559 research outputs found
Potential of the Surface Brightness Fluctuations method to measure distances to dwarf elliptical galaxies in nearby clusters
The potential of the Surface Brightness Fluctuations (SBF) method to
determine the membership of dwarf elliptical galaxies (dEs) in nearby galaxy
clusters is investigated. Extensive simulations for SBF measurements on dEs in
the I-band for various combinations of distance modulus, seeing and integration
time are presented, based on average VLT FORS1 and FORS2 zero points. These
show that for distances up to 20 Mpc (Fornax or Virgo cluster distance),
reliable membership determination of dEs can be obtained down to very faint
magnitudes -10<M_V<-12 mag (25 mag / arcsec^2) within integration times of the
order of 1 hour and with good seeing. Comparing the limiting magnitudes of the
method for the different simulated observing conditions we derive some simple
rules to calculate integration time and seeing needed to reach a determined
limiting magnitude at a given distance modulus for observing conditions
different to the ones adopted in the simulations. Our simulations show a small
offset of the order of 0.15 mag towards measuring too faint SBF. It is shown
that this is due to loss of fluctuation signal when recovering pixel-to-pixel
fluctuations from a seeing convolved image. To check whether our simulations
represent well the behaviour of real data, SBF measurements for a real and
simulated sample of bright Centaurus Cluster dEs are presented. They show that
our simulations are in good agreement with the achievable S/N of SBF
measurements on real galaxies.Comment: 12 pages, 8 figures, accepted by A&A, replaced by version with
non-abridged bottom page limi
HST ultraviolet spectral energy distributions for three ultraluminous infrared galaxies
We present HST Faint Object Camera ultraviolet (230 nm and 140 nm) images of
three ultraluminous infrared galaxies (ULIG: L_ir > 10^12 L_sun) selected from
the IRAS Revised Bright Galaxy Sample. The purpose is to estimate spectral
energy distributions (SEDs) to facilitate the identification of similar objects
at high redshift in deep optical, infrared, and submm surveys.
All three galaxies (VII Zw031 = IRAS F12112+0305, and IRAS F22491-1808) were
well detected at 230 nm. Two of the three were marginally detected at 140 nm.
The fluxes, together with ground-based optical and infrared photometry, are
used to compute SEDs over a wide wavelength range. The measured SEDs drop from
the optical to the ultraviolet, but the magnitude of the drop ranges from a
factor of ~3 in IRAS F22491-1808 to a factor of ~100 in VIIZw031. This is most
likely due to different internal extinctions. Such an interpretation is also
suggested by extrapolating to ultraviolet wavelengths the optical internal
extinction measured in VIIZw031. K-corrections are calculated to determine the
colors of the sample galaxies as seen at high redshifts. Galaxies like VIIZw031
have very low observed rest-frame UV fluxes which means that such galaxies at
high redshift will be extremely red or even missing in optical surveys. On the
other hand, galaxies like IRAS F12112+0305 and IRAS F22491-1808, if seen at
high redshift, would be sufficiently blue that they would not easily be
distinguished from normal field galaxies, and therefore, identified as ULIGs.
The implication is then that submillimeter surveys may be the only means of
properly identifying the majority of ULIGs at high redshift.Comment: AJ in press, TeX, 23 pages, 7 tab, 17 figs available also (at higher
resolution) from http://www.ast.cam.ac.uk~trentham/ufigs.htm
On the Formation of Galaxy Halos: Comparing NGC 5128 and the Local Group Members
The metallicity distribution function (MDF) for the old red-giant stars in
the halo of NGC 5128, the nearest giant elliptical galaxy, is virtually
identical with the MDF for the old-disk stars in the LMC and also strongly
resembles the halo MDF in M31. These galaxies all have high mean halo
metallicities ( ~ -0.4$) with very small proportions of low-metallicity
stars. These observations reinforce the view that metal-rich halos are quite
normal for large galaxies of all types. Such systems are unlikely to have built
up by accretion of pre-existing, gas-free small satellite galaxies, unless
these satellites had an extremely shallow mass distribution (d log N / d log M
> -1). We suggest that the halo of NGC 5128 is more likely to have assembled
from hierarchical merging of gas-rich lumps in which the bulk of star formation
took place during or after the merger stage.Comment: 10 pages, LaTeX, plus 3 figures in separate postscript files;
Astronomical Journal, in press for December 200
The r'-band luminosity function of Abell1367: a comparison with Coma
We made a large (approximately 1degr x 1degr) r'-band imaging survey of the
central regions of the two nearby clusters of galaxies, Abell1367 and Coma. The
data, presented as a catalog, are used to construct the r'-band luminosity
function (LF) of galaxies in these two clusters, by subtracting the Yasuda et
al. (2001) galaxy counts from our cluster counts. Our Coma luminosity function
is consistent with previous determinations, i.e. providing a faint end slope
alpha = -1.47_-0.09^+0.08, significantly steeper than the one we find for
Abell1367 (alpha = -1.07_-0.16^+0.20). The counts in Abell1367 show a relative
minimum at r' ~ 19, followed by a steep increase faintward. The difference
between the two clusters appears significant, given the consistency of the
experimental conditions in the two clusters. Whereas for Coma we find a
significant increase of the slope of the LF outwards, no such effect is found
for Abell1367.Comment: 12 pages, 8 figures, accepted for publication in A&
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Environmental exposures during windows of susceptibility for breast cancer: a framework for prevention research.
