779 research outputs found
Spectroscopy of Ultra-diffuse Galaxies in the Coma Cluster
We present spectra of 5 ultra-diffuse galaxies (UDGs) in the vicinity of the
Coma Cluster obtained with the Multi-Object Double Spectrograph on the Large
Binocular Telescope. We confirm 4 of these as members of the cluster,
quintupling the number of spectroscopically confirmed systems. Like the
previously confirmed large (projected half light radius 4.6 kpc) UDG, DF44,
the systems we targeted all have projected half light radii kpc. As
such, we spectroscopically confirm a population of physically large UDGs in the
Coma cluster. The remaining UDG is located in the field, about Mpc behind
the cluster. We observe Balmer and Ca II H \& K absorption lines in all of our
UDG spectra. By comparing the stacked UDG spectrum against stellar population
synthesis models, we conclude that, on average, these UDGs are composed of
metal-poor stars ([Fe/H] ). We also discover the first UDG with
[OII] and [OIII] emission lines within a clustered environment, demonstrating
that not all cluster UDGs are devoid of gas and sources of ionizing radiation.Comment: 5 pages, 4 figure
Constant peptidoglycan density in the sacculus of escherichia coli B/r growing at different rates
The determination and maintenance of bacterial cell shape are problems still far from being understood (reviewed [ 1,2]). Several models have been advanced during the last decade, for mechanisms governing th
The Caustic Ring Model of the Milky Way Halo
We present a proposal for the full phase space distribution of the Milky Way
halo. The model is axially and reflection symmetric and its time evolution is
self-similar. It describes the halo as a set of discrete dark matter flows with
stated densities and velocity vectors everywhere. We first discuss the general
conditions under which the time evolution of a cold collisionless
self-gravitating fluid is self-similar, and show that symmetry is not necessary
for self-similarity. When spherical symmetry is imposed, the model is the same
as described by Fillmore and Goldreich, and by Bertschinger, twenty-three years
ago. The spherically symmetric model depends on one dimensionless parameter
and two dimensionful parameters. We set = 0.3, a value
consistent with the slope of the power spectrum of density perturbations on
galactic scales. The dimensionful parameters are determined by the Galactic
rotation velocity (220 km/s) at the position of the Sun and by the age of the
Galaxy (13.7 Gyr). The properties of the outer caustics are derived in the
spherically symmetric model. The structure of the inner halo depends on the
angular momentum distribution of the dark matter particles. We assume that
distribution to be axial and reflection symmetric, and dominated by net overall
rotation. The inner caustics are rings whose radii are determined in terms of a
single additional parameter . We summarize the observational
evidence in support of the model. The evidence is consistent with
= 0.18 in Concordance Cosmology, equivalent to = 0.26 in
Einstein - de Sitter cosmology. We give formulas to estimate the flow densities
and velocity vectors anywhere in the Milky Way halo. The properties of the
first forty flows at the location of the Earth are listed.Comment: 35 pages, 6 figure
Quantifying the Drivers of Star Formation on Galactic Scales. I. The Small Magellanic Cloud
We use the star formation history of the Small Magellanic Cloud (SMC) to
place quantitative limits on the effect of tidal interactions and gas infall on
the star formation and chemical enrichment history of the SMC. The coincident
timing of two recent (< 4 Gyr) increases in the star formation rate and
SMC/Milky Way(MW) pericenter passages suggests that global star formation in
the SMC is driven at least in part by tidal forces due to the MW. The Large
Magellanic Cloud (LMC) is the other potential driver of star formation, but is
only near the SMC during the most recent burst. The poorly constrained LMC-SMC
orbit is our principal uncertainty. To explore the correspondence between
bursts and MW pericenter passages further, we model star formation in the SMC
using a combination of continuous and tidally-triggered star formation. The
behavior of the tidally-triggered mode is a strong inverse function of the
SMC-MW separation (preferred behavior ~ r^-5, resulting in a factor of ~100
difference in the rate of tidally-triggered star formation at pericenter and
apocenter). Despite the success of these closed-box evolutionary models in
reproducing the recent SMC star formation history and current chemical
abundance, they have some systematic shortcomings that are remedied by
postulating that a sizable infall event (~ 50% of the total gas mass) occured
about 4 Gyr ago. Regardless of whether this infall event is included, the
fraction of stars in the SMC that formed via a tidally triggered mode is > 10%
and could be as large as 70%.Comment: Accepted for publication in Ap
The Morphologies of the Small Magellanic Cloud
We compare the distribution of stars of different spectral types, and hence
mean age, within the central SMC and find that the asymmetric structures are
almost exclusively composed of young main sequence stars. Because of the
relative lack of older stars in these features, and the extremely regular
distribution of red giant and clump stars in the SMC central body, we conclude
that tides alone are not responsible for the irregular appearance of the
central SMC. The dominant physical mechanism in determining the current-day
appearance of the SMC must be star formation triggered by a hydrodynamic
interaction between gaseous components. These results extend the results of
population studies (cf. Gardiner and Hatzidimitriou) inward in radius and also
confirm the suggestion of the spheroidal nature of the central SMC based on
kinematic arguments (Dopita et al; Hardy, Suntzeff & Azzopardi). Finally, we
find no evidence in the underlying older stellar population for a ``bar'' or
``outer arm'', again supporting our classification of the central SMC as a
