Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe Spectral Slope and Escape Fraction of Bright Quasars at $z sim 3.8$: the Contribution to the Cosmic UV Background
123
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We use a sample of 1669 QSOs ($r<20.15$, $3.6<z<4.0$) from the BOSS survey to study the intrinsic shape of their continuum and the Lyman continuum photon escape fraction (f$_{esc}$), estimated as the ratio between the observed flux and the expected intrinsic flux (corrected for the intergalactic medium absorption) in the wavelength range 865-885 AA rest-frame. Modelling the intrinsic QSO continuum shape with a power-law, $F_{lambda}proptolambda^{-gamma}$, we find a median $gamma=1.30$ (with a dispersion of $0.38$, no dependence on the redshift and a mild intrinsic luminosity dependence) and a mean f$_{esc}=0.75$ (independent of the QSO luminosity and/or redshift). The f$_{esc}$ distribution shows a peak around zero and a long tail of higher values, with a resulting dispersion of $0.7$. If we assume for the QSO continuum a double power-law shape (also compatible with the data) with a break located at $lambda_{rm br}=1000$ AA and a softening $Deltagamma=0.72 $ at wavelengths shorter than $lambda_{rm br}$, the mean f$_{esc}$ rises to $=0.82$. Combining our $gamma$ and f$_{esc}$ estimates with the observed evolution of the AGN luminosity function (LF) we compute the AGN contribution to the UV ionizing background (UVB) as a function of redshift. AGN brighter than one tenth of the characteristic luminosity of the LF are able to produce most of it up $zsim 3$, if the present sample is representative of their properties. At higher redshifts a contribution of the galaxy population is required. Assuming an escape fraction of Lyman continuum photons from galaxies between $5.5$ and $7.6%$, independent of the galaxy luminosity and/or redshift, a remarkably good fit to the observational UVB data up to $zsim 6$ is obtained. At lower redshift the extrapolation of our empirical estimate agrees well with recent UVB observations, dispelling the so-called Photon Underproduction Crisis.
rate research
Read More
We study the escape of Ly-alpha photons from Ly-alpha emitting galaxies (LAEs) and the overall galaxy population using a sample of 99 LAEs at 1.9<z<3.8 detected through integral-field spectroscopy of blank fields by the HETDEX Pilot Survey. For 89 LAEs showing counterparts in deep broad-band images we measure the rest-frame UV luminosity and the UV slope, which we use to estimate E(B-V) under the assumption of a constant intrinsic UV slope for LAEs. These two quantities are used to measure the dust-corrected star formation rate (SFR). A comparison between the observed Ly-alpha luminosity and that predicted by the dust-corrected SFR yields the Ly-alpha escape fraction. We also measure the Ly-alpha luminosity function. Integration of the luminosity function provides a measurement of the Ly-alpha luminosity density across our redshift range. We combine our data with that from other surveys at 0.3<z<7.7 to trace the evolution of the Ly-alpha luminosity density. We then compare it to that expected from the star-formation history of the universe in order to characterize the evolution of the Ly-alpha escape fraction of the overall galaxy population [abriged]
Cosmic photons can be efficiently collected by broadband intensity mapping but information on their emission redshift and frequency is largely lost. We introduce a technique to statistically recover these otherwise collapsed dimensions by exploiting information in spatial fluctuations and apply it to the Galaxy Evolution Explorer (GALEX) All Sky and Medium Imaging Surveys. By spatially cross-correlating photons in the GALEX far-UV (1500A) and near-UV (2300A) bands with a million spectroscopic objects in the Sloan Digital Sky Survey as a function of redshift, we robustly detect the redshift-dependent intensity of the UV background (UVB) modulated by its clustering bias up to $zsim2$. These measurements clearly reveal the imprints of UVB spectral features redshifting through the filters. Using a simple parameterization, we simultaneously fit a UVB emissivity and clustering bias factor to these observations and constrain the main spectral features of the UVB spectrum: (i) the Lyman break, (ii) the non-ionizing UV continuum, which agrees with the Haardt & Madau model but does not rely on any assumption regarding the nature of the sources, and (iii) the Ly$alpha$ emission, whose luminosity density is consistent with estimates of the combined galaxy and AGN contributions at $zsim1$. Because the technique probes the total background including low surface brightness emission, we place constraints on the amount of UV light originating from the diffuse intergalactic medium (IGM). Finally, the clustering bias of UV photons is found to be chromatic and evolving. Our frequency- and redshift-dependent UVB measurement delivers a summary statistic of the universes net radiation output from stars, black holes, and the IGM combined.
