[en] We present U₃₃₆V₆₀₆J₁₂₅H₁₆₀ follow-up Hubble Space Telescope (HST) observations of 16 z ∼ 3 candidate Lyman continuum (LyC) emitters in the HS1549+1919 field. With these data, we obtain high spatial-resolution photometric redshifts of all sub-arcsecond components of the LyC candidates in order to eliminate foreground contamination and identify robust candidates for leaking LyC emission. Of the 16 candidates, we find one object with a robust LyC detection that is not due to foreground contamination. This object (MD5) resolves into two components; we refer to the LyC-emitting component as MD5b. MD5b has an observed 1500 Å to 900 Å flux-density ratio of ${(F_{\mathrm{U}\mathrm{V}}/F_{\mathrm{L}\mathrm{y}\mathrm{C}})}_{\mathrm{o}\mathrm{b}\mathrm{s}}=4.0\pm <!--\; ??\; -->2.0$, compatible with predictions from stellar population synthesis models. Assuming minimal IGM absorption, this ratio corresponds to a relative (absolute) escape fraction of $f_{\mathrm{e}\mathrm{s}\mathrm{c},\mathrm{r}\mathrm{e}\mathrm{l}}^{\mathrm{M}\mathrm{D}5\mathrm{b}}$ = 75%–100% ($f_{\mathrm{e}\mathrm{s}\mathrm{c},\mathrm{a}\mathrm{b}\mathrm{s}}^{\mathrm{M}\mathrm{D}5\mathrm{b}}$ = 14%–19%). The stellar population fit to MD5b indicates an age of ≲50 Myr, which is in the youngest 10% of the HST sample and the youngest third of typical z ∼ 3 Lyman break galaxies, and may be a contributing factor to its LyC detection. We obtain a revised, contamination-free estimate for the comoving specific ionizing emissivity at z = 2.85, indicating (with large uncertainties) that star-forming galaxies provide roughly the same contribution as QSOs to the ionizing background at this redshift. Our results show that foreground contamination prevents ground-based LyC studies from obtaining a full understanding of LyC emission from z ∼ 3 star-forming galaxies. Future progress in direct LyC searches is contingent upon the elimination of foreground contaminants through high spatial-resolution observations, and upon acquisition of sufficiently deep LyC imaging to probe ionizing radiation in high-redshift galaxies.

[en] We present a study of an extended Lyα nebula located in a known overdensity at z ∼ 2.38. The data include multiwavelength photometry covering the rest-frame spectral range from 0.1 to 250 μm, and deep optical spectra of the sources associated with the extended emission. Two galaxies are associated with the Lyα nebula. One of them is a dust enshrouded active galactic nucleus (AGN), while the other is a powerful starburst, forming stars at ∼>400 M_sun yr^-1. We detect the He II emission line at 1640 A in the spectrum of the obscured AGN, but detect no emission from other highly ionized metals (C IV or N V) as is expected from an AGN. One scenario that simultaneously reproduces the width of the detected emission lines, the lack of C IV emission, and the geometry of the emitting gas, is that the He II and the Lyα emission are the result of cooling gas that is being accreted on the dark matter halo of the two galaxies, Ly1 and Ly2. Given the complexity of the environment associated with our Lyα nebula it is possible that various mechanisms of excitation are at work simultaneously.

[en] We report the results of a Spitzer infrared (IR) study of the Cosmic Eye, a strongly lensed, L*_UV Lyman break galaxy (LBG) at z = 3.074. We obtained Spitzer mid-IR spectroscopy as well as MIPS 24 and 70 μm photometry. The Eye is detected with high significance at both 24 and 70 μm and, when including a flux limit at 3.5 mm, we estimate an IR luminosity of L _IR = 8.3^+4.7_-4.4 x 10¹¹ L _sun assuming a magnification of 28± 3. This L _IR is eight times lower than that predicted from the rest-frame ultraviolet properties assuming a Calzetti reddening law. This has also been observed in other young LBGs, and indicates that the dust reddening law may be steeper in these galaxies. The mid-IR spectrum shows strong polycyclic aromatic hydrocarbon (PAH) emission at 6.2 and 7.7 μm, with equivalent widths near the maximum values observed in star-forming galaxies at any redshift. The L _PAH-to-L _IR ratio lies close to the relation measured in local starbursts. Therefore, L _PAH or L _MIR may be used to estimate L _IR, and thus star formation rate, of LBGs, whose fluxes at longer wavelengths are typically below current confusion limits. We also report the highest redshift detection of the 3.3 μm PAH emission feature. The PAH ratio, L _6.2/L _3.3 = 5.1 ± 2.7, and the PAH-to-L_IR ratio, L _3.3/L _IR = 8.5 ± 4.7 x 10^-4, are both in agreement with measurements in local starbursts and ultraluminous infrared galaxies (ULIRGs), suggesting that this line may serve as a good proxy for L _PAH or L _IR at z > 3 with the James Webb Space Telescope.

