1,490 research outputs found
Identification and Analysis of Young Star Cluster Candidates in M31
We present a method for finding clusters of young stars in M31 using
broadband WFPC2 data from the HST data archive. Applying our identification
method to 13 WFPC2 fields, covering an area of ~60 arcmin^2, has revealed 79
new candidate young star clusters in these portions of the M31 disk. Most of
these clusters are small (~<5 pc) young (~10-200 Myr) star groups located
within large OB associations. We have estimated the reddening values and the
ages of each candidate individually by fitting isochrones to the stellar
photometry. We provide a catalog of the candidates including rough
approximations of their reddenings and ages. We also look for patterns of
cluster formation with galactocentric distance, but our rough estimates are not
precise enough to reveal any clear patterns.Comment: 32 pages, 9 figures, 5 tables, accepted to Ap
Matching structure and bargaining outcomes in buyerâseller networks
We examine the relationship between the matching structure of a bipartite (buyer-seller) network and the (expected) shares of the unit surplus that each connected pair in this network can create. We show that in different bargaining environments, these shares are closely related to the Gallai-Edmonds Structure Theorem. This theorem characterizes the structure of maximum matchings in an undirected graph. We show that the relationship between the (expected) shares and the tructure Theorem is not an artefact of a particular bargaining mechanism or trade centralization. However, this relationship does not necessarily generalize to non-bipartite networks or to networks with heterogeneous link values
Optical Selection of Galaxies at Redshifts 1<z<3
Few galaxies have been found between the redshift ranges z < ~1 probed by
magnitude-limited surveys and z > ~3 probed by Lyman-break surveys. Comparison
of galaxy samples at lower and higher redshift suggests that large numbers of
stars were born and the Hubble sequence began to take shape at the intermediate
redshifts 1<z<3, but observational challenges have prevented us from observing
the process in much detail. We present simple and efficient strategies that can
be used to find large numbers of galaxies throughout this important but
unexplored redshift range. All the strategies are based on selecting galaxies
for spectroscopy on the basis of their colors in ground-based images taken
through a small number of optical filters: GRi for redshifts 0.85<z<1.15, GRz
for 1<z<1.5, and UGR for 1.4<z<2.1 and 1.9<z<2.7. The performance of our
strategies is quantified empirically through spectroscopy of more than 2000
galaxies at 1<z<3.5. We estimate that more than half of the UV-luminosity
density at 1<z<3 is produced by galaxies that satisfy our color-selection
criteria. Our methodology is described in detail, allowing readers to devise
analogous selection criteria for other optical filter sets.Comment: 13 pages including 20 figures. Accepted for publication in the Ap
The Sightline to Q2343-BX415: Clues to Galaxy Formation in a Quasar Environment
(Abridged) We have discovered a strong DLA coincident in redshift with the
faint QSO Q2343-BX415 (R = 20.2, z_em = 2.57393). Follow-up observations at
intermediate spectral resolution reveal that the metal lines associated with
this 'proximate' DLA consist of two sets of absorption components. One set is
moving towards the quasar with velocities of ~ 150-600 km/s; this gas is highly
ionized and does not fully cover the continuum source, suggesting that it is
physically close to the active nucleus. The other, which accounts for most of
the neutral gas, is blueshifted relative to the QSO, with the strongest
component at ~ -160 km/s. We consider the possibility that the PDLA arises in
the outflowing interstellar medium of the host galaxy of Q2343-BX415, an
interpretation supported by strong C IV and N V absorption at nearby
velocities, and by the intense radiation field longward of the Lyman limit
implied by the high C II*/H I ratio. If Q2343-BX415 is the main source of these
UV photons, then the PDLA is located at either ~ 8 or ~ 37 kpc from the active
nucleus. Alternatively, the absorber may be a foreground star-forming galaxy
unrelated to the quasar and coincidentally at the same redshift, but our deep
imaging and follow-up spectroscopy of the field of Q2343-BX415 has not yet
produced a likely candidate. We measure the abundances of 14 elements in the
PDLA, finding an overall metallicity of ~ 1/5 solar and a normal pattern of
relative element abundances for this metallicity. Thus, in this PDLA there is
no evidence for the super-solar metallicities that have been claimed for some
proximate, high ionization, systems.Comment: Accepted for publication in the Astrophysical Journal. 27 pages, 8
tables, 21 postscript figure
New Observations of the Interstellar Medium in the Lyman Break Galaxy MS 1512-cB58
We present the results of a detailed study of the interstellar medium of MS
1512-cB58, an L* Lyman break galaxy at z = 2.7276, based on new spectral
observations obtained with the Echelle Spectrograph and Imager on the Keck II
telescope at 58 km/s resolution. We focus in particular on the chemical
abundances and kinematics of the interstellar gas and our main findings are as
follows. Even at this relatively early epoch, the ISM of this galaxy is already
highly enriched in elements released by Type II supernovae; the abundances of
O, Mg, Si, P, and S are all about 2/5 of their solar values. In contrast, N and
the Fe-peak elements Mn, Fe, and Ni are underabundant by a factor of about 3.
