1,255 research outputs found
An X-ray emitting black hole in a globular cluster
We present optical and X-ray data for the first object showing strong
evidence for being a black hole in a globular cluster. We show the initial
X-ray light curve and X-ray spectrum which led to the discovery that this is an
extremely bright, highly variable source, and thus must be a black hole. We
present the optical spectrum which unambiguously identifies the optical
counterpart as a globular cluster, and which shows a strong, broad [O III]
emission line, most likely coming from an outflow driven by the accreting
source.Comment: 5 pages, 2 figures, to appear in the proceedings of IAUS 246,
"Dynamical Evolution of Dense Stellar Systems", ed. Vesperini, Giersz and
Sill
Sensory neural pathways revisited to unravel the temporal dynamics of the Simon effect:A model-based cognitive neuroscience approach
The Diverse Infrared Properties of a Complete Sample of Star-Forming Dwarf Galaxies
We present mid-infrared Spitzer Space Telescope observations of a complete
sample of star-forming dwarf galaxies selected from the KPNO International
Spectroscopic Survey. The galaxies span a wide range in mid-infrared
properties. Contrary to expectations, some of the galaxies emit strongly at 8
micron indicating the presence of hot dust and/or PAHs. The ratio of this
mid-infrared dust emission to the stellar emission is compared with the
galaxies' luminosity, star-formation rate, metallicity, and optical reddening.
We find that the strength of the 8.0 micron dust emission to the stellar
emission ratio is more strongly correlated with the star-formation rate than it
is with the metallicity or the optical reddening in these systems. Nonetheless,
there is a correlation between the 8.0 micron luminosity and metallicity. The
slope of this luminosity-metallicity correlation is shallower than
corresponding ones in the B-band and 3.6 micron. The precise nature of the 8.0
micron emission seen in these galaxies (i.e., PAH versus hot dust or some
combination of the two) will require future study, including deep mid-IR
spectroscopy.Comment: 14 pages, accepted Ap
Deep multiband surface photometry on star forming galaxies: II. A volume limited sample of 21 emission lines galaxies
We present deep surface photometry of a volume--limited sample of 21 UM
emission line galaxies in broadband optical UBVRI and near infra-red (NIR) HKs
filters. The sample comprises 19 blue compact galaxies (BCGs) and two spirals.
For some targets the exposure times are the deepest to date. For the BCG UM462
we observe a previously undetected second disk component beyond a surface
brightness level of mu_B=26 mag arcsec^{-2}. This is a true low surface
brightness component with central surface brightness mu_0=24.1 mag arcsec^{-2}
and scale length h_r=1.5 kpc. All BCGs are dwarfs, with M_B>=-18, and very
compact, with an average scale length of h_r~1 kpc. We separate the burst and
host populations for each galaxy and compare them to stellar evolutionary
models with and without nebular emission contribution. We also measure the
A_{180} asymmetry in all filters and detect a shift from optical to NIR in the
average asymmetry of the sample. This shift seems to be correlated with the
morphological class of the BCGs. Using the color-asymmetry relation, we
identify five BCGs in the sample as mergers, which is confirmed by their
morphological class. Though clearly separated from normal galaxies in the
concentration-asymmetry parameter space, we find that it is not possible to
distinguish luminous starbursting BCGs from the merely star forming low
luminosity BCGs.Comment: 48 pages, 39 figures, submitte
The metallicities of UM151, UM408 and A1228+12 revisited
We present the results of new spectrophotometry and heavy element abundance
determinations for 3 dwarf galaxies UM151, UM408 and A1228+12 (RMB132). These
galaxies have been claimed in the literature to have very low metallicities,
corresponding to log(O/H)+12 < 7.65, that are in the metallicity range of some
candidate local young galaxies. We present higher S/N data for these three
galaxies. UM151 and UM408 have significantly larger metallicities: log(O/H)+12
= 8.5 and 7.93, respectively. For A1228+12 our new log(O/H)+12 = 7.73 is close
to that recalculated from earlier data (7.68). Thus, the rederived
metallicities allow us to remove these objects from the list of galaxies with Z
< 1/20 Z_Sun.Comment: LaTeX, 8 pages with 3 Postscript figures, A&A in pres
Metal Abundances of KISS Galaxies III. Nebular Abundances for Fourteen Galaxies and the Luminosity-Metallicity Relationship for HII Galaxies
We report results from the third in a series of nebular abundance studies of
emission-line galaxies from the KPNO International Spectroscopic Survey (KISS).
Galaxies with coarse metallicity estimates of 12 + log(O/H) less than 8.2 dex
were selected for observation. Spectra of 14 galaxies, which cover the full
optical region from [OII]3727,3729 to beyond [SII]6717,6731, are presented, and
abundance ratios of N, O, Ne, S, and Ar are computed. The auroral [OIII]4363
line is detected in all 14 galaxies. Oxygen abundances determined through the
direct electron temperature T_e method confirm that the sample is metal-poor
with 7.61 <= 12 + log(O/H) <= 8.32. By using these abundances in conjunction
with other T_e-based measurements from the literature, we demonstrate that HII
galaxies and more quiescent dwarf irregular galaxies follow similar
metallicity-luminosity (L-Z) relationships. The primary difference is a
zero-point shift between the correlations such that HII galaxies are brighter
by an average of 0.8 B magnitudes at a given metallicity. This offset can be
used as evidence to argue that low-luminosity HII galaxies typically undergo
factor of two luminosity enhancements, and starbursts that elevate the
luminosities of their host galaxies by 2 to 3 magnitudes are not as common. We
also demonstrate that the inclusion of interacting galaxies can increase the
scatter in the L-Z relation and may force the observed correlation towards
lower metallicities and/or larger luminosities. This must be taken into account
when attempting to infer metal abundance evolution by comparing local L-Z
relations with ones based on higher redshift samples since the fraction of
interacting galaxies should increase with look-back time.Comment: 36 pages, 5 figures. ApJ, in pres
On faint companions in the close environment of star-forming dwarf galaxies. Possible external star formation triggers ?
