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Register a new userCrucial Factors of Ly{alpha} Transmission in the Reionizing Intergalactic Medium: Infall Motion, HII Bubble Size, and Self-shielded Systems
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Using the CoDaII simulation, we study the Ly$alpha$ transmissivity of the intergalactic medium (IGM) during reionization. At $z>6$, a typical galaxy without an active galactic nuclei fails to form a proximity zone around itself due to the overdensity of the surrounding IGM. The gravitational infall motion in the IGM makes the resonance absorption extends to the red side of Ly$alpha$, suppressing the transmission up to roughly the circular velocity of the galaxy. In some sightlines, an optically thin blob generated by a supernovae in a neighboring galaxy results in a peak feature, which can be mistaken for a blue peak. Red-ward of the resonance absorption, the damping-wing opacity correlates with the global IGM neutral fraction and the UV magnitude of the source galaxy. Brighter galaxies tend to suffer lower opacity because they tend to reside in larger HII regions, and the surrounding IGM transmits redder photons, which are less susceptible to attenuation, owing to stronger infall velocity. The HII regions are highly nonspherical, causing both sightline-to-sightline and galaxy-to-galaxy variation in opacity. Also, self-shielded systems within HII regions strongly attenuate the emission for certain sightlines. All these factors adds to the transmissivity variation, requiring a large sample size to constrain the average transmission. The variation is largest for fainter galaxies at higher redshift. The 68% range of the transmissivity is similar to or greater than the median for galaxies with $M_{rm UV}ge-21$ at $zge7$, implying that more than a hundred galaxies would be needed to measure the transmission to 10% accuracy.
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The decline in abundance of Lyman-$alpha$ (Ly$alpha$) emitting galaxies at $z gtrsim 6$ is a powerful and commonly used probe to constrain the progress of cosmic reionization. We use the CoDaII simulation, which is a radiation hydrodynamic simulation featuring a box of $sim 94$ comoving Mpc side length, to compute the Ly$alpha$ transmission properties of the intergalactic medium (IGM) at $zsim 5.8$ to $7$. Our results mainly confirm previous studies, i.e., we find a declining Ly$alpha$ transmission with redshift and a large sightline-to-sightline variation. However, motivated by the recent discovery of blue Ly$alpha$ peaks at high redshift, we also analyze the IGM transmission on the blue side, which shows a rapid decline at $zgtrsim 6$ of the blue transmission. This low transmission can be attributed not only to the presence of neutral regions but also to the residual neutral hydrogen within ionized regions, for which a density even as low as $n_{rm HI}sim 10^{-9},mathrm{cm}^{-3}$ (sometimes combined with kinematic effects) leads to a significantly reduced visibility. Still, we find that $sim 5%$ of sightlines towards $M_{mathrm{1600AB}}sim -21$ galaxies at $zsim 7$ are transparent enough to allow a transmission of a blue Ly$alpha$ peak. We discuss our results in the context of the interpretation of observations.
We present spectroscopic observations of six high redshift ($z_{rm em}$ $>$ 2) quasars, which have been selected for their Lyman $alpha$ (Ly$alpha$) emission region being only partially covered by a strong proximate ($z_{rm abs}$ $sim$ $z_{rm em}$) coronagraphic damped Ly$alpha$ system (DLA). We detected spatially extended Ly$alpha$ emission envelopes surrounding these six quasars, with projected spatial extent in the range 26 $le$ $d_{rm Lyalpha}$ $le$ 51 kpc. No correlation is found between the quasar ionizing luminosity and the Ly$alpha$ luminosity of their extended envelopes. This could be related to the limited covering factor of the extended gas and/or due to the AGN being obscured in other directions than towards the observer. Indeed, we find a strong correlation between the luminosity of the envelope and its spatial extent, which suggests that the envelopes are probably ionized by the AGN. The metallicity of the coronagraphic DLAs is low and varies in the range $-$1.75 $<$ [Si/H] $<$ $-$0.63. Highly ionized gas is observed to be associated with most of these DLAs, probably indicating ionization by the central AGN. One of these DLAs has the highest AlIII/SiII ratio ever reported for any intervening and/or proximate DLA. Most of these DLAs are redshifted with respect to the quasar, implying that they might represent infalling gas probably accreted onto the quasar host galaxies through filaments.
