The Luminosity Distribution of Local Group Galaxies

From a rediscussion of Local Group membership, and of distances to individual galaxies, we obtain $M_V$ values for 35 probable and possible Local Group members. The luminosity function of these objects is well fitted by a Schechter function with faint end slope $\alpha = -1.1 \pm 0.1$. The probability that the luminosity distribution of the Local Group is a single Schechter function with $\alpha$ steeper than -1.3 is less than 1 per cent. However, more complicated luminosity functions, such as multi-component Schechter functions with steep faint-end slopes, cannot be ruled out. There is some evidence that the luminosity distribution of dwarf spheroidal galaxies in the Local Group is steeper than that of dwarf irregular galaxies.Comment: 13 pages, 2 figures, accepted for publication in The Astronomical Journal. Figure 2 replaced, conclusion based on this figure change

Photometry and the Metallicity Distribution of the Outer Halo of M31. II. The 30 Kpc Field

We present the results of a wide-field (V,I) photometric study of the red-giant branch (RGB) stars in the outer halo of M31, in a field located 30 to 35 kpc from the center of the galaxy along the southeast minor axis. At this remote location, we find that RGB stars belonging to M31 are sparsely but definitely present, after statistical subtraction of field contamination. We derive the metallicity distribution (MDF) for the halo stars using interpolation within a standard (I,V-I) grid of RGB evolutionary tracks. The halo MDF is quite broad but dominated by a moderately high-metallicity population peaking at [m/H] ~ -0.5, strikingly different from the [m/H] ~ -1.3 level which characterizes the outer halo of the Milky Way. However,the shape and peak metallicity for this region are entirely similar to those found in other studies for the inner regions of the M31 halo, particularly our previous study of a 20-kpc region (Durrell, Harris, & Pritchet 2001) employing similar data. In summary, we find no evidence for a metallicity gradient or systematic change in the MDF out to quite large distances in the M31 halo: it appears to be a homogeneous and moderately metal-rich subsystem of the galaxy at all locations. The star counts in the 30-kpc field are also consistent with the r^1/4 law that fits the interior regions of the M31 spheroid surface brightness profile. The metal-rich MDF and the r^1/4 spheroid suggests M31 more strongly resembles a giant elliptical galaxy than other, Milky-Way-like, spirals.Comment: 28 pages, including 9 figures; accepted for publication in the Astronomical Journa

The Globular Cluster Systems in the Coma Ellipticals. II: Metallicity Distribution and Radial Structure in NGC 4874, and Implications for Galaxy Formation

Deep HST/WFPC2 (V,I) photometry is used to investigate the globular cluster system (GCS) in NGC 4874, the central cD galaxy of the Coma cluster. The luminosity function of the clusters displays its normal Gaussian-like shape and turnover level. Other features of the system are surprising: the GCS is (a) spatially extended, with core radius r_c = 22 kpc, (b) entirely metal-poor (a narrow, unimodal metallicity distribution with mean [Fe/H] = -1.5), and (c) modestly populated, with specific frequency S_N = 3.7 +- 0.5. We suggest on the basis of some simple models that as much as half of this galaxy might have accreted from low-mass satellites, but no single one of the three classic modes of galaxy formation (accretion, disk mergers, in situ formation) can supply a fully satisfactory formation picture. Even when they are used in combination, strong challenges to these models remain. The principal anomaly in this GCS is essentially the complete lack of metal-rich clusters. If these were present in normal (M87-like) numbers in addition to the metal-poor ones that are already there, then the GCS in total would more closely resemble what we see in many other giant E galaxies.Comment: 27 pp. with 9 Figures. Astrophys.J. 533, in press (April 10, 2000

