ABSTRACT
We present a VLT/X-Shooter spectroscopy of the Lyman continuum (LyC) emitting galaxy Ion2 at z = 3.2121 and compare it to that of the recently discovered strongly lensed LyC emitter at ...z = 2.37, known as the Sunburst arc. Three main results emerge from the X-Shooter spectrum: (a) the Ly α has three distinct peaks with the central one at the systemic redshift, indicating a ionized tunnel through which both Ly α and LyC radiation escape; (b) the large O32 oxygen index (O iii λλ4959, 5007/O ii λλ3727, 3729) of $9.18_{-1.32}^{+1.82}$ is compatible to those measured in local (z ∼0.4) LyC leakers; (c) there are narrow nebular high-ionization metal lines with σv < 20 km s−1, which confirms the presence of young hot, massive stars. The He iiλ1640 appears broad, consistent with a young stellar component including Wolf–Rayet stars. Similarly, the Sunburst LyC emitter shows a triple-peaked Ly α profile and from VLT/MUSE spectroscopy the presence of spectral features arising from young hot and massive stars. The strong lensing magnification, (μ > 20), suggests that this exceptional object is a gravitationally bound star cluster observed at a cosmological distance, with a stellar mass M ≲ 107 M⊙ and an effective radius smaller than 20 pc. Intriguingly, sources like Sunburst but without lensing magnification might appear as Ion2-like galaxies, in which unresolved massive star clusters dominate the ultraviolet emission. This work supports the idea that dense young star clusters can contribute to the ionization of the IGM through holes created by stellar feedback.
The study of the space density of bright active galactic nuclei (AGNs) at z > 4 has been subject to extensive effort given its importance in the estimation of cosmological ionizing emissivity and ...growth of supermassive black holes. In this context we have recently derived high space densities of AGNs at z ∼ 4 and −25 < M1450 < −23 in the Cosmic Evolution Survey (COSMOS) field from a spectroscopically complete sample. In the present paper we attempt to extend the knowledge of the AGN space density at fainter magnitudes (−22.5 < M1450 < −18.5) in the 4 < z < 6.1 redshift interval by means of a multiwavelength sample of galaxies in the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) GOODS-South, GOODS-North, and EGS fields. We use an updated criterion to extract faint AGNs from a population of near-IR (rest-frame UV) selected galaxies at photometric z > 4 showing X-ray detection in deep Chandra images available for the three CANDELS fields. We have collected a photometric sample of 32 AGN candidates in the selected redshift interval, six of which having spectroscopic redshifts. Including our COSMOS sample as well as other bright QSO samples allows a first guess on the shape of the UV luminosity function (LF) at z ∼ 4.5. The resulting emissivity and photoionization rate appear consistent with that derived from the photoionization level of the intergalactic medium at z ∼ 4.5. An extrapolation to z ∼ 5.6 suggests an important AGN contribution to the ionization of intergalactic medium if there are no significant changes in the shape of the UV LF.
Abstract
We report on five compact, extremely young (<10 Myr) and blue (βUV < −2.5, Fλ = λβ) objects observed with VLT/Multi Unit Spectroscopic Explorer at redshifts 3.1169 and 3.235, in addition to ...three objects at z = 6.145. These sources are strongly magnified (3–40 times) by the Hubble Frontier Field galaxy clusters MACS J0416 and AS1063. Their delensed half-light radii (Re) are between 16 and 140 pc, the stellar masses are ≃1–20 × 106 M⊙, the magnitudes are mUV = 28.8–31.4 (−17 < MUV < −15) and specific star formation rates can be as large as ∼800 Gyr−1. Remarkably, the inferred physical properties of two objects are similar to those expected in some globular cluster formation scenarios, representing the best candidate proto-GCs discovered so far. Rest-frame optical high-dispersion spectroscopy of one of them at z = 3.1169 yields a velocity dispersion σv ≃ 20 km s−1, implying a dynamical mass dominated by the stellar mass. Another object at z = 6.145, with delensed MUV ≃ −15.3 (mUV ≃ 31.4), shows a stellar mass and a star formation rate surface density consistent with the values expected from popular GC formation scenarios. An additional star-forming region at z = 6.145, with delensed mUV ≃ 32, a stellar mass of 0.5 × 106 M⊙ and a star formation rate of 0.06 M⊙ yr−1 is also identified. These objects currently represent the faintest spectroscopically confirmed star-forming systems at z > 3, elusive even in the deepest blank fields. We discuss how proto-GCs might contribute to the ionization budget of the Universe and augment Lyα visibility during reionization. This work underlines the crucial role of JWST in characterizing the rest-frame optical and near-infrared properties of such low-luminosity high-z objects.
