Spatially Resolved Galaxy Star Formation and its Environmental Dependence

The role of star formation in galaxiesis clearly a fundamental component of their evolution, although itis becoming clear that galaxy environments may also play a significantrole. To explore the relationship between environment and star formationin galaxies, I use the photometric information contained in individualpixels of 44,964 galaxies (volume-limited) fromthe Fourth Data Release of the Sloan Digital Sky Survey. Iuse the pixel-z technique, which combines stellar population synthesis modelswith photometric redshift template fitting on the scale of individualpixels in galaxy images. Spectral energy distributions are constructed, sampling a wide range ofproperties such as age, star formation rate (SFR), dust obscuration andmetallicity. By summing the SFRs in the pixels, I show that, as found in other studies, thedistribution of total galaxy SFR shifts to lower values as the local densityof surrounding galaxies increases. The effectis most prominent in the galaxies with the highest SFR. Since the method enables an estimate to be made of the spatial distribution of star formation within galaxies, the mean SFR of each galaxy is then calculated asa function of radius. I find that, on average, the mean SFR is dominatedby star formation in the central regions of galaxies and it is this centralstar formation that is suppressed in high density environments. The mean SFR in the outskirts of galaxies is found to be largely independentof environmental effects. These trends are shared by galaxieswhich are highly star forming.I also investigate the impact of the density-morphology relation of galaxies on the observed trends. Early-type and late-type galaxies exhibit distinct radial SFR distributions.A suppression of star formation in the highest density environments is still found in the highest star forming galaxies within each type. I show that the density-morphology relation alone cannot accountfor this observed suppression. This points to a mechanism by which the environment governs the evolution of galaxies, affecting the star formation in the innermost regions in both early and late-type galaxies. I suggest that this is a natural consequence of "downsizing" in galaxies

Color Gradients in Galaxies Out to z~3: Dependence on Galaxy Properties

Using HST/ACS observations, we measure the color gradients of 3248 galaxies in the GOODS-South field out to z~3 and i_{AB}<25.5 and characterize their dependence on galaxy properties (luminosity, apparent magnitude, galaxy size, redshift and morphological type). The color gradient is measured by the difference of v-i color outside (R_{50}<r<2R_{50}) and inside the half light radius. The gradient shows little evolution with redshift up to z~1 but increases from z~1 to z~2 before flattening out. It also increases with apparent magnitude, with a median value of 0.24 magnitudes at i_{AB}~25.5. It has a strong color dependence, with the bluest galaxies (in terms of observed color) having cores that are bluer relative to their outskirts. We probe the redshift evolution by stacking galaxies and measuring the radial variation of v-i color within them. At low redshifts (z<0.5), the centres of galaxies (r<R_{50}) are slightly redder than their outskirts (1.5R_{50}<r<2R_{50}). Galaxies at z~1 and -22.0<M_I<-21.0 are bluer in their cores by 0.1 magnitudes, on average, compared to their outskirts. For z>1, galaxies show increasingly bluer cores while the color of the outskirts does not change as rapidly. At z~2.5 and -22.0<M_I<-21.0, we observe a difference, on average, of 0.4 magnitudes between the centre and the outskirts. The observed color gradients may indicate that strong star formation in galaxies at z>=2 is concentrated in their central regions. These color gradients and their dependence on galaxy properties could also have a significant impact on shear measurements in upcoming weak lensing cosmological surveys.Comment: 13 pages, 7 figures, submitted to Ap

