The Nearest Group of Galaxies

The small Antlia-Sextans clustering of galaxies is located at a distance of only 1.36 Mpc from the Sun, and 1.72 Mpc from the adopted barycenter of the Local Group. The latter value is significantly greater than the radius of the zero- velocity surface of the Local Group which, for an assumed age of 14 Gyr, has Ro = 1.18 " 0.15 Mpc. This, together with the observation that the members of the Ant-Sex group have a mean redshift of +114 " 12 km s-1 relative to the centroid of the Local Group, suggests that the Antlia-Sextans group is not bound to our Local Group, and that it is expanding with the Hubble flow. If this conclusion is correct, then Antlia-Sextans may be the nearest external clustering of galaxies. The total galaxian population of the Ant-Sex group is ~ 1/5 that of the Local Group. However, the integrated luminosity of Ant-Sex is two orders of magnitude lower than that of the Local Group. Subject headings: Galaxies - clusters: individual (Antlia-Sextans)Comment: Has been accepted for publication in the Astrophysical Journal Letter

A G1-like globular cluster in NGC 1023

The structure of a very bright (MV = -10.9) globular cluster in NGC 1023 is analyzed on two sets of images taken with the Hubble Space Telescope. From careful modeling of King profile fits to the cluster image, a core radius of 0.55+/-0.1 pc, effective radius 3.7+/-0.3 pc and a central V-band surface brightness of 12.9+/-0.5 mag / square arcsec are derived. This makes the cluster much more compact than Omega Cen, but very similar to the brightest globular cluster in M31, G1 = Mayall II. The cluster in NGC 1023 appears to be very highly flattened with an ellipticity of about 0.37, even higher than for Omega Cen and G1, and similar to the most flattened clusters in the Large Magellanic Cloud.Comment: 14 pages, 3 figures, 1 table. Accepted for AJ, Oct 200

Yet Another Model of Gamma-Ray Bursts

Sari and Piran have demonstrated that the time structure of gamma-ray bursts must reflect the time structure of their energy release. A model which satisfies this condition uses the electrodynamic emission of energy by the magnetized rotating ring of dense matter left by neutron star coalescence; GRB are essentially fast, high field, differentially rotating pulsars. The energy densities are large enough that the power appears as an outflowing equilibrium pair plasma, which produces the burst by baryon entrainment and subsequent internal shocks. I estimate the magnetic field and characteristic time scale for its rearrangement, which determines the observed time structure of the burst. There may be quasi-periodic oscillations at the rotational frequencies, which are predicted to range up to 5770 Hz (in a local frame). This model is one of a general class of electrodynamic accretion models which includes the Blandford and Lovelace model of AGN, and which can also be applied to black hole X-ray sources of stellar mass. The apparent efficiency of nonthermal particle acceleration is predicted to be 10--50%, but higher values are possible if the underlying accretion flow is super-Eddington. Applications to high energy gamma-ray observations of AGN are briefly discussed.Comment: 21pp, latex, uses aaspp4.st

Ozone detection by differential absorption spectroscopy at ambient pressure with a 9.6μm pulsed quantum-cascade laser

We report direct absorption spectroscopic detection of ozone at ambient pressure with a pulsed, DFB quantum-cascade laser (QCL) tuned within 1044-1050cm-1 by temperature scanning. Wavelength calibration curves were derived from FTIR and CO2 spectra and interpreted with respect to the heat transfer from the heterostructure to the sink. The laser linewidth (∼0.13cm-1 FWHM) was found to decrease with temperature, probably as a result of operation at constant current. Spurious spectral features due to baseline inaccuracies were successfully filtered out from the QCL O3 spectra using differential absorption. Reference O3 concentrations were obtained by applying the same method to UV spectra, simultaneously measured with a differential optical absorption spectrometer (DOAS). Column densities retrieved from QCL spectra are in fairly good agreement (±20%) with the DOAS values above 28ppm m. The estimated QCL lowest detectable, absolute and differential absorptions, (7×10-3 and 2×10-3, respectively), entail effective detection limits of 14 and 25ppm m, respectively. Ongoing improvements in the acquisition system should allow the achievement of detection limits at the level of commercial open-path DOAS systems (∼2ppm m) in the near future. Our results demonstrate the applicability of the differential absorption method to QCL spectroscopy at ambient pressure, and encourage its use for open path detectio

