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.

We present a sample of 2865 emission‐line galaxies with strong nebular emissions in the Sloan Digital Sky Survey Data Release 7 and use this sample to investigate the origin of this line in star‐forming galaxies. We show that star‐forming galaxies and galaxies dominated by an active galactic nucleus form clearly separated branches in the versus diagnostic diagram and derive an empirical classification scheme which separates the two classes. We also present an analysis of the physical properties of 189 star‐forming galaxies with strong emissions. These star‐forming galaxies provide constraints on the hard ionizing continuum of massive stars. To make a quantitative comparison with observation, we use photoionization models and examine how different stellar population models affect the predicted emission. We confirm previous findings that the models can predict emission only for instantaneous bursts of 20 per cent solar metallicity or higher, and only for ages of ∼4–5 Myr, the period when the extreme‐ultraviolet continuum is dominated by emission from Wolf–Rayet stars. We find, however, that 83 of the star‐forming galaxies (40 per cent) in our sample do not have Wolf–Rayet features in their spectra despite showing strong nebular emission. We discuss possible reasons for this and possible mechanisms for the emission in these galaxies.

Aims. Whereas the evolution of gas kinematics of massive galaxies is now relatively well established up to redshift z ∼ 3, little is known about the kinematics of lower mass (M ≤ 10 10 M ) galaxies. We use MUSE, a powerful wide-field, optical integral-field spectrograph (IFS) recently mounted on the VLT, to characterize this galaxy population at intermediate redshift.Methods. We made use of the deepest MUSE observations performed so far on the Hubble Deep Field South (HDFS). This data cube, resulting from 27 h of integration time, covers a 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 a final spatial resolution of ≈0.7 . We identified a sample of 28 resolved emission-line galaxies, extending over an area that is at least twice the seeing disk, spread over a redshift interval of 0.2 < z < 1.4. More than half of the galaxies are at z ∼ 0.3−0.7, which is a redshift range poorly studied so far with IFS kinematics. We used the public HST images and multiband photometry over the HDFS to constrain the stellar mass and star formation rate (SFR) of the galaxies and to perform a morphological analysis using Galfit, providing estimates of the disk inclination, disk scale length, and position angle of the major axis. We derived the resolved ionized gas properties of these galaxies from the MUSE data and model the disk (both in 2D and in 3D with GalPaK 3D ) to retrieve their intrinsic gas kinematics, including the maximum rotation velocity and velocity dispersion. Results. We build a sample of resolved emission-line galaxies of much lower stellar mass and SFR (by ∼1−2 orders of magnitude) than previous IFS surveys. The gas kinematics of most of the spatially resolved MUSE-HDFS galaxies is consistent with disk-like rotation, but about 20% have velocity dispersions that are larger than the rotation velocities and 30% are part of a close pair and/or show clear signs of recent gravitational interactions. These fractions are similar to what has been found in previous IFS surveys of more massive galaxies, indicating that the dynamical state of the ionized gas and the level of gravitational interactions of star-forming galaxies is not a strong function of their stellar mass. In the high-mass regime, the MUSE-HDFS galaxies follow the Tully-Fisher relation defined from previous IFS surveys in a similar redshift range. This scaling relation also extends to lower masses/velocities but with a higher dispersion. We find that 90% of the MUSE-HDFS galaxies with stellar masses below 10 9.5 M have settled gas disks. The MUSE-HDFS galaxies follow the scaling relations defined in the local Universe between the specific angular momentum and stellar mass. However, we find that intermediate-redshift, star-forming galaxies fill a continuum transition from the spiral to elliptical local scaling relations, according to the dynamical state (i.e., rotation-or dispersion-dominated) of the gas. This indicates that some galaxies may lose their angu...

We investigated AGN activity in low-mass galaxies, an important regime that can shed light onto BH formation and evolution, and their interaction with their host galaxies. We identified 336 AGN candidates from a parent sample of ∼ 48, 000 nearby low-mass galaxies (M 10 9.5 M , z < 0.1) in the SDSS. We selected the AGN using the classical BPT diagram, a similar optical emission line diagnostic based on the He ii λ4686 line, and mid-IR color cuts. Different criteria select host galaxies with different physical properties such as stellar mass and optical color and only 3 out of 336 sources fulfill all three criteria. This could be in part due to selection biases. The resulting AGN fraction of ∼ 0.7% is at least one order of magnitude below the one estimated for more massive galaxies. At optical wavelengths, the He ii -based AGN selection appears to be more sensitive to AGN hosted in star-forming galaxies than the classical BPT diagram, at least in the low-mass regime. The archival X-ray and radio data available for some of the optically selected AGN candidates seem to confirm their AGN nature, but follow-up observations are needed to confirm the AGN nature of the rest of the sample, especially in the case of mid-IR selection. Our sample will be important for future follow-up studies aiming to understand the relation between BHs and host galaxies in the low-mass regime.

The density of the warm ionized gas in high-redshift galaxies is known to be higher than what is typical in local galaxies on similar scales. At the same time, the mean global properties of the high-and low-redshift galaxies are quite different. Here, we present a detailed differential analysis of the ionization parameters of 14 star-forming galaxies at redshift 2.6-3.4, compiled from the literature. For each of those high-redshift galaxies, we construct a comparison sample of low-redshift galaxies closely matched in specific star formation rate (sSFR) and stellar mass, thus ensuring that their global physical conditions are similar to the high-redshift galaxy. We find that the median log[O iii] 5007/[O ii] 3727 line ratio of the high-redshift galaxies is 0.5 dex higher than their local counterparts. We construct a new calibration between the [O iii] 5007/[O ii] 3727 emission line ratio and ionization parameter to estimate the difference between the ionization parameters in the high and low-redshift samples. Using this, we show that the typical density of the warm ionized gas in star-forming regions decreases by a median factor of 7.1 +10.2 −5.4 from z ∼ 3.3 to z ∼ 0 at fixed mass and sSFR. We show that metallicity differences cannot explain the observed density differences. Because the high-and low-redshift samples are comparable in size, we infer that the relationship between star formation rate density and gas density must have been significantly less efficient at z ∼ 2 − 3 than what is observed in nearby galaxies with similar levels of star formation activity.