The Mass Function of the Stellar Component of Galaxies in the Sloan Digital Sky Survey

Using the MOPED algorithm we determine non-parametrically the Stellar Mass Function of 96,545 galaxies from the Sloan Digital Sky Survey data release one. By using the reconstructed spectrum due to starlight we can eliminate contamination from either emission lines or AGN components. Our results give excellent agreement with previous works, but extend their range by more than two decades in mass to 10^{7.5} < M_s/h^{-2}M_\odot < 10^{12}. We present both a standard Schechter fit and a fit modified to include an extra, high-mass contribution, possibly from cluster cD galaxies. The Schechter fit parameters are $\phi^*=(7.7\pm 0.8)\times 10^{-3} h^3 Mpc^{-3}$, $M^*=(7.53 \pm 0.04) \times 10^{10} h^{-2}M_\odot$ and $\alpha=-1.167\pm 0.004$. Our sample also yields an estimate for the contribution from baryons in stars to the critical density of $\Omega_{b*}h=(2.40 \pm 0.04) \times 10^{-3}$, in good agreement with other indicators. Error bars are statistical and a Salpeter IMF is assumed throughout. We find no evolution of the mass function in the redshift range $0.05 < z < 0.34$ indicating that almost all present day stars were already formed at $z \sim 0.34$ with little or no star formation activity since then and that the evolution seen in the luminosity function must be largely due to stellar fading.Comment: submitted to MNRA

Recovering galaxy star formation and metallicity histories from spectra using VESPA

We introduce VErsatile SPectral Analysis (VESPA): a new method which aims to recover robust star formation and metallicity histories from galactic spectra. VESPA uses the full spectral range to construct a galaxy history from synthetic models. We investigate the use of an adaptative parametrization grid to recover reliable star formation histories on a galaxy-by-galaxy basis. Our goal is robustness as opposed to high resolution histories, and the method is designed to return high time resolution only where the data demand it. In this paper we detail the method and we present our findings when we apply VESPA to synthetic and real Sloan Digital Sky Survey (SDSS) spectroscopic data. We show that the number of parameters that can be recovered from a spectrum depends strongly on the signal-to-noise, wavelength coverage and presence or absence of a young population. For a typical SDSS sample of galaxies, we can normally recover between 2 to 5 stellar populations. We find very good agreement between VESPA and our previous analysis of the SDSS sample with MOPED.Comment: In press MNRAS, minor revisions to match accepted version by the journa

Environment and the cosmic evolution of star formation

We present a mark correlation analysis of the galaxies in the Sloan Digital Sky Survey using weights provided by MOPED. The large size of the sample permits statistically significant statements about how galaxies with different metallicities and star formation histories are spatially correlated. Massive objects formed a larger fraction of their stars at higher redshifts and over shorter timescales than did less massive objects (sometimes called down-sizing). We find that those galaxies which dominated the cosmic star formation at z~3 are predominantly in clusters today, whereas galaxies which dominate the star formation at z~0 inhabit substantially lower mass objects in less dense regions today. Hence, our results indicate that star formation and chemical enrichment occured first in the denser regions of the Universe, and moved to less dense regions at later times.Comment: 4 pages, 4 figures, submitted to ApJ

The ages, metallicities and star formation histories of early-type galaxies in SDSS

We use the spectra of ~ 22,000 early-type galaxies, selected from the Sloan Digital Sky Survey, to infer the ages, metallicities and star formation histories of these galaxies. We find clear evidence of "downsizing", i.e. galaxies with larger velocity dispersion have older stellar populations. In particular, most early-type galaxies with velocity dispersion exceeding 200 km s-1 formed more than 90% of their current stellar mass at redshift z > 2.5. Therefore, star formation was suppressed around this redshift. We also show that chemical enrichment was rapid, lasting 1-2 Gyr and find evidence that [Fe/H] is sub-solar. We study the robustness of these results by comparing three different approaches: using (i) Lick absorption line indices; (ii) fitting a single-burst stellar population model to the whole spectrum (lines+continuum); and (iii) reconstructing the star formation and metallicity histories in multiple age-bins, providing a method to measure mass-weighted ages and metallicities. We find good agreement between the luminosity-weighted ages and metallicities computed with these three methods.Comment: Submitted to Ap