BackgroundThe long time from exposure to potentially harmful chemicals until breast cancer occurrence poses challenges for designing etiologic studies and for implementing successful prevention programs. Growing evidence from animal and human studies indicates that distinct time periods of heightened susceptibility to endocrine disruptors exist throughout the life course. The influence of environmental chemicals on breast cancer risk may be greater during several windows of susceptibility (WOS) in a woman's life, including prenatal development, puberty, pregnancy, and the menopausal transition. These time windows are considered as specific periods of susceptibility for breast cancer because significant structural and functional changes occur in the mammary gland, as well as alterations in the mammary micro-environment and hormone signaling that may influence risk. Breast cancer research focused on these breast cancer WOS will accelerate understanding of disease etiology and prevention.Main textDespite the plausible heightened mechanistic influences of environmental chemicals on breast cancer risk during time periods of change in the mammary gland's structure and function, most human studies of environmental chemicals are not focused on specific WOS. This article reviews studies conducted over the past few decades that have specifically addressed the effect of environmental chemicals and metals on breast cancer risk during at least one of these WOS. In addition to summarizing the broader evidence-base specific to WOS, we include discussion of the NIH-funded Breast Cancer and the Environment Research Program (BCERP) which included population-based and basic science research focused on specific WOS to evaluate associations between breast cancer risk and particular classes of endocrine-disrupting chemicals-including polycyclic aromatic hydrocarbons, perfluorinated compounds, polybrominated diphenyl ethers, and phenols-and metals. We outline ways in which ongoing transdisciplinary BCERP projects incorporate animal research and human epidemiologic studies in close partnership with community organizations and communication scientists to identify research priorities and effectively translate evidence-based findings to the public and policy makers.ConclusionsAn integrative model of breast cancer research is needed to determine the impact and mechanisms of action of endocrine disruptors at different WOS. By focusing on environmental chemical exposure during specific WOS, scientists and their community partners may identify when prevention efforts are likely to be most effective
Student Engagement in Peer Dialogue About Diversity and Inclusion
Student engagement in peer dialogue is a key aspect of a transformative learning process. However, the dynamics of peer dialogue become more complex when applied to concepts of diversity and inclusion, due to increased risk of student vulnerability and exclusion. This study examined how curricular content and contextual features in educational settings facilitate peer dialogue by analyzing the learning narratives of eleven occupational therapy graduate students. Considered within a transformative approach to learning, findings suggest the need to consider how students experience and name diversity. Critical questions are raised about the value of and potential caveats about exposing students to first person accounts, as well as pedagogical strategies aimed at creating respectful learning classroom spaces that acknowledge the many intersecting social identities that students bring. Insights from this study informed the development of the linked concept of critical intersectional peer dialogue (CIPD)
Did VV~29 collide with a dark Dark-Matter halo?
