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Register a new userThe Lyman Continuum escape fraction of faint galaxies at z~3.3 in the CANDELS/GOODS-North, EGS, and COSMOS fields with LBC
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The reionization of the Universe is one of the most important topics of present day astrophysical research. The most plausible candidates for the reionization process are star-forming galaxies, which according to the predictions of the majority of the theoretical and semi-analytical models should dominate the HI ionizing background at z~3. We aim at measuring the Lyman continuum escape fraction, which is one of the key parameters to compute the contribution of star-forming galaxies to the UV background. We have used ultra-deep U-band imaging (U=30.2mag at 1sigma) by LBC/LBT in the CANDELS/GOODS-North field, as well as deep imaging in COSMOS and EGS fields, in order to estimate the Lyman continuum escape fraction of 69 star-forming galaxies with secure spectroscopic redshifts at 3.27<z<3.40 to faint magnitude limits (L=0.2L*, or equivalently M1500~-19). We have measured through stacks a stringent upper limit (<1.7% at 1sigma) for the relative escape fraction of HI ionizing photons from bright galaxies (L>L*), while for the faint population (L=0.2L*) the limit to the escape fraction is ~10%. We have computed the contribution of star-forming galaxies to the observed UV background at z~3 and we have found that it is not enough to keep the Universe ionized at these redshifts, unless their escape fraction increases significantly (>10%) at low luminosities (M1500>-19). We compare our results on the Lyman continuum escape fraction of high-z galaxies with recent estimates in the literature and discuss future prospects to shed light on the end of the Dark Ages. In the future, strong gravitational lensing will be fundamental to measure the Lyman continuum escape fraction down to faint magnitudes (M1500~-16) which are inaccessible with the present instrumentation on blank fields.
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The Lyman continuum (LyC) flux escaping from high-z galaxies into the IGM is a fundamental quantity to understand the physical processes involved in the reionization epoch. We have investigated a sample of star-forming galaxies at z~3.3 in order to search for possible detections of LyC photons escaping from galaxy halos. UV deep imaging in the COSMOS field obtained with the prime focus camera LBC at the LBT telescope was used together with a catalog of spectroscopic redshifts obtained by the VIMOS Ultra Deep Survey (VUDS) to build a sample of 45 galaxies at z~3.3 with L>0.5L*. We obtained deep LBC images of galaxies with spectroscopic redshifts in the interval 3.27<z<3.40 both in the R and deep U bands. A sub-sample of 10 galaxies apparently shows escape fractions>28% but a detailed analysis of their properties reveals that, with the exception of two marginal detections (S/N~2) in the U band, all the other 8 galaxies are most likely contaminated by the UV flux of low-z interlopers located close to the high-z targets. The average escape fraction derived from the stacking of the cleaned sample was constrained to fesc_rel<2%. The implied HI photo-ionization rate is a factor two lower than that needed to keep the IGM ionized at z~3, as observed in the Lyman forest of high-z QSO spectra or by the proximity effect. These results support a scenario where high redshift, relatively bright (L>0.5L*) star-forming galaxies alone are unable to sustain the level of ionization observed in the cosmic IGM at z~3. Star-forming galaxies at higher redshift and at fainter luminosities (L<<L*) can be the major contributors to the reionization of the Universe only if their physical properties are subject to rapid changes from z~3 to z~6-10. Alternatively, ionizing sources could be discovered looking for fainter sources among the AGN population at high-z.
We present our analysis of the LyC emission and escape fraction of 111 spectroscopically verified galaxies with and without AGN from $2.26<z<4.3$. We extended our ERS sample from Smith et al. (2018; arXiv:1602.01555) with 64 galaxies in the GOODS North and South fields using WFC3/UVIS F225W, F275W, and F336W mosaics we independently drizzled using the HDUV, CANDELS, and UVUDF data. Among the 17 AGN from the 111 galaxies, one provided a LyC detection in F275W at $m_{AB}=23.19$ mag (S/N $simeq$ 133) and $GALEX$ NUV at $m_{AB}=23.77$ mag (S/N $simeq$ 13). We simultaneously fit $SDSS$ and $Chandra$ spectra of this AGN to an accretion disk and Comptonization model and find $f_{esc}$ values of $f_{esc}^{F275W}simeq 28^{+20}_{-4}$% and $f_{esc}^{NUV}simeq 30^{+22}_{-5}$%. For the remaining 110 galaxies, we stack image cutouts that capture their LyC emission using the F225W, F275W, and F336W data of the GOODS and ERS samples, and both combined, as well as subsamples of galaxies with and without AGN, and $all$ galaxies. We find the stack of 17 AGN dominate the LyC production from $langle zranglesimeq 2.3-4.3$ by a factor of $sim$10 compared to all 94 galaxies without AGN. While the IGM of the early universe may have been reionized mostly by massive stars, there is evidence that a significant portion of the ionizing energy came from AGN.
