The VIMOS VLT Deep Survey Vergani, D.; Scodeggio, M.; Pozzetti, L. ...
Astronomy & astrophysics,
08/2008, Volume:
487, Issue:
1
Journal Article
Peer reviewed
Open access
Aims. Our aim is to investigate the history of mass assembly for galaxies of different stellar masses and types. Methods. We selected a mass-limited sample of 4048 objects from the VIMOS VLT Deep ...Survey (VVDS) in the redshift interval $0.5 \le z \le 1.3$. We then used an empirical criterion, based on the amplitude of the 4000 ÅBalmer break (Dn4000), to separate the galaxy population into spectroscopically early- and late-type systems. The equivalent width of the OII3727 line is used as proxy for the star formation activity. We also derived a type-dependent stellar mass function in three redshift bins. Results. We discuss to what extent stellar mass drives galaxy evolution, showing for the first time the interplay between stellar ages and stellar masses over the past 8 Gyr. Low-mass galaxies have small Dn4000 and at increasing stellar mass, the galaxy distribution moves to higher Dn4000 values as observed in the local Universe. As cosmic time goes by, we witness an increasing abundance of massive spectroscopically early-type systems at the expense of the late-type systems. This spectral transformation of late-type systems into old massive galaxies at lower redshift is a process started at early epochs (z > 1.3) and continuing efficiently down to the local Universe. This is also confirmed by the evolution of our type-dependent stellar mass function. The underlying stellar ages of late-type galaxies apparently do not show evolution, most likely as a result of a continuous and efficient formation of new stars. All star formation activity indicators consistently point towards a star formation history peaked in the past for massive galaxies, with little or no residual star formation taking place in the most recent epochs. In contrast, most of the low-mass systems show just the opposite characteristics, with significant star formation present at all epochs. The activity and efficiency of forming stars are mechanisms that depend on galaxy stellar mass, and the stellar mass assembly becomes progressively less efficient in massive systems as time elapses. The concepts of star formation downsizing and mass assembly downsizing describe a single scenario that has a top-down evolutionary pattern in how the star formation is quenched, as well as how the stellar mass is grown. The role of (dry) merging events seems to be only marginal at z < 1.3, as our estimated efficiency in stellar mass assembly can possibly account for the progressive accumulation of observed passively evolving galaxies.
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This paper presents the findings from a cross-national study of expert mental health social workers in Melbourne, Australia and San Jose, California, USA. In this study, expert social workers were ...selected based on a peer-nomination process and they were invited to participate in two-hour focus group interviews in which they described memorable practice situations. Detailed accounts of these practice situations were gathered from nineteen Australian and American expert practitioners employed in community-based mental health settings. The study findings corroborate many of the dimensions that are explicated in a theory of professional expertise developed by Fook et al. (2000). Further, the findings describe the skilful and ethical comportment of these social workers in the context of often complex and challenging practice situations. Narrative accounts are used to demonstrate the distinguishing dimensions and features of expert practice described by Fook and her colleagues, especially when providing services to people with chronic mental illness. In-depth articulation of practice expertise using cross-national data enhances social work research and pedagogy by providing important exemplars that inform more fully the development of empirically based practice theories.
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BFBNIB, NMLJ, NUK, OILJ, PNG, UL, UM, UPUK, VSZLJ
VIMOS (Visible Multi‐Object Spectrograph) is a multiobject imaging spectrograph installed at the VLT (Very large Telescope) at the ESO (European Southern Observatory) Paranal Observatory that is ...especially suited for survey work. VIMOS is characterized by its very high multiplexing factor: it is possible to take up to 800 spectra with 10
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long slits in a single exposure. To fully exploit its multiplexing potential, we designed and implemented a dedicated software tool: the VIMOS Mask Preparation Software (VMMPS), which allows the astronomer to select the objects to be spectroscopically observed, and provides for automatic slit positioning and slit number maximization within the instrumental constraints. The output of VMMPS is used to manufacture the slit masks to be mounted in the instrument for spectroscopic observations.
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Measuring the build-up of stellar mass is one of the main objectives of studies of galaxy evolution. Traditionally, the mass in stars and the star formation rates have been measured by different ...indicators, such as photometric colours, emission lines, and the UV and IR emission. We intend to show that it is possible to derive the physical parameters of galaxies from their broad-band spectral energy distribution out to a redshift of 1.2. This method has the potential to yield the physical parameters of all galaxies in a single field in a homogeneous way, thus overcoming problems with the sample size that particularly plague methods relying on spectroscopy. We use an extensive dataset, assembled in the context of the VVDS survey, which reaches from the UV to the IR and covers a sample of 84 073 galaxies over an area of 0.89 deg2. We also use a library of 100 000 model galaxies with a wide variety of star formation histories (in particular including late bursts of star formation). We find that we can determine the physical parameters stellar mass, age, and star formation rate with good confidence. We validate the star formation rate determination in particular by comparing it to a sample of spectroscopically observed galaxies with an emission-line measurement. While the attenuation in the galaxies shows more scatter, the mean over the sample is unbiased. Metallicity, however, cannot be measured from rest-frame optical photometry alone. As a first application we use our sample to build the number density function of galaxies as a function of stellar mass, specific star formation rate, and redshift. We are then able to study whether the stellar mass function at a later time can be predicted from the stellar mass function and star formation rate distribution at an earlier time. We find that, between redshifts of 1.02 and 0.47, the predicted growth in stellar mass from star formation agrees with the observed one. However, the predicted stellar mass density for massive galaxies is lower than observed, while the mass density of intermediate mass galaxies is overpredicted. This apparent discrepancy can be explained by major and minor mergers. Indeed, when comparing with a direct measurement of the major merger rate from the VVDS survey, we find that major mergers can account for about half of the mass build-up at the massive end. Minor mergers are very likely to contribute the missing fraction.
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