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Register a new userThe star-formation history of mass-selected galaxies from the VIDEO survey
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We measure star-formation rates (SFRs) and specific SFRs (SSFRs) of Ks-selected galaxies from the VIDEO survey by stacking 1.4-GHz Very Large Array data. We split the sample, which spans 0 < z < 3 and stellar masses 10**8.0 < Mstellar/Msol < 10**11.5, into elliptical, irregular or starburst galaxies based on their spectral-energy distributions. We find that SSFR falls with stellar mass, in agreement with the `downsizing paradigm. We consider the dependence of the SSFR-mass slope on redshift: for our full and elliptical samples the slope flattens, but for the irregular and starburst samples the slope is independent of redshift. The rate of SSFR evolution reduces slightly with stellar mass for ellipticals, but irregulars and starbursts co-evolve across stellar masses. Our results for SSFR as a function of stellar mass and redshift are in agreement with those derived from other radio-stacking measurements of mass-selected passive and star-forming galaxies, but inconsistent with those generated from semi-analytic models, which tend to underestimate SFRs and SSFRs. There is a need for deeper high-resolution radio surveys such as those from telescopes like the next-generation MeerKAT in order to probe lower masses at earlier times and to permit direct detections, i.e. to study individual galaxies in detail.
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We have studied the uJy radio properties of K-selected galaxies detected in the Ultra-Deep Survey portion of UKIDSS using 610- and 1,400-MHz images from the VLA and GMRT. These deep radio mosaics, combined with the largest and deepest K image currently available, allow high-S/N detections of many K-selected sub-populations, including sBzK and pBzK star-forming and passive galaxies. We find a strong correlation between the radio and K fluxes and a linear relationship between SFR and K luminosity. We find no evidence, from either radio spectral indices or a comparison with submm-derived SFRs, that the full sample is strongly contaminated by AGN. The sBzK and pBzK galaxies have similar levels of radio flux, SFR and specific SFR (SSFR) at z < 1.4, suggesting there is strong contamination of the pBzK sample by star-forming galaxies. At z > 1.4, pBzK galaxies become difficult to detect in the radio stack, though the implied SFRs are still much higher than expected for passively evolving galaxies. Their radio emission may come from low-luminosity AGN. EROs straddle the passive and star-forming regions of the BzK diagram and also straddle the two groups in terms of their radio properties. K-bright ERO samples are dominated by passive galaxies and faint ERO samples contain more star-forming galaxies. The star-formation history (SFH) from stacking all K sources in the UDS agrees well with that derived for other wavebands and other radio surveys, at least out to z ~ 2. The radio-derived SFH then appears to fall more steeply than that measured at other wavelengths. The SSFR for K-selected sources rises strongly with redshift at all stellar masses, and shows a weak dependence on stellar mass. High- and low-mass galaxies show a similar decline in SSFR since z ~ 2 (abridged).
The Calar Alto Legacy Integral Field Area (CALIFA) is an ongoing 3D spectroscopic survey of 600 nearby galaxies of all kinds. This pioneer survey is providing valuable clues on how galaxies form and evolve. Processed through spectral synthesis techniques, CALIFA datacubes allow us to, for the first time, spatially resolve the star formation history of galaxies spread across the color-magnitude diagram. The richness of this approach is already evident from the results obtained for the first 107 galaxies. Here we show how the different galactic spatial sub-components (bulge and disk) grow their stellar mass over time. We explore the results stacking galaxies in mass bins, finding that, except at the lowest masses, galaxies grow inside-out, and that the growth rate depends on a galaxys mass. The growth rate of inner and outer regions differ maximally at intermediate masses. We also find a good correlation between the age radial gradient and the stellar mass density, suggesting that the local density is a main driver of galaxy evolution.
