333 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
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
IUE observations of luminous blue star associations in irregular galaxies
Two regions of recent star formation in blue irregular galaxies were observed with the IUE in the short wavelength, low dispersion mode. The spectra indicate that the massive star content is similar in these regions and is best fit by massive stars formed in a burst and now approximately 2.5 to 3.0 million years old
- …