27 research outputs found
Red Giants in the Halo of the S0 Galaxy NGC 3115: A Distance and a Bimodal Metallicity Distribution
Using the Hubble Space Telescope, we resolve the red giant branch in the halo
of the S0 galaxy NGC 3115. We measure magnitudes and colours for stars
down to 1.5 magnitudes below the tip of the red giant branch. From the
brightest stars we estimate a distance modulus ,
corresponding to a distance of Mpc. This is in excellent
agreement with the value determined from the planetary
nebula luminosity function. Our results rule out the shorter distance modulus
determined from surface brightness fluctuations. A histogram of
colours shows a clear bimodality, indicating the presence of two
distinct halo populations of roughly equal size. One has [Fe/H] and
one has [Fe/H]. This is the most distant galaxy in which a
Population II halo has been resolved, and it is the first time a colour
bimodality has been observed among the halo stars of any early-type galaxy.Comment: 15 pages, 4 figures. MNRAS, in pres
HST Star Counts at High Galactic Latitudes
We use star counts from 13 deep HST fields imaged with the {\it Wide Field
Camera - 2} in order to constrain the amount of dark matter in the Galaxy that
can be in the form of low-mass main sequence stars or white-dwarfs. Based on
the number of red stars found in our fields, we exclude the possibility that
more than 25\% of the massive dark halo is made up of M dwarfs or subdwarfs;
fairly massive () and yet extremely faint ( \gtsima
) stellar candidates would have to be invoked in order to make the
observed number of stars compatible with that predicted by a stellar dark halo.
White dwarfs must also be intrinsically very faint ( \gtsima ) in
order to be consistent with the observed number of faint stars in the HST
fields. We also rule out an increasing or flat stellar luminosity function
beyond . The inferred slope of the disk luminosity function is
intermediary between local, volume-limited surveys and ground-based photometric
ones. Finally, the magnitude counts are well fitted by existing models for the
structure of the Galaxy, with only small changes in the fiducial values of the
model parameters. The colour distribution, however, is not well reproduced by
the models. It is unclear at present if this reflects inadequacies of the
available models or uncertainties in the colour-magnitude diagrams for low
metallicity stars and in the photometric calibration.Comment: 18 pages plus 6 figure
HST Observations of the Field Star Population in the Large Magellanic Cloud
We present and photometry, obtained with the Hubble Space Telescope,
for stars in a field in the inner disk of the Large Magellanic
Cloud. We confirm previous results indicating that an intense star formation
event, probably corresponding to the formation of the LMC disk, occurred a few
times years ago. We find a small but real difference between our field
and one further out in the disk observed by Gallagher et al (1996): either star
formation in the inner disk commenced slightly earlier, or the stars are
slightly more metal rich. We also find evidence for a later burst, around 1 Gyr
ago, which may correspond to the formation of the LMC bar. About 5% of the
stars in our field are substantially older than either burst, and are probably
members of an old disk or halo population with age Gyr.Comment: 18 pages, 7 figures: only 3 available ellectronically - complete
copies by request from [email protected]
HST Imaging of the Globular Clusters in the Fornax Cluster: NGC 1399 and NGC 1404
The Fornax cluster galaxies NGC 1399 and NGC 1404 are ideal for studying the
effects of a cluster environment on globular cluster systems. Here we present
new optical imaging of these two galaxies from both the Hubble Space
Telescope's Wide Field and Planetary Camera 2 and the Cerro Tololo
Inter-American Observatory's 1.5m telescope. The combination of both data sets
provides unique insight on the spatial and colour distribution of globular
clusters. From B-I colours, we find that both galaxies have a broad globular
cluster metallicity distribution that is inconsistent with a single population.
