function up to redshift z ~ 4. We make use of the theoretical merger rates as a function of mass and redshift from the Millennium simulation to build galaxy merger trees. We derive a new galaxy [Fe/H]-M 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. We perform a series of Monte Carlo simulations of the galaxy hierarchical assembly, and study the properties of the final globular cluster population as a function of galaxy mass, assembly and star formation history, and under different assumptions for the evolution of the galaxy mass-metallicity relation. 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. (2) The model reproduces the observed relations by Peng et al. for the metallicities of the metal-rich and metal-poor globular cluster subpopulations as a function of galaxy mass; the positions of the metal-poor and metal-rich peaks depend exclusively on the evolution of the galaxy mass-metallicity relation and the [O/Fe], both of which can be constrained by this method. In particular, we find that the galaxy [O/Fe] evolves linearly with redshift from a value of ~0.5 at redshift z ~ 4 to a value of ~0.1 at z = 0. (3) For a given galaxy mass, the relative strength of the metal-rich and metal-poor peaks depends exclusively on the galaxy assembly and star formation history, where galaxies living in denser environments and/or early-type galaxies show a larger fraction of metal-poor clusters, while galaxies with a sparse merger history and/or late-type galaxies are dominated by metal-rich clusters. (4) The globular cluster metallicity bimodality disappears for galaxy masses around and below M star ~ 109 M ☉, and for redshifts z > 2.

The SAMI Galaxy Survey: cubism and covariance, putting round pegs into square holes

We present a methodology for the regularization and combination of sparse sampled and irregularly gridded observations from fibre-optic multiobject integral field spectroscopy. The approach minimizes interpolation and retains image resolution on combining subpixel dithered data. We discuss the methodology in the context of the Sydney–AAO multiobject integral field spectrograph (SAMI) Galaxy Survey underway at the Anglo-Australian Telescope. The SAMI instrument uses 13 fibre bundles to perform high-multiplex integral field spectroscopy across a 1° diameter field of view. The SAMI Galaxy Survey is targeting ∼3000 galaxies drawn from the full range of galaxy environments. We demonstrate the subcritical sampling of the seeing and incomplete fill factor for the integral field bundles results in only a 10 per cent degradation in the final image resolution recovered. We also implement a new methodology for tracking covariance between elements of the resulting data cubes which retains 90 per cent of the covariance information while incurring only a modest increase in the survey data volume.

The effect of thermally pulsating asymptotic giant branch stars on the evolution of the rest‐frame near‐infrared galaxy luminosity function

We address the fundamental question of matching the rest-frame K-band luminosity function (LF) of galaxies over the Hubble time using semi-analytic models after modification of the stellar population modelling. We include the Maraston evolutionary synthesis models, which feature a higher contribution by the thermally pulsating asymptotic giant branch (TP-AGB) stellar phase, into three different semi-analytic models, namely the De Lucia and Blaizot version of the Munich model, morgana and the Menci model. We leave all other input physics and parameters unchanged. We find that the modification of the stellar population emission can solve the mismatch between models and the observed rest-frame K-band luminosity from the brightest galaxies derived from UKIRT Infrared Deep Sky Survey data at high redshift. For all explored semi-analytic models, this holds at the redshifts – between 2 and 3 – where the discrepancy was recently pointed out. The reason for the success is that at these cosmic epochs the model galaxies have the right age (∼1 Gyr) to contain a well-developed TP-AGB phase, which makes them redder without the need of changing their mass or age. We have also computed a version of the Munich model using the Charlot and Bruzual models that adopt the Marigo TP-AGB prescription and find the same result as that with the Maraston models. At the same time, the known overestimation of the faint end is enhanced in the K band when including the TP-AGB contribution. At lower redshifts (z < 2) some of the explored models deviate from the data. This is due to short merging time-scales and inefficient ‘radio-mode’ active galactic nucleus feedback. Our results show that a strong evolution in mass predicted by hierarchical models is compatible with no evolution on the bright end of the K-band LF from z= 3 to the local universe. This means that, at high redshifts and contrary to what is commonly accepted, K-band emission is not necessarily a good tracer of galaxy mass.