We study the average Ly$alpha$ emission associated with high-$z$ strong (log $N$(H I) $ge$ 21) damped Ly$alpha$ systems (DLAs). We report Ly$alpha$ luminosities ($L_{rm Lyalpha}$) for the full as well as various sub-samples based on $N$(H I), $z$, $(r-i)$ colours of QSOs and rest equivalent width of Si II$lambda$1526 line (i.e., $W_{1526}$). For the full sample, we find $L_{rm Lyalpha}$$< 10^{41} (3sigma) rm erg s^{-1}$ with a $2.8sigma$ level detection of Ly$alpha$ emission in the red part of the DLA trough. The $L_{rm Lyalpha}$ is found to be higher for systems with higher $W_{1526}$ with its peak, detected at $geq 3sigma$, redshifted by about 300-400 $rm km s^{-1}$ with respect to the systemic absorption redshift, as seen in Lyman Break Galaxies (LBGs) and Ly$alpha$ emitters. A clear signature of a double-hump Ly$alpha$ profile is seen when we consider $W_{1526} ge 0.4$ AA and $(r-i) < 0.05$. Based on the known correlation between metallicity and $W_{1526}$, we interpret our results in terms of star formation rate (SFR) being higher in high metallicity (mass) galaxies with high velocity fields that facilitates easy Ly$alpha$ escape. The measured Ly$alpha$ surface brightness requires local ionizing radiation that is 4 to 10 times stronger than the metagalactic UV background at these redshifts. The relationship between the SFR and surface mass density of atomic gas seen in DLAs is similar to that of local dwarf and metal poor galaxies. We show that the low luminosity galaxies will contribute appreciably to the stacked spectrum if the size-luminosity relation seen for H I at low-$z$ is also present at high-$z$. Alternatively, large Ly$alpha$ halos seen around LBGs could also explain our measurements.
We compare a sample of five high-resolution, high S/N Ly$alpha$ forest spectra of bright $6<z lesssim 6.5$ QSOs aimed at spectrally resolving the last remaining transmission spikes at $z>5$ with those obtained from mock absorption spectra from the Sherwood and Sherwood-Relics suites of hydrodynamical simulations of the intergalactic medium (IGM). We use a profile fitting procedure for the inverted transmitted flux, $1-F$, similar to the widely used Voigt profile fitting of the transmitted flux $F$ at lower redshifts, to characterise the transmission spikes that probe predominately underdense regions of the IGM. We are able to reproduce the width and height distributions of the transmission spikes, both with optically thin simulations of the post-reionization Universe using a homogeneous UV background and full radiative transfer simulations of a late reionization model. We find that the width of the fitted components of the simulated transmission spikes is very sensitive to the instantaneous temperature of the reionized IGM. The internal structures of the spikes are more prominant in low temeperature models of the IGM. The width distribution of the observed transmission spikes, which require high spectral resolution ($leq $ 8 km/s) to be resolved, is reproduced for optically thin simulations with a temperature at mean density of $T_0= (11000 pm 1600,10500pm 2100,12000 pm 2200)$ K at $z= (5.4,5.6,5.8)$. This is weakly dependent on the slope of the temperature-density relation, which is favoured to be moderately steeper than isothermal. In the inhomogeneous, late reionization, full radiative transfer simulations where islands of neutral hydrogen persist to $zsim5.3$, the width distribution of the observed transmission spikes is consistent with the range of $T_0$ caused by spatial fluctuations in the temperature-density relation.
Tidal debris which are rich in HI gas, formed in interacting and merging systems, are suitable laboratories to study star formation outside galaxies. Recently, several such systems were observed, which contained many young star forming regions outside the galaxies. In previous works, we have studied young star forming regions outside galaxies in different systems with optical and/or gaseous tidal debris, all of them with available archive GALEX/UV images, in order to understand how often they occur and in which type of environments. In this paper we searched for star forming regions around the galaxy NGC2865, a shell galaxy which is circled by a ring of HI, with a total mass of 1.2 x 10$^9$ M$_odot$. Using the Multi-Slit Imaging Spectroscopy Technique with the Gemini telescope, we detected all H$alpha$ emitting sources in the surroundings of the galaxy NGC2865, down to a flux limit of 10$^{-18}$ erg cm$^{-2}$ s$^{-1}$ AA$^{-1}$. Together with Near and Far-Ultraviolet flux information we characterize the star formation rates, masses, ages, and metallicities for these HII regions. In total, we found 26 emission-line sources in a 60 $times$ 60 Kpc field centered over the southeastern tail of the HI gas present around the galaxy NGC2865. Out of the 26 H$alpha$ emitters, 19 are in the satellite galaxy FGCE 0745 and seven are intergalactic HII regions scattered over the south tail of the HI gas around NGC2865. We found that the intergalactic HII regions are young ($<$200 Myr) with stellar masses in the range 4 X 10$^3$M$_odot$ to 17x10$^6$ M$_odot$. These are found in a region of low HI gas density, where the probability of forming stars is expected to be low. For one of the intergalactic HII regions we estimated a solar oxygen abundance of 12 + log(O/H) $sim$ 8.7. We also were able to estimate the metallicity for the satellite galaxy FGCE0745 to be 12 + log(O/H) ~ 8.0.
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