Based on current ideas of stellar nucleosynthesis, these results can be
understood if most of the metal enrichment in cB58 has taken place within the
last 300 million years, the timescale for the release of N from intermediate
mass stars. cB58 appears to be an example of a galaxy in the process of
converting its gas into stars on a few dynamical timescales; quite possibly we
are witnessing the formation of a galactic bulge or an elliptical galaxy. The
energetic star formation activity has stirred the interstellar medium to high
velocities of up to 1000 km/s. The net effect is a bulk outflow of the ISM at a
speed of 255 km/s and at a rate which exceeds the star formation rate. It is
unclear whether this gas will be lost or retained by the galaxy. We point out
that the chemical and kinematic properties of cB58 are markedly different from
those of most damped Lyman alpha systems at the same redshift.Comment: 38 pages, LaTeX, 9 Postscript Figures. Accepted for publication in
the Astrophysical Journal. Sections 3.3 and 5.3 expanded, and two additional
figures included, following referee's repor
The Stellar Populations of the Cetus Dwarf Spheroidal Galaxy
We present Hubble Space Telescope Wide Field Planetary Camera 2 photometry in
the V and I passbands of the recently discovered Local Group dwarf spheroidal
galaxy in Cetus. Our color-magnitude diagram extends from above the first
ascent red giant branch (RGB) tip to approximately half a magnitude below the
horizontal branch (HB). Adopting a reddening of E(B-V) = 0.03, the magnitude of
the RGB tip yields a distance modulus of (m-M)o = 24.46 +/- 0.14. After
applying the reddening and distance modulus, we have utilized the color
distribution of RGB stars to determine a mean metal abundance of [Fe/H] = -1.7
on the Zinn & West scale with an intrinsic internal abundance dispersion of
+/-0.2 dex. An indirect calculation of the HB morphology of Cetus based on the
mean dereddened HB color yields (B-R)/(B+V+R) = -0.91 +/- 0.09, which
represents an HB that is redder than what can be attributed solely to Cetus'
metal abundance. As such, Cetus suffers from the `second parameter effect' in
which another parameter besides metallicity is controlling the HB morphology.
If we adopt the conventional `age hypothesis' explanation for the second
parameter effect, then this implies that Cetus is 2-3 Gyr younger than Galactic
globular clusters at its metallicity.Comment: 13 pages, 8 figures, Accepted for publication in the March 10, 2002
Ap
Lyman Alpha Imaging of a Proto-Cluster Region at <z>=3.09
We present very deep imaging observations, through a narrow-band filter tuned
to Lyman alpha at , of a volume containing a significant over-density
of galaxies at this redshift previously discovered in our survey for
continuum-selected Lyman break galaxies (LBGs). The new observations are used
in conjunction with our spectroscopic results on LBGs to compare the
effectiveness of continuum and emission line searches for star forming galaxies
at high redshift, and to extend the search for members of the structure at =
3.09 to much fainter continuum luminosities. We find that only 20-25 percent of
all galaxies at a given UV continuum luminosity would be flagged as narrow-band
excess objects subject to the typical limits W_Lya > 80 Angstroms in the
observed frame. The density enhancement of strong Lyman alpha emitters in this
field is consistent with that inferred from the analysis of the spectroscopic
Lyman break galaxy sample in the same region (6+/-1), but extends to continuum
luminosities up to 2 magnitudes fainter. There is no evidence for a
significantly higher fraction of large Lyman alpha line equivalent widths at
faint continuum luminosities. By combining the 24 spectroscopic members of the
z=3.09 ``spike'' with the narrow-band candidates, we are able to produce a
sample of 162 objects which are either known or likely members of this large
structure. We have also discovered two extremely large and diffuse Lyman alpha
emitting ``blobs'', which have physical extents >100/h kpc and Lyman alpha line
fluxes 20-40 times larger than the typical line emitters at the same redshifts
in the field. The possible nature of the ``blobs'' is discussed. (abstract
abridged)Comment: To appear in the ApJ, 31 pages, aaste
Consequences of a Change in the Galactic Environment of the Sun
The interaction of the heliosphere with interstellar clouds has attracted
interest since the late 1920's, both with a view to explaining apparent
quasi-periodic climate "catastrophes" as well as periodic mass extinctions.
