1,663 research outputs found
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
Dwarf Spheroidal Galaxies : Keystones of Galaxy Evolution
Dwarf spheroidal galaxies are the most insignificant extragalactic stellar
systems in terms of their visibility, but potentially very significant in terms
of their role in the formation and evolution of much more luminous galaxies. We
discuss the present observational data and their implications for theories of
the formation and evolution of both dwarf and giant galaxies. The putative dark
matter content of these low-surface-brightness systems is of particular
interest, as is their chemical evolution. Surveys for new dwarf spheroidals
hidden behind the stars of our Galaxy and those which are not bound to giant
galaxies may give new clues as to the origins of this unique class of galaxy.Comment: 41 pages, plain tex (no figures included -- available by snail mail).
review to appear in PAS
Ionized interstellar froth in irregular galaxies
The warm interstellar medium of galaxies is a complicated place. It is often full of holes, neutral and ionized loops and shells, and diffuse ionized gas. Deep H alpha images of Magellanic-type irregular galaxies also reveal complex spatial structures consisting of loops and filaments in the interstellar gas outside of the boundaries of traditional HII regions. Researchers refer to these ionized structures as froth. Such structures could mark paths over which newly produced heavy elements are dispersed in irregular galaxies, and they could be the signatures of a feedback process related to star formation. In order to investigate the physical nature of the froth, researchers obtained narrow-band images and high and low dispersion spectra from Kitt Peak National Observatory (KPNO) and deep blue-passband plates from the Canada-France-Hawaii Observatory (CFHO)
Measuring star formation rates in blue galaxies
The problems associated with measurements of star formation rates in galaxies are briefly reviewed, and specific models are presented for determinations of current star formation rates from H alpha and Far Infrared (FIR) luminosities. The models are applied to a sample of optically blue irregular galaxies, and the results are discussed in terms of star forming histories. It appears likely that typical irregular galaxies are forming stars at nearly constant rates, although a few examples of systems with enhanced star forming activity are found among HII regions and luminous irregular galaxies
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