167 research outputs found
Observational Test of Environmental Effects on The Local Group Dwarf Spheroidal Galaxies
In this paper, we examine whether tidal forces exerted by the Galaxy or M31
have an influence on the Local Group dwarf spheroidal galaxies (dSphs) which
are their companions. We focus on the surface brightness profiles of the dSphs,
especially their core radii because it is suggested based on the numerical
simulations that tidal disturbance can make core radii extended. We examine the
correlation for the dSphs between the distances from their parent galaxy (the
Galaxy or M31) and the compactnesses of their surface brightness profiles by
using a parameter ``C'' defined newly in this paper. Consequently, we find no
significant correlation. We make some remarks on the origin of this result by
considering three possible scenarios; tidal picture, dark matter picture, and
heterogeneity of the group of dSphs, each of which has been often discussed to
understand fundamental properties and formation processes of dSphs.Comment: 14 pages LaTeX, 2 PostScript figures, to appear in ApJ Letter
The infrared-dark dust content of high redshift galaxies
We present a theoretical model aimed at explaining the IRX- relation
for high redshift (z >5) galaxies. Recent observations (Capak+2015;
Bouwens+2016) have shown that early Lyman Break Galaxies, although
characterized by a large UV attenuation (e.g. flat UV beta slopes), show a
striking FIR deficit, i.e. they are "infrared-dark". This marked deviation from
the local IRX-beta relation can be explained by the larger molecular gas
content of these systems. While dust in the diffuse ISM attains relatively high
temperatures (Td = 45 K for typical size a=0.1 um; smaller grains can reach Td
= 60 K), a sizable fraction of the dust mass is embedded in dense gas, and
therefore remains cold. If confirmed, the FIR deficit might represent a novel,
powerful indicator of the molecular content of high-z galaxies which can be
used to pre-select candidates for follow-up deep CO observations. Thus, high-z
CO line searches with ALMA might be much more promising than currently thought.Comment: 8 pages, 4 Figures, MNRAS Submitte
Evolution of Interstellar Clouds in Local Group Dwarf Spheroidal Galaxies in the Context of Their Star Formation Histories
We consider evolution of interstellar clouds in Local Group dwarf spheroidal
galaxies (dSphs) in the context of their observed star formation histories. The
Local Group dSphs generally experienced initial bursts of star formations in
their formation epochs ( Gyr ago), when hot gas originating from the
supernovae can make the cold interstellar clouds evaporate. We find that the
maximum size of evaporating cloud is 10 pc. Thus, clouds larger than 10 pc can
survive during the initial star formation. These surviving clouds can
contribute to the second star formation to produce ``intermediate-age (
3--10 Gyr ago) stellar populations.'' Assuming that collisions between clouds
induce star formation and that the timescale of the second star formation is a
few Gyr, we estimate the total mass of the clouds. The total mass is about
, which is 1--3 orders of magnitude smaller than the typical
stellar mass of a present dSph. This implies that the initial star formation is
dominant over the second star formation, which is broadly consistent with the
observed star formation histories. However, the variety of the dSphs in their
star formation histories suggests that the effects of environments on the dSphs
may be important.Comment: 14 pages LaTeX, no figures, to appear in Ap
Dust-to-Gas Ratio and Metallicity in Dwarf Galaxies
We examine the dust-to-gas ratio as a function of metallicity for dwarf
galaxies [dwarf irregular galaxies (dIrrs) and blue compact dwarf galaxies
(BCDGs)]. Using a one-zone model and adopting the instantaneous recycling
approximation, we prepare a set of basic equations which describes processes of
dust formation and destruction in a galaxy. Four terms are included for the
processes: dust formation from heavy elements ejected by stellar mass loss,
dust destruction in supernova remnants, dust destruction in star-forming
regions, and accretion of heavy elements onto preexisting dust grains. Solving
the equations, we compare the result with observational data of nearby dIrrs
and BCDGs. The solution is consistent with the data within the reasonable
ranges of model parameters constrained by the previous examinations. This means
that the model is successful in understanding the dust amount of nearby
galaxies. We also show that the accretion rate of heavy element onto
preexisting dust grains is less effective than the condensation of heavy
elements in dwarf galaxies.Comment: 14 pages LaTeX, 4 figures, to appear in Ap
Mass-Metallicity Relation for the Local Group Dwarf Spheroidal Galaxies: A New Picture for the Chemical Enrichment of Galaxies in the Lowest Mass Range
The virial mass ()-metallicity relation among the Local Group
dwarf spheroidal galaxies (dSphs) is examined. Hirashita, Takeuchi, & Tamura
showed that the dSphs can be divided into two distinct classes with respect to
the relation between their virial masses and luminosities: low-mass (M_{\rm
vir} \la 10^8 M_\odot) and high-mass (M_{\rm vir} \ga 10^8 M_\odot) groups.
