258 research outputs found
X-Ray Emission from M32: X-Ray Binaries or a micro-AGN?
We have analysed archival {\it ROSAT} PSPC data for M32 in order to study the
x-ray emission from this nearest elliptical galaxy. We fit spectra from three
long exposures with Raymond-Smith, thermal bremsstrahlung, and power-law
models. All models give excellent fits. The thermal fits have kT4 keV,
the Raymond-Smith iron abundance is Solar, the power-law
fit has =1.60.1, and all fits have consistent with the
Galactic column. The source is centered on M32 to an accuracy of 9, and
unresolved at 27 FWHM (90 pc). M32 is x-ray variable by a factor of
3--5 on timescales of a decade down to minutes, with evidence for a possible
period of 1.3 days.
There are two plausible interpretations for these results: 1) Emission due to
low-mass x-ray binaries; 2) Emission due to accretion onto a massive central
black hole. Both of these possibilities are supported by arguments based on
previous studies of M32 and other old stellar systems; the {\it ROSAT} PSPC
data do not allow us to unambiguously choose between them. Observations with
the {\it ROSAT} HRI and with {\it ASCA} are required to determine which of
these two very different physical models is correct.Comment: 9 pages, 5 PostScript figures, uses AASTeX style files, Accepted for
publication in Astrophysical Journal Letter
ARE SPATIAL MODELS NEEDED WITH ADEQUATELY BLOCKED FIELD TRIALS?
The use of nearest neighbors and spatial models (SPAT) to analyze field trial data has become commonplace in recent years. These two types of analyses improve precision compared to ANOVA when trials are poorly blocked, but results are less clear in well-blocked trials. We examined data from wheat trials containing 60 cultivars, conducted at five locations, where each location was set up as an alpha lattice design. We compared the relative efficiency of detecting cultivar differences for spatial models and nearest neighbors analyses (NNA) to ANOVA, fit of the models, and correlations of ranked cultivars. Though the SPAT and NNA generally outperformed the ANOVA, the selection of desirable cultivars remained relatively unchanged when using a well-blocked design analyzed with an ANOVA
Analysis of a Proper-Motion Selected Sample of Stars in the Ursa Minor Dwarf Spheroidal Galaxy
We have studied the stellar population and internal structure of the core of
the Ursa Minor dwarf spheroidal galaxy, using a sample of stars selected to be
members based on their proper motions. In agreement with previous studies, we
find Ursa Minor to be dominated by an old, metal-poor stellar population. A
small number of stars with high membership probabilities lie redward of the red
giant branch. The brightest (V <= 18) such stars are known to be Carbon stars,
rather than metal-rich first-ascent giants. A number of stars with high
membership probabilities lie blueward of the red giant branch, and are more
luminous than the horizontal branch. We speculate that these are
post-horizontal branch stars. There may also be one or two stars in the
post-AGB phase. Spectroscopy of the candidate post-HB and post-AGB stars is
required to determine their nature. We recover the internal substructure in
Ursa Minor that has been noted by several authors in the last 15 years. Using a
variety of two- and three-dimensional statistical tests, we conclude that this
substructure is statistically significant at the 0.005 level. There is no
evidence that the regions of density excess have stellar populations that
differ from the main body of Ursa Minor. The crossing time for a typical
density excess is only ~5 million years. They are therefore clearly not due to
intermediate age star-forming bursts. We conclude that they are instead due to
tidal interactions between the Galaxy and Ursa Minor.Comment: LaTeX with AASTeX style file, 22 pages with 7 figures. Accepted for
publication in The Astronomical Journal (Dec. 2001
The Absence of Extra-Tidal Structure in the Sculptor Dwarf Spheroidal Galaxy
The results of a wide-field survey of the Sculptor dwarf spheroidal galaxy
are presented. Our aims were to obtain an accurate map of the outer structure
of Sculptor, and to determine the level of interaction between this system and
the Galaxy. Photometry was obtained in two colours down to the magnitude limits
of V=20 and I=19, covering a 3.1 times 3.1 square deg area centred on Sculptor.
