Is the interstellar gas of starburst galaxies well mixed?

The extent to which the ISM in galaxies is well mixed is not yet settled. Measured metal abundances in the diffuse neutral gas of star--forming gas--rich dwarf galaxies are deficient with respect to that of the ionized gas. The reasons, if real, are not clear and need to be based on firm grounds. Far-UV spectroscopy of giant HII regions such as NGC604 in the spiral galaxy M33 using FUSE allows us to investigate possible systematic errors in the metallicity derivation. We still find underabundances of nitrogen, oxygen, argon, and iron in the neutral phase by a factor of~6. This could either be explained by the presence of less chemically evolved gas pockets in the sightlines or by dense clouds out of which HIIregions form. Those could be more metallic than the diffuse medium.Comment: 4 pages, 2 figures;contribution to Starbursts: from 30 Dor to Lyman Break Galaxies, 6 -10 September 2004, Institute of Astronomy, University of Cambridge, U

How do dwarf galaxies acquire their mass & when do they form their stars?

We apply a simple, one-equation, galaxy formation model on top of the halos and subhalos of a high-resolution dark matter cosmological simulation to study how dwarf galaxies acquire their mass and, for better mass resolution, on over 10^5 halo merger trees, to predict when they form their stars. With the first approach, we show that the large majority of galaxies within group- and cluster-mass halos have acquired the bulk of their stellar mass through gas accretion and not via galaxy mergers. We deduce that most dwarf ellipticals are not built up by galaxy mergers. With the second approach, we constrain the star formation histories of dwarfs by requiring that star formation must occur within halos of a minimum circular velocity set by the evolution of the temperature of the IGM, starting before the epoch of reionization. We qualitatively reproduce the downsizing trend of greater ages at greater masses and predict an upsizing trend of greater ages as one proceeds to masses lower than m_crit. We find that the fraction of galaxies with very young stellar populations (more than half the mass formed within the last 1.5 Gyr) is a function of present-day mass in stars and cold gas, which peaks at 0.5% at m_crit=10^6-8 M_Sun, corresponding to blue compact dwarfs such as I Zw 18. We predict that the baryonic mass function of galaxies should not show a maximum at masses above 10^5.5, M_Sun, and we speculate on the nature of the lowest mass galaxies.Comment: 6 pages, to appear in "A Universe of Dwarf Galaxies: Observations, Theories, Simulations", ed. M. Koleva, P. Prugniel & I. Vauglin, EAS Series (Paris: EDP

Diffuse light and galaxy interactions in the core of nearby clusters

The kinematics of the diffuse light in the densest regions of the nearby clusters can be unmasked using the planetary nebulae (PNs) as probes of the stellar motions. The position-velocity diagrams around the brightest cluster galaxies (BCGs) identify the relative contributions from the outer halos and the intracluster light (ICL), defined as the light radiated by the stars floating in the cluster potential. The kinematics of the ICL can then be used to asses the dynamical status of the nearby cluster cores and to infer their formation histories. The cores of the Virgo and Coma are observed to be far from equilibrium, with mergers currently on-going, while the ICL properties in the Fornax and Hydra clusters show the presence of sub-components being accreted in their cores, but superposed to an otherwise relaxed population of stars. Finally the comparison of the observed ICL properties with those predicted from Lambda-CDM simulations indicates a qualitative agreement and provides insights on the ICL formation. Both observations and simulations indicate that BCG halos and ICL are physically distinct components, with the ``hotter" ICL dominating at large radial distances from the BCGs halos as the latter become progressively fainter.Comment: 14 pages, 5 figures. Invited review to appear in the proceedings of "Galaxies and their masks" eds. Block, D.L., Freeman, K.C. and Puerari, I., 2010, Springer (New York

Radiative Decay of a Long-Lived Particle and Big-Bang Nucleosynthesis

The effects of radiatively decaying, long-lived particles on big-bang nucleosynthesis (BBN) are discussed. If high-energy photons are emitted after BBN, they may change the abundances of the light elements through photodissociation processes, which may result in a significant discrepancy between the BBN theory and observation. We calculate the abundances of the light elements, including the effects of photodissociation induced by a radiatively decaying particle, but neglecting the hadronic branching ratio. Using these calculated abundances, we derive a constraint on such particles by comparing our theoretical results with observations. Taking into account the recent controversies regarding the observations of the light-element abundances, we derive constraints for various combinations of the measurements. We also discuss several models which predict such radiatively decaying particles, and we derive constraints on such models.Comment: Published version in Phys. Rev. D. Typos in figure captions correcte

Neutral Hydrogen Observations of the Extremely Metal-Poor Blue Compact Dwarf Galaxy SBS 0335-052

otherPeer reviewe

Near infrared photometry of faint radio galaxies

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Einstein X-ray observations of optical-radio selected areas

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Detection of a second optical component in the H I cloud of the young dwarf galaxy SBS 0335-052 : New observations with the 6-m telescope

The Blue Compact Dwarf (BCD) galaxy SBS 0335–052 is one of the most metal–deficient galaxies known and one of the best candidates for a young dwarf galaxy in the process of formation. A VLA HI map reveals an unusual structure: the neutral gas is distributed in a very extended disk, about 15 times larger than the Holmberg optical diameter of the BCG. There are two peaks of high density. The eastern peak is close to the position of SBS 0335–052 whereas the western HI peak is associated with a faint compact optical galaxy. A 6–m telescope spectrum of this object shows H, H, and [OIII]5007 emission with a redshift close to that of SBS 0335–052. We suggest that it may be a young and chemically unevolved dwarf galaxy.Peer reviewe