Archival research on absorption lines in violently star-forming galaxies

A computerized analysis of a starburst model is discussed. The model proposes that the absorption line equivalent width should scale with the level of star forming activity. Archival International Ultraviolet Explorer (IUE) data on IUE spectra of luminous blue galaxies were compared with previous IUE observations of extragalactic HII regions and low luminosity galaxies. The comparisons are summarized and causes for offsets are discussed

The small-scale Structure of the Magellanic Stream as a Foundation for Galaxy Evolution

The Magellanic Stream (MS) is the nearest example of a gaseous trail formed by interacting galaxies. While the substantial gas masses in these kinds of circumgalactic structures are postulated to represent important sources of fuel for future star formation, the mechanisms whereby this material might be accreted back into galaxies remain unclear. Recent neutral hydrogen (HI) observations have demonstrated that the northern portion of the MS, which probably has been interacting with the Milky Way's hot gaseous halo for close to 1000~Myr, has a larger spatial extent than previously recognized, while also containing significant amounts of small-scale structure. After a brief consideration of the large-scale kinematics of the MS as traced by the recently-discovered extension of the MS, we explore the aging process of the MS gas through the operation of various hydrodynamic instabilities and interstellar turbulence. This in turn leads to consideration of processes whereby MS material survives as cool gas, and yet also evidently fails to form stars. Parallels between the MS and extragalactic tidal features are briefly discussed with an emphasis on steps toward establishing what the MS reveals about the critical role of local processes in determining the evolution of these kinds of systems

Near-IR Spectroscopy of a Young Super-Star Cluster in NGC 6946: Chemical Abundances and Abundance Patterns

Using the NIRSPEC spectrograph at Keck II, we have obtained H and K-band echelle spectra for a young (10-15 Myr), luminous (MV=-13.2) super-star cluster in the nearby spiral galaxy NGC 6946. From spectral synthesis and equivalent width measurements we obtain for the first time accurate abundances and abundance patterns in an extragalactic super-star cluster. We find [Fe/H]=-0.45+/-0.08 dex, an average alpha-enhancement of +0.22+/-0.1 dex, and a relatively low 12C/13C~ 8+/-2 isotopic ratio. We also measure a velocity dispersion of ~9.1 km/s, in agreement with previous estimates. We conclude that integrated high-dispersion spectroscopy of massive star clusters is a promising alternative to other methods for abundance analysis in extragalactic young stellar populations.Comment: 5 pages, incl. 2 figures. Accepted for publication in MNRAS Letters. The definitive version will be available at http://www.blackwell-synergy.co

Galaxy Aggregates in the Coma Cluster

We present evidence for a new morphologically defined form of small-scale substructure in the Coma Cluster, which we call galaxy aggregates. These aggregates are dominated by a central galaxy, which is on average three magnitudes brighter than the smaller aggregate members nearly all of which lie to one side of the central galaxy. We have found three such galaxy aggregates dominated by the S0 galaxies RB 55, RB 60, and the star-bursting SBb, NGC 4858. RB 55 and RB 60 are both equi-distant between the two dominate D galaxies NGC 4874 and NGC 4889, while NGC 4858 is located next to the larger E0 galaxy NGC 4860. All three central galaxies have redshifts consistant with Coma Cluster membership. We describe the spatial structures of these unique objects and suggest several possible mechanisms to explain their origin. These include: chance superpositions from background galaxies, interactions between other galaxies and with the cluster gravitational potential, and ram pressure. We conclude that the most probable scenario of creation is an interaction with the cluster through its potential.Comment: Ten Pages with six figures; submitted to MNRAS letter

Stellar Populations and Ages of M82 Super Star Clusters

We present high signal-to-noise optical spectra of two luminous super star clusters in the starburst galaxy M82. The data for cluster F and the nearby, highly reddened cluster L were obtained with the William Herschel Telescope (WHT) at a resolution of 1.6A. The blue spectrum (3250-5540A) of cluster F shows features typical of mid-B stars. The red spectra (5730-8790A) of clusters F and L show the Ca II triplet and numerous F and G-type absorption features. Strong Ca II and Na I interstellar absorption lines arising in M82 are also detected, and the 6283A diffuse interstellar band appears to be present. The quality of the WHT spectra allows us to considerably improve previous age estimates for cluster F. By comparing the blue spectrum with theoretical model cluster spectra using the PEGASE spectral synthesis code (Fioc & Rocca-Volmerange 1997), we derive an age of 60+/-20 Myr. The strength of the Ca II triplet is also in accord with this age. Cluster L appears to have a similar age, although this is much less certain. The measured radial velocities for the two clusters differ substantially, indicating that they are located in different regions of the M82 disk. Cluster F appears to be deep in M82, slightly beyond the main starburst region while the highly obscured cluster L lies near the outer edges of the disk. We derive an absolute V magnitude of -16.5 for F indicating that it is an extremely massive cluster. The presence of such a luminous super star cluster suggests that the M82 starburst experienced an episode of intense star formation approximately 60 Myr ago.Comment: 10 pages and 5 figures for publication in MNRA