205 research outputs found
An optical and HI study of the dwarf Local Group galaxy VV124=UGC4879. A gas-poor dwarf with a stellar disk?
We present a detailed study of the dwarf galaxy VV124, recently recognized as
a isolated member of the Local Group. We have obtained deep (r=26.5) wide-field
g,r photometry of individual stars with the LBT under sub-arcsec seeing
conditions. The Color-Magnitude Diagram suggests that the stellar content of
the galaxy is dominated by an old, metal-poor population, with a significant
metallicity spread. A very clean detection of the RGB tip allows us to derive
an accurate distance of D=1.3 +/- 0.1 Mpc. Combining surface photometry with
star counts, we are able to trace the surface brightness profile of VV124 out
to ~ 5' = 1.9 kpc radius (where mu_r=30 mag/arcsec^2), showing that it is much
more extended than previously believed. Moreover, the surface density map
reveals the presence of two symmetric flattened wings emanating from the
central elongated spheroid and aligned with its major axis, resembling a
stellar disk seen nearly edge-on. We also present HI observations obtained with
the WSRT, the first ever of this object. A total amount of 10^6 M_sun of HI gas
is detected in VV124. Compared to the total luminosity, this gives a value of
M_HI/L_V=0.11, which is particularly low for isolated Local Group dwarfs. The
spatial distribution of the gas does not correlate with the observed stellar
wings. The systemic velocity of the HI in the region superposed to the stellar
main body of the galaxy is V_h=-25 km/s. The velocity field shows substructures
typical of galaxies of this size but no sign of rotation. The HI spectra
indicates the presence of a two-phase interstellar medium, again typical of
many dwarf galaxies.Comment: Accepted for publication in A&A. 19 pages, 20 reduced-resolution
figures, pdflatex, A&A style. The full resolution pdf file can be be
downloaded from http://www.bo.astro.it/SGR
A high fraction of Be stars in young massive clusters: evidence for a large population of near-critically rotating stars
Recent photometric analysis of the colour-magnitude diagrams (CMDs) of young massive clusters (YMCs) have found evidence for splitting in the main sequence and extended main sequence turn-offs, both of which have been suggested to be caused by stellar rotation. Comparison of the observed main sequence splitting with models has led various authors to suggest a rather extreme stellar rotation distribution, with a minority (10−30\%) of stars with low rotational velocities and the remainder (70−90\%) of stars rotating near the critical rotation (i.e., near break-up). We test this hypothesis by searching for Be stars within two YMCs in the LMC (NGC 1850 and NGC 1856), which are thought to be critically rotating stars with decretion disks that are (partially) ionised by their host stars. In both clusters we detect large populations of Be stars at the main sequence turn-off (∼30−60\% of stars), which supports previous suggestions of large populations of rapidly rotating stars within massive clusters
When Do Stars Go BOOM?
The maximum mass of a star that can produce a white dwarf (WD) is an
important astrophysical quantity. One of the best approaches to establishing
this limit is to search for WDs in young star clusters in which only massive
stars have had time to evolve and where the mass of the progenitor can be
established from the cooling time of the WD together with the age of the
cluster. Searches in young Milky Way clusters have not thus far yielded WD
members more massive than about 1.1, well below the Chandrasekhar
mass of , nor progenitors with masses in excess of about
. However, the hunt for potentially massive WDs that escaped their
cluster environs is yielding interesting candidates. To expand the cluster
sample further, we used HST to survey four young and massive star clusters in
the Magellanic Clouds for bright WDs that could have evolved from stars as
massive as 10. We located five potential WD candidates in the
oldest of the four clusters examined, the first extragalactic single WDs thus
far discovered. As these hot WDs are very faint at optical wavelengths, final
confirmation will likely have to await spectroscopy with 30-metre class
telescopes.Comment: 10 pages, 5 figures, accepted to the Astrophysical Journal Letter
The merger history, AGN and dwarf galaxies of Hickson Compact Group 59
Compact group galaxies often appear unaffected by their unusually dense
environment. Closer examination can, however, reveal the subtle, cumulative
effects of multiple galaxy interactions. Hickson Compact Group (HCG) 59 is an
excellent example of this situation. We present a photometric study of this
group in the optical (HST), infrared (Spitzer) and X-ray (Chandra) regimes
aimed at characterizing the star formation and nuclear activity in its
constituent galaxies and intra-group medium. We associate five dwarf galaxies
with the group and update the velocity dispersion, leading to an increase in
the dynamical mass of the group of up to a factor of 10 (to 2.8e13 Msun), and a
subsequent revision of its evolutionary stage. Star formation is proceeding at
a level consistent with the morphological types of the four main galaxies, of
which two are star-forming and the other two quiescent. Unlike in some other
compact groups, star-forming complexes across HCG 59 closely follow mass-radius
scaling relations typical of nearby galaxies. In contrast, the ancient globular
cluster populations in galaxies HCG 59A and B show intriguing irregularities,
and two extragalactic HII regions are found just west of B. We age-date a faint
stellar stream in the intra-group medium at ~1 Gyr to examine recent
interactions. We detect a likely low-luminosity AGN in HCG 59A by its ~10e40
erg/s X-ray emission; the active nucleus rather than star formation can account
for the UV+IR SED. We discuss the implications of our findings in the context
of galaxy evolution in dense environments.Comment: 38 pages, 17 figures. Please visit "http://tinyurl.com/isk-hcg59" for
a full-resolution PDF. Accepted for publication in the Astrophysical Journa
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