The impact of thermally pulsing asymptotic giant branch stars on hierarchical galaxy formation models

The spectro-photometric properties of galaxies in galaxy formation models are obtained by combining the predicted history of star formation and mass accretion with the physics of stellar evolution through stellar population models. In the recent literature, significant differences have emerged regarding the implementation of the thermally pulsing asymptotic giant branch phase of stellar evolution. The emission in the TP-AGB phase dominates the bolometric and near-IR spectrum of intermediate-age (∼1Gyr) stellar populations, hence it is crucial for the correct modelling of the galaxy luminosities and colours. In this paper, for the first time, we incorporate a full prescription of the TP-AGB phase in a semi-analytic model of galaxy formation. We find that the inclusion of the TP-AGB in the model spectra dramatically alters the predicted colour–magnitude relation and its evolution with redshift. When the TP-AGB phase is active, the rest-frame V−K galaxy colours are redder by almost 2 mag in the redshift range z∼ 2–3 and by 1 mag at z∼ 1. Very red colours are produced in disc galaxies, so that the V−K colour distributions of disc and spheroids are virtually undistinguishable at low redshifts. We also find that the galaxy K-band emission is more than 1 mag higher in the range z∼ 1–3. This may alleviate the difficulties met by the hierarchical clustering scenario in predicting the red galaxy population at high redshifts. The comparison between simulations and observations has to be revisited in the light of our results.

The SAMI Galaxy Survey: the link between angular momentum and optical morphology

We investigate the relationship between stellar and gas specific angular momentum j, stellar mass M-* and optical morphology for a sample of 488 galaxies extracted from the Sydney-AAO Multi-object Integral field Galaxy Survey. We find that j, measured within one effective radius, monotonically increases with M-* and that, for M-* > 10(9.5) M-aS (TM), the scatter in this relation strongly correlates with optical morphology (i.e. visual classification and S,rsic index). These findings confirm that massive galaxies of all types lie on a plane relating mass, angular momentum and stellar-light distribution, and suggest that the large-scale morphology of a galaxy is regulated by its mass and dynamical state. We show that the significant scatter in the M-*-j relation is accounted for by the fact that, at fixed stellar mass, the contribution of ordered motions to the dynamical support of galaxies varies by at least a factor of 3. Indeed, the stellar spin parameter (quantified via lambda(R)) correlates strongly with S,rsic and concentration indices. This correlation is particularly strong once slow rotators are removed from the sample, showing that late-type galaxies and early-type fast rotators form a continuous class of objects in terms of their kinematic properties.

THE SAMI GALAXY SURVEY: TOWARD A UNIFIED DYNAMICAL SCALING RELATION FOR GALAXIES OF ALL TYPES

We take advantage of the first data from the Sydney-AAO Multi-object Integral field Galaxy Survey to investigate the relation between the kinematics of gas and stars, and stellar mass in a comprehensive sample of nearby galaxies. We find that all 235 objects in our sample, regardless of their morphology, lie on a tight relation linking stellar mass (M-*) to internal velocity quantified by the S-0.5 parameter, which combines the contribution of both dispersion (sigma) and rotational velocity (V-rot) to the dynamical support of a galaxy (S-0.5 = root 0.5 V-rot(2) + sigma(2)). Our results are independent of the baryonic component from which sigma and V-rot are estimated, as the S-0.5 of stars and gas agree remarkably well. This represents a significant improvement compared to the canonical M-* versus Vrot and M-* versus s relations. Not only is no sample pruning necessary, but also stellar and gas kinematics can be used simultaneously, as the effect of asymmetric drift is taken into account once V-rot and sigma are combined. Our findings illustrate how the combination of dispersion and rotational velocities for both gas and stars can provide us with a single dynamical scaling relation valid for galaxies of all morphologies across at least the stellar mass range 8.5 < log (M-*/M-circle dot) < 11. Such relation appears to be more general and at least as tight as any other dynamical scaling relation, representing a unique tool for investigating the link between galaxy kinematics and baryonic content, and a less biased comparison with theoretical models.