[en] We present the optical spectroscopic follow-up of 31 z = 0.3 Lyα emitters, previously identified by Deharveng et al. We find that 17% of the Lyα emitters have line ratios that require the hard ionizing continuum produced by an active galactic nucleus. The uniform dust screen geometry traditionally used in studies similar to ours is not able to simultaneously reproduce the observed high Lyα/Hα and Hα/Hβ line ratios. We consider different possibilities for the geometry of the dust around the emitting sources. We find that also a uniform mixture of sources and dust does not reproduce the observed line ratios. Instead, these are well reproduced by a clumpy dust screen. This more realistic treatment of the geometry results in extinction corrected (Lyα/Hα) _C values consistent with case B recombination theory, whereas a uniform dust screen model would imply values (Lyα/Hα) _C higher than 8.7. Our analysis shows that there is no need to invoke ad hoc multiphase media in which the Lyα photons only scatter between the dusty clouds and eventually escape.

[en] The first statistically significant detection of the cosmic γ-ray horizon (CGRH) that is independent of any extragalactic background light (EBL) model is presented. The CGRH is a fundamental quantity in cosmology. It gives an estimate of the opacity of the universe to very high energy (VHE) γ-ray photons due to photon-photon pair production with the EBL. The only estimations of the CGRH to date are predictions from EBL models and lower limits from γ-ray observations of cosmological blazars and γ-ray bursts. Here, we present homogeneous synchrotron/synchrotron self-Compton (SSC) models of the spectral energy distributions of 15 blazars based on (almost) simultaneous observations from radio up to the highest energy γ-rays taken with the Fermi satellite. These synchrotron/SSC models predict the unattenuated VHE fluxes, which are compared with the observations by imaging atmospheric Cherenkov telescopes. This comparison provides an estimate of the optical depth of the EBL, which allows us a derivation of the CGRH through a maximum likelihood analysis that is EBL-model independent. We find that the observed CGRH is compatible with the current knowledge of the EBL.

[en] We present the WFC3 Infrared Spectroscopic Parallel (WISP) Survey. WISP is obtaining slitless, near-infrared grism spectroscopy of ∼90 independent, high-latitude fields by observing in the pure-parallel mode with the Wide Field Camera Three on the Hubble Space Telescope for a total of ∼250 orbits. Spectra are obtained with the G₁₀₂ (λ = 0.8-1.17 μm, R ∼210) and G₁₄₁ grisms (λ = 1.11-1.67 μm, R ∼130), together with direct imaging in the J and H bands (F110W and F140W, respectively). In the present paper, we present the first results from 19 WISP fields, covering approximately 63 arcmin². For typical exposure times (∼6400 s in G₁₀₂ and ∼2700 s in G₁₄₁), we reach 5σ detection limits for emission lines of f ∼ 5 x 10^-17 erg s^-1 cm^-2 for compact objects. Typical direct imaging 5σ limits are 26.3 and 26.1 mag. (AB) in F110W and F140W, respectively. Restricting ourselves to the lines measured with the highest confidence, we present a list of 328 emission lines, in 229 objects, in a redshift range 0.3 < z < 3. The single-line emitters are likely to be a mix of Hα and [O III]5007,4959 A, with Hα predominating. The overall surface density of high-confidence emission-line objects in our sample is approximately 4 per arcmin². These first fields show high equivalent width sources, active galactic nucleus, and post-starburst galaxies. The median observed star formation rate (SFR) of our Hα-selected sample is 4 M_sun yr^-1. At intermediate redshifts, we detect emission lines in galaxies as faint as H₁₄₀ ∼ 25, or M_R < -19, and are sensitive to SFRs down to less than 1 M_sun yr^-1. The slitless grisms on WFC3 provide a unique opportunity to study the spectral properties of galaxies much fainter than L* at the peak of the galaxy assembly epoch.

[en] The WFC3 Infrared Spectroscopic Parallel Survey uses the Hubble Space Telescope (HST) infrared grism capabilities to obtain slitless spectra of thousands of galaxies over a wide redshift range including the peak of star formation history of the universe. We select a population of very strong emission-line galaxies with rest-frame equivalent widths (EWs) higher than 200 Å. A total of 176 objects are found over the redshift range 0.35 < z < 2.3 in the 180 arcmin² area that we have analyzed so far. This population consists of young and low-mass starbursts with high specific star formation rates (sSFR). After spectroscopic follow-up of one of these galaxies with Keck/Low Resolution Imaging Spectrometer, we report the detection at z = 0.7 of an extremely metal-poor galaxy with 12 + log(O/H) =7.47 ± 0.11. After estimating the active galactic nucleus fraction in the sample, we show that the high-EW galaxies have higher sSFR than normal star-forming galaxies at any redshift. We find that the nebular emission lines can substantially affect the total broadband flux density with a median brightening of 0.3 mag, with some examples of line contamination producing brightening of up to 1 mag. We show that the presence of strong emission lines in low-z galaxies can mimic the color-selection criteria used in the z ∼ 8 dropout surveys. In order to effectively remove low-redshift interlopers, deep optical imaging is needed, at least 1 mag deeper than the bands in which the objects are detected. Without deep optical data, most of the interlopers cannot be ruled out in the wide shallow HST imaging surveys. Finally, we empirically demonstrate that strong nebular lines can lead to an overestimation of the mass and the age of galaxies derived from fitting of their spectral energy distribution (SED). Without removing emission lines, the age and the stellar mass estimates are overestimated by a factor of 2 on average and up to a factor of 10 for the high-EW galaxies. Therefore, the contribution of emission lines should be systematically taken into account in SED fitting of star-forming galaxies at all redshifts.

