Is there Evidence for Flat Cores in the Halos of Dwarf Galaxies?: The Case of NGC 3109 and NGC 6822

Two well studied dwarf galaxies -- NGC 3109 and NGC 6822 -- present some of the strongest observational support for a flat core at the center of galactic dark matter (DM) halos. We use detailed cosmologically motivated numerical models to investigate the systematics and the accuracy of recovering parameters of the galaxies. Some of our models match the observed structure of the two galaxies remarkably well. Our analysis shows that the rotation curves of these two galaxies are instead quite compatible with their DM halos having steep cuspy density profiles. The rotation curves in our models are measured using standard observational techniques. The models reproduce the rotation curves of both galaxies, the disk surface brightness profiles as well as the profile of isophotal ellipticity and position angle. The models are centrally dominated by baryons; however, the dark matter component is globally dominant. The simulated disk mass is marginally consistent with a stellar mass-to-light ratio in agreement with the observed colors. We show that non-circular motions combined with gas pressure support and projection effects results in a large underestimation of the circular velocity in the central $\sim 1$ kpc region, creating the illusion of a constant density core. Although the systematic effects mentioned above are stronger in barred systems, they are also present in axisymetric disks. Our results strongly suggest that there is no contradiction between the observed rotation curves in dwarf galaxies and the cuspy central dark matter density profiles predicted by Cold Dark Matter models.Comment: Accepted for publication in the ApJ. New discussion, figures and one appendix. High resolution version at:http://www.astro.washington.edu/octavio/N3109_paper.ps.g

The Metallicity-Luminosity Relation, Effective Yields, and Metal Loss in Spiral and Irregular Galaxies

I present results on the correlation between galaxy mass, luminosity, and metallicity for a sample of spiral and irregular galaxies having well-measured abundance profiles, distances, and rotation speeds. Additional data for low surface brightness galaxies from the literature are also included for comparison. These data are combined to study the metallicity-luminosity and metallicity-rotation speed correlations for spiral and irregular galaxies. The metallicity luminosity correlation shows its familiar form for these galaxies, a roughly uniform change in the average present-day O/H abundance of about a factor 100 over 11 magnitudes in B luminosity. However, the O/H - V(rot) relation shows a change in slope at a rotation speed of about 125 km/sec. At faster V(rot), there appears to be no relation between average metallicity and rotation speed. At lower V(rot), the metallicity correlates with rotation speed. This change in behavior could be the result of increasing loss of metals from the smaller galaxies in supernova-driven winds. This idea is tested by looking at the variation in effective yield, derived from observed abundances and gas fractions assuming closed box chemical evolution. The effective yields derived for spiral and irregular galaxies increase by a factor of 10-20 from V(rot) approximately 5 km/sec to V(rot) approximately 300 km/sec, asympotically increasing to approximately constant y(eff) for V(rot) > 150 km/sec. The trend suggests that galaxies with V(rot) < 100-150 km/sec may lose a large fraction of their SN ejecta, while galaxies above this value tend to retain metals.Comment: 40 pages total, including 7 encapsulated postscript figures. Accepted for publication in 20 Dec 2002 Ap

Dust-to-gas ratio and metal abundance in dwarf galaxies

We have compared the metallicity (represented by oxygen abundance), and the dust-to-gas ratio, in a sample of dwarf galaxies. For dwarf irregulars (dIrrs) we find a good correlation between the two quantities, with a power-law index 0.54±0.2. Blue Compact Dwarf (BCD) galaxies do not show such a good correlation; in addition both the dust-to-gas ratio and the metallicity tend to be higher than for dIrrs. We have then developed a simple but physical analytical model for the above relation. Comparing the model results with the data, we conclude that: (i) for low values of the dust-to-gas ratio, the relation between the dust-to-gas ratio and the metallicity is quasi-linear, whereas for higher values the curve strongly deviates from the linear behavior, implying that the commonly used power-law approximation is very poor; (ii) the deviation from the linear behavior depends critically on the parameter χ, the ``differential'' mass outflow rate from the galaxy in units of the star formation rate (SFR); (iii) the shape of the curve representing the relation between the dust-to-gas ratio and the metallicity does not depend on the SFR, but only on χ; however, the specific location of a given galaxy on the curve does depend on ψ; (iv) the BCD metallicity segregation is due to a higher ψ, together with a significant differential mass outflow. Thus, the lack of correlation can be produced by largely different star formation rates and values of χ in these objects.U. L. gratefully acknowledges the receipt of a grant by the Deutsche Forschungsgemeinschaft (DFG) and by the Comisi´on Interministerial de Ciencia y Technolog´ıa (Spain)

2MASS Extended Sources in the Zone of Avoidance

The Two Micron All Sky Survey (2MASS) is now underway and will provide a complete census of galaxies as faint as 13.5 mag (3 mJy) at 2.2 microns for most of the sky, and ~12.1 mag (10 mJy) for regions veiled by the Milky Way. This census has already discovered nearby galaxies previously hidden behind our Galaxy, and will allow delineation of large-scale structures in the distribution of galaxies across the whole sky. Here we report the detection and discovery of new extended sources from this survey for fields incorporating the Galactic plane at longitudes between 40 and 70 deg. The area-normalized detection rate is ~1-2 galaxies per deg^2 brighter than 12.1 mag (10 mJy), roughly constant with Galactic latitude throughout the "Zone of Avoidance", of which 85-95% are newly discovered sources. In conjunction with the deep HI surveys, 2MASS will greatly increase the current census of galaxies hidden behind the Milky Way.Comment: Astronomial Journal, in press. See http://spider.ipac.caltech.edu/staff/jarrett/papers/ZoA