Search

Context.Luminous blue compact galaxies, common at z~ 1 but now relatively rare, show disturbed kinematics in emission lines.

Aims.We study the two-dimensional distribution and kinematics of the stellar and gaseous components in the centre of the blue compact dwarf galaxy He 2-10. The aim is to compare the kinematics of gas and stars in order to determine whether they are consistent with one another, or if stars and gas can be decoupled due to gravitational perturbations and feedback from star formation.

Luminous Compact Galaxies (LCGs) are enigmatic sources in many aspects. They can reach the luminosity of the Milky Way within a radius of only a few kpc. They also represent one of the most rapidly evolving populations of galaxies since they represent up to 1/5 of the luminous galaxies at redshift $z= 0.7$, while being almost absent in the local Universe. The measurement of their dynamics is crucial to our understanding of LCGs since this has the potential of telling us which physical process(es) drive(s) them and ultimately link them to the existing present-day galaxies. Here, we derive the 3-dimensional velocity fields and velocity dispersion (σ) maps of 17 LCGs selected from the Canada France Redshift Survey and the Hubble Deep Field South with redshifts ranging from $z=0.4$to $z=0.75$. We find that only 18% of them show rotational velocity fields typical of rotating disks while the others show more complex kinematics. Assuming that LCGs are not too far from equilibrium, about half of LCGs then appear to be either non-relaxed objects, or objects that are not supported by velocity dispersion alone. This supports the view that an important fraction of LCGs are probably mergers. It brings additional support to the “spiral rebuilding scenario” in which LCGs correspond to a previous or post-merger phase before the disk re-building.

We present BVIphotometry and long-slit Hαrotation curve data obtained with ESO VLT/FORS2 for six low surface brightness galaxies with extremely blue colours and very faint central regions. We find no evidence for a steep central density cusp of the type predicted by many N-body simulations of cold dark matter (CDM) halos. Our observations are instead consistent with dark matter halos characterized by cores of roughly constant density, in agreement with previous investigations. While unremarkable in terms of the central density slope, these galaxies appear unusually challenging for existing CDM halo models in terms of average central halo density, as measured by the $\Delta_{V/2}$parameter. Since most of our target galaxies are bulgeless disks, our observations also disfavour a recently suggested mechanism for lowering the central mass concentration of the halo by means of a fast collapse phase, as this scenario predicts that the original CDM profile should still be detectable in bulgeless galaxies. Other potential ways of reconciling the CDM predictions with these observations are discussed.

Although controversial, the scenario of microlensing as the dominant mechanism for the long-term optical variability of quasars does provide a natural explanation for both the statistical symmetry, achromaticity and lack of cosmological time dilation in quasar light curves. Here, we investigate to what extent dark matter populations of compact objects allowed in the currently favored $\Omega_\mathrm{M}=0.3$, $\Omega_\Lambda=0.7$cosmology really can explain the quantitative statistical features of the observed variability. We find that microlensing reasonably well reproduces the average structure function of quasars, but fails to explain both the high fraction of objects with amplitudes higher than 0.35 mag and the mean amplitudes observed at redshifts below one. Even though microlensing may still contribute to the long-term optical variability at some level, another significant mechanism must also be involved. This severely complicates the task of using light-curve statistics from quasars which are not multiply imaged to isolate properties of any cosmologically significant population of compact objects which may in fact be present.

We have studied the afterglow of the gamma-ray burst (GRB) of February 18, 2006. This is a nearby long GRB, with a very low peak energy, and is therefore classified as an X-ray Flash (XRF). XRF?060218 is clearly associated with a supernova ? dubbed SN?2006aj. We present early spectra for SN?2006aj as well as optical lightcurves reaching out to 50 days past explosion. Our optical lightcurves define the rise times, the lightcurve shapes and the absolute magnitudes in the U, V and R?bands, and we compare these data with data for other relevant supernovae. SN?2006aj evolved quite fast, somewhat similarly to SN?2002ap, but not as fast as SN?1994I. Our spectra show the evolution of the supernova over the peak, when the U-band portion of the spectrum rapidly fades due to extensive line blanketing. We compare to similar spectra of very energetic type?Ic supernovae. Our first spectra are earlier than spectra for any other GRB-SN. The spectrum taken 12?days after burst in the rest frame is similar to somewhat later spectra of both SN?1998bw and SN?2003dh, implying a rapid early evolution. This is consistent with the fast lightcurve. From the narrow emission lines from the host galaxy we derive a redshift of z=0.0331?0.0007. This makes XRF?060218 the second closest gamma-ray burst detected. The flux of these emission lines indicate a high-excitation state, and a modest metallicity and star formation rate of the host galaxy.