Red giant branch stars as probes of stellar populations. II. Properties of the newly discovered globular cluster GLIMPSE-C01

Deep near infrared photometry of the newly discovered Galactic globular Cluster GLIMPSE-C01 is reported. We derived for the first time the metal abundance of this object from the slope of the RGB: [Fe/H]=-1.61+/-0.14 in the scale of Zinn (as implemented in Harris 1996), [Fe/H]=-1.44+/-0.12 in the scale of Caretta & Gratton (1997), and [Fe/H]=-1.12+/-0.12 in the scale of Ferraro et al. (1999). The tip and the clump of the red giant branch were used to confirm the estimates of Kobulnicky et al. (2005), placing the cluster at D~3.7+/-0.8 kpc, behind A_v~15 mag of visual extinction. The best fit to the radial surface brightness profile with a single-mass King's model (1962) yielded core radius r_c=0.78 arcmin, tidal radius r_t=27 arcmin, and central concentration c=1.54. Finally, we estimate the number of the "missing" globulars in the central region of the Milky Way. Based on the spatial distribution of the known clusters, and assuming radial symmetry around the Galactic center, we conclude that the Milky Way contains at least 10+/-3 undiscovered objects. The distribution of known clusters in the bulge seem to resemble the orientation of the Milky Way bar.Comment: A&A accepted; 6 pages, 7 figures (one in jpg format, available upon request from the authors as postscript

Investigation of the New Local Group Galaxy VV 124

We present the results of our stellar photometry and spectroscopy for the new Local Group galaxy VV 124 (UGC 4879) obtained with the 6-m BTA telescope. The presence of a few bright supergiants in the galaxy indicates that the current star formation process is weak. The apparent distribution of stars with different ages in VV 124 does not differ from the analogous distributions of stars in irregular galaxies, but the ratio of the numbers of young and old stars indicates that VV 124 belongs to the rare Irr/Sph type of galaxies. The old stars (red giants) form the most extended structure, a thick disk with an exponential decrease in the star number density to the edge. Definitely, the young population unresolvable in images makes a great contribution to the background emission from the central galactic regions. The presence of young stars is also confirmed by the [O III] emission line visible in the spectra that belongs to extensive diffuse galactic regions. The mean radial velocity of several components (two bright supergiants, the unresolvable stellar population, and the diffuse gas) is v_h = -70+/-15 km/s and the velocity with which VV 124 falls into the Local Group is v_LG = -12+/-15 km/s. We confirm the distance to the galaxy D = 1.1+/-0.1 Mpc and the metallicity of red giants ([Fe/H] = -1.37) found by Kopylov et al. (2008).VV 124 is located on the periphery of the Local Group approximately at the same distance from M 31 and our Galaxy and is isolated from other galaxies. The galaxy LeoA nearest to it is 0.5 Mpc away.Comment: 10 pages, 8 figures. Accepted for publication in Astronomy Letters (2010, Vol. 36, No. 5, pp. 309-318

Ca II Triplet Spectroscopy of Giants in SMC Star Clusters: Abundances, Velocities and the Age-Metallicity Relation

