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- ItemNew criteria for the selection of galaxy close pairs from cosmological simulations: Evolution of the major and minor merger fraction in MUSE deep fields(Les Ulis : EDP Sciences, 2019) Ventou, E.; Contini, T.; Bouché, N.; Epinat, B.; Brinchmann, J.; Inami, H.; Richard, J.; Schroetter, I.; Soucail, G.; Steinmetz, M.; Weilbacher, P.M.It remains a challenge to assess the merger fraction of galaxies at different cosmic epochs in order to probe the evolution of their mass assembly. Using the Illustris cosmological simulation project, we investigate the relation between the separation of galaxies in a pair, both in velocity and projected spatial separation space, and the probability that these interacting galaxies will merge in the future. From this analysis, we propose a new set of criteria to select close pairs of galaxies along with a new corrective term to be applied to the computation of the galaxy merger fraction. We then probe the evolution of the major and minor merger fraction using the latest Multi-Unit Spectroscopic Explorer (MUSE) deep observations over the Hubble Ultra Deep Field, Hubble Deep Field South, COSMOS-Gr30, and Abell 2744 regions. From a parent sample of 2483 galaxies with spectroscopic redshifts, we identify 366 close pairs spread over a large range of redshifts (0:2 < z < 6) and stellar masses (107-1011 M ). Using the stellar mass ratio between the secondary and primary galaxy as a proxy to split the sample into major, minor, and very minor mergers, we found a total of 183 major, 142 minor, and 47 very minor close pairs corresponding to a mass ratio range of 1:1-1:6, 1:6-1:100, and lower than 1:100, respectively. Due to completeness issues, we do not consider the very minor pairs in the analysis. Overall, the major merger fraction increases up to z ≈2-3 reaching 25% for pairs where the most massive galaxy has a stellar mass M· = 109:5 M . Beyond this redshift, the fraction decreases down to ∼5% at z≈6. The major merger fraction for lower-mass primary galaxies with M· = 109:5 M seems to follow a more constant evolutionary trend with redshift. Thanks to the addition of new MUSE fields and new selection criteria, the increased statistics of the pair samples allow us to significantly shorten the error bars compared to our previous analysis. The evolution of the minor merger fraction is roughly constant with cosmic time, with a fraction of 20% at z < 3 and a slow decrease to 8-13% in the redshift range 3 ≤ z ≤ 6.
- ItemEvidence for ram-pressure stripping in a cluster of galaxies at z = 0.7(Les Ulis : EDP Sciences, 2019) Boselli, A.; Epinat, B.; Contini, T.; Abril-Melgarejo, V.; Boogaard, L. A.; Pointecouteau, E.; Ventou, E.; Brinchmann, J.; Carton, D.; Finley, H.; Michel-Dansac, L.; Soucail, G.; Weilbacher, P.M.Multi-Unit Spectroscopic Explorer (MUSE) observations of the cluster of galaxies CGr32 (M200≅ 2×1014 M⊙) at = 0.73 reveal the presence of two massive star-forming galaxies with extended tails of diffuse gas detected in the [O II]λλ3727-3729 Å emission-line doublet. The tails, which have a cometary shape with a typical surface brightness of a few 10-18 erg s-1 cm-2 arcsec-2, extend up to ≅ 100 kpc (projected distance) from the galaxy discs, and are not associated with any stellar component. All this observational evidence suggests that the gas was removed during a ram-pressure stripping event. This observation is thus the first evidence that dynamical interactions with the intracluster medium were active when the Universe was only half its present age. The density of the gas derived using the observed [O II]λ3729/[O II]λ3726 line ratio implies a very short recombination time, suggesting that a source of ionisation is necessary to keep the gas ionised within the tail.
- ItemFaint end of the z ∼ 3-7 luminosity function of Lyman-alpha emitters behind lensing clusters observed with MUSE(Les Ulis : EDP Sciences, 2019) de La Vieuville, G.; Bina, D.; Pello, R.; Mahler, G.; Richard, J.; Drake, A.B.; Herenz, E. C.; Bauer, F.E.; Clément, B.; Lagattuta, D.; Laporte, N.; Martinez, J.; PatrÃcio, V.; Wisotzki, L.; Zabl, J.; Bouwens, R.J.; Contini, T.; Garel, T.; Guiderdoni, B.; Marino, R.A.; Maseda, M.V.; Matthee, J.; Schaye, J.; Soucail, G.Contact. This paper presents the results obtained with the Multi-Unit Spectroscopic Explorer (MUSE) at the ESO Very Large Telescope on the faint end of the Lyman-alpha luminosity function (LF) based on deep observations of four lensing clusters. The goal of our project is to set strong constraints on the relative contribution of the Lyman-alpha emitter (LAE) population to cosmic reionization. Aims. The precise aim of the present study is to further constrain the abundance of LAEs by taking advantage of the magnification provided by lensing clusters to build a blindly selected sample of galaxies which is less biased than current blank field samples in redshift and luminosity. By construction, this sample of LAEs is complementary to those built from deep blank fields, whether observed by MUSE or by other facilities, and makes it possible to determine the shape of the LF at fainter levels, as well as its evolution with redshift. Methods. We selected a sample of 156 LAEs with redshifts between 2.9 ≤ z ≤ 6.7 and magnification-corrected luminosities in the range 39 ≳ log LLyα [erg s-1] ≲ 43. To properly take into account the individual differences in detection conditions between the LAEs when computing the LF, including lensing configurations, and spatial and spectral morphologies, the non-parametric 1/Vmax method was adopted. The price to pay to benefit from magnification is a reduction of the effective volume of the survey, together with a more complex analysis procedure to properly determine the effective volume Vmax for each galaxy. In this paper we present a complete procedure for the determination of the LF based on IFU detections in lensing clusters. This procedure, including some new methods for masking, effective volume integration and (individual) completeness determinations, has been fully automated when possible, and it can be easily generalized to the analysis of IFU observations in blank fields. Results. As a result of this analysis, the Lyman-alpha LF has been obtained in four different redshift bins: 2.9 < z < 6, 7, 2.9 < z < 4.0, 4.0 < z < 5.0; and 5.0 < z < 6.7 with constraints down to log LLyα = 40.5. From our data only, no significant evolution of LF mean slope can be found. When performing a Schechter analysis also including data from the literature to complete the present sample towards the brightest luminosities, a steep faint end slope was measured varying from α = -1.69+0.08-0.08 to α = -1.87+0.12-0.12 between the lowest and the highest redshift bins. Conclusions. The contribution of the LAE population to the star formation rate density at z z ∼ 6 is ≲50% depending on the luminosity limit considered, which is of the same order as the Lyman-break galaxy (LBG) contribution. The evolution of the LAE contribution with redshift depends on the assumed escape fraction of Lyman-alpha photons, and appears to slightly increase with increasing redshift when this fraction is conservatively set to one. Depending on the intersection between the LAE/LBG populations, the contribution of the observed galaxies to the ionizing flux may suffice to keep the universe ionized at z ∼ 6.