157 research outputs found
An updated survey of globular clusters in M31. II Newly discovered bright and remote clusters
We present the first results of a large spectroscopic survey of candidate
globular clusters located in the extreme outskirts of the nearby M31 galaxy. We
obtained low resolution spectra of 48 targets selected from the XSC of 2MASS,
as in Galleti et al. (2005). The observed candidates have been robustly
classified according to their radial velocity and by verifying their
extended/point-source nature from ground-based optical images. Among the 48
observed candidates clusters we found 5 genuine remote globular clusters. One
of them has been already identified independently by Mackey et al. (2007),
their GC1; the other four are completely new discoveries: B516, B517, B518,
B519. The newly discovered clusters lie at projected distance 40 kpc<~R_p<~100
kpc from the center of M31, and have absolute integrated magnitude
-9.5<M_V<-7.5. For all the observed clusters we have measured the strongest
Lick indices and we have obtained spectroscopic metallicity estimates.
Mackey-GC1, Martin-GC1, B517 and B518 have spectra typical of old and metal
poor globular clusters ([Fe/H]<~ -1.3); B519 appears old but quite metal-rich
([Fe/H]~-0.5); B516 presents very strong Balmer absorption lines: if this is
indeed a cluster it should have a relatively young age (likely <2 Gyr). The
present analysis nearly doubles the number of M31 globulars at R_p> 40 kpc. At
odds with the Milky Way, M31 appears to have a significant population of very
bright globular clusters in its extreme outskirts.Comment: 16 pages including 6 pages published only in the electronic edition
of the Journal. Accepted for publication in A&
Primordial Nucleosynthesis: Theory and Observations
We review the Cosmology and Physics underlying Primordial Nucleosynthesis and
survey current observational data in order to compare the predictions of Big
Bang Nucleosynthesis with the inferred primordial abundances. From this
comparison we report on the status of the consistency of the standard hot big
bang model, we constrain the universal density of baryons (nucleons), and we
set limits to the numbers and/or effective interactions of hypothetical new
"light" particles (equivalent massless neutrinos).Comment: 25 pages, latex, 4 ps figures, to be published in a special memorial
volume of Physics Reports in honor of David Schram
Listen to Nice
In describing Humphrey Jennings’ wartime documentary propaganda film, 'Listen to Britain' (1942), a film with an overtly poetic sensibility and dominantly musical soundtrack, John Corner asserts that ‘through listening to
Britain, we are enabled to properly look at it'. This idea of sound leading our attention to the images has underpinned much of the collaborative
work between composer and sound designer, Geoffrey Cox, and documentary filmmaker, Keith Marley. It is in this context that the article will analyse an extract of A Film About Nice (Marley and Cox 2010), a contemporary
re-imagining of Jean Vigo’s silent documentary, 'A propos de Nice' (1930). Reference will be made throughout to the historical context, and the filmic and theoretical influences that have informed the way music and creative sound design have been used to place emphasis on hearing a place, as much as seeing it
The Origin of the Hubble Sequence in Lambda-CDM Cosmology
The Galform semi-analytic model of galaxy formation is used to explore the
mechanisms primarily responsible for the three types of galaxies seen in the
local universe: bulge, bulge+disk and disk, identified with the visual
morphological types E, S0/a-Sbc, and Sc-Scd, respectively. With a suitable
choice of parameters the Galform model can accurately reproduce the observed
local K_s-band luminosity function (LF) for galaxies split by visual
morphological type. The successful set of model parameters is used to populate
the Millennium Simulation with 9.4 million galaxies and their dark matter
halos. The resulting catalogue is then used to explore the evolution of
galaxies through cosmic history. The model predictions concur with recent
observational results including the galaxy merger rate, the star formation rate
and the seemingly anti-hierarchical evolution of ellipticals. However, the
model also predicts significant evolution of the elliptical galaxy LF that is
not observed. The discrepancy raises the possibility that samples of z~1
galaxies which have been selected using colour and morphological criteria may
be contaminated with galaxies that are not actually ellipticals.Comment: Accepted for publication in MNRAS. Missing reference adde
Galaxy Zoo 1 : Data Release of Morphological Classifications for nearly 900,000 galaxies
Morphology is a powerful indicator of a galaxy's dynamical and merger
history. It is strongly correlated with many physical parameters, including
mass, star formation history and the distribution of mass. The Galaxy Zoo
project collected simple morphological classifications of nearly 900,000
galaxies drawn from the Sloan Digital Sky Survey, contributed by hundreds of
thousands of volunteers. This large number of classifications allows us to
exclude classifier error, and measure the influence of subtle biases inherent
in morphological classification. This paper presents the data collected by the
project, alongside measures of classification accuracy and bias. The data are
now publicly available and full catalogues can be downloaded in electronic
format from http://data.galaxyzoo.org.Comment: Accepted by MNRAS, 14 pages. Updated to match final version; problem
with table 7 header fixed. Full tables available at http://data.galaxyzoo.or
The Environmental Dependence of the Luminosity-Size Relation for Galaxies
We have examined the luminosity-size relationship as a function of
environment for 12150 SDSS galaxies with precise visual classifications from
the catalog of Nair & Abraham (2010a). Our analysis is subdivided into
investigations of early-type galaxies and late-type galaxies. Early-type
galaxies reveal a surprisingly tight luminosity-size relation. The dispersion
in luminosity about the fiducial relation is only ~0.14 dex (0.35 mag), even
though the sample contains galaxies which differ by a factor of almost 100 in
luminosity. The dispersion about the luminosity-size relation is comparable to
the dispersion about the fundamental plane, even though the luminosity-size
relation is fundamentally simpler and computed using purely photometric