We have searched for companion galaxies in the close environment of 98
star-forming dwarf galaxies (SFDGs) from field and low density environments,
using the NASA Extragalactic Database. Most of the companions are dwarf
galaxies which due to observational selection effects were previously
disregarded in environmental studies of SFDGs. A subsample at low redshift,
cz<2000 km/s, was chosen to partially eliminate the observational bias against
distant dwarf companions. We find companion candidates for approximately 30% of
the objects within a projected linear separation s_p<100 kpc and a redshift
difference (Delta cz)<500 km/s. The limited completeness of the available data
sets, together with the non-negligible frequency of HI clouds in the vicinity
of SFDGs indicated by recent radio surveys, suggest that a considerably larger
fraction of these galaxies may be accompanied by low-mass systems. This casts
doubt on the hypothesis that the majority of them can be considered truly
isolated. The velocity differences between companion candidates and sample
SFDGs amount typically to (Delta cz)<250 km/s, and show a rising distribution
towards lower (Delta cz). This is similarly found for dwarf satellites of
spiral galaxies, suggesting a physical association between the companion
candidates and the sample SFDGs. SFDGs with a close companion do not show
significant differences in their Hbeta equivalent widths and B-V colours as
compared to isolated ones. However, the available data do not allow us to rule
out that interactions with close dwarf companions can influence the star
formation activity in SFDGs.Comment: 9 pages, 5 figures, to appear in A&A; also available at
http://www.uni-sw.gwdg.de/~knoeske/PUB_LIST/sfdg_comps.ps.g
A Test of the Standard Hypothesis for the Origin of the HI Holes in Holmberg II
The nearby irregular galaxy Holmberg II has been extensively mapped in HI
using the Very Large Array (VLA), revealing intricate structure in its
interstellar gas component (Puche et al. 1992). An analysis of these structures
shows the neutral gas to contain a number of expanding HI holes. The formation
of the HI holes has been attributed to multiple supernova events occurring
within wind-blown shells around young, massive star clusters, with as many as
10-200 supernovae required to produce many of the holes. From the sizes and
expansion velocities of the holes, Puche et al. assigned ages of ~10^7 to 10^8
years. If the supernova scenario for the formation of the HI holes is correct,
it implies the existence of star clusters with a substantial population of
late-B, A and F main sequence stars at the centers of the holes. Many of these
clusters should be detectable in deep ground-based CCD images of the galaxy. In
order to test the supernova hypothesis for the formation of the HI holes, we
have obtained and analyzed deep broad-band BVR and narrow-band H-alpha images
of Ho II. We compare the optical and HI data and search for evidence of the
expected star clusters in and around the HI holes. We also use the HI data to
constrain models of the expected remnant stellar population. We show that in
several of the holes the observed upper limits for the remnant cluster
brightness are strongly inconsistent with the SNe hypothesis described in Puche
et al. Moreover, many of the HI holes are located in regions of very low
optical surface brightness which show no indication of recent star formation.
Here we present our findings and explore possible alternative explanations for
the existence of the HI holes in Ho II, including the suggestion that some of
the holes were produced by Gamma-ray burst events.Comment: 30 pages, including 6 tables and 3 images. To appear in Astron.
Journal (June 1999
New southern blue compact dwarf galaxies in the 2dF Galaxy Redshift Survey
Aiming to find new extremely metal-deficient star-forming galaxies we
extracted from the Two-Degree Field Galaxy Redshift Survey (2dFGRS) 100K Data
Release 14 emission-line galaxies with relatively strong [OIII] 4363 emission.
Spectroscopic and photometric studies of this sample and, in addition, of 7
Tololo and 2 UM galaxies were performed on the basis of observations with the
ESO 3.6m telescope. All sample galaxies qualify with respect to their
photometric and spectroscopic properties as blue compact dwarf (BCD) galaxies.
Additionally, they show a good overlap with a comparison sample of 100
well-studied emission-line galaxies on the 12+log(O/H) vs. log(Ne/O), log(Ar/O)
and log(Fe/O) planes. From the analysis of the 2dFGRS subsample we report the
discovery of two new extremely metal-deficient BCDs with an oxygen abundance
12+log(O/H) < 7.6 and of another seven galaxies with 12+log(O/H) < 7.8.
Furthermore, we confirm previous oxygen abundance determinations for the BCDs
Tol 1304-353, Tol 2146-391, UM 559 and UM 570 to be 12+log(O/H) < 7.8.Comment: 26 pages, 65 figures, 5 tables, uses psfig.sty, Accepted for
publication in Astronomy and Astrophysics. The paper with high-resolution
figures can be downloaded at
http://www.uni-sw.gwdg.de/~papade/Publications/Papaderos2006_2dF.pd
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