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Register a new userLow-Redshift Lyman Limit Systems as Diagnostics of Cosmological Inflows and Outflows
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Z. Hafen
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We use cosmological hydrodynamic simulations with stellar feedback from the FIRE project to study the physical nature of Lyman limit systems (LLSs) at z<1. At these low redshifts, LLSs are closely associated with dense gas structures surrounding galaxies, such as galactic winds, dwarf satellites, and cool inflows from the intergalactic medium. Our analysis is based on 14 zoom-in simulations covering the halo mass range M_h~10^9-10^13 Msun at z=0, which we convolve with the dark matter halo mass function to produce cosmological statistics. We find that the majority of cosmologically-selected LLSs are associated with halos in the mass range 10^10 < M_h < 10^12 Msun. The incidence and HI column density distribution of simulated absorbers with columns 10^16.2 < N_HI < 2x10^20 cm^-2 are consistent with observations. High-velocity outflows (with radial velocity exceeding the halo circular velocity by a factor >~2) tend to have higher metallicities ([X/H] ~ -0.5) while very low metallicity ([X/H] < -2) LLSs are typically associated with gas infalling from the intergalactic medium. However, most LLSs occupy an intermediate region in metallicity-radial velocity space, for which there is no clear trend between metallicity and radial kinematics. Metal-enriched inflows arise in the FIRE simulations as a result of galactic winds that fall back onto galaxies at low redshift. The overall simulated LLS metallicity distribution has a mean (standard deviation) [X/H] = -0.9 (0.4) and does not show significant evidence for bimodality, in contrast to recent observational studies but consistent with LLSs arising from halos with a broad range of masses and metallicities.
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We searched quasar spectra from the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS) for the rare occurrences where a strong damped Lyman-alpha absorber (DLA) blocks the Broad Line Region emission from the quasar and acts as a natural coronagraph to reveal narrow Lyalpha emission from the host galaxy. We define a statistical sample of 31 DLAs in Data Release 9 (DR9) with log N(HI) > 21.3 cm^-2 located at less than 1500 km s^-1 from the quasar redshift. In 25% (8) of these DLAs, a strong narrow Lyalpha emission line is observed with flux ~25 x 10^-17 erg s^-1 cm^-2 on average. For DLAs without this feature in their troughs, the average 3-sigma upper limit is < 0.8 x 10^-17 erg s^-1 cm^-2. Our statistical sample is nearly 2.5 times larger than the anticipated number of intervening DLAs in DR9 within 1500 km s^-1 of the quasar redshift. We also define a sample of 26 DLAs from DR9 and DR10 with narrow Lyalpha emission detected and no limit on the HI column density to better characterize properties of the host galaxy emission. Analyzing the statistical sample, we do not find substantial differences in the kinematics, metals, or reddening for the two populations with and without emission detected. The highly symmetric narrow Lyalpha emission line profile centered in the HI trough indicates that the emitting region is separate from the absorber. The luminosity of the narrow Lyalpha emission peaks is intermediate between that of Lyman-alpha emitters and radio galaxies, implying that the Lyalpha emission is predominantly due to ionizing radiation from the AGN. Galaxies neighboring the quasar host are likely responsible for the majority (> 75%) of these DLAs, with only a minority (< 25%) arising from HI clouds located in the AGN host galaxy.
Lyman Limit systems (LLSs) trace the low-density circumgalactic medium and the most dense regions of the intergalactic medium, so their number density and evolution at high redshift, just after reionisation, are important to constrain. We present a survey for LLSs at high redshifts, $z_{rm LLS} =3.5$--5.4, in the homogeneous dataset of 153 optical quasar spectra at $z sim 5$ from the Giant Gemini GMOS survey. Our analysis includes detailed investigation of survey biases using mock spectra which provide important corrections to the raw measurements. We estimate the incidence of LLSs per unit redshift at $z approx 4.4$ to be $ell(z) = 2.6 pm 0.4$. Combining our results with previous surveys at $z_{rm LLS} <4$, the best-fit power-law evolution is $ell(z) = ell_* [(1+z)/4]^alpha$ with $ell_* = 1.46 pm 0.11$ and $alpha = 1.70 pm 0.22$ (68% confidence intervals). Despite hints in previous $z_{rm LLS} <4$ results, there is no indication for a deviation from this single power-law soon after reionization. Finally, we integrate our new results with previous surveys of the intergalactic and circumgalactic media to constrain the hydrogen column density distribution function, $f(N_{rm HI},X)$, over 10 orders of magnitude. The data at $z sim 5$ are not well described by the $f(N_{rm HI},X)$ model previously reported for $z sim 2$--3 (after re-scaling) and a 7-pivot model fitting the full $z sim 2$--5 dataset is statistically unacceptable. We conclude that there is significant evolution in the shape of $f(N_{rm HI},X)$ over this $sim$2 billion year period.
