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Register a new userA Hubble Space Telescope Study of Lyman Limit Systems: Census and Evolution
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We present a survey for optically thick Lyman limit absorbers at z<2.6 using archival Hubble Space Telescope observations with the Faint Object Spectrograph and Space Telescope Imaging Spectrograph. We identify 206 Lyman limit systems (LLSs) increasing the number of catalogued LLSs at z<2.6 by a factor of ~10. We compile a statistical sample of 50 tau_LLS > 2 LLSs drawn from 249 QSO sight lines that avoid known targeting biases. The incidence of such LLSs per unit redshift, l(z)=dn/dz, at these redshifts is well described by a single power law, l(z) = C1 (1+z)^gamma, with gamma=1.33 +/- 0.61 at z<2.6, or with gamma=1.83 +/- 0.21 over the redshift range 0.2 < z < 4.9. The incidence of LLSs per absorption distance, l(X), decreases by a factor of ~1.5 over the ~0.6 Gyr from z=4.9 to 3.5; l(X) evolves much more slowly at low redshifts, decreasing by a similar factor over the ~8 Gyr from z=2.6 to 0.25. We show that the column density distribution function, f(N(HI)), at low redshift is not well fitted by a single power law index (f(N(HI)) = C2 N(HI)^(-beta)) over the column density range 13 < log N(HI) < 22 or log N(HI) >17.2. While low and high redshift f(N(HI)) distributions are consistent for log N(HI)>19.0, there is some evidence that f(N(HI)) evolves with z for log N(HI) < 17.7, possibly due to the evolution of the UV background and galactic feedback. Assuming LLSs are associated with individual galaxies, we show that the physical cross section of the optically thick envelopes of galaxies decreased by a factor of ~9 from z~5 to 2 and has remained relatively constant since that time. We argue that a significant fraction of the observed population of LLSs arises in the circumgalactic gas of sub-L* galaxies.
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We present a census of z(abs) < 2, intrinsic (those showing partial coverage) and associated [z(abs) ~ z(em)] quasar absorption-line systems detected in the Hubble Space Telescope archive of Space Telescope Imaging Spectrograph echelle spectra. This work complements the Misawa et al. (2007) survey of 2 < z(em) < 4 quasars that selects systems using similar techniques. We confirm the existence of so-called strong N V intrinsic systems (where the equivalent width of H I Ly alpha is small compared to N V 1238) presented in that work, but find no convincing cases of strong C IV intrinsic systems at low redshift/luminosity. Moreover, we also report on the existence of strong O VI systems. From a comparison of partial coverage results as a function of ion, we conclude that systems selected by the N V ion have the highest probability of being intrinsic. By contrast, the C IV and O VI ions are poor selectors. Of the 30 O VI systems tested, only two of the systems in the spectrum on 3C 351 show convincing evidence for partial coverage. However, there is a 3-sigma excess in the number of absorbers near the quasar redshift (|Delta v| <= 5000 km/s) over absorbers at large redshift differences. In at least two cases, the associated O VI systems are known not to arise close to the accretion disk of the quasar.
We present the highest redshift detections of resolved Lyman alpha emission, using Hubble Space Telescope/ACS F658N narrowband-imaging data taken in parallel with the Wide Field Camera 3 Early Release Science program in the GOODS CDF-S. We detect Lyman alpha emission from three spectroscopically confirmed z = 4.4 Lyman alpha emitting galaxies (LAEs), more than doubling the sample of LAEs with resolved Lyman alpha emission. Comparing the light distribution between the rest-frame ultraviolet continuum and narrowband images, we investigate the escape of Lyman alpha photons at high redshift. While our data do not support a positional offset between the Lyman alpha and rest-frame ultraviolet (UV) continuum emission, the half-light radii in two out of the three galaxies are significantly larger in Lyman alpha than in the rest-frame UV continuum. This result is confirmed when comparing object sizes in a stack of all objects in both bands. Additionally, the narrowband flux detected with HST is significantly less than observed in similar filters from the ground. These results together imply that the Lyman alpha emission is not strictly confined to its indigenous star-forming regions. Rather, the Lyman alpha emission is more extended, with the missing HST flux likely existing in a diffuse outer halo. This suggests that the radiative transfer of Lyman alpha photons in high-redshift LAEs is complicated, with the interstellar-medium geometry and/or outflows playing a significant role in galaxies at these redshifts.
