Star Forming Objects in the Tidal Tails of Compact Groups

A search for star forming objects belonging to tidal tails has been carried out in a sample of deep Halpha images of 16 compact groups of galaxies. A total of 36 objects with Halpha luminosity larger than 10^38 erg s-1 have been detected in five groups. The fraction of the total Halpha luminosity of their respective parent galaxies shown by the tidal objects is always below 5% except for the tidal features of HCG95, whose Halpha luminosity amounts to 65% of the total luminosity. Out of this 36 objects, 9 star forming tidal dwarf galaxy candidates have been finally identified on the basis of their projected distances to the nuclei of the parent galaxies and their total Halpha luminosities. Overall, the observed properties of the candidates resemble those previously reported for the so-called tidal dwarf galaxies.Comment: 5 gif figures. Accepted for publication in Astrophysical Journa

b anti-b Higgs production at the LHC: Yukawa corrections and the leading Landau singularity

At tree-level Higgs production in association with a b-quark pair proceeds through the small Yukawa bottom coupling in the Standard Model. Even in the limit where this coupling vanishes, electroweak one-loop effects, through the top-Higgs Yukawa coupling in particular, can still trigger this reaction. This contribution is small for Higgs masses around 120GeV but it quickly picks up for higher Higgs masses especially because the one-loop amplitude develops a leading Landau singularity and new thresholds open up. These effects can be viewed as the production of a pair of top quarks which rescatter to give rise to Higgs production through WW fusion. We study the leading Landau singularity in detail. Since this singularity is not integrable when the one-loop amplitude is squared, we regulate the cross section by taking into account the width of the internal top and W particles. This requires that we extend the usual box one-loop function to the case of imaginary masses. We show how this can be implemented analytically in our case. We study in some detail the cross section at the LHC as a function of the Higgs mass and show how some distributions can be drastically affected compared to the tree-level result.Comment: 48 pages, 20 figures. Phys.Rev.D accepted version. Conclusions unchanged, minor changes and references adde

Star formation associated with neutral hydrogen in the outskirts of early-type galaxies

About 20 percent of all nearby early-type galaxies ($M_{\star} \gtrsim 6 \times 10^{9}$ M$_{\odot}$) outside the Virgo cluster are surrounded by a disc or ring of low-column-density neutral hydrogen (HI) gas with typical radii of tens of kpc, much larger than the stellar body. In order to understand the impact of these gas reservoirs on the host galaxies, we analyse the distribution of star formation out to large radii as a function of HI properties using GALEX UV and SDSS optical images. Our sample consists of 18 HI-rich galaxies as well as 55 control galaxies where no HI has been detected. In half of the HI-rich galaxies the radial UV profile changes slope at the position of the HI radial profile peak. To study the stellar populations, we calculate the FUV-NUV and UV-optical colours in two apertures, 1-3 and 3-10 R$_{eff}$ . We find that HI -rich galaxies are on average 0.5 and 0.8 mag bluer than the HI-poor ones, respectively. This indicates that a significant fraction of the UV emission traces recent star formation and is associated with the HI gas. Using FUV emission as a proxy for star formation, we estimate the integrated star formation rate in the outer regions (R > 1R$_{eff}$) to be on average $6 \times 10^{-3}$ M$_{\odot}$ yr$^{-1}$ for the HI-rich galaxies. This rate is too low to build a substantial stellar disc and, therefore, change the morphology of the host. We find that the star formation efficiency and the gas depletion time are similar to those at the outskirts of spirals.Comment: 27 pages (13 without appendices). 9 figures, 5 tables, 2 appendix tables and 12 appendix figures. Accepted for publication in MNRA

