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We have initiated a search for extended ultraviolet disk (XUV-disk) galaxies in the local universe. Herein, we compare GALEX UV and visible--NIR images of 189 nearby (D$<$40 Mpc) S0--Sm galaxies included in the GALEX Atlas of Nearby Galaxies and present the first catalogue of XUV-disk galaxies. We find that XUV-disk galaxies are surprisingly common but have varied relative (UV/optical) extent and morphology. Type~1 objects ($\ga$20% incidence) have structured, UV-bright/optically-faint emission features in the outer disk, beyond the traditional star formation threshold. Type~2 XUV-disk galaxies ($\sim$10% incidence) exhibit an exceptionally large, UV-bright/optically-low-surface-brightness (LSB) zone having blue $UV-K_s$ outside the effective extent of the inner, older stellar population, but not reaching extreme galactocentric distance. If the activity occuring in XUV-disks is episodic, a higher fraction of present-day spirals could be influenced by such outer disk star formation. Type~1 disks are associated with spirals of all types, whereas Type~2 XUV-disks are predominantly found in late-type spirals. Type~2 XUV-disks are forming stars quickly enough to double their [presently low] stellar mass in the next Gyr (assuming a constant SF rate). XUV-disk galaxies of both types are systematically more gas-rich than the general galaxy population. Minor external perturbation may stimulate XUV-disk incidence, at least for Type~1 objects. XUV-disks are the most actively evolving galaxies growing via inside-out disk formation in the current epoch, and may constitute a segment of the galaxy population experiencing significant, continued gas accretion from the intergalactic medium or neighboring objects., Comment: 83 pages, 16 figures, 2 tables. Appearing in the GALEX special issue of ApJS. (A version with high quality figures and proof corrections can be found at http://www.journals.uchicago.edu/toc/apjs/173/2)

We present an X-ray study of the massive edge-on Sa galaxy, Sombrero (M~104; NGC 4594), based on XMM-Newton and Chandra observations. A list of 62 XMM-Newton and 175 Chandra discrete X-ray sources is provided. At energies above 2 keV, the source-subtracted X-ray emission is distributed similarly as the stellar K-band light and is primarily due to the residual emission from discrete sources. At lower energies, however, a substantial fraction of the source-subtracted emission arises from diffuse hot gas extending to ~20 kpc from the galactic center. The observed diffuse X-ray emission from the galaxy shows a steep spectrum that can be characterized by an optically-thin thermal plasma with temperatures of ~0.6-0.7 keV, varying little with radius. The diffuse emission has a total luminosity of ~3e39 erg/s in the 0.2-2 keV energy range. This luminosity is significantly smaller than the prediction by current numerical simulations for galaxies as massive as Sombrero. We argue that the stellar feedback likely plays an essential role in regulating the physical properties of hot gas. Indeed, the observed diffuse X-ray luminosity of Sombrero accounts for at most a few percent of the expected mechanical energy input from Type Ia supernovae. The inferred gas mass and metal content are also substantially less than those expected from stellar ejecta. We speculate that a galactic bulge wind, powered primarily by Type Ia supernovae, has removed much of the 'missing' energy and metal-enriched gas from regions revealed by the X-ray observations., Comment: 25 pages, 11 figures, accepted to MNRAS

New results, based on one of the most comprehensive H-alpha imaging surveys of nearby Sa-Sab spirals completed to date, reveals early-type spirals to be a diverse group of galaxies that span a wide range in massive star formation rates. While the majority of Sa-Sab galaxies in our sample are forming stars at a modest rate, a significant fraction (~29%) exhibit star formation rates greater than 1 M(solar/yr), rivaling the most prolifically star forming late-type spirals. A similar diversity is apparent in the star formation history of Sa-Sab spirals as measured by their H-alpha equivalent widths. Consistent with our preliminary results presented in the first paper in this series, we find giant HII regions (L(H-alpha)>10^{39}erg/s) in the disks of 37% of early-type spirals. We suspect that recent minor mergers or past interactions are responsible for the elevated levels of H-alpha emission and perhaps, for the presence of giant HII regions in these galaxies., Comment: 42 pages and 17 figures (6 in jpg format, available upon request from the authors as postscript); to appear in the June issue of AJ

The nearby Seyfert galaxy NGC 7213 has been imaged in H-alpha and HI with the CTIO 1.5 m telescope and with the Australia Telescope Compact Array (ATCA), respectively. Optically NGC 7213 looks undisturbed and relatively featureless but the continuum-subtracted H-alpha image shows a 19 kpc long filament located approximately 18.6 kpc from the nucleus. The H-alpha filament could be neutral gas photo-ionized by the active nucleus, as has been suggested for the Seyfert galaxy NGC 5252, or shock-ionized by a jet interacting with the surrounding HI, as has been suggested for the radio galaxy PKS 2240-41. The HI map reveals NGC 7213 to be a highly disturbed system suggesting a past merging event., Comment: 14 pages including 4 figures and 1 table. Figures 1-4 are in jpeg format; Better quality images can be retrieved in postscript format at ftp://charon.nmsu.edu/pub/shameed/ ; Accepted for publication in ApJ Letters

H-alpha and continuum images are presented for 27 nearby early-type(Sa-Sab) spiral galaxies. Contrary to popular perception, the images reveal copious massive star formation in some of these galaxies. A determination of the H-alpha morphology and a measure of the H-alpha luminosity suggests that early-type spirals can be classified into two broad categories based on the luminosity of largest HII region in the disk. The first category includes galaxies for which the individual HII regions have L(H-alpha) < 10^(39) erg/s. Most of the category 1 galaxies appear to be morphologically undisturbed, but show a wide diversity in nuclear H-alpha properties. The second category includes galaxies which have at least one HII region in the disk with L(H-alpha) > 10^(39) erg/s. All category 2 galaxies show either prominent dust lanes or other morphological peculiarities such as tidal tails which suggests that the anomalously luminous HII regions in category 2 galaxies may have formed as a result of a recent interaction. The observations, which are part of an on-going H-alpha survey, reveal early-type spirals to be a heterogeneous class of galaxies that are evolving in the current epoch. We have also identified some systematic differences between the classifications of spiral galaxies in the Second General Catalog (RC2) and the Revised Shapley-Ames Catalog (RSA) which may be traced to subtle variations in the application of the criteria used for classifying spiral galaxies. An examination of earlier studies suggests that perceptions concerning the Hubble type dependence of star formation rates among spiral galaxies depends on the choice of catalog., Comment: 35 pages including 16 figures and 5 tables; figures 5a-5f,6a-6c, and 7 are images in jpeg format; Better quality images can be retrieved in postscript format at ftp://astro.nmsu.edu/pub/shameed/ ; Accepted for publication in AJ (August 1999)

A recent analysis of the IRAS database indicates that the massive star formation rates in early-type (Sa-Sab) spirals are comparable to the massive star formation rates in late-type spirals. $H\alpha$ imaging of some of the infrared luminous early-type spirals reveals two types of galaxies. One type shows clear signs of interaction, whereas the other type appears to host a nuclear starburst. The occurence of nuclear starbursts in early-type spirals may be related to the propensity for such galaxies to also host Seyfert nuclei. The evidence for interactions suggests that early-type spirals are evolving in the current epoch., Comment: 4 pages including two postscript figures. Contribution to the Conference proceedings of the 7th Annual Astrophysics Conference in Maryland, STAR FORMATION, NEAR AND FAR, eds. Stephen S. Holt and Lee G. Mundy