Following our first detection reported in Izotov et al. (2016), we present the detection of Lyman continuum (LyC) radiation of four other compact star-forming galaxies observed with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST). These galaxies, at redshifts of z~0.3, are characterized by high emission-line flux ratios [OIII]5007/[OII]3727 > 5. The escape fractions of the LyC radiation fesc(LyC) in these galaxies are in the range of ~6%-13%, the highest values found so far in low-redshift star-forming galaxies. Narrow double-peaked Lyalpha emission lines are detected in the spectra of all four galaxies, compatible with predictions for Lyman continuum leakers. We find escape fractions of Lyalpha, fesc(Lyalpha) ~20%-40%, among the highest known for Lyalpha emitters (LAEs). Surface brightness profiles produced from the COS acquisition images reveal bright star-forming regions in the center and exponential discs in the outskirts with disc scale lengths alpha in the range ~0.6-1.4 kpc. Our galaxies are characterized by low metallicity, ~1/8-1/5 solar, low stellar mass ~(0.2 - 4)e9 Msun, high star formation rates SFR~14-36 Msun/yr, and high SFR densities Sigma~2-35 Msun/yr/kpc^2. These properties are comparable to those of high-redshift star-forming galaxies. Finally, our observations, combined with our first detection reported in Izotov et al. (2016), reveal that a selection for compact star-forming galaxies showing high [OIII]5007/[OII]3727 ratios appears to pick up very efficiently sources with escaping Lyman continuum radiation: all five of our selected galaxies are LyC leakers.
We report a deep search for redshifted HI 21 cm emission from three damped and sub-damped Lyman-$alpha$ absorbers (DLAs) at $z approx 0.1$ with the Green Bank Telescope (GBT). No evidence for a redshifted HI 21 cm emission signal was obtained in the GBT spectra of two absorbers, with the data on the third rendered unusable by terrestrial interference. The non-detections of HI 21 cm emission yield strong constraints on the HI masses of the associated galaxies, M$_{rm HI} < 2.3 times 10^9 times (Delta V/100)^{1/2}$ M$_odot$ for the sub-DLA at $z = 0.0830$ towards J1553+3548, and M$_{rm HI} < 2.7 times 10^9 times (Delta V/100)^{1/2}$ M$_odot$ for the DLA at $z = 0.0963$ towards J1619+3342, where $Delta V$ is the HI 21 cm line width, in km s$^{-1}$. This continues the trend of low HI masses found in all low-$z$ DLAs and sub-DLAs that have been searched for redshifted HI 21 cm emission. Low-redshift absorbers with relatively low HI column densities, $lesssim few times 10^{20}$ cm$^{-2}$, thus do not typically arise in massive gas-rich galaxies.
Recent stacked ALMA observations have revealed that normal, star-forming galaxies at $zapprox 6$ are surrounded by extended ($approx 10,mathrm{kpc}$) [CII] emitting halos which are not predicted by the most advanced, zoom-in simulations. We present a model in which these halos are the result of supernova-driven cooling outflows. Our model contains two free parameters, the outflow mass loading factor, $eta$, and the parent galaxy dark matter halo circular velocity, $v_c$. The outflow model successfully matches the observed [CII] surface brightness profile if $eta = 3.20 pm 0.10$ and $v_c = 170 pm 10{,rm km,s^{-1}}$, corresponding to a dynamical mass of $approx 10^{11}, mathrm{M}_odot$. The predicted outflow rate and velocity range are $128 pm 5 ,mathrm{M}_odot {rm yr}^{-1}$ and $300-500 {,rm km,s^{-1}}$, respectively. We conclude that: (a) extended halos can be produced by cooling outflows; (b) the large $eta$ value is marginally consistent with starburst-driven outflows, but it might indicate additional energy input from AGN; (c) the presence of [CII] halos requires an ionizing photon escape fraction from galaxies $f_{rm esc} ll 1$. The model can be readily applied also to individual high-$z$ galaxies, as those observed, e.g., by the ALMA ALPINE survey now becoming available.
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