Spatially Resolved Galaxy Star Formation and its Environmental Dependence I

We use the photometric information contained in individual pixels of 44,964 (0.019<z<0.125 and -23.5<M_r<-20.5) galaxies in the Fourth Data Release (DR4) of the Sloan Digital Sky Survey to investigate the effects of environment on galaxy star formation (SF). We use the pixel-z technique, which combines stellar population synthesis models with photometric redshift template fitting on the scale of individual pixels in galaxy images. Spectral energy distributions are constructed, sampling a wide range of properties such as age, star formation rate (SFR), dust obscuration and metallicity. By summing the SFRs in the pixels, we demonstrate that the distribution of total galaxy SFR shifts to lower values as the local density of surrounding galaxies increases, as found in other studies. The effect is most prominent in the galaxies with the highest star formation, and we see the break in the SFR-density relation at a local galaxy density of $\approx 0.05$(Mpc/h)$^{-3}$. Since our method allows us to spatially resolve the SF distribution within galaxies, we can calculate the mean SFR of each galaxy as a function of radius. We find that on average the mean SFR is dominated by SF in the central regions of galaxies, and that the trend for suppression of SFR in high density environments is driven by a reduction in this nuclear SF. We also find that the mean SFR in the outskirts is largely independent of environmental effects. This trend in the mean SFR is shared by galaxies which are highly star forming, while those which are weakly star forming show no statistically significant correlation between their environment and the mean SFR at any radius.Comment: 37 pages, 11 figures. Referee's comments included and matches version accepted for publication in the Astrophysical Journal. For high resolution figures, see http://www.phyast.pitt.edu/~welikala/pixelz/paper1

Pixel-z: Studying Substructure and Stellar Populations in Galaxies out to z~3 using Pixel Colors I. Systematics

We perform a pixel-by-pixel analysis of 467 galaxies in the GOODS-VIMOS survey to study systematic effects in extracting properties of stellar populations (age, dust, metallicity and SFR) from pixel colors using the pixel-z method. The systematics studied include the effect of the input stellar population synthesis model, passband limitations and differences between individual SED fits to pixels and global SED-fitting to a galaxy's colors. We find that with optical-only colors, the systematic errors due to differences among the models are well constrained. The largest impact on the age and SFR e-folding time estimates in the pixels arises from differences between the Maraston models and the Bruzual&Charlot models, when optical colors are used. This results in systematic differences larger than the 2{\sigma} uncertainties in over 10 percent of all pixels in the galaxy sample. The effect of restricting the available passbands is more severe. In 26 percent of pixels in the full sample, passband limitations result in systematic biases in the age estimates which are larger than the 2{\sigma} uncertainties. Systematic effects from model differences are reexamined using Near-IR colors for a subsample of 46 galaxies in the GOODS-NICMOS survey. For z > 1, the observed optical/NIR colors span the rest frame UV-optical SED, and the use of different models does not significantly bias the estimates of the stellar population parameters compared to using optical-only colors. We then illustrate how pixel-z can be applied robustly to make detailed studies of substructure in high redshift galaxies such as (a) radial gradients of age, SFR, sSFR and dust and (b) the distribution of these properties within subcomponents such as spiral arms and clumps. Finally, we show preliminary results from the CANDELS survey illustrating how the new HST/WFC3 data can be exploited to probe substructure in z~1-3 galaxies.Comment: 37 pages, 21 figures, submitted to Ap

Spatially Resolved Galaxy Star Formation and its Environmental Dependence II. Effect of the Morphology-Density Relation

In this second of a series of papers on spatially resolved star formation, we investigate the impact of the density-morphology relation of galaxies on the spatial variation of star formation (SF) and its dependence on environment. We find that while a density-morphology relation is present for the sample, it cannot solely explain the observed suppression of SF in galaxies in high-density environments. We also find that early-type and late-type galaxies exhibit distinct radial star formation rate (SFR) distributions, with early-types having a SFR distribution that extends further relative to the galaxy scale length, compared to late-types at all densities. We find that a suppression of SF in the highest density environments is found in the highest star forming galaxies for both galaxy types. This suppression occurs in the innermost regions in late-types (r <= 0.125 Petrosian radii), and further out in radius in early-types (0.125< r <= 0.25 Petrosian radii). When the full sample is considered no clear suppression of SF is detected, indicating that the environmental trends are driven only by the highest SF galaxies. We demonstrate that the density-morphology relation alone cannot account for the suppression of SF in the highest density environments. This points to an environmentally-governed evolutionary mechanism that affects the SF in the innermost regions in both early and late-type galaxies. We suggest that this is a natural consequence of the "downsizing" of SF in galaxies.Comment: 31 pages, 8 figures, replaced with accepted version, added reference