If we are to develop a comprehensive and predictive theory of galaxy formation and evolution, it is essential that we obtain an accurate assessment of how and when galaxies assemble their stellar populations, and how this assembly varies with environment. There is strong observational support for the hierarchical assembly of galaxies, but our insight into this assembly comes from sifting through the resolved field populations of the surviving galaxies we see today, in order to reconstruct their star formation histories, chemical evolution, and kinematics. To obtain the detailed distribution of stellar ages and metallicities over the entire life of a galaxy, one needs multi-band photometry reaching solar-luminosity main sequence stars. The Hubble Space Telescope can obtain such data in the low-density regions of Local Group galaxies. To perform these essential studies for a fair sample of the Local Universe, we will require observational capabilities that allow us to extend the study of resolved stellar populations to much larger galaxy samples that span the full range of galaxy morphologies, while also enabling the study of the more crowded regions of relatively nearby galaxies. With such capabilities in hand, we will reveal the detailed history of star formation and chemical evolution in the universe.
We study the radial structure of the stellar mass surface density ($mu$) and stellar population age as a function of the total stellar mass and morphology for a sample of 107 galaxies from the CALIFA survey. We use the fossil record to recover the star formation history (SFH) in spheroidal and disk dominated galaxies with masses from 10$^9$ to 10$^{12}$ M$_odot$. We derive the half mass radius, and we find that galaxies are on average 15% more compact in mass than in light. HMR/HLR decreases with increasing mass for disk galaxies, but is almost constant in spheroidal galaxies. We find that the galaxy-averaged stellar population age, stellar extinction, and $mu$ are well represented by their values at 1 HLR. Negative radial gradients of the stellar population ages support an inside-out formation. The larger inner age gradients occur in the most massive disk galaxies that have the most prominent bulges; shallower age gradients are obtained in spheroids of similar mass. Disk and spheroidal galaxies show negative $mu$ gradients that steepen with stellar mass. In spheroidal galaxies $mu$ saturates at a critical value that is independent of the galaxy mass. Thus, all the massive spheroidal galaxies have similar local $mu$ at the same radius (in HLR units). The SFH of the regions beyond 1 HLR are well correlated with their local $mu$, and follow the same relation as the galaxy-averaged age and $mu$; suggesting that local stellar mass surface density preserves the SFH of disks. The SFH of bulges are, however, more fundamentally related to the total stellar mass, since the radial structure of the stellar age changes with galaxy mass even though all the spheroid dominated galaxies have similar radial structure in $mu$. Thus, galaxy mass is a more fundamental property in spheroidal systems while the local stellar mass surface density is more important in disks.
We present the first results of a pilot study aimed at understanding the influence of bars on the evolution of galaxy discs through the study of their stellar content. We examine here the kinematics, star formation history, mass-weighted, luminosity-weighted, and single stellar population (SSP) equivalent ages and metallicities for four galaxies ranging from lenticulars to late-type spirals. The data employed extends to 2-3 disc scalelengths, with S/N(A)>50. Several techniques are explored to derive star formation histories and SSP-equivalent parameters, each of which are shown to be compatible. We demostrate that the age-metallicity degeneracy is highly reduced by using spectral fitting techniques --instead of indices-- to derive these parameters. We found that the majority of the stellar mass in our sample is composed of old (~10 Gyr) stars. This is true in the bulge and the disc region, even beyond two disc scalelengths. In the bulge region, we find that the young, dynamically cold, structures produced by the presence of the bar (e.g., nuclear discs or rings) are responsible for shaping the bulges age and metallicity gradients. In the disc region, a larger fraction of young stars is present in the external parts of the disc compared with the inner disc. The disc growth is, therefore, compatible with a moderate inside-out formation scenario, where the luminosity weighted age changes from ~10 Gyrs in the centre, to ~4 Gyrs at two disc scalelengths, depending upon the galaxy. For two galaxies, we compare the metallicity and age gradients of the disc major axis with that of the bar, finding very important differences. In particular, the stellar population of the bar is more similar to the bulge than to the disc, indicating that, at least in those two galaxies, bars formed long ago and have survived to the present day. (abridged)
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