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Register a new userLyman continuum leakage in faint star-forming galaxies at redshift z=3-3.5 probed by gamma-ray bursts
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Added by
Jean-Baptiste Vielfaure
Publication date
2020
fields
Physics
and research's language is
English
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We present the observations of Lyman continuum (LyC) emission in the afterglow spectra of GRB 191004B at $z=3.5055$, together with those of the other two previously known LyC-emitting long gamma-ray bursts (LGRB) (GRB 050908 at $z=3.3467$, and GRB 060607A at $z=3.0749$), to determine their LyC escape fraction and compare their properties. From the afterglow spectrum of GRB 191004B we determine a neutral hydrogen column density at the LGRB redshift of $log(N_{rm HI}/cm^{-2})= 17.2 pm 0.15$, and negligible extinction ($A_{rm V}=0.03 pm 0.02$ mag). The only metal absorption lines detected are CIV and SiIV. In contrast to GRB 050908 and GRB 060607A, the host galaxy of GRB 191004B displays significant Ly$alpha$ emission. From its Ly$alpha$ emission and the non-detection of Balmer emission lines we constrain its star-formation rate (SFR) to $1 leq$ SFR $leq 4.7$ M$_{odot} yr^{-1}$. We fit the Ly$alpha$ emission with a shell model and find parameters values consistent with the observed ones. The absolute LyC escape fractions we find for GRB 191004B, GRB 050908 and GRB 060607A are of $0.35^{+0.10}_{-0.11}$, $0.08^{+0.05}_{-0.04}$ and $0.20^{+0.05}_{-0.05}$, respectively. We compare the LyC escape fraction of LGRBs to the values of other LyC emitters found from the literature, showing that LGRB afterglows can be powerful tools to study LyC escape for faint high-redshift star-forming galaxies. Indeed we could push LyC leakage studies to much higher absolute magnitudes. The host galaxies of the three LGRB presented here have all $M_{rm 1600} > -19.5$ mag, with the GRB 060607A host at $M_{rm 1600} > -16$ mag. LGRB hosts may therefore be particularly suitable for exploring the ionizing escape fraction in galaxies that are too faint or distant for conventional techniques. Furthermore the time investment is very small compared to galaxy studies. [Abridged]
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Motivated by the discovery of the ultra-strong emission line starburst galaxies (EELGs) known as green pea galaxies, we consider here their contribution to the intergalactic flux of ionizing UV at high redshifts. Most galaxies that have been observed show a precipitous drop in their flux blueward of the Lyman limit. However, recent observations of EELGs have discovered that many more Lyman continuum photons escape from them into intergalactic space than was previously suspected. We calculate their contribution to the extragalactic background light (EBL). We also calculate the effect of these photons on the absorption of high energy $gamma$-rays. For the more distant $gamma$-ray sources, particularly at $z ge 3$, the intergalactic opacity above a few GeV is significantly higher than previous estimates which ignored the Lyman continuum photons. We calculate the results of this increased opacity on observed $gamma$-ray spectra, which produces a high-energy turnover starting at lower energies than previously thought, and a gradual spectral steepening that may also be observable.