Photometry and the Metallicity Distribution of the Outer Halo of M31

We have conducted a wide-field CCD-mosaic study of the resolved red-giant branch (RGB) stars of M31, in a field located 20 kpc from the nucleus along the SE minor axis. In our (I, V-I) color-magnitude diagram, RGB stars in the top three magnitudes of the M31 halo are strongly present. Photometry of a more distant control field to subtract field contamination is used to derive the `cleaned' luminosity function and metallicity distribution function (MDF) of the M31 halo field. From the color distribution of the foreground Milky Way halo stars, we find a reddening E(V-I)= 0.10 +/- 0.02 for this field, and from the luminosity of the RGB tip, we determine a distance modulus (m-M)_o = 24.47 +/- 0.12 (= 783 +/- 43 kpc). The MDF is derived from interpolation within an extensive new grid of RGB models (Vandenberg et al. 2000). The MDF is dominated by a moderately high-metallicity population ([m/H]~ -0.5) found previously in more interior M31 halo/bulge fields, and is much more metal-rich than the [m/H]~ -1.5 level in the Milky Way halo. A significant (~30% - 40%, depending on AGB star contribution) metal-poor population is also present. To first order, the shape of the MDF resembles that predicted by a simple, single-component model of chemical evolution starting from primordial gas with an effective yield y=0.0055. It strongly resembles the MDF recently found for the outer halo of the giant elliptical NGC 5128 (Harris et al. 2000), though NGC 5128 has an even lower fraction of low-metallicity stars. Intriguingly, in both NGC 5128 and M31, the metallicity distribution of the globular clusters in M31 does not match the halo stars; the clusters are far more heavily weighted to metal-poor objects. We suggest similarities in the formation and early evolution of massive, spheroidal stellar systems.Comment: to appear in the Astronomical Journal; 43 pages, including 15 figure

The type Ia supernova SNLS-03D3bb from a super-Chandrasekhar-mass white dwarf star

The acceleration of the expansion of the universe, and the need for Dark Energy, were inferred from the observations of Type Ia supernovae (SNe Ia). There is consensus that SNe Ia are thermonuclear explosions that destroy carbon-oxygen white dwarf stars that accrete matter from a companion star, although the nature of this companion remains uncertain. SNe Ia are thought to be reliable distance indicators because they have a standard amount of fuel and a uniform trigger -- they are predicted to explode when the mass of the white dwarf nears the Chandrasekhar mass -- 1.4 solar masses. Here we show that the high redshift supernova SNLS-03D3bb has an exceptionally high luminosity and low kinetic energy that both imply a super-Chandrasekhar mass progenitor. Super-Chandrasekhar mass SNe Ia should preferentially occur in a young stellar population, so this may provide an explanation for the observed trend that overluminous SNe Ia only occur in young environments. Since this supernova does not obey the relations that allow them to be calibrated as standard candles, and since no counterparts have been found at low redshift, future cosmology studies will have to consider contamination from such events.Comment: 9 pages, 4 figures. To appear in Nature Sept. 21. Accompanying News & Views in same issue. Supplementary information available at www.nature.com/natur

Evidence for Type Ia Supernova Diversity from Ultraviolet Observations with the Hubble Space Telescope

We present ultraviolet (UV) spectroscopy and photometry of four Type Ia supernovae (SNe 2004dt, 2004ef, 2005M, and 2005cf) obtained with the UV prism of the Advanced Camera for Surveys on the Hubble Space Telescope. This dataset provides unique spectral time series down to 2000 Angstrom. Significant diversity is seen in the near maximum-light spectra (~ 2000--3500 Angstrom) for this small sample. The corresponding photometric data, together with archival data from Swift Ultraviolet/Optical Telescope observations, provide further evidence of increased dispersion in the UV emission with respect to the optical. The peak luminosities measured in uvw1/F250W are found to correlate with the B-band light-curve shape parameter dm15(B), but with much larger scatter relative to the correlation in the broad-band B band (e.g., ~0.4 mag versus ~0.2 mag for those with 0.8 < dm15 < 1.7 mag). SN 2004dt is found as an outlier of this correlation (at > 3 sigma), being brighter than normal SNe Ia such as SN 2005cf by ~0.9 mag and ~2.0 mag in the uvw1/F250W and uvm2/F220W filters, respectively. We show that different progenitor metallicity or line-expansion velocities alone cannot explain such a large discrepancy. Viewing-angle effects, such as due to an asymmetric explosion, may have a significant influence on the flux emitted in the UV region. Detailed modeling is needed to disentangle and quantify the above effects.Comment: 17 pages, 13 figures, accepted by Ap