[en] We present the discovery of three late-type (≥T4.5) brown dwarfs, including a probable Y dwarf, in the WFC3 Infrared Spectroscopic Parallels (WISP) survey. We use the G141 grism spectra to determine the spectral types of the dwarfs and derive distance estimates based on a comparison with nearby T dwarfs with known parallaxes. These are the most distant spectroscopically confirmed T/Y dwarfs, with the farthest at an estimated distance of ∼400 pc. We compare the number of cold dwarfs found in the WISP survey with simulations of the brown dwarf mass function. The number found is generally consistent with an initial stellar mass function dN/dM∝M^–α with α = 0.0-0.5, although the identification of a Y dwarf is somewhat surprising and may be indicative of either a flatter absolute magnitude/spectral-type relation than previously reported or an upturn in the number of very-late-type brown dwarfs in the observed volume.

[en] Spectroscopic observations of Hα and Hβ emission lines of 128 star-forming galaxies in the redshift range 0.75 ≤ z ≤ 1.5 are presented. These data were taken with slitless spectroscopy using the G102 and G141 grisms of the Wide-Field-Camera 3 (WFC3) on board the Hubble Space Telescope as part of the WFC3 Infrared Spectroscopic Parallel survey. Interstellar dust extinction is measured from stacked spectra that cover the Balmer decrement (Hα/Hβ). We present dust extinction as a function of Hα luminosity (down to 3 × 10⁴¹ erg s^–1), galaxy stellar mass (reaching 4 × 10⁸ M _☉), and rest-frame Hα equivalent width. The faintest galaxies are two times fainter in Hα luminosity than galaxies previously studied at z ∼ 1.5. An evolution is observed where galaxies of the same Hα luminosity have lower extinction at higher redshifts, whereas no evolution is found within our error bars with stellar mass. The lower Hα luminosity galaxies in our sample are found to be consistent with no dust extinction. We find an anti-correlation of the [O III] λ5007/Hα flux ratio as a function of luminosity where galaxies with L _Hα < 5 × 10⁴¹ erg s^–1 are brighter in [O III] λ5007 than Hα. This trend is evident even after extinction correction, suggesting that the increased [O III] λ5007/Hα ratio in low-luminosity galaxies is likely due to lower metallicity and/or higher ionization parameters.

[en] We combine Hubble Space Telescope (HST) G102 and G141 near-IR (NIR) grism spectroscopy with HST/WFC3-UVIS, HST/WFC3-IR, and Spitzer/IRAC [3.6 μm] photometry to assemble a sample of massive (log (M _star/M _☉) ∼ 11.0) and quenched (specific star formation rate <0.01 Gyr^–1) galaxies at z ∼ 1.5. Our sample of 41 galaxies is the largest with G102+G141 NIR spectroscopy for quenched sources at these redshifts. In contrast to the local universe, z ∼ 1.5 quenched galaxies in the high-mass range have a wide range of stellar population properties. We find that their spectral energy distributions (SEDs) are well fitted with exponentially decreasing star formation histories and short star formation timescales (τ ≤ 100 Myr). Quenched galaxies also show a wide distribution in ages, between 1 and 4 Gyr. In the (u – r)₀-versus-mass space quenched galaxies have a large spread in rest-frame color at a given mass. Most quenched galaxies populate the z ∼ 1.5 red sequence (RS), but an important fraction of them (32%) have substantially bluer colors. Although with a large spread, we find that the quenched galaxies on the RS have older median ages (3.1 Gyr) than the quenched galaxies off the RS (1.5 Gyr). We also show that a rejuvenated SED cannot reproduce the observed stacked spectra of (the bluer) quenched galaxies off the RS. We derive the upper limit on the fraction of massive galaxies on the RS at z ∼ 1.5 to be <43%. We speculate that the young quenched galaxies off the RS are in a transition phase between vigorous star formation at z > 2 and the z ∼ 1.5 RS. According to their estimated ages, the time required for quenched galaxies off the RS to join their counterparts on the z ∼ 1.5 RS is of the order of ∼1 Gyr.