Westerbork Radio Synthesis Telescope observation of the galaxy
VV29=Arp188=UGC10214 shows that there are at least three distinct dynamical
components whose kinematics can be traced in 21cm line emission. The system
appears to be the result of a galaxy-galaxy interaction. We identify a
sufficient number of dynamical elements containing baryons (stars and neutral
gas) that there is no compelling reason to postulate the presence of an
additional dark matter halo that is devoid of detectable baryons. The central
galaxy VV29a is massive (V_rot = 330 km/s) and gas rich (M_HI} = 6x10^9
Msolar). The distinctive optical plume (VV29b), which extends eastward from the
main galaxy, is also gas rich (M_HI = 3x10^9 Msolar) and has a very low
gradient in line of sight velocity (<30 km/s) over 70kpc. On the western side,
there is an HI feature of M_HI = 4x10^8 Msolar that participates strongly in
orbital motion about the host in the same sense of rotation as the VV29a
itself. A blue, less massive, gas-rich galaxy "VV29c" (M_HI = 9x10^8 Msolar)
appears clearly in the HI maps as an 170 km/s wide spectral feature, seen in
projection against or, more likely, behind the west side of the host disk. Its
high recessional velocity is counter to the host rotation direction. The
optical images of Trentham et al (2001) show signs of this blue dwarf against
the redder VV29a disk. The companion galaxy CGCG27-021=MGC09-26-54 (at
projected distance 115 kpc) is not detected in 21cm line emission
(M_HI<10^9Msolar).Comment: 7 pages, 13 figures, accepted for publication in Astronomy and
Astrophysic
Chemical evolution of the intra-cluster medium
The high metallicity of the intra-cluster medium (ICM) is generally
interpreted on the base of the galactic wind scenario for elliptical galaxies.
In this framework, we develop a toy-model to follow the chemical evolution of
the ICM, formulated in analogy to chemical models for individual galaxies. The
model computes the galaxy formation history (GFH) of cluster galaxies,
connecting the final luminosity function (LF) to the corresponding metal
enrichment history of the ICM. The observed LF can be reproduced with a smooth,
Madau-plot like GFH peaking at z~ 1-2, plus a "burst" of formation of dwarf
galaxies at high redshift. The model is used to test the response of the
predicted metal content and abundance evolution of the ICM to varying input
galactic models. The chemical enrichment is computed from "galactic yields"
based on models of elliptical galaxies with a variable initial mass function
(IMF), favouring the formation of massive stars at high redshift and/or in more
massive galaxies. For a given final galactic luminosity, these model
ellipticals eject into the ICM a larger quantity of gas and of metals than do
standard models based on the Salpeter IMF. However, a scenario in which the IMF
varies with redshift as a consequence of the effect of the the cosmic
background temperature on the Jeans mass scale, appears to be too mild to
account for the observed metal production in clusters. The high
iron-mass-to-luminosity-ratio of the ICM can be reproduced only by assuming a
more dramatic variation of the typical stellar mass, in line with other recent
findings. The mass in the wind-ejected gas is predicted to exceed the mass in
galaxies by a factor of 1.5-2 and to constitute roughly half of the
intra-cluster gas.Comment: 25 pages, 17 figures, accepted for publication in A&
Dissecting the luminosity function of the Coma cluster of galaxies using CFHT wide field images
[Abridged] We determined the luminosity function (LF) of the Coma cluster
galaxies selected by luminosity, and the LF bi-variate in central brightness.
The Coma cluster and control fields were imaged using the CFH12K (42x28 arcmin)
and UH8K (28x28 arcmin) wide-field cameras at the CFHT. Selected HST images
were used for testing. Quantities were derived from measurements in at least
two colors, which have the following features: (1) Galaxies as faint as three
times the luminosity of the brightest globular clusters are in the completeness
region of our data. (2) We have a complete census (in the explored region) of
low surface brightness galaxies with central surface brightness galaxies almost
as low as the faintest so far cataloged ones. (3) The explored area is among
the largest ever sampled with CCDs at comparable depth for any cluster of
galaxies. (4) The error budget includes all sources of errors known to date.
Using HST images we also discovered that blends of globular clusters, not
resolved in individual components due to seeing, look like dwarf galaxies when
observed from the ground and are numerous and bright. The derived Coma LF is
relatively steep (alpha=-1.4) over the 11 magnitudes sampled, but the slope and
shape depend on color. A large population of faint low surface brightness
galaxies was discovered, representing the largest contributor (in number) to
the LF at faint magnitudes. We found a clear progression for a faintening of
the LF from high surface brightness galaxies (mu~20 mag/arcsec2) to galaxies of
very faint central brightnesses (mu~24.5 mag/arcsec2), and some evidence for a
steepening. Compact galaxies, usually classified as stars and therefore not
included in the LF, are found to be a minor population in Coma.Comment: ApJ, in pres
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