parameters. The key contributors to the dispersion about the luminosity-size
relation are found to be color and central concentration. Expanding our
analysis to the full range of morphological types, we show that the slope, zero
point, and scatter about the luminosity-size relation is independent of
environmental density. Our study thus indicates that whatever process is
building galaxies is doing so in a way that preserves fundamental scaling laws
even as the typical luminosity of galaxies changes with environment. However,
the distribution of galaxies along the luminosity-size relation is found to be
strongly dependent on galaxy environment. This variation is in the sense that,
at a given morphology, larger and more luminous galaxies are rarer in sparser
environments. Our analysis of late-type galaxy morphologies reveals that
scatter increases towards later Hubble types. Taken together, these results
place strong constraints on conventional hierarchical models in which galaxies
are built up in an essentially stochastic way.Comment: 20 pages, 10 figures, 5 tables, Submitted Nov 5, 2009; Accepted by
ApJ April 6, 2010 Higher resolution versions of the figures can be found at:
http://www.bo.astro.it/~nair/Morphology
Current star formation in early-type galaxies and the K+A phenomenon
We present the results of an effort to identify and study a sample of the
likely progenitors of elliptical (E) and lenticular (S0) K+A galaxies. To
achieve this, we have searched a sample ~11,000 nearby (m(r)<16) early-type
galaxies selected by morphology from the Sloan Digital Sky Survey (SDSS) Main
spectroscopic sample for actively star-forming E and S0 galaxies. Using
emission line ratios and visual inspection of SDSS g-band images, we have
identified 335 galaxies from the SDSS Fourth Data Release (DR4) as actively
star-forming E and S0 galaxies. These galaxies make up about 3% of the total
early-type sample and less than 1% of all Main galaxies with m(r)<16. We also
identified a sample of ~400 K+A galaxies from DR4 with m(r)<16; more than half
of these are E and S0 galaxies. We find that star-forming early-type galaxies
and K+A galaxies have similar mass distributions; they are on average less
massive than typical early-type galaxies but more massive than the average
star-forming galaxy. Both of these types of galaxies are found in higher
fractions among all galaxies in lower density environments. The fractions of
star-forming E and S0 galaxies and E and S0 K+A galaxies depend on environment
in nearly the same way. Model spectra fit to the stellar continua of the
star-forming E and S0 galaxies showed that their properties are consistent with
star formation episodes of <1 Gyr in duration. The modelling results imply that
on average, the star formation episodes will increase the stellar masses by
about 4%. There is also evidence that the star-forming regions within these
galaxies are rotationally supported.Comment: Accepted to MNRAS, 19 pages, 14 figure
Are disk galaxies the progenitors of giant ellipticals?
A popular formation scenario for giant elliptical galaxies proposes that they
might have formed from binary mergers of disk galaxies. Difficulties with the
scenario that emerged from earlier studies included providing the necessary
stellar mass and metallicity, maintaining the tight color-magnitude relation
and avoiding phase space limits. In this paper we revisit the issue and put
constraints on the binary disc merger scenario based on the stellar populations
of disc galaxies. We draw the following conclusions: Low redshift collisionless
or gaseous mergers of present day Milky Way like disc galaxies do not form
present day elliptical galaxies. Binary mergers of the progenitors of present
day Milky Way like disc galaxies can have evolved into intermediate mass
elliptical galaxies () if they have merged earlier than 3-4
Gyrs ago. Assuming that most present day disk galaxies formed in a similar way
to the Milky Way model presented here, more massive giant ellipticals in
general can not have formed from binary mergers of the progenitors of present
day disc galaxies. A major reason for these conclusions is that the mass in
metals of typical disk galaxy is approximately a factor of 4-8 smaller than the
mass in metals of a typical early-type galaxy and this ratio grows to larger
values with increasing redshift.Comment: accepted by ApJ, to appear in Jan 200
The Different Physical Mechanisms that Drive the Star-Formation Histories of Giant and Dwarf Galaxies
We present an analysis of star-formation and nuclear activity in galaxies as
a function of both luminosity and environment in the SDSS DR4 dataset. Using a
sample of 27753 galaxies at 0.00590% complete to Mr=-18.0 we
find that the EW(Ha) distribution is strongly bimodal, allowing galaxies to be
robustly separated into passive and star-forming populations about a value
EW(Ha)=2A. In high-density regions ~70% of galaxies are passive independent of
luminosity. In the rarefied field however, the fraction of passively-evolving
galaxies is a strong function of luminosity, dropping from ~50% for Mr<-21 to
zero by Mr~-18. Indeed for the lowest luminosity range covered (-18<Mr<-16)
none of the ~600 galaxies in the lowest density quartile are passive. The few
passively-evolving dwarf galaxies in field regions appear as satellites to
bright (~L*) galaxies. The fraction of galaxies with optical AGN signatures
decreases steadily from ~50% at Mr~-21 to ~0% by Mr~-18 closely mirroring the
luminosity-dependence of the passive galaxy fraction in low-density
environments. This result reflects the increasing importance of AGN feedback
with galaxy mass for their evolution, such that the star-formation histories of
massive galaxies are primarily determined by their past merger history. In
contrast, the complete absence of passively-evolving dwarf galaxies more than
~2 virial radii from the nearest massive halo (i.e. cluster, group or massive
galaxy) indicates that internal processes, such as merging, AGN feedback or gas
consumption through star-formation, are not responsible for terminating
star-formation in dwarf galaxies. Instead the evolution of dwarf galaxies is
primarily driven by the mass of their host halo, probably through the combined
effects of tidal forces and ram-pressure stripping.Comment: 29 pages, 11 figures. Accepted for publication in MNRA
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