We see that both the mass-metallicity and the mass-luminosity relations of the
high-mass dSphs are understood as a low-mass extension of giant ellipticals. On
the contrary, we find that the classical galactic-wind model is problematic to
apply to the low-mass dSphs, whose low binding energy is comparable to that
released by several supernova explosions. A strongly regulated star formation
in their formation phase is required to reproduce their observed metallicity.
Such regulation is naturally expected in a gas cloud with the primordial
elemental abundance according to Nishi & Tashiro. A significant scatter in the
mass-metallicity relation for the low-mass dSphs is also successfully explained
along with the scenario of Hirashita and coworkers. We not only propose a new
picture for a chemical enrichment of the dSphs, but also suggest that the
mass-metallicity and the mass-luminosity relations be understood in a
consistent context.Comment: 14 pages LaTeX, 1 PostScript figure, to appear in ApJ Lette
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
Chemical Evolution of the Galaxy Based on the Oscillatory Star Formation History
We model the star formation history (SFH) and the chemical evolution of the
Galactic disk by combining an infall model and a limit-cycle model of the
interstellar medium (ISM). Recent observations have shown that the SFH of the
Galactic disk violently variates or oscillates. We model the oscillatory SFH
based on the limit-cycle behavior of the fractional masses of three components
of the ISM. The observed period of the oscillation ( Gyr) is reproduced
within the natural parameter range. This means that we can interpret the
oscillatory SFH as the limit-cycle behavior of the ISM. We then test the
chemical evolution of stars and gas in the framework of the limit-cycle model,
since the oscillatory behavior of the SFH may cause an oscillatory evolution of
the metallicity. We find however that the oscillatory behavior of metallicity
is not prominent because the metallicity reflects the past integrated SFH. This
indicates that the metallicity cannot be used to distinguish an oscillatory SFH
from one without oscillations.Comment: 21 pages LaTeX, to appear in Ap
Star Formation Efficiency in the Central 1 kpc Region of Early-Type Spiral Galaxies
It has been reported recently that there are some early-type spiral (Sa--Sab)
galaxies having evident star-forming regions which concentrate in their own
central 1-kpc. In such central region, is the mechanism of the star formation
distinct from that in disks of spiral galaxies? To reveal this, we estimate the
star formation efficiency (SFE) in this central 1-kpc star-forming region of
some early-type spiral galaxies, taking account of the condition for this 1-kpc
region to be self-gravitating. Using two indicators of present star formation
rate (H and infrared luminosity), we estimate the SFE to be a few
percents. This is equivalent to the observational SFE in the disks of late-type
spiral (Sb--) galaxies. This coincidence may support the universality of the
mean SFE of spiral galaxies reported in the recent studies. That is, we find no
evidence of distinct mechanism of the star formation in the central 1-kpc
region of early-type galaxies. Also, we examine the structure of the central
star-forming region, and discuss a method for estimating the mass of
star-forming regions.Comment: accepted by A
Stars of the Draco Dwarf Spheroidal Galaxy Beyond its Measured Tidal Boundary
We report R- and V-band photometry derived from CCD imaging for objects in
nine fields in and around the Draco dwarf spheroidal galaxy. The most distant
fields are about 1.3 degrees from the center. We use these data to search for
Draco stars outside of its measured tidal boundary. The search involves three
methods: 1) Plotting color-magnitude diagrams for individual fields, for
sections of fields, and for combined fields and sections. A color-magnitude
diagram can reveal a population of Draco stars by the presence of the expected
principal sequences. 2) Measuring field-to-field fluctuations and 3) measuring
intra-field fluctuations in the surface density of objects located near the
Draco principal sequences in the color-magnitude diagram. We find evidence for
the presence of Draco stars immediately beyond the measured tidal boundary of
Draco and place an upper limit on the number of such stars in more distant
fields that lie close to the extension of its major axis. The best evidence is
the presence of the Draco principal sequences in the color-magnitude diagram
for some combined fields and sections of fields. The measurements of the
field-to-field fluctuations in the stellar surface density confirm this result.Comment: To appear in the Feb 2001 Astronomical Journal, 30 pages, 16 figures,
and 7 tables, higher resolution postscript figures available at
http://www.physics.rutgers.edu/~pryor/halo.htm
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