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Register a new userCaII and DLA absorption line systems: dust, metals and star formation at 0.4<z<1.3
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Vivienne Wild MPA
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Absorption line studies of galaxies along the line-of-sight to distant quasars allow a direct observational link between the properties of the extended gaseous disk/halo and of the star forming region of galaxies. In these proceedings we review recent work on CaII absorbers detected in the SDSS at 0.4<z<1.3 which, because of their dust content and chemical properties, may lie spatially closer to the central host galaxy than most DLAs. We present direct evidence for the presence of star formation, through observation of the [OII]3727,3730 emission line, in both CaII absorbers and MgII-selected Damped Lyman-alpha (DLA) systems. The measured star formation rate (SFR) from light falling within the SDSS fibre apertures (corresponding to physical radii of 6-9 h^{-1}kpc) is 0.11-0.48 Msol/yr for the CaII-absorbers and 0.11-0.14 Msol/yr for the MgII-selected DLAs. The contribution of both CaII absorbers and DLAs to the total observed SFR density, in the redshift range 0.4 < z < 1.3, is small, <10%. Our result contrasts with recent conclusions, based on the Schmidt law, that DLA absorbers can account for the majority of the total observed SFR density in the same redshift range.
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[abridged] Using stacked Sloan Digital Sky Survey spectra, we present the detection of [OII]3727,3730 nebular emission from galaxies hosting CaII and MgII absorption line systems. Both samples of absorbers, 345 CaII systems and 3461 MgII systems, span the redshift interval 0.4 < z < 1.3; all of the former and half the latter sample are expected to be bona-fide damped Lyman-alpha (DLA) absorbers. The measured star formation rate (SFR) per absorber from light falling within the SDSS fibre apertures (corresponding to physical radii of 6-9 h^-1 kpc) is 0.11-0.14 Msol/yr for the MgII-selected DLAs and 0.11-0.48 Msol/yr for the CaII absorbers. These results represent the first estimates of the average SFR in an absorption-selected galaxy population from the direct detection of nebular emission. Adopting the currently favoured model in which DLAs are large, with radii >9h^-1 kpc, and assuming no attenuation by dust, leads to the conclusion that the SFR per unit area of MgII-selected DLAs falls an order of magnitude below the predictions of the Schmidt law, which relates the SFR to the HI column density at z~0. The contribution of both DLA and CaII absorbers to the total observed star formation rate density in the redshift range 0.4 < z < 1.3, is small, <10% and <3% respectively. The result contrasts with the conclusions of Hopkins et al. that DLA absorbers can account for the majority of the total observed SFR density in the same redshift range. Our results effectively rule out a picture in which DLA absorbers are the sites in which a large fraction of the total SFR density at redshifts z < 1 occurs.
We present observations of CaII, ZnII, and CrII absorption lines in 16 DLAs and 6 subDLAs at 0.6 < z < 1.3, obtained for the dual purposes of: (i) clarifying the relationship between DLAs and absorbers selected via strong CaII lines, and (ii) increasing the still limited sample of Zn and Cr abundances in this redshift range. We find only partial overlap between current samples of intermediate-z DLAs (which are drawn from magnitude limited surveys) and strong CaII absorbers: approximately 25% of known DLAs at these redshifts have an associated CaII 3935 line with REW>0.35A, the threshold of the SDSS sample assembled by Wild and her collaborators. The lack of the strongest systems (with REW>0.5A) is consistent with these authors conclusion that such absorbers are often missed in current DLA surveys because they redden/dim the light of the background QSOs. We rule out the suggestion that strong CaII absorption is associated exclusively with the highest-N(HI) DLAs. Furthermore, we find no correlation between the strength of the CaII lines and either the metallicity or depletion, although the strongest CaII absorber in our sample is also the most metal-rich DLA yet discovered, with [Zn/H] ~ solar. We conclude that a complex mix of parameters determine the strengths of the CaII lines, including the density of particles and UV photons in the ISM of the galaxies hosting the DLAs. We find tentative evidence (given the small size of our sample) that strong CaII systems may preferentially sample regions of high gas density, perhaps akin to the DLAs exhibiting molecular hydrogen absorption at redshifts z>2. If this connection is confirmed, strong CaII absorbers would trace possibly metal-rich, H2-bearing columns of cool, dense gas at distances up to tens of kpc from normal galaxies. (abridged)
We present new imaging and spectroscopic observations of the fields of five QSOs with very strong intervening CaII absorption systems at redshifts z<0.5 selected from the Sloan Digital Sky Survey. Recent studies of these very rare absorbers indicate that they may be related to damped Lyman alpha systems (DLAs). In all five cases we identify a galaxy at the redshift of the CaII system with impact parameters up to ~24 kpc. In four out of five cases the galaxies are luminous (L ~L*), metal-rich (Z ~Zsun), massive (velocity dispersion, sigma ~100 km/s) spirals. Their star formation rates, deduced from Halpha emission, are high, in the range SFR = 0.3 - 30 Msun/yr. In our analysis, we paid particular attention to correcting the observed emission line fluxes for stellar absorption and dust extinction. We show that these effects are important for a correct SFR estimate; their neglect in previous low-z studies of DLA-selected galaxies has probably led to an underestimate of the star formation activity in at least some DLA hosts. We discuss possible links between CaII-selected galaxies and DLAs and outline future observations which will help clarify the relationship between these different classes of QSO absorbers.
