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Register a new userMeasuring the HI content of individual galaxies out to the epoch of reionization with [CII]
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The HI gas content is a key ingredient in galaxy evolution, the study of which has been limited to moderate cosmological distances for individual galaxies due to the weakness of the hyperfine HI 21-cm transition. Here we present a new approach that allows us to infer the HI gas mass $M_{rm HI}$ of individual galaxies up to $zapprox 6$, based on a direct measurement of the [CII]-to-HI conversion factor in star-forming galaxies at $zgtrsim 2$ using $gamma$-ray burst afterglows. By compiling recent [CII]-158 $mu$m emission line measurements we quantify the evolution of the HI content in galaxies through cosmic time. We find that the HI mass starts to exceed the stellar mass $M_star$ at $zgtrsim 1$, and increases as a function of redshift. The HI fraction of the total baryonic mass increases from around $20%$ at $z = 0$ to about $60%$ at $zsim 6$. We further uncover a universal relation between the HI gas fraction $M_{rm HI}/M_star$ and the gas-phase metallicity, which seems to hold from $zapprox 6$ to $z=0$. The majority of galaxies at $z>2$ are observed to have HI depletion times, $t_{rm dep,HI} = M_{rm HI}/{rm SFR}$, less than $approx 2$ Gyr, substantially shorter than for $zsim 0$ galaxies. Finally, we use the [CII]-to-HI conversion factor to determine the cosmic mass density of HI in galaxies, $rho_{rm HI}$, at three distinct epochs: $zapprox 0$, $zapprox 2$, and $zsim 4-6$. These measurements are consistent with previous estimates based on 21-cm HI observations in the local Universe and with damped Lyman-$alpha$ absorbers (DLAs) at $zgtrsim 2$, suggesting an overall decrease by a factor of $approx 5$ in $rho_{rm HI}(z)$ from the end of the reionization epoch to the present.
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We present new results on [CII]158$mu$ m emission from four galaxies in the reionization epoch. These galaxies were previously confirmed to be at redshifts between 6.6 and 7.15 from the presence of the Ly$alpha$ emission line in their spectra. The Ly$alpha$ emission line is redshifted by 100-200 km/s compared to the systemic redshift given by the [CII] line. These velocity offsets are smaller than what is observed in z~3 Lyman break galaxies with similar UV luminosities and emission line properties. Smaller velocity shifts reduce the visibility of Ly$alpha$ and hence somewhat alleviate the need for a very neutral IGM at z~7 to explain the drop in the fraction of Ly$alpha$ emitters observed at this epoch. The galaxies show [CII] emission with L[CII]=0.6-1.6 x10$^8 L_odot$: these luminosities place them consistently below the SFR-L[CII] relation observed for low redshift star forming and metal poor galaxies and also below z =5.5 Lyman break galaxies with similar star formation rates. We argue that previous undetections of [CII] in z~7 galaxies with similar or smaller star formation rates are due to selection effects: previous targets were mostly strong Ly$alpha$ emitters and therefore probably metal poor systems, while our galaxies are more representative of the general high redshift star forming population .
Recent observations of galaxies at $z gtrsim 7$, along with the low value of the electron scattering optical depth measured by the Planck mission, make galaxies plausible as dominant sources of ionizing photons during the epoch of reionization. However, scenarios of galaxy-driven reionization hinge on the assumption that the average escape fraction of ionizing photons is significantly higher for galaxies in the reionization epoch than in the local Universe. The NIRSpec instrument on the James Webb Space Telescope (JWST) will enable spectroscopic observations of large samples of reionization-epoch galaxies. While the leakage of ionizing photons will not be directly measurable from these spectra, the leakage is predicted to have an indirect effect on the spectral slope and the strength of nebular emission lines in the rest-frame ultraviolet and optical. Here, we apply a machine learning technique known as lasso regression on mock JWST/NIRSpec observations of simulated $z=7$ galaxies in order to obtain a model that can predict the escape fraction from JWST/NIRSpec data. Barring systematic biases in the simulated spectra, our method is able to retrieve the escape fraction with a mean absolute error of $Delta f_{mathrm{esc}} approx 0.12$ for spectra with $S/Napprox 5$ at a rest-frame wavelength of 1500 {AA} for our fiducial simulation. This prediction accuracy represents a significant improvement over previous similar approaches.
The neutral hydrogen (HI) and its 21 cm line are promising probes to the reionization process of the intergalactic medium (IGM). To use this probe effectively, it is imperative to have a good understanding on how the neutral hydrogen traces the underlying matter distribution. Here we study this problem using semi-numerical modeling by combining the HI in the IGM and the HI from halos during the epoch of reionization (EoR), and investigate the evolution and the scale-dependence of the neutral fraction bias as well as the 21 cm line bias. We find that the neutral fraction bias on large scales is negative during reionization, and its absolute value on large scales increases during the early stage of reionization and then decreases during the late stage. During the late stage of reionization, there is a transition scale at which the HI bias transits from negative on large scales to positive on small scales, and this scale increases as the reionization proceeds to the end.
The detection of the Epoch of Reionization (EoR) in the redshifted 21-cm line is a challenging task. Here we formulate the detection of the EoR signal using the drift scan strategy. This method potentially has better instrumental stability as compared to the case where a single patch of sky is tracked. We demonstrate that the correlation time between measured visibilities could extend up to 1-2 hr for an interferometer array such as the Murchison Widefield Array (MWA), which has a wide primary beam. We estimate the EoR power based on cross-correlation of visibilities across time and show that the drift scan strategy is capable of the detection of the EoR signal with comparable/better signal-to-noise as compared to the tracking case. We also estimate the visibility correlation for a set of bright point sources and argue that the statistical inhomogeneity of bright point sources might allow their separation from the EoR signal.
We present the Obelisk project, a cosmological radiation-hydrodynamics simulation following the assembly and reionization of a protocluster progenitor during the first two billions of years from the big bang, down to $z = 3.5$. The simulation resolves haloes down to the atomic cooling limit, and tracks the contribution of different sources of ionization: stars, active galactic nuclei, and collisions. The Obelisk project is designed specifically to study the coevolution of high redshift galaxies and quasars in an environment favouring black hole growth. In this paper, we establish the relative contribution of these two sources of radiation to reionization and their respective role in establishing and maintaining the high redshift ionizing background. Our volume is typical of an overdense region of the Universe and displays star formation rate and black hole accretion rate densities similar to high redshift protoclusters. We find that hydrogen reionization happens inside-out and is completed by $z sim 6$ in our overdensity, and is predominantly driven by galaxies, while accreting black holes only play a role at $z sim 4$.
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