We report the detection of extended Lyα emission around individual star-forming galaxies at redshifts z = 3−6 in an ultradeep exposure of the Hubble Deep Field South obtained with MUSE on the ESO-VLT. The data reach a limiting surface brightness (1σ) of ∼1 × 10 −19 erg s −1 cm −2 arcsec −2 in azimuthally averaged radial profiles, an order of magnitude improvement over previous narrowband imaging. Our sample consists of 26 spectroscopically confirmed Lyα-emitting, but mostly continuum-faint (m AB > ∼ 27) galaxies. In most objects the Lyα emission is considerably more extended than the UV continuum light. While five of the faintest galaxies in the sample show no significantly detected Lyα haloes, the derived upper limits suggest that this is due to insufficient S/N. Lyα haloes therefore appear to be ubiquitous even for low-mass (∼10 8 −10 9 M ) star-forming galaxies at z > 3. We decompose the Lyα emission of each object into a compact component tracing the UV continuum and an extended halo component, and infer sizes and luminosities of the haloes. The extended Lyα emission approximately follows an exponential surface brightness distribution with a scale length of a few kpc. While these haloes are thus quite modest in terms of their absolute sizes, they are larger by a factor of 5−15 than the corresponding rest-frame UV continuum sources as seen by HST. They are also much more extended, by a factor ∼5, than Lyα haloes around low-redshift star-forming galaxies. Between ∼40% and > ∼ 90% of the observed Lyα flux comes from the extended halo component, with no obvious correlation of this fraction with either the absolute or the relative size of the Lyα halo. Our observations provide direct insights into the spatial distribution of at least partly neutral gas residing in the circumgalactic medium of low to intermediate mass galaxies at z > 3.

We observed Hubble Deep Field South with the new panoramic integral-field spectrograph MUSE that we built and have just commissioned at the VLT. The data cube resulting from 27 h of integration covers one arcmin 2 field of view at an unprecedented depth with a 1σ emission-line surface brightness limit of 1 × 10 −19 erg s −1 cm −2 arcsec −2 , and contains ∼90 000 spectra. We present the combined and calibrated data cube, and we performed a first-pass analysis of the sources detected in the Hubble Deep Field South imaging. We measured the redshifts of 189 sources up to a magnitude I 814 = 29.5, increasing the number of known spectroscopic redshifts in this field by more than an order of magnitude. We also discovered 26 Lyα emitting galaxies that are not detected in the HST WFPC2 deep broad-band images. The intermediate spectral resolution of 2.3 Å allows us to separate resolved asymmetric Lyα emitters, [O ]3727 emitters, and C ]1908 emitters, and the broad instantaneous wavelength range of 4500 Å helps to identify single emission lines, such as [O ]5007, Hβ, and Hα, over a very wide redshift range. We also show how the three-dimensional information of MUSE helps to resolve sources that are confused at ground-based image quality. Overall, secure identifications are provided for 83% of the 227 emission line sources detected in the MUSE data cube and for 32% of the 586 sources identified in the HST catalogue. The overall redshift distribution is fairly flat to z = 6.3, with a reduction between z = 1.5 to 2.9, in the well-known redshift desert. The field of view of MUSE also allowed us to detect 17 groups within the field. We checked that the number counts of [O ]3727 and Lyα emitters are roughly consistent with predictions from the literature. Using two examples, we demonstrate that MUSE is able to provide exquisite spatially resolved spectroscopic information on the intermediate-redshift galaxies present in the field. This unique data set can be used for a wide range of follow-up studies. We release the data cube, the associated products, and the source catalogue with redshifts, spectra, and emission-line fluxes.

No abstract

The Lyman alpha (Lyα) line of Hydrogen is a prominent feature in the spectra of star-forming galaxies, usually redshifted by a few hundreds of km s −1 compared to the systemic redshift. This large offset hampers follow-up surveys, galaxy pair statistics and correlations with quasar absorption lines when only Lyα is available. We propose diagnostics that can be used to recover the systemic redshift directly from the properties of the Lyα line profile. We use spectroscopic observations of Lyman-Alpha Emitters (LAEs) for which a precise measurement of the systemic redshift is available. Our sample contains 13 sources detected between z ≈ 3 and z ≈ 6 as part of various Multi Unit Spectroscopic Explorer (MUSE) Guaranteed Time Observations (GTO). We also include a compilation of spectroscopic Lyα data from the literature spanning a wide redshift range (z ≈ 0 − 8). First, restricting our analysis to double-peaked Lyα spectra, we find a tight correlation between the velocity offset of the red peak with respect to the systemic redshift, V red peak , and the separation of the peaks. Secondly, we find a correlation between V red peak and the full width at half maximum of the Lyα line. Fitting formulas, to estimate systemic redshifts of galaxies with an accuracy of ≤ 100 km s −1 when only the Lyα emission line is available, are given for the two methods.

We present rest-frame Lyα equivalent widths (EW 0 ) of 417 Lyα emitters (LAEs) detected with Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT) at 2.9 < z < 6.6 in the Hubble Ultra Deep Field. Based on the deep MUSE spectroscopy and ancillary Hubble Space Telescope (HST) photometry data, we carefully measured EW 0 values taking into account extended Lyα emission and UV continuum slopes (β). Our LAEs reach unprecedented depths, both in Lyα luminosities and UV absolute magnitudes, from log (L Lyα /erg s −1 ) ∼ 41.0 to 43.0 and from M UV ∼ −16 to −21 (0.01 − 1.0 L * z=3 ). The EW 0 values span the range of ∼ 5 to 240 Å or larger, and their distribution can be well fitted by an exponential law N = N 0 exp(−EW 0 /w 0 ). Owing to the high dynamic range in M UV , we find that the scale factor, w 0 , depends on M UV in the sense that including fainter M UV objects increases w 0 , i.e., the Ando effect. The results indicate that selection functions affect the EW 0 scale factor. Taking these effects into account, we find that our w 0 values are consistent with those in the literature within 1σ uncertainties at 2.9 < z < 6.6 at a given threshold of M UV and L Lyα . Interestingly, we find 12 objects with EW 0 > 200 Å above 1σ uncertainties. Two of these 12 LAEs show signatures of merger or AGN activity: the weak Civ λ1549 emission line. For the remaining 10 very large EW 0 LAEs, we find that the EW 0 values can be reproduced by young stellar ages (< 100 Myr) and low metallicities ( 0.02 Z ). Otherwise, at least part of the Lyα emission in these LAEs needs to arise from anisotropic radiative transfer effects, fluorescence by hidden AGN or quasi-stellar object activity, or gravitational cooling.