Gas and Stars in an HI-Selected Galaxy Sample

We present the results of a J-band study of the HI-selected Arecibo Dual-Beam Survey and Arecibo Slice Survey galaxy samples using the 2 Micron All-Sky Survey data. We find that these galaxies span a wide range of stellar and gas properties. However, despite the diversity within the samples, we find a very tight correlation between luminosity and size in the J-band, similar to that we previously found (Rosenberg & Schneider 2003) between the HI mass and size. We also find that the correlation between the baryonic mass and the J-band diameter is even tighter than between the baryonic mass and the rotational velocity.Comment: AJ in press, 17 pages (including tables and figures) + 6 additional jpg figure

Chandra Observations of the X-ray Point Source Population in NGC 4636

We present the X-ray point source population in the nearby Virgo elliptical galaxy NGC 4636 from 3 Chandra/ACIS observations, totaling 193 ks, taken over 3 years. Using wavelet decomposition, we detect 318 point sources. Here, we use a subset of the 277 sources with >10 net cts (>1.2E37 erg/s, 0.5-2 keV, >1.5 arcmin of center). Between 1.5-6 arcmin from the center, 25% are likely background sources (AGN) and the rest are LMXBs within the galaxy, while at >6 arcmin, background sources dominate. We present a color-color diagram based on ratios of X-ray flux, which yields a tighter source distribution, and shows a large population of sources which are likely LMXBs and black hole candidates. We explore short and long-term variability (over time-scales of 1 day to 3 yr): 54 sources (24%) in the common ACIS fields show significant variability between observations. Of these, 37 are detected with >10 net counts only once and thus may be "transient". About 10% of sources in each observation show significant short-term variability. The cumulative luminosity function for the point sources can be represented as a power-law of slope 1.14. Between 1.5-6 arcmin of the center, 48 of the 129 X-ray point sources (37%) with >10 net counts are matched with optical globular cluster (GC) candidates (matches could be as high as 50% since many of the optical sources are AGN). 70% of matched sources are associated with the redder GC candidates, those that are thought to have near-solar metal abundance. The fraction of GC candidates matched with point sources decreases with decreasing GC luminosity. The X-ray luminosities of the matched point sources are not correlated with the luminosity or color of the host GC candidates. Finally, we estimate an upper limit of 4.5E37 erg/s to the current X-ray luminosity of the historical supernova SN1939A.Comment: Accepted for publication in the Astrophysical Journal; 19 pages, 15 figures, 3 tables. Version with full quality figures can be obtained at http://hea-www.harvard.edu/~jpbrown/ngc4636_astroph.pd

Searching for the pulsar in G18.95-1.1: Discovery of an X-ray point source and associated synchrotron nebula with Chandra

Using the Chandra X-ray Observatory, we have pinpointed the location of a faint X-ray point source (CXOUJ182913.1-125113) and an associated diffuse nebula in the composite supernova remnant G18.95-1.1. These objects appear to be the long-sought pulsar and its wind nebula. The X-ray spectrum of the point source is best described by an absorbed powerlaw model with Gamma=1.6 and an N_H of ~1x10^(22) cm^(-2). This model predicts a relatively low unabsorbed X-ray luminosity of about L_X (0.5-8.0keV) = 4.1x10^(31)D_2^2 erg s^(-1), where D_2 is the distance in units of 2kpc. The best-fitted model of the diffuse nebula is a combination of thermal (kT = 0.48keV) and non-thermal (1.4 < Gamma < 1.9) emission. The unabsorbed X-ray luminosity of L_X = 5.4x10^(33)D_2^2 erg s^(-1) in the 0.5-8keV energy band seems to be largely dominated by the thermal component from the SNR, providing 87% of L_X in this band. No radio or X-ray pulsations have been reported for CXOUJ182913.1-125113. If we assume an age of ~5300yr for G18.95-1.1 and use the X-ray luminosity for the pulsar and the wind nebula together with the relationship between spin-down luminosity (via magnetic dipole radiation) and period, we estimate the pulsar's period to be P = 0.4s. Compared to other rotation-powered pulsars, a magnetic field of 2.2x10^(13)G is implied by its location in the P-Pdot diagram, a value which is close to that of the quantum critical field.Comment: 8 pages, 3 Figures, accepted for publication in Ap

Stellar kinematics and populations out to 1.5 effective radius in the elliptical galaxy NGC4636

We present high quality long slit spectra along the major and minor axes out to 1.5 effective radius ($R_e$) of the massive galaxy NGC4636 taken by Hobby-Eberly Telescope (HET). Using Fourier Correlation Quotient (FCQ) method, we measured the stellar line-of-sight velocity distribution along the axes. Furthermore, six Lick/IDS indices ($H\beta,Mgb,Fe_{5015},Fe_{5270},Fe_{5335},Fe_{5406}$) are derived from the clean spectrum. By comparing the measured absorption line strengths with the predictions of Simple Stellar Populations (SSP) models, we derived ages, total metallicity and $\alpha$ abundance profiles of the galaxy. This galaxy presents old and $[\alpha/Fe]$ over abundant stellar populations. Indeed, using the SSP model, we obtained the broadband color profiles. The theoretical colors match well with the measured colors and present red sharp peaks at the galaxy center. The sharp peaks of the colors are mainly shaped by the high metallicity in the galaxy center. Interestingly, the galaxy has steep negative metallicity gradients, but trend flattens outwards. This result likly suggests that the center and outer regions of the galaxy formed through different formation process.Comment: 14 pages, 7 figures, accepted by RA

