Models of disk galaxy formation commonly predict the existence of an extended reservoir of accreted hot gas surrounding massive spirals at low redshift. As a test of these models, we use X-ray and Hα data of the two massive, quiescent edge-on spirals NGC 5746 and NGC 5170 to investigate the amount and origin of any hot gas in their halos. Contrary to our earlier claim, the Chandra analysis of NGC 5746, employing more recent calibration data, does not reveal any significant evidence for diffuse X-ray emission outside the optical disk, with a 3σ upper limit to the halo X-ray luminosity of 4×10 39 erg s −1 . An identical study of the less massive NGC 5170 also fails to detect any extraplanar Xray emission. By extracting hot halo properties of disk galaxies formed in cosmological hydrodynamical simulations, we compare these results to expectations for cosmological accretion of hot gas by spirals. For Milky Way-sized galaxies, these high-resolution simulations predict hot halo X-ray luminosities which are lower by a factor of ∼ 2 compared to our earlier results reported by Toft et al. (2002). We find the new simulation predictions to be consistent with our observational constraints for both NGC 5746 and NGC 5170, while also confirming that the hot gas detected so far around more actively star-forming spirals is in general probably associated with stellar activity in the disk. Observational results on quiescent disk galaxies at the high-mass end are nevertheless providing powerful constraints on theoretical predictions, and hence on the assumed input physics in numerical studies of disk galaxy formation and evolution.

Context. Lyα-emitters have proven to be excellent probes of faint, star-forming galaxies in the high redshift universe. However, although the sample of known emitters is increasingly growing, their nature (e.g. stellar masses, ages, metallicities, star-formation rates) is still poorly constrained. Aims. We aim to study the nature of Lyα-emitters, to find the properties of a typical Lyα-emitting galaxy and to compare these properties with the properties of other galaxies at similar redshift, in particular Lyman-break galaxies. Methods. We have performed narrow-band imaging at the VLT, focused on Lyα at redshift z ≈ 3.15, in the GOODS-S field. We have identified a sample of Lyα-emitting candidates, and we have studied their Spectral Energy Distributions (SEDs). Results. We find that the emitters are best fit by an SED with low metallicity (Z/Z = 0.005), low dust extinction (A V ≈ 0.26) and medium stellar masses of approximately 10 9 M . The age is not very well constrained. One object out of 24 appears to be a high redshift Lyα-emitting dusty starburst galaxy. We find filamentary structure as traced by the Lyα-emitters at the 4σ level. The restframe UV SED of these galaxies is very similar to that of Lyman Break Galaxies (LBGs) and comply with the selection criteria for U-band drop-outs, except they are intrinsically fainter than the current limit for LBGs. Conclusions. Lyα-emitters are excellent probes of galaxies in the distant universe, and represent a class of star-forming, dust and AGN free, medium mass objects.

SN 2008D was discovered while following up an unusually bright X-ray transient (XT) in the nearby spiral galaxy NGC 2770. We present early optical spectra (obtained 1.75 days after the XT) which allowed the first identification of the object as a supernova (SN) at redshift z = 0.007. These spectra were acquired during the initial declining phase of the light curve, likely produced in the stellar envelope cooling after shock breakout, and rarely observed. They exhibit a rather flat spectral energy distribution with broad undulations, and a strong, W-shaped feature with minima at 3980 and 4190Å (rest frame). We also present extensive spectroscopy and photometry of the SN during the subsequent photospheric phase. Unlike SNe associated with gamma-ray bursts, SN 2008D displayed prominent He features and is therefore of Type Ib. Subject headings: supernovae: individual (SN 2008D) OBSERVATIONS OF SN 2008DOn 2008 January 9.56 UT, while observing the supernova (SN) 2007uy in the nearby spiral galaxy NGC 2770 (z = 0.007), the X-Ray Telescope onboard Swift detected a bright X-ray transient (XT), with a peak luminosity of 6 × 10 43 erg s −1 and a duration of about 10 minutes

Context. We have substantial information about the kinematics and abundances of galaxies at z ≈ 3 studied in absorption against the light of background QSOs. At the same time we have already studied 1000s of galaxies detected in emission mainly through the Lyman-break selection technique; however, we know very little about how to make the connection between the two data sets. Aims. We aim at bridging the gap between absorption-selected and emission-selected galaxies at z ≈ 3 by probing the faint end of the luminosity function of star-forming galaxies at z ≈ 3. Methods. Narrow-band surveys for Lyman-α (Lyα) emitters have proven to be an efficient probe of faint, star-forming galaxies in the high-redshift universe. We performed narrow-band imaging in three fields with intervening QSO absorbers (a damped Lyα absorber and two Lyman-limit systems) using the VLT. We target Lyα at redshifts 2.85, 3.15, and 3.20. Results. We find a consistent surface density of about 10 Lyα-emitters per square arcmin per unit redshift in all three fields down to our detection limit of about 3 × 10 41 erg s −1 . The luminosity function is consistent with what has been found by other surveys at similar redshifts. About 85% of the sources are fainter than the canonical limit of R = 25.5 for most Lyman-break galaxy surveys. In none of the three fields do we detect the emission counterparts of the QSO absorbers. In particular we do not detect the counterpart of the z = 2.85 damped Lyα absorber towards Q2138−4427. This implies that the DLA galaxy is either not a Lyα emitter or is fainter than our flux limit. Conclusions. Narrow-band surveys for Lyα emitters are excellent for probing the faint end of the luminosity function at z ≈ 3. There is a very high surface density of this class of objects; yet, we only detect galaxies with Lyα in emission, so the density of galaxies with similar broad band magnitudes will be substantially higher. This is consistent with a very steep slope of the faint end of the luminosity function as has been inferred by other studies. This faint population of galaxies is playing a central role in the early Universe. There is evidence that this popualtion is dominating the intergrated star-formation activity, responsible for the bulk of the ionising photons at z 3 and likely also responsible for the bulk of the enrichment of the intergalactic medium.

Aims. We investigate the gain of added leverage and completeness of the constructed cluster catalogue, of applying the matched-filter detection algorithm to multiple passbands. In particular, we investigate the gain from having both i -and z -band data available when searching for galaxy clusters at z > ∼ 1.Methods. We applied a matched filter detection method to the CFHTLS r -and z -band data of the four Deep fields and compared the cluster catalogues with the one extracted from the i -band data presented in a previous paper. We also applied the matched filter to the Deep fields but with the limiting magnitudes appropriate for the much larger Wide survey in order to understand the best combination of i -and z -band depth for the most efficient cluster searches based on this algorithm. Results. The density of clusters identified in the Deep r -and z -band catalogues are 36 and 80 per square degree, respectively. The estimated densities of false detections are 12 and 20 per square degree in the two bands. We find that the recovered properties are in good agreement between the different bands and also that the efficiency of each band is consistent with the expectations based on the shift of the 4000 Å break through the filters. When comparing r -and i -band, we do not find any significant additions to the i -band catalogue. On the contrary, we find a large number of high redshift detections in the z -band not in i . These detections add ∼60% to the number of high-redshift detections in the i -band. Conclusions. We conclude that, for cluster searches to redshifts > ∼ 1, it is important to include sufficiently deep data redward of the i -band, which in this work is provided by the z -band coverage. The combination of catalogues extracted from two different passbands does not provide a cluster sample with greater purity.