We analyse a unique sample of 517 bright ($L>L^{*}$) LBGs at redshift z$sim$3 in order to characterise the distribution of their UV slopes $beta$ and infer their dust extinction under standard assumptions. We exploited multi-band observations over 750 arcmin$^2$ of the COSMOS field that were acquired with three different ground-based facilities: the Large Binocular Camera (LBC) on the Large Binocular Telescope (LBT), the Suprime-Cam on the SUBARU telescope, and the VIRCAM on the VISTA telescope (ULTRAVISTA DR2). Our multi-band photometric catalogue is based on a new method that is designed to maximise the signal-to-noise ratio in the estimate of accurate galaxy colours from images with different point spread functions (PSF). We adopted an improved selection criterion based on deep Y-band data to isolate a sample of galaxies at $zsim 3 $ to minimise selection biases. We measured the UV slopes ($beta$) of the objects in our sample and then recovered the intrinsic probability density function of $beta$ values (PDF($beta$)), taking into account the effect of observational uncertainties through detailed simulations. The galaxies in our sample are characterised by mildly red UV slopes with $<beta>simeq -1.70$ throughout the enitre luminosity range that is probed by our data ($-24lesssim M_{1600}lesssim -21$). The resulting dust-corrected star formation rate density (SFRD) is $log(SFRD)simeq-1.6 M_{odot}/yr/Mpc^{3}$, corresponding to a contribution of about 25% to the total SFRD at z$sim$3 under standard assumptions. Ultra-bright LBGs at $z sim 3$ match the known trends, with UV slopes being redder at decreasing redshifts, and brighter galaxies being more highly dust extinct and more frequently star-forming than fainter galaxies. [abridged]
Recent detections of Lyman alpha (Ly$alpha$) emission from $z>7.5$ galaxies were somewhat unexpected given a dearth of previous non-detections in this era when the intergalactic medium (IGM) is still highly neutral. But these detections were from UV bright galaxies, which preferentially live in overdensities which reionize early, and have significantly Doppler-shifted Ly$alpha$ line profiles emerging from their interstellar media (ISM), making them less affected by the global IGM state. Using a combination of reionization simulations and empirical ISM models we show, as a result of these two effects, UV bright galaxies in overdensities have $>2times$ higher transmission through the $zsim7$ IGM than typical field galaxies, and this boosted transmission is enhanced as the neutral fraction increases. The boosted transmission is not sufficient to explain the observed high Ly$alpha$ fraction of $M_mathrm{UV} lesssim -22$ galaxies (Stark et al. 2017), suggesting Ly$alpha$ emitted by these galaxies must be stronger than expected due to enhanced production and/or selection effects. Despite the bias of UV bright galaxies to reside in overdensities we show Ly$alpha$ observations of such galaxies can accurately measure the global neutral hydrogen fraction, particularly when Ly$alpha$ from UV faint galaxies is extinguished, making them ideal candidates for spectroscopic follow-up into the cosmic Dark Ages.
[Abridged] We present a physical model for the evolution of the ultraviolet (UV) luminosity function (LF) of high-z galaxies taking into account in a self-consistent way their chemical evolution and the associated evolution of dust extinction. The model yields good fits of the UV and Lyman-alpha LFs at z>~2. The weak evolution of both LFs between z=2 and z=6 is explained as the combined effect of the negative evolution of the halo mass function, of the increase with redshift of the star formation efficiency, and of dust extinction. The slope of the faint end of the UV LF is found to steepen with increasing redshift, implying that low luminosity galaxies increasingly dominate the contribution to the UV background at higher and higher redshifts. The observed range of UV luminosities at high-z implies a minimum halo mass capable of hosting active star formation M_crit <~ 10^9.8 M_odot, consistent with the constraints from hydrodynamical simulations. From fits of Lyman-alpha LFs plus data on the luminosity dependence of extinction and from the measured ratios of non-ionizing UV to Lyman-continuum flux density for samples of z=~3 Lyman break galaxies and Lyman-alpha emitters, we derive a simple relationship between the escape fraction of ionizing photons and the star formation rate, impling larger escape fraction for less massive galaxies. Galaxies already represented in the UV LF (M_UV <~ -18) can keep the universe fully ionized up to z=~6, consistent with (uncertain) data pointing to a rapid drop of the ionization degree above z~6. On the other side, the electron scattering optical depth, tau_es, inferred from CMB experiments favor an ionization degree close to unity up to z=~9-10. Consistency with CMB data can be achieved if M_crit =~ 10^8.5 M_odot, implying that the UV LFs extend to M_UV =~ -13, although the corresponding tau_es is still on the low side of CMB-based estimates.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