The Fossil Record of Star Formation from Galaxy Spectra

In this thesis I present work using the MOPED algorithm to extract in a non-parametric fashion star formation histories and galaxy masses from the spectra of galaxies in the Sloan Digital Sky Survey. The recovered parameters for all galaxies are combined to give insight into the processes of star and galaxy formation on both individual galaxy and cosmic scales. The MOPED algorithm allows use of the entire spectral range, rather than concentrating on specific features, and can be used to estimate the complete star formation history without prior assumptions about its form. By combining the star formation histories of 96,545 galaxies in the redshift range 0 < z < 0:34 the cosmic star formation rate is determined from the present day to z ~ 6. The results show that the peak of star formation occurred at z ~ 0:6, and that 26% of the mass of stars in the present-day Universe was formed at z ~ 2. The average metallicity rises from Z/Z= 0:44 at high redshift to a peak of 0:8 at z ~ 1 before declining to a level around 0.25 atthe present day. Although the peak in star formation is more recent than previously thought, the sample used includes galaxies with a range of masses not accessible to traditional studies, down to a limit of L ~ 2 x 10-3L*. By cutting the sample into ranges of mass it can be seen that the redshift at which starformation activity peaks is an essentially monotonically increasing function of final stellar mass. The time of the peak in star formation ranges from z > 2 for the highest mass galaxies (MS < 1012M) to z ~ 0:2 for the lowest (MS < 1010M). A typical L* galaxy appears to have its peak at around z » 0:8. These differences in star formation with mass reconcile the redshift of the peak found in this work with the previous estimates, generally deep surveys only probe the SFR of galaxies with MS < ML*. The stellar mass calculated using the reconstructed spectra eliminates contamination from either emission lines or AGN components. Using these masses it is possible to construct the mass function for the stellar mass component of galaxies which give excellent agreement with previous works, but extend their range by more than two decades in mass to 10 7.5 < Ms/h-2M < 1012. I present both a standard Schechter fit and a fit modified to include an extra, high-mass contribution, possibly from cluster cD galaxies. The Schechter fit parameters are phi* = (7:8 +/- 0:1) £ 10-3h3Mpc-3, M* = (7.64 +/- 0.09) x 10*10h-2M and alpha = -1.159 +/- 0.008. The sample also yields an estimate for the contribution from baryons in stars to the critical density of omega b*h = (2.39+/-0.08)x10-3, in good agreement with other indicators. No evolution of the mass function in the redshift range 0:05 < z < 0:34 is apparent, indicating that almost all stars were already formed at z » 0:34 with little or no star formation activity since then and that the evolution seen in the luminosity function must be largely due to stellar fading. The star formation history can be interpreted as a measure of how gas was transformed into stars as a function of time and stellar mass: the Baryonic Conversion Tree (BCT). There is a clear correlation between early star formation activity and present-day stellar mass: the more massive galaxies have formed about 80% of their stars at z > 1, while for the less massive ones the value is only about 20%. Comparing the BCT to the dark matter merger tree indicates that star formation efficiency at z > 1 had to be high (as much as 10%) in galaxies with present-day stellar mass larger than 2 x 10*11M, if this early star formation occurred in the main progenitor. The LCDM paradigm can accommodate a large number of red objects; it is the high efficiency in the conversion from gas to stars that needs to be explained. On the other hand, in galaxies with present-day stellar mass less than 10*11M, efficient star formation seems to have been triggered at z ~ 0:2. This work shows that there is a characteristic mass (M » 10*10M) for feedback efficiency (or lack of star formation). For galaxies with masses lower than this, feedback (or star formation suppression) is very efficient while for higher masses it is not. The BCT, determined here for the first time, should be an important observable with which to confront theoretical models of galaxy formation

Physical Classification of Galaxies with MOPED/VESPA

The availability of high-quality spectra for a large number of galaxies in the SDSS survey allows for a more sophisticated extraction of information about their stellar populations than, e.g., the luminosity weighted age. Indeed, sophisticated and robust techniques to fully analyze galaxy spectra have now reached enough maturity as to trust their results and findings. By reconstructing the star formation and metallicity history of galaxies from the SDSS fossil record and analyzing how it relates to its environment, we have learned how to classify galaxies: to first order the evolution of a galaxy is determined by its present stellar mass, which in turn seems to be governed by the merger rate of dark halos.Comment: Invited talk at the class2008 meeting in Ringber