Two Gaussians provide a reasonable representation of the metallicity
distribution in each galaxy. The metal-rich subpopulation is more centrally
concentrated than the metal-poor one. We show that the radial metallicity
gradient can be explained by the changing relative mix of the two globular
cluster subpopulations. We derive globular cluster surface density profiles,
and find that they are flatter (i.e. more extended) than the underlying
starlight. The total number of globular clusters and specific frequency are
calculated to be N = 5700 +/- 500, S_N = 11.5 +/- 1.0 for NGC 1399 and N = 725
+/- 145, S_N = 2.0 +/- 0.5 for NGC 1404. Our results are compared to the
expectations of globular cluster formation scenarios.Comment: 21 pages, Latex, 13 figures, submitted to MNRA
Two serendipitous low-mass LMC clusters discovered with HST
We present V and I photometry of two open clusters in the LMC down to V ~ 26.
The clusters were imaged with the Wide Field and Planetary Camera - 2 on board
of the Hubble Space Telescope, as part of the Medium Deep Survey Key-Project.
Both are low luminosity (Mv ~ -3.5), low mass systems (M ~ 10^3 Msolar). The
chance discovery of these two clusters in two parallel WFPC2 fields suggests a
significant incompleteness in the LMC cluster census near the bar. One of the
clusters is roughly elliptical and compact, with a steep light profile, a
central surface brightness Mu_v(0) 20.2 mag/arcsec2, half-light radius Rhl ~
0.9 pc (total visual major diameter D ~ 3 pc) and an estimated mass M ~ 1500
Msolar. From the colour-magnitude diagram and isochrone fits we estimate its
age as t ~ 2-5 10^8 years. Its mass function has a fitted slope of Gamma =
\Delta log \phi (M) / \Delta log M = -1.8 +/- 0.7 in the range probed (0.9 <
M/Msolar < 4.5). The other cluster is more irregular and sparser, having
shallower density and surface brightness profiles. We obtain Gamma = -1.2 +/-
0.4, and estimate its mass as M ~ 400 Msolar. A derived upper limit for its age
is t < 5 10^8 years. Both clusters have mass functions with slopes similar to
that of R136, a massive LMC cluster, for which HST results indicate Gamma ~
-1.2. They also seem to be relaxed in their cores and well contained in their
tidal radii.Comment: 16 pages plus 9 figures (2 large figs not included
Globular Cluster Abundances from High-Resolution Integrated Light Spectra, I: 47 Tuc
We describe the detailed chemical abundance analysis of a high-resolution
(R~35,000), integrated-light (IL), spectrum of the core of the Galactic
globular cluster 47 Tuc, obtained using the du Pont echelle at Las Campanas. We
develop an abundance analysis strategy that can be applied to spatial
unresolved extra- galactic clusters. We have computed abundances for Na, Mg,
Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Ba, La, Nd and Eu. For an
analysis with the known color-magnitude diagram (cmd) for 47 Tuc we obtain a
mean [Fe/H] value of -0.75 +/-0.026+/-0.045 dex (random and systematic error),
in good agreement with the mean of 5 recent high resolution abundance studies,
at -0.70 dex. Typical random errors on our mean [X/Fe] ratios are 0.07-0.10
dex, similar to studies of individual stars in 47 Tuc, although Na and Al
appear enhanced, perhaps due to proton burning in the most luminous cluster
stars. Our IL abundance analysis with an unknown cmd employed theoretical
Teramo isochrones; however, we apply zero-point abundance corrections to
account for the factor of 3 underprediction of stars at the AGB bump
luminosity. While line diagnostics alone provide only mild constraints on the
cluster age (ruling-out ages younger than ~2 Gyr), when theoretical IL B-V
colors are combined with metallicity derived from the Fe I lines, the age is
constrained to 10--15 Gyr and we obtain [Fe/H]=-0.70 +/-0.021 +/-0.052 dex. We
find that Fe I line diagnostics may also be used to constrain the horizontal
branch morphology of an unresolved cluster. Lastly, our spectrum synthesis of
5.4 million TiO lines indicates that the 7300-7600A TiO window should be useful
for estimating the effect of M giants on the IL abundances, and important for
clusters more metal-rich than 47 Tuc.Comment: 40 pages text & references, 4 tables, 19 figures (72 pages total).