The Globular Cluster System of the Virgo Giant Elliptical Galaxy NGC 4636: I. Subaru/FOCAS Spectroscopy and Database

We present a spectroscopic study of the globular clusters (GCs) in the giant elliptical galaxy NGC 4636 in the Virgo cluster. We selected target GC candidates using the Washington photometry derived from the deep CCD images taken at the KPNO 4m. Then we obtained the spectra of 164 target objects in the field of NGC 4636 using the Multi-Object Spectroscopy (MOS) mode of Faint Object Camera and Spectrograph (FOCAS) on the SUBARU 8.2m Telescope. We have measured the velocities for 122 objects: 105 GCs in NGC 4636, the nucleus of NGC 4636, 11 foreground stars, 2 background galaxies, and 3 probable intracluster GCs in the Virgo cluster. The GCs in NGC 4636 are located in the projected galactocentric radius within 10arcmin (corresponding to 43 kpc). The measured velocities for the GCs range from 300km/s to 1600km/s, with a mean value of 932_{-22}^{+25} km/s, which is in good agreement with the velocity for the nucleus of NGC 4636, 928\pm 45 km/s. The velocity dispersion of the GCs in NGC 4636 is derived to be 231_{-17}^{+15} km/s and the velocity dispersion of the blue GCs is slightly larger than that of the red GCs. Combining our results with data in the literature, we produce a master catalog of radial velocities for 238 GCs in NGC 4636. The velocity dispersion of the GCs in the master catalog is found to be 225_{-9}^{+12} km/s for the entire sample, 251_{-12}^{+18} km/s for 108 blue GCs, and 205_{-13}^{+11} km/s for 130 red GCs.Comment: 27 pages, 9 figues, Accepted by Ap

X-ray emission from the Sombrero galaxy: discrete sources

We present a study of discrete X-ray sources in and around the bulge-dominated, massive Sa galaxy, Sombrero (M104), based on new and archival Chandra observations with a total exposure of ~200 ks. With a detection limit of L_X = 1E37 erg/s and a field of view covering a galactocentric radius of ~30 kpc (11.5 arcminute), 383 sources are detected. Cross-correlation with Spitler et al.'s catalogue of Sombrero globular clusters (GCs) identified from HST/ACS observations reveals 41 X-rays sources in GCs, presumably low-mass X-ray binaries (LMXBs). We quantify the differential luminosity functions (LFs) for both the detected GC and field LMXBs, whose power-low indices (~1.1 for the GC-LF and ~1.6 for field-LF) are consistent with previous studies for elliptical galaxies. With precise sky positions of the GCs without a detected X-ray source, we further quantify, through a fluctuation analysis, the GC LF at fainter luminosities down to 1E35 erg/s. The derived index rules out a faint-end slope flatter than 1.1 at a 2 sigma significance, contrary to recent findings in several elliptical galaxies and the bulge of M31. On the other hand, the 2-6 keV unresolved emission places a tight constraint on the field LF, implying a flattened index of ~1.0 below 1E37 erg/s. We also detect 101 sources in the halo of Sombrero. The presence of these sources cannot be interpreted as galactic LMXBs whose spatial distribution empirically follows the starlight. Their number is also higher than the expected number of cosmic AGNs (52+/-11 [1 sigma]) whose surface density is constrained by deep X-ray surveys. We suggest that either the cosmic X-ray background is unusually high in the direction of Sombrero, or a distinct population of X-ray sources is present in the halo of Sombrero.Comment: 11 figures, 5 tables, ApJ in pres

A census of X-ray nuclear activity in nearby galaxies

We have studied the X-ray nuclear activity of 187 nearby (distance < 15 Mpc) galaxies observed with Chandra/ACIS. We found that 86 of them have a point-like X-ray core, consistent with an accreting black hole (BH). We argue that the majority of them are nuclear BHs, rather than X-ray binaries. The fraction of galaxies with an X-ray detected nuclear BH is higher (~60 per cent) for ellipticals and early-type spirals (E to Sb), and lower (~30 per cent) for late-type spirals (Sc to Sm). There is no preferential association of X-ray cores with a large-scale bar; in fact, strongly barred galaxies appear to have slightly lower detection fraction and luminosity for their nuclear X-ray sources, compared with non-barred or weakly barred galaxies of similar Hubble types. The cumulative luminosity distribution of the nuclear sources in the 0.3-8 keV band is a power-law with slope ~-0.5, from ~2 x 10^{38} erg/s to ~10^{42} erg/s. The Eddington ratio is lower for ellipticals (L_{X}/L_{Edd} ~ 10^{-8}) and higher for late-type spirals (up to L_{X}/L_{Edd} ~ 10^{-4}), but in all cases, the accretion rate is low enough to be in the radiatively-inefficient regime. The intrinsic NH is generally low, especially for the less luminous sources: there appear to be no Type-2 nuclear BHs at luminosities <~ 10^{39} erg/s. The lack of a dusty torus or of other sources of intrinsic absorption (e.g., an optically-thick disk wind) may be directly related to the lack of a standard accretion disk around those faint nuclear BHs. The fraction of obscured sources increases with the nuclear BH luminosity: 2/3 of the sources with L_{X} > 10^{40} erg/s have a fitted NH > 10^{22} cm^{-2}. This is contrary to the declining trend of the obscured fraction with increasing luminosities, observed in more luminous AGN and quasars.Comment: 27 pages, 353 kB, accepted by Ap