The Structure and Metallicity Gradient in the Extreme Outer Disk of NGC 7793

Studies of outer regions of spirals disks are fundamental to our understanding of both the process of galaxy assembly and the subsequent secular evolution of galaxies. In an earlier series of papers we explored the extent and abundance gradient in the outer disk of NGC 300 and found an extended purely exponential disk with a metallicity gradient which flattens off in the outermost regions. We now continue the study of outskirts of pure disk spirals with another Sculptor Group spiral, NGC 7793. Using Gemini Multi Object Spectrograph camera at Gemini South, we trace the disk of NGC 7793 with star counts out to ~9 scale lengths, corresponding to 11.5 kpc at our calibrated distance of 3.61+/-0.53 Mpc. The outer disk of NGC 7793 shows no evidence of a break in its light profile down to an effective surface brightness of ~30 mag/arcsec^2 (~3 mag/arcsec^2 deeper than what has been achieved with surface photometry) and exhibits a non-negative abundance gradient within the radial extent of our data.Comment: 11 pages, 9 figures, ApJ accepte

Astroarchaeology

The goal of astroarchaeology or Galactic archaeology is to unravel the history of assembly of the Milky Way, using fossil remnants of ancient star formation events which have disrupted and are now dispersed around the Galaxy. Recent studies of chemical a

Isolated Star Formation: A Compact HII Region in the Virgo Cluster

We report on the discovery of an isolated, compact HII region in the Virgo cluster. The object is located in the diffuse outer halo of NGC 4388, or could possibly be in intracluster space. Star formation can thus take place far outside the main star forming regions of galaxies. This object is powered by a small starburst with an estimated mass of \sim 400\msun and age of \sim 3\myr. From a total sample of 17 HII region candidates, the present rate of isolated star formation estimated in our Virgo field is small, \sim 10^{-6} Msun arcmin}^{-2} yr^{-1}. However, this mode of star formation might have been more important at higher redshifts and be responsible for a fraction of the observed intracluster stars and total cluster metal production. This object is relevant also for distance determinations with the planetary nebula luminosity function from emission line surveys, for high-velocity clouds and the in situ origin of B stars in the Galactic halo, and for local enrichment of the intracluster gas by Type II supernovae.Comment: 5 pages, LaTeX, 1 figure. ApJ Letters, in press (scheduled Dec 1, 2002

The Properties and Gaseous Environments of Powerful Classical Double Radio Galaxies

The properties of a sample of 31 very powerful classical double radio galaxies with redshifts between zero and 1.8 are studied. The source velocities, beam powers, ambient gas densities, total lifetimes, and total outflow energies are presented and discussed. The rate of growth of each side of each source were obtained using a spectral aging analysis. The beam power and ambient gas density were obtained by applying the strong shock jump conditions to the ends of each side of the source. The total outflow lifetime was obtained by applying the power-law relationship between the beam power and the total source lifetime derived elsewhere for sources of this type, and the total outflow energy was obtained by combining the beam power and the total source lifetime. Composite profiles were constructed by combining results obtained from each side of each source. The composite profiles indicate that the ambient gas density falls with distance from the central engine. The source velocities, beam powers, total lifetimes, and total energies seem to be independent of radio source size. This is consistent with the standard model in which each source grows at a roughly constant rate during which time the central engine puts out a roughly constant beam power. The fact that the total source lifetimes and energies are independent of radio source size indicates that the sources are being sampled at random times during their lifetimes.Comment: 7 pages, 5 figures, to appear in "Extragalactic Jets: Theory and Observation from Radio to Gamma Ray", eds. T. A. Rector and D. S. De Young, ASP conference series, Replaced version has minor textual correction

A four-lidar view of Cirrus from the FIRE IFO: 27-28 October 1986

The four ground-based lidar systems that participated in the 1986 FIRE IFO were configured in a diamond-shaped array across central and southern Wisconsin. Data were generally collected in the zenith pointing mode, except for the Doppler lidar, which regularly operated in a scanning mode with intermittent zenith observations. As a component of the cirrus case study of 27 and 28 October 1986 selected for initial analysis, data collected by the remote sensor ensemble from 1600 (on the 27th) to 2400 UTC (on the 28th) is described and compared. In general, the cirrus studied on the 27th consisted of intermittent layers of thin and subvisual cirrus clouds. Particularly at Wausau, subvisual cirrus was detected from 11.0 to 11.5 km MSL, just below the tropopause. At lower levels, occasional cirrus clouds between approx. 8.0 to 9.5 km were detected from all ground sites. Preliminary analysis of the four-lidar dataset reveals the passage of surprisingly consistent cloud features across the experiment area. A variety of types and amounts of middle and high level clouds occurred, ranging from subvisual cirrus to deep cloud bands. It is expected that the ground-based lidar measurements from this case study, as well as the airborne observations, will provide an excellent data base for comparison to satellite observations