The Discovery of a Molecular Complex in the Tidal Arms near NGC 3077

We present the discovery of a giant molecular complex (r=350 pc, M_ vir=10^7 M_sun) in the tidal arms south-east of NGC 3077, a member of the M 81 triplet. The complex is clearly detected in the 12CO (J=1-0) transition at five independent positions. The position relative to NGC 3077, the systemic velocity (v_hel=14km/s) and the cloud averaged line width (15km/s) indicate that the object is not related to Galactic cirrus but is extragalactic. The tidal HI arm where the molecular complex is located has an total HI mass of M_HI=3x10^8 M_sun. This tidal material was presumably stripped off the outer parts of NGC 3077 during the closest encounter with M 81, about 3x10^8 years ago. After the complex detected along a torn-out spiral arm of M 81 by Brouillet et al., it is the second of its kind reported so far. Based on published optical observations, we have no evidence for on--going star formation in the newly detected molecular complex. Since the system has all the ingredients to form stars in the future, we speculate that it might eventually resemble the young dwarf galaxies in the M 81 group.Comment: 12 pages (including 3 figures), accepted for publication in the ApJ Letter

17O NMR study of the intrinsic magnetic susceptibility and spin dynamics of the quantum kagome antiferromagnet ZnCu3(OH)6Cl2

We report through 17O NMR, an unambiguous local determination of the intrinsic kagome lattice spin susceptibility as well as that created around non-magnetic defects issued from natural Zn/ Cu exchange in the S=1/2 (Cu2+) herbertsmithite ZnCu3(OH)6Cl2 compound. The issue of a singlet-triplet gap is addressed. The magnetic response around a defect is found to markedly differ from that observed in non-frustrated antiferromagnetic materials. Finally, we discuss our relaxation measurements in the light of Cu and Cl NMR data [cond-mat 070314] and suggest a flat q-dependence of the excitations.Comment: Accepted for publication in Phys. Rev. Lett., 3 jan. 2008 Figure 1 has been modified to include a two-components fit of the 17O NMR spectru

The orbital poles of Milky Way satellite galaxies: a rotationally supported disc-of-satellites

Available proper motion measurements of Milky Way (MW) satellite galaxies are used to calculate their orbital poles and projected uncertainties. These are compared to a set of recent cold dark-matter (CDM) simulations, tailored specifically to solve the MW satellite problem. We show that the CDM satellite orbital poles are fully consistent with being drawn from a random distribution, while the MW satellite orbital poles indicate that the disc-of-satellites of the Milky Way is rotationally supported. Furthermore, the bootstrapping analysis of the spatial distribution of theoretical CDM satellites also shows that they are consistent with being randomly drawn. The theoretical CDM satellite population thus shows a significantly different orbital and spatial distribution than the MW satellites, most probably indicating that the majority of the latter are of tidal origin rather than being DM dominated sub-structures. A statistic is presented that can be used to test a possible correlation of satellite galaxy orbits with their spatial distribution.Comment: Accepted for publication in Ap

A top-down scenario for the formation of massive Tidal Dwarf Galaxies

Among those objects formed out of collisional debris during galaxy mergers, the prominent gaseous accumulations observed near the tip of some long tidal tails are the most likely to survive long enough to form genuine recycled galaxies. Using simple numerical models, Bournaud, Duc & Masset (2003) claimed that tidal objects as massive as 10^9 Msun could only form, in these simulations, within extended dark matter (DM) haloes. We present here a new set of simulations of galaxy collisions to further investigate the structure of tidal tails. First of all, we checked that massive objects are still produced in full N-body codes that include feedback and a large number of particles. Using a simpler N-body code with rigid haloes, we noticed that dissipation and self-gravity in the tails, although important, are not the key factors. Exploiting toy models, we found that, for truncated DM haloes, material is stretched along the tail, while, within extended haloes, the tidal field can efficiently carry away from the disk a large fraction of the gas, while maintaining its surface density to a high value. This creates a density enhancement near the tip of the tail. Only later-on, self-gravity takes over; the gas clouds collapse and start forming stars. Thus, such objects were fundamentally formed following a kinematical process, according to a top-down scenario, contrary to the less massive Super Star Clusters that are also present around mergers. This conclusion leads us to introduce a restrictive definition for Tidal Dwarf Galaxies (TDGs) and their progenitors, considering only the most massive ones, initially mostly made of gas, that were able to pile up in the tidal tails. More simulations will be necessary to precisely determine the fate of these proto--TDGs and estimate their number.Comment: 13 pages, 11 figures, accepted for publication in A&