Mass and environment as drivers of galaxy evolution in SDSS and zCOSMOS and the origin of the Schechter function

We explore the inter-relationships between mass, star-formation rate and environment in the SDSS, zCOSMOS and other surveys. The differential effects of mass and environment are completely separable to z ~ 1, indicating that two distinct processes are operating, "mass-quenching" and "environment-quenching". Environment-quenching, at fixed over-density, evidently does not change with epoch to z ~ 1, suggesting that it occurs as large-scale structure develops in the Universe. The observed constancy of the mass-function shape for star-forming galaxies, demands that the mass-quenching of galaxies around and above M*, must be proportional to their star-formation rates at all z < 2. We postulate that this simple mass-quenching law also holds over a much broader range of stellar mass and epoch. These two simple quenching processes, plus some additional quenching due to merging, then naturally produce (a) a quasi-static Schechter mass function for star-forming galaxies with a value of M* that is set by the proportionality between the star-formation and mass-quenching rates, (b) a double Schechter function for passive galaxies with two components: the dominant one is produced by mass-quenching and has exactly the same M* as the star-forming galaxies but an alpha shallower by +1, while the other is produced by environment effects and has the same M* and alpha as the star-forming galaxies, and is larger in high density environments. Subsequent merging of quenched galaxies modifies these predictions somewhat in the denser environments, slightly increasing M* and making alpha more negative. All of these detailed quantitative relationships between the Schechter parameters are indeed seen in the SDSS, lending strong support to our simple empirically-based model. The model naturally produces for passive galaxies the "anti-hierarchical" run of mean ages and alpha-element abundances with mass.Comment: 66 pages, 19 figures, 1 movie, accepted for publication in ApJ. The movie is also available at http://www.exp-astro.phys.ethz.ch/zCOSMOS/MF_simulation_d1_d4.mo

The Science Case for Multi-Object Spectroscopy on the European ELT

This White Paper presents the scientific motivations for a multi-object spectrograph (MOS) on the European Extremely Large Telescope (E-ELT). The MOS case draws on all fields of contemporary astronomy, from extra-solar planets, to the study of the halo of the Milky Way and its satellites, and from resolved stellar populations in nearby galaxies out to observations of the earliest 'first-light' structures in the partially-reionised Universe. The material presented here results from thorough discussions within the community over the past four years, building on the past competitive studies to agree a common strategy toward realising a MOS capability on the E-ELT. The cases have been distilled to a set of common requirements which will be used to define the MOSAIC instrument, entailing two observational modes ('high multiplex' and 'high definition'). When combined with the unprecedented sensitivity of the E-ELT, MOSAIC will be the world's leading MOS facility. In analysing the requirements we also identify a high-multiplex MOS for the longer-term plans for the E-ELT, with an even greater multiplex (>1000 targets) to enable studies of large-scale structures in the high-redshift Universe. Following the green light for the construction of the E-ELT the MOS community, structured through the MOSAIC consortium, is eager to realise a MOS on the E-ELT as soon as possible. We argue that several of the most compelling cases for ELT science, in highly competitive areas of modern astronomy, demand such a capability. For example, MOS observations in the early stages of E-ELT operations will be essential for follow-up of sources identified by the James Webb Space Telescope (JWST). In particular, multi-object adaptive optics and accurate sky subtraction with fibres have both recently been demonstrated on sky, making fast-track development of MOSAIC feasible.Comment: Significantly expanded and updated version of previous ELT-MOS White Paper, so there is some textual overlap with arXiv:1303.002