Changes include addition of B-V color to help constrain GC age. To appear in
Ap
Discovery of a luminous white dwarf in a young star cluster in the Large Magellanic Cloud
We have identified a candidate 1-2 x 10^5 year old luminous white dwarf in
NGC 1818, a young star cluster in the Large Magellanic Cloud. This discovery
strongly constrains the boundary mass M_c at which stars stop forming neutron
stars and start forming white dwarfs, to M_c > 7.6 Msun.Comment: 4 pages, 2 figures, greyscale image available by ftp from
[email protected]. ApJLetters, accepted 17 March 199
The Metal-rich Globular Cluster NGC6553: Observations with WFPC2, STIS, and NICMOS
We present a HST study of the metal-rich globular cluster NGC6553 using
WFPC2, NICMOS and STIS. Our primary motivation is to calibrate the STIS
broad-band LP magnitude against and magnitudes for stars of
known metallicity and absolute (visual) magnitude, for application to our study
of LMC globular clusters. NGC6553 has been shown in earlier studies to have a
very unusual colour-magnitude diagram, so we also use our data to investigate
the reddening, distance, luminosity function and structure of this cluster. We
deduce a higher metallicity and smaller distance modulus than did some previous
studies, but emphasise that very large patchy extinction on small angular
scales prohibits accurate determination of the parameters of this cluster. The
horizontal branch of NGC6553 in () is tilted at an angle close to that
of the reddening vector. We show that extinction does not, however, explain the
tilt, which is presumably a metallicity effect. The colour-magnitude diagram
shows an apparent second turnoff some 1.5 magnitudes fainter than that of the
cluster. We show that this is most likely the background Galactic bulge:
however, in that case, the colour-magnitude diagram of NGC6553 is not a good
match to that of the field bulge population. The cluster is probably more
metal-rich than is the mean field bulge star.Comment: 29 pages (Latex), 13 figs (PS, in document), 10 figs (JPEG format,
outside document, degraded from original to save download time), accepted for
pub. in A
The re-discovery of contemplation through science : with Tom McLeish, âThe Re-Discovery of Contemplation through Science: Boyle Lecture 2021â; Rowan Williams, âThe Re-Discovery of Contemplation through Science: A Response to Tom McLeishâ; Fraser Watts, âDiscussion of the Boyle Lecture 2021â; and Tom McLeish, âResponse to Boyle Lecture 2021 Panel and Participant Discussion.â
Some of the early-modern changes in the social framing of science, while often believed to be essential, are shown to be contingent. They contribute to the flawed public narrative around science today, and especially to the misconceptions around science and religion. Four are examined in detail, each of which contributes to the demise of the contemplative stance that science both requires and offers. They are: (1) a turn from an immersed subject to the pretense of a pure objectivity, (2) a turn from imagination as a legitimate pathway to knowledge, (3) a turn from shared and participative science to a restricted professionalism, and (4) an overprosaic reading of the metaphor of the âBook of Nature.â All four, but especially the imperative to consider reading nature as poetry, and a deeper examination of the entanglements between poetry and theoretical science, draw unavoidably on theological ideas, and contribute to a developing âtheology of science.â
The Globular Clusters in M87: A Bimodal Colour Distribution 1
Based on observations obtained as part of the Medium Deep Survey. We present V and I photometry for ⌠150 globular clusters with 20 < V < 27 ⌠2.5 arcmin from the centre of M87, the cD galaxy in the Virgo cluster. The data were acquired with the Hubble Space Telescope which, with effective resolution 0.1 arcsec, allows us for the first time to distinguish between globular clusters and most background galaxies on the basis of morphology, and to obtain accurate photometry even at faint magnitudes. The (V â I) distribution of the clusters is clearly bimodal, implying a corresponding bimodality in the metallicity distribution, with peaks at [Fe/H] â â1.5 and â0.5. We also find that the brightest clusters are predominantly metal poor, while the fainter clusters are divided roughly eaually between metal poor and metal rich. Our sample is essentially complete to V â 25, a full magnitude past the expected peak of a âuniversalâ luminosity function, and the observed luminosity function is well represented by the standard gaussian distribution with < MV> = â7.2 and Ï = 1.4