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Register a new userA Super-Damped Lyman-alpha QSO Absorber at z=2.2
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We report the discovery of a super-damped Lyman-alpha absorber at $z_{abs}=2.2068$ toward QSO Q1135-0010 in the Sloan Digital Sky Survey, and follow-up VLT UVES spectroscopy. Voigt profile fit to the DLA line indicates log $N_{rm H I} = 22.05 pm 0.1$. This is the second QSO DLA discovered to date with such high $N_{rm H I}$. We derive element abundances [Si/H] = $-1.10 pm 0.10$, [Zn/H] = $-1.06 pm 0.10$, [Cr/H] = $-1.55 pm 0.10$, [Ni/H] = $-1.60 pm 0.10$, [Fe/H] = $-1.76 pm 0.10$, [Ti/H] = $-1.69 pm 0.11$, [P/H] = $-0.93 pm 0.23$, and [Cu/H] = $-0.75 pm 0.14$. Our data indicate detection of Ly-$alpha$ emission in the DLA trough, implying a star formation rate of $sim$10 $M_{odot}$ yr$^{-1}$ in the absence of dust attenuation. C II$^{*} , lambda 1336$ absorption is also detected, suggesting SFR surface density $-2 < {rm log} , dot{psi_{*}} < 0$ $M_{odot}$ yr$^{-1}$ kpc$^{-2}$. We estimate electron density in the range $3.5 times 10^{-4}$ to 24.7 cm$^{-3}$ from C II$^{*}$/C II, and $sim$0.5-0.9 cm$^{-3}$ from Si II$^{*}$/Si II. Overall, this is a robustly star-forming, moderately enriched absorber, but with relatively low dust depletion. Fitting of the SDSS spectrum yields low reddening for Milky Way, LMC, or SMC extinction curves. No CO absorption is detected, and C I absorption is weak. The low dust and molecular content, reminiscent of some SMC sight-lines, may result from the lower metallicity, and a stronger radiation field (due to higher SFR). Finally, we compare this absorber with other QSO and GRB DLAs.
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We report a detection of the 9.7 micrometer silicate absorption feature in a damped Lyman-alpha (DLA) system at z_{abs} = 0.524 toward AO0235+164, using the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope. The feature shows a broad shallow profile over about 8-12 micrometers in the absorber rest frame and appears to be > 15 sigma significant in equivalent width. The feature is fit reasonably well by the silicate absorption profiles for laboratory amorphous olivine or diffuse Galactic interstellar clouds. To our knowledge, this is the first indication of 9.7 micrometer silicate absorption in a DLA. We discuss potential implications of this finding for the nature of the dust in quasar absorbers. Although the feature is relatively shallow (tau_{9.7} = 0.08-0.09), it is about 2 times deeper than expected from extrapolation of the tau_{9.7} vs. E(B-V) relation known for diffuse Galactic interstellar clouds. Further studies of the 9.7 micrometer silicate feature in quasar absorbers will open a new window on the dust in distant galaxies.
We present deep ground based imaging of the environments of five QSOs that contain sub-Damped Lyman-alpha systems at z<1 with the SOAR telescope and SOI camera. We detect a clear surplus of galaxies in these small fields, supporting the assumption that we are detecting the galaxies responsible for the absorption systems. Assuming these galaxies are at the redshift of the absorption line systems, we detect luminous L>L* galaxies for four of the five fields within 10 of the QSO. In contrast to previous imaging surveys of DLA systems at these redshifts, which indicate a range of morphological types and luminosities for the host galaxies of the systems, the galaxies we detect in these sub-DLA fields appear to be luminous (L>L*). In the case of the absorber towards Q1009-0026 at z=0.8866 we have spectroscopic confirmation that the candidate galaxy is at the redshift of the absorber, at an impact parameter of ~35 kpc with a luminosity of 3 < L/L* < 8 depending on the magnitude of the K-correction. These observations are in concordance with the view that sub-DLAs may be more representative of massive galaxies than DLA systems. The environments of the absorbers span a range of types, from the inner disk of a galaxy, the periphery of a luminous galaxy, and the outskirts of interacting galaxies. The large impact parameters to some of the candidate galaxies suggest that galactic outflows or tidal tails are likely responsible for the material seen in absorption. We find a weak correlation between N(HI) and the impact parameter at the 2 sigma level, which may be expected from the heterogeneous population of galaxies hosting the absorption line systems and random orientation angles. In addition, we detect a possible gravitationally lensed image of the BL-Lac object Q0826-2230.
Measuring rest-frame ultraviolet rotational transitions from the Lyman and Werner bands in absorption against a bright background continuum is one of the few ways to directly measure molecular hydrogen (H2). Here we report the detection of Lyman-Werner absorption from H2 at z=0.56 in a sub-damped Ly-alpha system with neutral hydrogen column density N(HI) = 10^(19.5 +/- 0.2) cm^-2. This is the first H2 system analysed at a redshift < 1.5 beyond the Milky Way halo. It has a surprisingly high molecular fraction: log f(H2) > -1.93 +/- 0.36 based on modelling the line profiles, with a robust model-independent lower limit of f(H2) > 10^-3. This is higher than f(H2) values seen along sightlines with similar N(HI) through the Milky Way disk and the Magellanic clouds. The metallicity of the absorber is 0.19 +0.21 -0.10 solar, with a dust-to-gas ratio < 0.36 times the value in the solar neighbourhood. Absorption from associated low-ionisation metal transitions such as OI and FeII is observed in addition to OVI. Using Cloudy models we show that there are three phases present; a ~100 K phase giving rise to H2, a ~10^4 K phase where most of the low-ionisation metal absorption is produced; and a hotter phase associated with OVI. Based on similarities to high velocity clouds in the Milky Way halo showing H2 and the presence of two nearby galaxy candidates with impact parameters of ~10 kpc, we suggest that the absorber may be produced by a tidally-stripped structure similar to the Magellanic Stream.
The amplitude of fluctuations in the Ly-a forest on small spatial scales is sensitive to the temperature of the IGM and its spatial fluctuations. The temperature of the IGM and its spatial variations contain important information about hydrogen and helium reionization. We present a new measurement of the small-scale structure in the Ly-a forest from 40 high resolution, high signal-to-noise, VLT spectra at z=2.2-4.2. We convolve each Ly-a forest spectrum with a suitably chosen wavelet filter, which allows us to extract the amount of small-scale structure in the forest as a function of position across each spectrum. We compare these measurements with high resolution hydrodynamic simulations of the Ly-a forest which track more than 2 billion particles. This comparison suggests that the IGM temperature close to the cosmic mean density (T_0) peaks near z=3.4, at which point it is greater than 20,000 K at 2-sigma confidence. The temperature at lower redshift is consistent with the fall-off expected from adiabatic cooling ($T_0 propto (1+z)^2$), after the peak temperature is reached near z=3.4. At z=4.2 our results favor a temperature of T_0 = 15-20,000 K. However, owing mostly to uncertainties in the mean transmitted flux at this redshift, a cooler IGM model with T_0 = 10,000 K is only disfavored at the 2-sigma level here, although such cool IGM models are strongly discrepant with the z ~ 3-3.4 measurement. We do not detect large spatial fluctuations in the IGM temperature at any redshift covered by our data set. The simplest interpretation of our measurements is that HeII reionization completes sometime near z ~ 3.4, although statistical uncertainties are still large [Abridged].
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