Improving Photometric Redshifts using GALEX Observations for the SDSS Stripe 82 and the Next Generation of SZ Cluster Surveys

Four large-area Sunyaev-Zeldovich (SZ) experiments -- APEX-SZ, SPT, ACT, and Planck -- promise to detect clusters of galaxies through the distortion of Cosmic Microwave Background photons by hot (> 10^6 K) cluster gas (the SZ effect) over thousands of square degrees. A large observational follow-up effort to obtain redshifts for these SZ-detected clusters is under way. Given the large area covered by these surveys, most of the redshifts will be obtained via the photometric redshift (photo-z) technique. Here we demonstrate, in an application using ~3000 SDSS stripe 82 galaxies with r<20, how the addition of GALEX photometry (FUV, NUV) greatly improves the photometric redshifts of galaxies obtained with optical griz or ugriz photometry. In the case where large spectroscopic training sets are available, empirical neural-network-based techniques (e.g., ANNz) can yield a photo-z scatter of $\sigma_z = 0.018 (1+z)$. If large spectroscopic training sets are not available, the addition of GALEX data makes possible the use simple maximum likelihood techniques, without resorting to Bayesian priors, and obtains $\sigma_z=0.04(1+z)$, accuracy that approaches the accuracy obtained using spectroscopic training of neural networks on ugriz observations. This improvement is especially notable for blue galaxies. To achieve these results, we have developed a new set of high resolution spectral templates based on physical information about the star formation history of galaxies. We envision these templates to be useful for the next generation of photo-z applications. We make our spectral templates and new photo-z catalogs available to the community at http://www.ice.csic.es/personal/jimenez/PHOTOZ .Comment: 10 pages, 8 figure

Changes to commute mode: The role of life events, spatial context and environmental attitude

© 2016 The Authors. It has been suggested that commuting behaviours become habitual and that changes to commute mode are more likely at the time of major life events. However, evidence to support this has so far been limited to analyses of small-scale samples. To address this evidence gap, we use two waves of panel data from the UK Household Longitudinal Study (2009/10 and 2010/11) to identify and explain the prevalence of individual change in commute mode from year to year amongst a representative sample of the English working population (n = 15,200). One third of those that cycle or get the bus to work, and one quarter of those that walk to work, are shown to change commuting mode by the following year. Car commuting is more stable, with only one in ten car commuters changing mode by the following year. Commute mode changes are found to be primarily driven by alterations to the distance to work which occur in association with changing job or moving home. Switching to non-car commuting becomes much more likely (9.2 times) as the distance to work drops below three miles. High quality public transport links to employment centres are shown to encourage switches away from car commuting and mixed land uses are shown to encourage switches to active commuting (walking and cycling). Switches away from car commuting are found to be more likely (1.3 times) for those with a pro-environmental attitude. The attitude orientation is shown to precede the behaviour change, demonstrating evidence of 'cause and effect'. Overall, the study shows that changes in commuting behaviour are strongly influenced by life events, spatial context and environmental attitude

The role of spin in the formation and evolution of galaxies

Using the SDSS spectroscopic sample, we estimate the dark matter halo spin parameter lambda for ~53,000 disk galaxies for which MOPED star formation histories are available. We investigate the relationship between spin and total stellar mass, star formation history, and environment. First, we find a clear anti-correlation between stellar mass and spin, with low mass galaxies generally having high dark matter spins. Second, galaxies which have formed more than ~5% of their stars in the last 0.2 Gyr have more broadly distributed and typically higher spins (including a significant fraction with lambda > 0.1) than galaxies which formed a large fraction of their stars more than 10 Gyr ago. Finally, we find little or no correlation between the value of spin of the dark halo and environment as determined both by proximity to a new cluster catalog and a marked correlation study. This agrees well with the predictions from linear hierarchical torquing theory and numerical simulations.Comment: Accepted to MNRAS after moderate revisio