Until recently, however, models describing the solar wind - local interstellar
medium (LISM) interaction self-consistently had not been developed. Here, we
describe the results of a two-dimensional (2D) simulation of the interaction
between the heliosphere and an interstellar cloud with the same properties as
currently, except that the neutral H density is increased from the present
value of n(H) ~ 0.2 cm^-3 to 10 cm^-3. The mutual interaction of interstellar
neutral hydrogen and plasma is included. The heliospheric cavity is reduced
considerably in size (approximately 10 - 14 AU to the termination shock in the
upstream direction) and is highly dynamical. The interplanetary environment at
the orbit of the Earth changes markedly, with the density of interstellar H
increasing to ~2 cm^-3. The termination shock itself experiences periods where
it disappears, reforms and disappears again. Considerable mixing of the shocked
solar wind and LISM occurs due to Rayleigh-Taylor-like instabilities at the
nose, driven by ion-neutral friction. Implications for two anomalously high
concentrations of 10Be found in Antarctic ice cores 33 kya and 60 kya, and the
absence of prior similar events, are discussed in terms of density enhancements
in the surrounding interstellar cloud. The calculation presented here supports
past speculation that the galactic environment of the Sun moderates the
interplanetary environment at the orbit of the Earth, and possibly also the
terrestrial climate.Comment: 23 pages, 2 color plates (jpg), 3 figures (eps
Physical properties of z~4 LBGs: differences between galaxies with and without Ly-alpha emission
We have analysed the physical properties of z~4 Lyman Break Galaxies observed
in the GOODS-S survey, in order to investigate the possible differences between
galaxies where the Ly-alpha is present in emission, and those where the line is
absent or in absorption. The objects have been selected from their optical
color and then spectroscopically confirmed by Vanzella et al. (2005). From the
public spectra we assessed the nature of the Ly-alpha emission and divided the
sample into galaxies with Ly-alpha in emission and objects without Ly-alpha
line (i.e. either absent or in absorption). We have then used the complete
photometry, from U band to mid infrared from the GOODS-MUSIC database, to study
the observational properties of the galaxies, such as UV spectral slopes and
optical to mid-infrared colors, and the possible differences between the two
samples. Finally through standard spectral fitting tecniques we have determined
the physical properties of the galaxies, such as total stellar mass, stellar
ages and so on, and again we have studied the possible differences between the
two samples. Our results indicate that LBG with Ly-alpha in emission are on
average a much younger and less massive population than the LBGs without
Ly-alpha emission. Both populations are forming stars very actively and are
relatively dust free, although those with line emission seem to be even less
dusty on average. We briefly discuss these results in the context of recent
models for the evolution of Lyman break galaxies and Ly-alpha emitters.Comment: Accepted for publication in A&A. Uses aa.cls, 6 pages, 3 figure
Deep mid-infrared observations of Lyman-break galaxies
As part of the In-Orbit Checkout activities for the Spitzer Space Telescope,
the IRAC team carried out a deep observation (average integration time ~8
hours) of a field surrounding the bright QSO HS 1700+6416. This field contains
several hundred z~3 Lyman-break galaxy candidates, and we report here on their
mid-infrared properties, including the IRAC detection rate, flux densities and
colors, and the results of fitting population synthesis models to the optical,
near-infrared, and IRAC magnitudes. The results of the model-fitting show that
previous optical/near-infrared studies of LBGs were not missing large, hidden
old stellar populations. The LBG candidates' properties are consistent with
those of massive, star-forming galaxies at z~3. Other IRAC sources in the same
field have similar properties, so IRAC selection may prove a promising method
of finding additional high-redshift galaxies.Comment: ApJS in press (Spitzer special issue); 13 pages, 3 figure
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