We present a new constraint on the Lyman Continuum (LyC) escape fraction at z~1.3. We obtain deep, high sensitivity far-UV imaging with the Advanced Camera for Surveys (ACS) Solar Blind Channel (SBC) on the Hubble Space Telescope (HST), targeting 11 star-forming galaxies at 1.2<z<1.4. The galaxies are selected from the 3D-HST survey to have high H$alpha$ equivalent width (EW) with EW > 190 AA, low stellar mass (M* < 10^10 M_sun) and U-band magnitude of U<24.2. These criteria identify young, low metallicity star bursting populations similar to the primordial star-forming galaxies believed to have reionized the universe. We do not detect any LyC signal (with S/N >3) in the individual galaxies or in the stack in the far-UV images. We place $3sigma$ limits on the relative escape fraction of individual galaxies to be f_{esc,rel}<[0.10-0.22] and a stacked $3sigma$ limit of f_{esc,rel}<0.07. Comparing to the confirmed LyC emitters from the literature, the galaxies in our sample span similar ranges of various galaxy properties including stellar mass, dust attenuation, and star formation rate (SFR). In particular, we compare the distribution of H$alpha$ and [OIII] EWs of confirmed LyC emitters and non-detections including the galaxies in this study. Finally, we discuss if a dichotomy seen in the distribution of H$alpha$ EWs can perhaps distinguish the LyC emitters from the non-detections.
We use the wealth of deep archival optical spectroscopy on the GOODS-South field from Keck, the VLT, and other facilities to select candidate high-redshift Lyman continuum (LyC) leakers in the Hubble Deep UV Legacy Survey (HDUV) dataset. We select sources at $2.35 < z < 3.05$, where the HST/WFC3 F275W filter probes only the redshifted LyC. We find five moderately F275W-bright sources (four detected at $gtrsim3sigma$ significance) in this redshift range. However, two of these show evidence in their optical spectra for contamination by foreground galaxies along the line-of-sight. We then perform an F275W error-weighted sum of the fluxes of all 129 galaxies at $2.35 < z < 3.05$ in both the GOODS-N and GOODS-S HDUV areas to estimate the total ionizing flux. The result is dominated by just five candidate F275W-bright LyC sources. Lastly, we examine the contributions to the metagalactic ionizing background, finding that, at the sensitivity of the HDUV F275W data and allowing for the effects of LyC transmission in the intergalactic medium (IGM), star-forming galaxies can match the UV flux required to maintain an ionized IGM at $z sim 2.5$.
Simulations have indicated that most of the escaped Lyman continuum photons escape through a minority of solid angles with near complete transparency, with the remaining majority of the solid angles largely opaque, resulting in a very broad and skewed probability distribution function (PDF) of the escape fraction when viewed at different angles. Thus, the escape fraction of Lyman continuum photons of a galaxy observed along a line of sight merely represents the properties of the interstellar medium along that line of sight, which may be an ill-representation of true escape fraction of the galaxy averaged over its full sky. Here we study how Lyman continuum photons escape from galaxies at $z=4-6$, utilizing high-resolution large-scale cosmological radiation-hydrodynamic simulations. We compute the PDF of the mean escape fraction ($left<f_{rm esc,1D}right>$) averaged over mock observational samples, as a function of the sample size, compared to the true mean (had you an infinite sample size). We find that, when the sample size is small, the apparent mean skews to the low end. For example, for a true mean of 6.7%, an observational sample of (2,10,50) galaxies at $z=4$ would have have 2.5% probability of obtaining the sample mean lower than $left<f_{rm esc,1D}right>=$(0.007%, 1.8%, 4.1%) and 2.5% probability of obtaining the sample mean being greater than (43%, 18%, 11%). Our simulations suggest that at least $sim$ 100 galaxies should be stacked in order to constrain the true escape fraction within 20% uncertainty.
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