The dark matter halos of the bluest low surface brightness galaxies

We present BVI photometry and long-slit Halpha rotation 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 very 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.Comment: 14 pages, 9 figures, accepted for publication in Astronomy & Astrophysic

The impact of nebular emission on the broadband fluxes of high-redshift galaxies

A substantial fraction of the light emitted from young or star-forming galaxies at ultraviolet to near-infrared wavelengths comes from the ionized interstellar medium in the form of emission lines and a nebular continuum. At high redshifts, star formation rates are on average higher and stellar populations younger than in the local Universe. Both of these effects act to boost the impact of nebular emission on the overall spectrum of galaxies. Even so, the broadband fluxes and colours of high-redshift galaxies are routinely analyzed under the assumption that the light observed originates directly from stars. Here, we assess the impact of nebular emission on broadband fluxes in Johnson/Cousins BVRIJHK, Sloan Digital Sky Survey griz and Spitzer IRAC/MIPS filters as a function of observed redshift (up to z=15) for galaxies with different star formation histories. We find that nebular emission may account for a non-negligible fraction of the light received from high-redshift galaxies. The ages and masses inferred for such objects through the use of spectral evolutionary models that omit the nebular contribution are therefore likely to contain systematic errors. We argue that a careful treatment of the nebular component will be essential for the interpretation of the rest-frame ultraviolet-to-infrared properties of the first galaxies formed, like the ones expected to be detected with the James Webb Space Telescope.Comment: 5 pages, 3 figures, accepted for publication in ApJ

Constraining Dark Matter with the Long-Term Variability of Quasars

By comparing the results of numerical microlensing simulations to the observed long-term variability of quasars, strong upper limits on the cosmological density of compact objects in the mass range 0.01 to 0.0001 solar masses may be imposed. Using recently developed methods to better approximate the amplification of large sources, we investigate in what way the constraints are affected by assumptions concerning the size of the optical continuum-emitting region of quasars in the currently favored (Omega_M=0.3, Omega_Lambda=0.7) cosmology.Comment: 3 pages, 2 figures, to appear in the proceedings from the conference "Lighthouses of the Universe: The Most Luminous Celestial Objects and Their Use for Cosmology", Garching, August 2001 (Springer-Verlag

The temporal and spatial evolution of the starburst in ESO 338-IG04 as probed by its star clusters

In this paper we use ultra-violet (UV) and optical HST photometry in five bands, and an extensive set of spectral evolutionary synthesis scenarios to investigate the age and masses of 124 star clusters in the luminous blue compact galaxy ESO338-IG04 (Tololo 1924-416). The very small internal reddening makes ESO 338-IG04 an excellent laboratory for studying the formation of massive star clusters. We have used the star clusters to trace the temporal and spatial evolution of the starburst, and to put constraints on the star formation activity over a cosmological time-scale. The present starburst has been active for about 40 Myr. A standard Salpeter initial mass function (IMF) extending up to 120 solar masses provides the best fit to the data, although a flatter IMF cannot be excluded. The compact star clusters provide 30-40 percent of the UV luminosity and star formation activity. We find no evidence for dust obscuration even among the youngest (< 1 Myr) clusters. The fraction of stellar mass contained in compact star clusters is found to be several percent, which is an unusually high value. The intermediate age clusters show a flattened space distribution which agrees with the isophotal shape of the galaxy, whereas the oldest clusters seem to have a spherical distribution.(abridged)Comment: Accepted for publication in A&

The stellar populations of the bluest low surface brightness galaxies

Using optical/near-IR broadband photometry together with Halpha emission line data, we attempt to constrain the star formation histories, ages, total stellar masses and stellar mass-to-light ratios for a sample of extremely blue low surface brightness galaxies. We find that, under standard assumptions about the stellar initial mass function, the Halpha equivalent widths of these objects appear inconsistent with recently suggested scenarios including constant or increasing star formation rates over cosmological time scales. In a critical assessment of the prospects of obtaining ages from integrated broadband photometry, we conclude that even with near-IR data, the ages are poorly constrained and that current observations cannot rule out the possibility that these objects formed as recently as 1-2 Gyr ago. Methods which could potentially improve the age estimates are discussed. The stellar masses of these galaxies are inferred to lie below 10^10 solar masses. This, in combination with low ages, could constitute a problem for current hierarchical models of galaxy formation, which predict objects of this mass to form predominantly early in the history of the universe. The possibility to use the ages of the bluest low surface brightness galaxies as a test of such models is demonstrated.Comment: 13 pages, 14 figures, accepted for publication in A&

A model of spectral galaxy evolution including the effects of nebular emission

This paper presents a new spectral evolutionary model of galaxies, properly taking the effects of nebular emission and pre-main sequence evolution into account. The impact of these features in different photometric filters is evaluated, along with the influence that variations in the physical conditions of the gas may have on broadband colours, line ratios and equivalent widths. Inclusion of nebular emission is demonstrated to radically change the predicted ultraviolet, optical and near-infrared colours during active star formation. Pre-main sequence evolution is also seen to give a non-negligible contribution to the luminosity in the near-infrared during the first few millions years of evolution and should not be omitted when very young systems are being modelled. Finally, we present a comparison of our predictions to observations and two other recent codes of evolutionary synthesis.Comment: 13 pages, 10 figures, accepted for publication in A&

The starburst phenomenon from the optical/near-IR perspective

The optical/near-IR stellar continuum carries unique information about the stellar population in a galaxy, its mass function and star-formation history. Star-forming regions display rich emission-line spectra from which we can derive the dust and gas distribution, map velocity fields, metallicities and young massive stars and locate shocks and stellar winds. All this information is very useful in the dissection of the starburst phenomenon. We discuss a few of the advantages and limitations of observations in the optical/near-IR region and focus on some results. Special attention is given to the role of interactions and mergers and observations of the relatively dust-free starburst dwarfs. In the future we expect new and refined diagnostic tools to provide us with more detailed information about the IMF, strength and duration of the burst and its triggering mechanisms.Comment: 6 pages, 3 figures, to appear in "Starbursts: from 30 Doradus to Lyman Break Galaxies" 2005, eds. R. de Grijs and R. M. Gonzalez Delgado (Kluwer