This paper uses radial colour profiles to infer the distributions of dust, gas and star formation in z=0.4-1.4 star-forming main sequence galaxies. We start with the standard UVJ-based method to estimate dust extinction and specific star formation rate (sSFR). By replacing J with I band, a new calibration method suitable for use with ACS+WFC3 data is created (i.e. UVI diagram). Using a multi-wavelength multi-aperture photometry catalogue based on CANDELS, UVI colour profiles of 1328 galaxies are stacked in stellar mass and redshift bins. The resulting colour gradients, covering a radial range of 0.2--2.0 effective radii, increase strongly with galaxy mass and with global $A_V$. Colour gradient directions are nearly parallel to the Calzetti extinction vector, indicating that dust plays a more important role than stellar population variations. With our calibration, the resulting $A_V$ profiles fall much more slowly than stellar mass profiles over the measured radial range. sSFR gradients are nearly flat without central quenching signatures, except for $M_*>10^{10.5} M_{odot}$, where central declines of 20--25 per cent are observed. Both sets of profiles agree well with previous radial sSFR and (continuum) $A_V$ measurements. They are also consistent with the sSFR profiles and, if assuming a radially constant gas-to-dust ratio, gas profiles in recent hydrodynamic models. We finally discuss the striking findings that SFR scales with stellar mass density in the inner parts of galaxies, and that dust content is high in the outer parts despite low stellar-mass surface densities there.
The goal of the Gemini Deep Deep Survey (GDDS) is to study an unbiased sample of K<20.6 galaxies in the redshift range 0.8<z<2.0. Here we determine the statistical properties of the heavy element enrichment in the interstellar medium (ISM) of a subsample of 13 galaxies with 1.34<z<1.97 and UV absolute magnitude M_2000 < -19.65. The sample contains 38% of the total number of identified galaxies in the first two fields of the survey with z>1.3. The selected objects have colors typical of irregular and Sbc galaxies. Strong [OII] emission indicates high star formation activity in the HII regions (SFR~13-106 M_sun/yr). The high S/N composite spectrum shows strong ISM MgII and FeII absorption, together with weak MnII and MgI lines. The FeII column density, derived using the curve of growth analysis, is logN_FeII = 15.54^{+0.23}_{-0.13}. This is considerably larger than typical values found in damped Ly-alpha systems (DLAs) along QSO sight lines, where only 10 out of 87 (~11%) have logN_FeII > 15.2. High FeII column densities are observed in the z=2.72 Lyman break galaxy cB58 (logN_FeII ~ 15.25) and in gamma-ray burst host galaxies (logN_FeII ~ 14.8-15.9). Given our measured FeII column density and assuming a moderate iron dust depletion (delta_Fe ~ 1 dex), we derive an optical dust extinction A_V ~ 0.6. If the HI column density is log N(HI)<21.7 (as in 98% of DLAs), then the mean metallicity is Z/Z_sun > 0.2. The high completeness of the GDDS sample implies that these results are typical of star-forming galaxies in the 1<z<2 redshift range, an epoch which has heretofore been particularly challenging for observational programs.
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