The resulting colour-magnitude data were used as a mask to select candidate
horizontal branch and red giant branch stars for this system. Previous work has
shown that the red horizontal branch (HB) stars are more concentrated than the
blue HB stars. We have determined the radial distributions of these two
populations and show that the overall Sculptor density profile is well
described by a two component model based on a combination of these radial
distributions. Additionally, spectra of the Ca ii triplet region were obtained
for over 700 candidate red giant stars over the 10 square deg region using the
2dF instrument on the Anglo-Australian Telescope. These spectra were used to
remove foreground Galactic stars based on radial velocity and Ca ii triplet
strength. The final list of Sculptor members contained 148 stars, seven of
which are located beyond the nominal tidal radius. Both the photometric and
spectroscopic datasets indicate no significant extra-tidal structure. These
results support at most a mild level of interaction between this system and the
Galaxy, and we have measured an upper mass limit for extra-tidal material to be
2.3 +/- 0.6% of the Sculptor luminous mass. This lack of tidal interaction
indicates that previous velocity dispersion measurements (and hence the amount
of dark matter detected) in this system are not strongly influenced by the
Galactic tidal field.Comment: 53 pages, 23 figures. Accepted for publication in the Astronomical
Journal. Some figures are reduced in size, and a full version is available
at: ftp://ftp.mso.anu.edu.au/pub/coleman/sculptor.pd
A Dust-Penetrated Classification Scheme for Bars as Inferred from their Gravitational Force Fields
The division of galaxies into ``barred'' (SB) and ``normal'' (S) spirals is a
fundamental aspect of the Hubble galaxy classification system. This ``tuning
fork'' view was revised by de Vaucouleurs, whose classification volume
recognized apparent ``bar strength'' (SA, SAB, SB) as a continuous property of
galaxies called the ``family''. However, the SA, SAB, and SB families are
purely visual judgments that can have little bearing on the actual bar strength
in a given galaxy. Until very recently, published bar judgments were based
exclusively on blue light images, where internal extinction or star formation
can either mask a bar completely or give the false impression of a bar in a
nonbarred galaxy. Near-infrared camera arrays, which principally trace the old
stellar populations in both normal and barred galaxies, now facilitate a
quantification of bar strength in terms of their gravitational potentials and
force fields. In this paper, we show that the maximum value, Qb, of the ratio
of the tangential force to the mean radial force is a quantitative measure of
the strength of a bar. Qb does not measure bar ellipticity or bar shape, but
rather depends on the actual forcing due to the bar embedded in its disk. We
show that a wide range of true bar strengths characterizes the category ``SB'',
while de Vaucouleurs category ``SAB'' corresponds to a much narrower range of
bar strengths. We present Qb values for 36 galaxies, and we incorporate our bar
classes into a dust-penetrated classification system for spiral galaxies.Comment: Accepted for publication in the Astrophysical Journal (LaTex, 30
pages + 3 figures); Figs. 1 and 3 are in color and are also available at
http://bama.ua.edu/~rbuta/bars
The Progenitors of Dwarf Spheroidal Galaxies
Dwarf spheroidal (dSph) galaxies present an evolutionary puzzle that we
explore in 40 early- and late-type dwarfs in the Local Group and nearby field.
Although dSphs formed stars over extended periods, today all but one are free
of detectable interstellar matter (ISM), even in the Fornax dSph, where stars
still formed 100 Myr ago. Combining metallicities for red giants with HI data
from the literature, we show that the well-known offset in
luminosity-metallicity (L-Z) relations for dSphs and dwarf irregular (dIrr)
galaxies exists also when comparing only their old stellar populations: dSphs
have higher mean stellar metallicities for a fixed luminosity. Evidently
younger dSphs experienced more efficient enrichment than young dIrrs. Dwarf
galaxies, whose locus in the L-Z diagram is consistent with that of dSphs even
for baryonic luminosities, are the ``transition-type dwarfs'' Phoenix, DDO210,
LGS3, Antlia, and KKR25. They have mixed dIrr/dSph morphologies, low stellar
masses, low angular momentum, and HI contents of less than a few 10^6 solar
masses. Unlike dIrrs, many transition-type dwarfs would closely resemble dSphs
if their gas were removed; they are likely dSph progenitors. As gas removal is
key, we consider the empirical evidence for various gas removal processes. We
suggest that internal gas removal mechanisms are inadequate and favor ram
pressure stripping to make dSphs. A combination of initial conditions and
environment seems to support the formation of dSphs, which appear to form from
small galaxies with active early star formation, whose evolution halts due to
externally induced gas loss. Transition-type dwarfs then are dSphs that kept
their ISM, and therefore should replace dSphs in isolated locations where
stripping is ineffective. (Abridged)Comment: 25 pages in AASTeX two-column preprint style, 1 table, 3 figures.
Accepted for publication in the Astronomical Journal (April 2003 issue
The Star Formation History of the Local Group dwarf galaxy Leo I
We present a quantitative analysis of the star formation history (SFH) of the
Local Group dSph galaxy Leo I, from the information in its HST [(V-I),I]
color-magnitude diagram (CMD). The method we use is based in comparing, via
synthetic CMDs, the expected distribution of stars in the CMD for different
evolutionary scenarios, with the observed distribution. We consider the SFH to
be composed by the SFR(t), the Z(t), the IMF, and a function ,
controlling the fraction and mass ratio distribution of binary stars.
The comparison between the observed CMD and the model CMDs is done through
chi-square minimization of the differences in the number of stars in a set of
regions of the CMD.
Our solution for the SFH of Leo I defines a minimum of chi-square in a well
defined position of the parameter space, and the derived SFR(t) is robust, in
the sense that its main characteristics are unchanged for different
combinations of the remaining parameters. However, only a narrow range of
assumptions for Z(t), IMF and result in a good agreement between
the data and the models, namely: Z=0.0004, a Kroupa et al. (1993) IMF or
slightly steeper, and a relatively large fraction of binary stars. Most star
formation activity (70% to 80%) occurred between 7 and 1 Gyr ago. At 1 Gyr ago,
it abruptly dropped to a negligible value, but seems to have been active until
at least ~ 300 Myr ago. Our results don't unambiguously answer the question of
whether Leo I began forming stars around 15 Gyr ago, but it appears that the
amount of this star formation, if existing at all, would be small.Comment: 25 pages + 14 figures. Accepted by The Astronomical Journa
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