We present the results of CANDELSz7, a European Southern Observatory (ESO) Large Program aimed at spectroscopically confirming a homogeneous sample of z ≃ 6 and z ≃ 7 star forming galaxies. The ...candidates were selected in the GOODS-South, UDS, and COSMOS fields using the official CANDELS catalogs based on H160-band detections. Standard color criteria, which were tailored depending on the ancillary multi-wavelength data available for each field, were applied to select more than 160 candidate galaxies at z ≃ 6 and z ≃ 7. Deep, medium-resolution FORS2 spectroscopic observations were then conducted with integration times ranging from 12 to 20 h to reach a Lyα flux limit of approximately 1 − 3 × 10−18 erg s−1 cm−2 at 3σ. We could determine a spectroscopic redshift for about 40% of the galaxies, mainly through the detection of a single emission line that we interpret as Lyα emission, or for some of the brightest objects (H160 ≤ 25.5) from the presence of faint continuum and a sharp drop that we interpret as a Lyα break. In this paper we present the redshifts and main properties of 65 newly confirmed high-redshift galaxies. Adding previous proprietary and archival data we assemble a sample of ≃260 galaxies that we use to explore the evolution of the Lyα fraction in Lyman break galaxies and the change in the shape of the emission line between z ∼ 6 and z ∼ 7. We also discuss the accuracy of the CANDELS photometric redshifts in this redshift range.
We investigate the strongly lensed (
μ
≃ ×10 − 100) Lyman continuum (LyC) galaxy, dubbed Sunburst, at
z
= 2.37, taking advantage of a new accurate model of the lens. A characterization of the ...intrinsic (delensed) properties of the system yields a size of ≃3 sq. kpc, a luminosity of
M
UV
= −20.3, and a stellar mass of
M
≃ 10
9
M
⊙
; 16% of the ultraviolet light is located in a 3 Myr old gravitationally bound young massive star cluster (YMC), with an effective radius of ∼8 pc (corresponding to 1 milliarcsec without lensing) and a dynamical mass of ∼10
7
M
⊙
(similar to the stellar mass) – from which LyC radiation is detected (
λ
< 912 Å). The star formation rate and stellar mass surface densities for the YMC are Log
10
(Σ
SFR
M
⊙
yr
−1
kpc
−2
) ≃ 3.7 and Log
10
(Σ
M
M
⊙
pc
−2
) ≃ 4.1, with sSFR > 330 Gyr
−1
, consistent with the values observed in local young massive star clusters. The inferred outflowing gas velocity (> 300 km s
−1
) exceeds the escape velocity of the cluster. The resulting relative escape fraction of the ionizing radiation emerging from the entire galaxy is higher than 6−12%, whilst it is ≳46 − 93% if inferred from the YMC multiple line of sights. At least 12 additional unresolved star-forming knots with radii spanning the interval 3 − 20 pc (the majority of them likely gravitationally bound star clusters) are identified in the galaxy. A significant fraction (40−60%) of the ultraviolet light of the entire galaxy is located in such bound star clusters. In adopting a formation timescale of the star clusters of 20 Myr, a cluster formation efficiency Γ ≳ 30%. The star formation rate surface density of the Sunburst galaxy (Log
10
(Σ
SFR
) = 0.5
−0.2
+0.3
) is consistent with the high inferred Γ, as observed in local galaxies experiencing extreme gas physical conditions. Overall, the presence of a bursty event (i.e., the 3 Myr old YMC with large sSFR) significantly influences the morphology (nucleation), photometry (photometric jumps), and spectroscopic output (nebular emission) of the entire galaxy. Without lensing magnification, the YMC would be associated to an unresolved 0.5 kpc–size star-forming clump. The delensed LyC and UV magnitude
m
1600
(at 1600 Å) of the YMC are ≃30.6 and ≃26.9, whilst the entire galaxy has
m
1600
≃ 24.8. The Sunburst galaxy shows a relatively large rest-frame equivalent width of EW
rest
(H
β
+ O
III
λλ
4959, 5007) ≃ 450 Å, with the YMC contributing to ∼30% (having a local EW
rest
≃ 1100 Å) and ∼1% of the total stellar mass. If O-type (ionizing) stars are mainly forged in star clusters, then such engines were the key ionizing agents during reionization and the increasing occurrence of high equivalent width lines (H
β
+ O
III
) observed at
z
> 6.5 might be an indirect signature of a high frequency of forming massive star clusters (or high Γ) at reionization. Future facilities, which will perform at few tens milliarcsec resolution (e.g., VLT/MAVIS or ELT), will probe bound clusters on moderately magnified (
μ
< 5 − 10) galaxies across cosmic epochs up to reionization.