We have obtained spectra at the Ca II triplet of individual red giants in seven SMC star clusters whose ages range from ~4 to 12 Gyr. The spectra have been used to determine mean abundances for six of the star clusters to a typical precision of 0.12 dex. When combined with existing data for other objects, the resulting SMC age-metallicity relation is generally consistent with that for a simple model of chemical evolution, scaled to the present-day SMC mean abundance and gas mass fraction. Two of the clusters (Lindsay 113 and NGC 339), however, have abundances that ~0.5 dex lower than that expected from the mean age-metallicity relation. It is suggested that the formation of these clusters, which have ages of ~5 Gyr, may have involved the infall of uneriched gas, perhaps from the Magellanic Stream. The spectra also yield radial velocities for the seven clusters. The resulting velocity dispersion is 16 +/- 4 km/sec, consistent with those of the SMC planetary nebula and carbon star populations.Comment: 28 pages including 4 figure

Supernovae and their host galaxies. I. The SDSS DR8 database and statistics

(Abridged) In this first paper of a series, we report the creation of large and well-defined database that combines extensive new measurements and a literature search of 3876 supernovae (SNe) and their 3679 host galaxies located in the sky area covered by the Sloan Digital Sky Survey (SDSS) Data Release 8 (DR8). This database should be much larger than previous ones, and should contain a homogenous set of global parameters of SN hosts, including morphological classifications and measures of nuclear activity. The measurements of apparent magnitudes, diameters (D25), axial ratios (b/a), and position angles (PA) of SN host galaxies were made using the images extracted from the SDSS g-band. For each host galaxy, we analyzed RGB images of the SDSS to accurately measure the position of its nucleus to provide the SDSS name. With these images, we also provide the host galaxy's morphological type, and note if it has a bar, a disturbed disk, and whether it is part of an interacting or merging system. In addition, the SDSS nuclear spectra were analyzed to diagnose the central power source of the galaxies. Special attention was paid to collect accurate data on the spectroscopic classes, coordinates, offsets of SNe, and heliocentric redshifts of the host galaxies. The creation of this large database will help to better understand how the different types of SNe are correlated with the properties of the nuclei and global physical parameters of the host galaxies, and minimize possible selection effects and errors that often arise when data are selected from different sources and catalogs.Comment: 20 pages, 15 figures, 7 table

The statistical investigation of type Ib/c and II supernovae and their host galaxies

This is a statistical study of the properties of type Ib/c and II supernovae and of the integral parameters of their spiral host galaxies. The methods of one-dimensional and multivariate statistics were applied to the data sample. It was found that the Ib/c supernovae are more concentrated radially toward the centers of the galaxies than those of type II. The distributions of the radial distances R(SN)/R(25) for the type Ib/c and II supernovae in active galaxies are more concentrated toward the center than in normal galaxies. This effect is stronger for type Ib/c than for type II supernovae.Comment: 8 pages, 2 figure

The Environments of Supernovae in Post-Refurbishment Hubble Space Telescope Images

The locations of supernovae in the local stellar and gaseous environment in galaxies contain important clues to their progenitor stars. Access to this information, however, has been hampered by the limited resolution achieved by ground-based observations. High spatial resolution Hubble Space Telescope (HST) images of galaxy fields in which supernovae had been observed can improve the situation considerably. We have examined the immediate environments of a few dozen supernovae using archival post-refurbishment HST images. Although our analysis is limited due to signal-to-noise ratio and filter bandpass considerations, the images allow us for the first time to resolve individual stars in, and to derive detailed color-magnitude diagrams for, several environments. We are able to place more rigorous constraints on the masses of these supernovae. A search was made for late-time emission from supernovae in the archival images, and for the progenitor stars in presupernova images of the host galaxies. We have detected SN 1986J in NGC 891 and, possibly, SN 1981K in NGC 4258. We have also identified the progenitor of the Type IIn SN 1997bs in NGC 3627. By removing younger resolved stars in the environments of SNe Ia, we can measure the colors of the unresolved stellar background and attribute these colors generally to an older, redder population. HST images ``accidentally'' caught the Type Ia SN 1994D in NGC 4526 shortly after its outburst; we measure its brightness. Finally, we add to the statistical inferences that can be made from studying the association of SNe with recent star-forming regions.Comment: 20 pages, 29 figures, to appear in A