spheroidal body with highly irregular recent star formation.Comment: 8 pages, accepted for publication in ApJ Letters (higher quality
figures available at http://ngala.as.arizona.edu/dennis/mcsurvey.html
A Deeper Look at the New Milky Way Satellites: Sagittarius II, Reticulum II, Phoenix II, and Tucana III
We present deep Magellan/Megacam stellar photometry of four recently
discovered faint Milky Way satellites: Sagittarius II (Sgr II), Reticulum II
(Ret II), Phoenix II (Phe II), and Tucana III (Tuc III). Our photometry reaches
~2-3 magnitudes deeper than the discovery data, allowing us to revisit the
properties of these new objects (e.g., distance, structural properties,
luminosity measurements, and signs of tidal disturbance). The satellite
color-magnitude diagrams show that they are all old (~13.5 Gyr) and metal-poor
([Fe/H]). Sgr II is particularly interesting as it sits in an
intermediate position between the loci of dwarf galaxies and globular clusters
in the size-luminosity plane. The ensemble of its structural parameters is more
consistent with a globular cluster classification, indicating that Sgr II is
the most extended globular cluster in its luminosity range. The other three
satellites land directly on the locus defined by Milky Way ultra-faint dwarf
galaxies of similar luminosity. Ret II is the most elongated nearby dwarf
galaxy currently known for its luminosity range. Our structural parameters for
Phe II and Tuc III suggest that they are both dwarf galaxies. Tuc III is known
to be associated with a stellar stream, which is clearly visible in our
matched-filter stellar density map. The other satellites do not show any clear
evidence of tidal stripping in the form of extensions or distortions. Finally,
we also use archival HI data to place limits on the gas content of each object.Comment: Accepted for publication in ApJ. Minor updates to match accepted
versio
AGB stars in the Magellanic Clouds. III. The rate of star formation across the SMC
This article compares the Ks magnitude distribution of Small Magellanic Cloud
asymptotic giant branch stars obtained from the DENIS and 2MASS data with
theoretical distributions. Theoretical Ks magnitude distributions have been
constructed using up-to-date stellar evolution calculations for low and
intermediate-mass stars, and in particular for thermally pulsing asymptotic
giant branch stars. Separate fits of the magnitude distributions of carbon- and
oxygen-rich stars allowed us to constrain the metallicity distribution across
the galaxy and its star formation rate. The Small Magellanic Cloud stellar
population is found to be on average 7-9 Gyr old but older stars are present at
its periphery and younger stars are present in the direction of the companion
galaxy the Large Magellanic Cloud. The metallicity distribution traces a
ring-like structure that is more metal rich than the inner region of the
galaxy. The C/M ratio discussed in Paper I is a tracer of the metallicity
distribution only if the underlying stellar population is of intermediate-age.Comment: 8 pages, 7 figures, accepted by A&
Abundance determinations in HII regions: model fitting versus Te-method
The discrepancy between the oxygen abundances in high-metallicity HII regions
determined through the Te-method (and/or through the corresponding "strong
lines - oxygen abundance" calibration) and that determined through the model
fitting (and/or through the corresponding "strong lines - oxygen abundance"
calibration) is discussed. It is suggested to use the interstellar oxygen
abundance in the solar vicinity, derived with very high precision from the
high-resolution observations of the weak interstellar absorption lines towards
the stars, as a "Rosetta stone" to verify the validity of the oxygen abundances
derived in HII regions with the Te-method at high abundances. The agreement
between the value of the oxygen abundance at the solar galactocentric distance
traced by the abundances derived in HII regions through the Te-method and that
derived from the interstellar absorption lines towards the stars is strong
evidence in favor of that i) the two-zone model for Te seems to be a realistic
interpretation of the temperature structure within HII regions, and ii) the
classic Te-method provides accurate oxygen abundances in HII regions. It has
been concluded that the "strong lines - oxygen abundance" calibrations must be
based on the HII regions with the oxygen abundances derived with the Te-method
but not on the existing grids of the models for HII regions.Comment: 5 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
AGB stars in the Magellanic Clouds II. The rate of star formation across the LMC
Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO)This article compares the distribution of Ks magnitudes of Large Magellanic Cloud (LMC) asymptotic giant branch (AGB) stars obtained from the DENIS and 2MASS data with theoretical distributions. These have been constructed using up-to-date stellar evolution calculations for low and intermediate-mass stars, and in particular for thermally pulsing AGB stars. A fit of the magnitude distribution of both carbon- and oxygen-rich AGB stars allowed us to constrain the metallicity distribution across the LMC and its star formation rate (SFR). The LMC stellar population is found to be on average 5 â 6 Gyr old and is consistent with a mean metallicity corresponding to Z = 0.006. These values may however be affected by systematic errors in the underlying stellar models, and by the limited exploration of the possible SFR histories. Instead our method should be particularly useful for detecting variations in the mean metallicity and SFR across the LMC disk. There are well defined regions where both the metallicity and the mean-age of the underlying stellar population span the whole range of grid parameters. The C/M ratio discussed in paper I is a tracer of the metallicity distribution if the underlying stellar population is older than about a few Gyr. A similar study across the Small Magellanic Cloud is given in paper III of this series.Peer reviewe
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