Abstract
Star-forming galaxies have long been considered the dominant sources of the cosmic ultraviolet background radiation at early epochs. However, observing and characterizing the galaxy ...population with significant ionizing emission have proven to be challenging. In particular, the fraction of ionizing radiation that escapes the local environment to the intergalactic medium is poorly known. We investigate the relation between the escape fraction and galaxy luminosity. We combine deep ultraviolet observations of Hubble Ultra Deep Field (UVUDF) with deep Multi Unit Spectroscopic Explorer (MUSE) observations of the same field, collecting a sample of 165 faint star-forming galaxies in the 3 < z < 4 redshift range with deep rest-frame observations of the Lyman continuum (LyC). In our sample, we do not find any galaxy with significant emission of LyC radiation. We bin the galaxies in various redshift and brightness intervals and stack their images. From stacked images, we estimate the relative escape fraction upper limits as a function of the luminosity. Thanks to the depth of the sample, we measure meaningful 1σ upper limits of f
esc, rel < 0.07, 0.2 and 0.6 at
$L \sim L_{z=3}^{*}, 0.5L_{z=3}^{*}$
and
$0.1L_{z=3}^{*}$
, respectively. We use our estimates and theoretical predictions from the literature to study a possible dependence of the escape fraction on galaxy luminosity by modelling the ionizing background with different prescriptions of f
esc(M
UV). We show that the understanding of the luminosity dependence hinges on the ability to constrain the escape fraction down to M
UV ∼ −18 mag in the future.
Context. Establishing the number of faint active galactic nuclei (AGNs) at z = 4−6 is crucial to understanding their cosmological importance as main contributors to the reionization of the Universe. ...Aims. In order to derive the AGN contribution to the cosmological ionizing emissivity we have selected faint AGN candidates at z> 4 in the CANDELS GOODS-South field, which is one of the deepest fields with extensive multiwavelength coverage from Chandra, HST, Spitzer, and various ground-based telescopes. Methods. We have adopted a relatively novel criterion. As a first step, high redshift galaxies are selected in the NIR H band down to very faint levels (H ≤ 27) using reliable photometric redshifts. At z> 4 this corresponds to a selection criterion based on the galaxy rest-frame UV flux. AGN candidates are then picked up from this parent sample if they show X-ray fluxes above a threshold of FX ∼ 1.5 × 10-17 erg cm-2 s-1 (0.5−2 keV), corresponding to a probability of spurious detections of 2 × 10-4 in the deep X-ray 4 Ms Chandra image. Results. We have found 22 AGN candidates at z> 4 and we have derived the first estimate of the UV luminosity function in the redshift interval 4 <z< 6.5 and absolute magnitude interval − 22.5 ≲ M1450 ≲ −18.5 typical of local Seyfert galaxies. The faint end of the derived luminosity function is about two to four magnitudes fainter at z ∼ 4−6 than that derived from previous UV surveys. We estimated ionizing emissivities and hydrogen photoionization rates in the same redshift interval under reasonable assumptions and after discussion of possible caveats, the most important being the large uncertainties involved in the estimate of photometric redshift for sources with featureless, almost power-law SEDs and/or low average escape fraction of ionizing photons from the AGN host galaxies. Both effects could, in principle, significantly reduce the estimated average volume densities and/or ionizing emissivities, especially at the highest redshifts. Conclusions. At z = 4−6.5 we argue that, under reasonable evaluations of possible biases, the probed AGN population can produce photoionization rates consistent with that required to keep the intergalactic medium observed in the Lyman-α forest of high redshift QSO spectra highly ionized, providing an important contribution to the cosmic reionization.