We present $U_{336}V_{606}J_{125}H_{160}$ follow-up $HST$ observations of 16 $zsim3$ candidate LyC emitters in the HS1549+1919 field. With these data, we obtain high spatial-resolution photometric redshifts of all sub-arcsecond components of the LyC candidates in order to eliminate foreground contamination and identify robust candidates for leaking LyC emission. Of the 16 candidates, we find one object with a robust LyC detection that is not due to foreground contamination. This object (MD5) resolves into two components; we refer to the LyC-emitting component as MD5b. MD5b has an observed 1500AA to 900AA flux-density ratio of $(F_{UV}/F_{LyC})_{obs}=4.0pm2.0$, compatible with predictions from stellar population synthesis models. Assuming minimal IGM absorption, this ratio corresponds to a relative (absolute) escape fraction of $f_{esc,rel}^{MD5b}=75-100$% ($f_{esc,abs}^{MD5b}=14-19$%). The stellar population fit to MD5b indicates an age of $lesssim50$Myr, which is in the youngest 10% of the $HST$ sample and the youngest third of typical $zsim3$ Lyman break galaxies, and may be a contributing factor to its LyC detection. We obtain a revised, contamination-free estimate for the comoving specific ionizing emissivity at $z=2.85$, indicating (with large uncertainties) that star-forming galaxies provide roughly the same contribution as QSOs to the ionizing background at this redshift. Our results show that foreground contamination prevents ground-based LyC studies from obtaining a full understanding of LyC emission from $zsim3$ star-forming galaxies. Future progress in direct LyC searches is contingent upon the elimination of foreground contaminants through high spatial-resolution observations, and upon acquisition of sufficiently deep LyC imaging to probe ionizing radiation in high-redshift galaxies.
We report observations of four sub-damped Lyman-alpha (sub-DLA) quasar absorbers at z<0.5 obtained with the Hubble Space Telescope Cosmic Origins Spectrograph. We measure the available neutrals or ions of C, N, O, Si, P, S, Ar, Mn, Fe, and/or Ni. Our data have doubled the sub-DLA metallicity samples at z<0.5 and improved constraints on sub-DLA chemical evolution. All four of our sub-DLAs are consistent with near-solar or super-solar metallicities and relatively modest ionization corrections; observations of more lines and detailed modeling will help to verify this. Combining our data with measurements from the literature, we confirm previous suggestions that the N(HI)-weighted mean metallicity of sub-DLAs exceeds that of DLAs at all redshifts studied, even after making ionization corrections for sub-DLAs. The absorber toward PHL 1598 shows significant dust depletion. The absorbers toward PHL 1226 and PKS 0439-433 show the S/P ratio consistent with solar, i.e., they lack a profound odd-even effect. The absorber toward Q0439-433 shows super-solar Mn/Fe. For several sub-DLAs at z<0.5, [N/S] is below the level expected for secondary N production, suggesting a delay in the release of the secondary N or a tertiary N production mechanism. We constrain the electron density using Si II* and C II* absorption. We also report different metallicity vs. Delta V_90 relations for sub-DLAs and DLAs. For two sub-DLAs with detections of emission lines from the underlying galaxies, our measurements of the absorption-line metallicities are consistent with the emission-line metallicities, suggesting that metallicity gradients are not significant in these galaxies.
We present a quantitative morphological analysis using HST NICMOS H160- and ACS I775- band imaging of 25 spectroscopically confirmed submillimetre galaxies (SMGs) which have redshifts between z=0.7-3.4. Our analysis also employs a comparison sample of more typical star-forming galaxies at similar redshifts (such as LBGs) which have lower far-infrared luminosities. This is the first large-scale study of the morphologies of SMGs in the near-infrared at ~0.1 resolution (<1kpc). We find that the half light radii of the SMGs (r_h=2.3+/-0.3 and 2.8+/-0.4kpc in the observed I- and H-bands respectively) and asymmetries are not statistically distinct from the comparison sample of star-forming galaxies. However, we demonstrate that the SMG morphologies differ more between the rest-frame UV and optical-bands than typical star-forming galaxies and interpret this as evidence for structured dust obscuration. We show that the composite observed H-band light profile of SMGs is better fit with a Sersic index with n~2, suggesting the stellar structure of SMGs is best described by a spheroid/elliptical galaxy light distribution. We also compare the sizes and stellar masses of SMGs to local and high-redshift populations, and find that the SMGs have stellar densities which are comparable to local early-type galaxies, as well as luminous, red and dense galaxies at z~1.5 which have been proposed as direct SMG descendants, although the SMG stellar masses and sizes are systematically larger. Overall, our results suggest that the physical processes occuring within the galaxies are too complex to be simply characterised by the rest-frame UV/optical morphologies which appear to be essentially decoupled from all other observables, such as bolometric luminosity, stellar or dynamical mass.
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