We present Lyman continuum (LyC) radiation escape fraction $f_{rm{esc}}$ measurements for 183 spectroscopically confirmed star-forming galaxies in the redshift range $3.11 < z < 3.53$ in the textit{Chandra} Deep Field South. We use ground-based imaging to measure $f_{rm{esc}}$, and use ground- and space-based photometry to derive galaxy physical properties using spectral energy distribution (SED) fitting. We additionally derive [O,textsc{iii}],+,H$beta$ equivalent widths (that fall in the observed $K$ band) by including nebular emission in the SED fitting. After removing foreground contaminants, we report the discovery of 11 new candidate LyC leakers, with absolute LyC escape fractions, $f_{rm{esc}}$ in the range $0.07-0.52$. Most galaxies in our sample ($approx94%$) do not show any LyC leakage, and we place $1sigma$ upper limits of $f_{rm{esc}} < 0.07$ through weighted averaging, where the Lyman-break selected galaxies have $f_{rm{esc}} < 0.07$ and `blindly discovered galaxies with no prior photometric selection have $f_{rm{esc}} < 0.10$. We additionally measure $f_{rm{esc}} < 0.09$ for extreme emission line galaxies in our sample with rest-frame [O,textsc{iii}],+,H$beta$ equivalent widths $>300$,AA. For the candidate LyC leakers, we do not find a strong dependence of $f_{rm{esc}}$ on their stellar masses and/or specific star-formation rates, and no correlation between $f_{rm{esc}}$ and EW$_0$([O,textsc{iii}],+,H$beta$). We suggest that this lack of correlations may be explained by viewing angle and/or non-coincident timescales of starburst activity and periods of high $f_{rm{esc}}$. Alternatively, escaping radiation may predominantly occur in highly localised star-forming regions, thereby obscuring any global trends with galaxy properties. Both hypotheses have important consequences for models of reionisation.
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.
To identify the galaxies responsible for the reionization of the Universe, we must rely on the investigation of the Lyman Continuum (LyC) properties of z<5 star-forming galaxies, where we can still directly observe their ionizing radiation. We selected a sample of 201 star-forming galaxies from the Vimos Ultra Deep Survey at 3.5<z<4.3 to explore the validity of some of the proposed indirect indicators of LyC radiation. We created subsamples of galaxies with EWLya>70{AA}, Lya(ext)<5.7kpc, rUV<0.3kpc and |Dv Lya|<200km/s, stacked all the galaxies in each subsample and measured the flux density ratio fnu(895)/fnu(1470), that we consider to be a proxy for LyC emission. We compared these ratios to those obtained for the complementary samples. We find that the stacks of galaxies which are UV compact (rUV<0.3kpc) and have bright Lya emission (EWLya>70{AA}), have much higher LyC fluxes compared to the rest of the galaxy population in agreement with theoretical studies and previous observational works. We also find that galaxies with low Lya(ext) have the highest LyC flux: this new correlation seems even stronger than the correlations with high EWLya and small rUV. These results assume that the stacks from all the subsamples present the same statistical contamination from lower redshift interlopers. If we subtract a statistical contamination from low redshift interlopers obtained with dedicated Monte Carlo simulations, from the flux density ratios (fnu(895)/fnu(1470)) of the significant subsamples we find that these samples contain real LyC leaking flux with a very high probability, but the true average escape fractions remain uncertain. Our work indicates that galaxies with very high EWLya, small Lya(ext) and small rUV are very likely the best candidates to show LyC radiation at z=4 and could therefore be the galaxies that contributed more to reionization.
Questions as to what drove the bulk reionization of the Universe, how that reionization proceeded, and how the hard ionizing radiation reached the intergalactic medium remain open and debated. Observations probing that epoch are severely hampered by the increasing amounts of neutral gas with increasing redshift, so a small, but growing number of experiments are targeting star forming galaxies ($zsim3$) as proxies. However, these studies, while providing fantastic detail, are time intensive, contain relatively few targets, and can suffer from selection biases. As a complementary alternative, we investigate whether stacking the already vast (and growing) numbers of low-resolution ($Delta lambda / lambda = 800$) Lyman-$alpha$ Emitting (LAE) galaxy spectra from the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) can be used to measure ionizing photons (restframe 880-910AA) escaping their galaxy hosts. As a blind survey, HETDEX avoids the biases from continuum selected galaxies and its planned 540 square degree coverage promotes the statistical power of large numbers. In this paper, we confirm the feasibility of Lyman continuum detection by carefully selecting a sample of lyccount high redshift ($zsim$3) LAEs from a subset of HETDEX observations, stacking their spectra and measuring a $gtrsim$3$sigma$ detection of $0.10 mu$Jy restframe Lyman continuum emission, uncorrected for attenuation in the intergalactic medium, over the full sample stack ($3.0 < z < 3.5$ and $-22.0 lesssim M_{text{UV}} lesssim -19.0$).
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