Context. One of the main challenges in observational cosmology is related to the redshift evolution of the average hydrogen (HI) ionization in the Universe, as evidenced by the changing in ionization ...level of the intergalactic medium (IGM) through cosmic time. Starting from the first cosmic reionization, the rapid evolution of the IGM physical properties in particular poses severe constraints for the identification of the sources responsible for maintaining its high level of ionization up to lower redshifts. Aims. In order to probe the ionization level of the IGM and the ionization capabilities of bright quasi-stellar objects (QSOs) at z = 4, we selected a sample of 2508 QSOs drawn from the Sloan Digital Sky Survey (SDSS, DR14) in the redshift interval 3.6 ≤ z ≤ 4.6 and absolute magnitude range −29.0 ≲ M1450 ≲ −26.0. Particularly, we focus on the estimate of the escape fraction of HI-ionizing photons and their mean free path (MFP), which are fundamental for characterizing the surrounding IGM. Methods. Starting from UV/optical rest-frame spectra of the whole QSO sample from the SDSS survey, we estimated the escape fraction and free path individually for each of the QSOs. We calculated the Lyman continuum (LyC) escape fraction as the flux ratio blueward (∼900 Å rest frame) and redward (∼930 Å rest frame) of the Lyman limit. We then obtained the probability distribution function (PDF) of the individual free paths of the QSOs in the sample and studied its evolution in luminosity and redshift, comparing our results with those in literature. Results. We find a lower limit to the mean LyC escape fraction of 0.49, in agreement with the values obtained for both brighter and fainter sources at the same redshift. We show that the free paths of ionizing photons are characterized by a skewed distribution function that peaks at low values, with an average of ∼49 − 59 proper Mpc at z ∼ 4, after possible associated absorbers (AAs) were excluded. This value is higher than the one obtained at the same redshift by many authors in the literature using different techniques. Moreover, the PDF of free path gives information that is complementary to the MFP derived through the stacking technique. Finally, we also find that the redshift evolution of this parameter might be milder than previously thought. Conclusions. Our new determination of the MFP at z ∼ 4 implies that previous estimates of the HI photoionization rate ΓHI available in the literature should be corrected by a factor of 1.2−1.7. These results have important implications when they are extrapolated at the epoch of reionization.
ABSTRACT
We present the goals, design, and first results of the MUSE Ultra Deep Field (MUDF) survey, a large programme using the Multi Unit Spectroscopic Explorer (MUSE) instrument at the ESO Very ...Large Telescope. The MUDF survey is collecting ≈150 h on-source of integral field optical spectroscopy in a 1.5 × 1.2 arcmin2 region which hosts several astrophysical structures along the line of sight, including two bright z ≈ 3.2 quasars with close separation (≈500 kpc). Following the description of the data reduction procedures, we present the analysis of the galaxy environment and gaseous properties of seven groups detected at redshifts 0.5 < z < 1.5, spanning a large dynamic range in halo mass, $\log (M_h/\rm {M_\odot }) \approx 11 - 13.5$. For four of the groups, we find associated Mg ii absorbers tracing cool gas in high-resolution spectroscopy of the two quasars, including one case of correlated absorption in both sightlines at distance ≈480 kpc. The absorption strength associated with the groups is higher than what has been reported for more isolated galaxies of comparable mass and impact parameters. We do not find evidence for widespread cool gas giving rise to strong absorption within these groups. Combining these results with the distribution of neutral and ionized gas seen in emission in lower redshift groups, we conclude that gravitational interactions in the group environment strip gas from the galaxy haloes into the intragroup medium, boosting the cross-section of cool gas and leading to the high fraction of strong Mg ii absorbers that we detect.
Abstract
We report on the serendipitous discovery of a z = 4.0, M1500 = −22.20 star-forming galaxy (Ion3) showing copious Lyman continuum (LyC) leakage (∼60 per cent escaping), a remarkable multiple ...peaked Ly α emission, and significant Ly α radiation directly emerging at the resonance frequency. This is the highest redshift confirmed LyC emitter in which the ionizing and Ly α radiation possibly share a common ionized channel (with NH I < 1017.2 cm−2). Ion3 is spatially resolved, it shows clear stellar winds signatures like the P-Cygni N vλ1240 profile, and has blue ultraviolet continuum (β = −2.5 ± 0.25, Fλ ∼ λβ) with weak low-ionization interstellar metal lines. Deep VLT/HAWKI Ks and Spitzer/IRAC 3.6 and 4.5μm imaging show a clear photometric signature of the H α line with equivalent width of 1000 Å rest-frame emerging over a flat continuum (Ks − 4.5μm ≃ 0). From the SED fitting, we derive a stellar mass of 1.5 × 109 M⊙, SFR of 140 M⊙ yr−1 and age of ∼10 Myr, with a low dust extinction, E(B − V) ≲ 0.1, placing the source in the starburst region of the SFR−M* plane. Ion3 shows similar properties of another LyC emitter previously discovered (z = 3.21, Ion2, Vanzella et al. 2016). Ion3 (and Ion2) represents ideal high-redshift reference cases to guide the search for reionizing sources at z > 6.5 with JWST.
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