1,674 research outputs found
The Millennium Galaxy Catalogue: morphological classification and bimodality in the colour-concentration plane
Using 10 095 galaxies (B < 20 mag) from the Millennium Galaxy Catalogue, we
derive B-band luminosity distributions and selected bivariate brightness
distributions for the galaxy population. All subdivisions extract highly
correlated sub-sets of the galaxy population which consistently point towards
two overlapping distributions. A clear bimodality in the observed distribution
is seen in both the rest-(u-r) colour and log(n) distributions. The rest-(u-r)
colour bimodality becomes more pronounced when using the core colour as opposed
to global colour. The two populations are extremely well separated in the
colour-log(n) plane. Using our sample of 3 314 (B < 19 mag) eyeball classified
galaxies, we show that the bulge-dominated, early-type galaxies populate one
peak and the bulge-less, late-type galaxies occupy the second. The early- and
mid-type spirals sprawl across and between the peaks. This constitutes
extremely strong evidence that the fundamental way to divide the luminous
galaxy population is into bulges and discs and that the galaxy bimodality
reflects the two component nature of galaxies and not two distinct galaxy
classes. We argue that these two-components require two independent formation
mechanisms/processes and advocate early bulge formation through initial
collapse and ongoing disc formation through splashback, infall and
merging/accretion. We calculate the B-band luminosity-densities and
stellar-mass densities within each subdivision and estimate that the z ~ 0
stellar mass content in spheroids, bulges and discs is 35 +/- 2 per cent, 18
+/- 7 and 47 +/- 7 per cent respectively. [Abridged]Comment: Accepted for publication in MNRAS, 23 pages, 17 figures. Comments
welcome. MGC website is at: http://www.eso.org/~jliske/mgc
Resolving the age bimodality of galaxy stellar populations on kpc scales
Galaxies in the local Universe are known to follow bimodal distributions in
the global stellar populations properties. We analyze the distribution of the
local average stellar-population ages of 654,053 sub-galactic regions resolved
on ~1-kpc scales in a volume-corrected sample of 394 galaxies, drawn from the
CALIFA-DR3 integral-field-spectroscopy survey and complemented by SDSS imaging.
We find a bimodal local-age distribution, with an old and a young peak
primarily due to regions in early-type galaxies and star-forming regions of
spirals, respectively. Within spiral galaxies, the older ages of bulges and
inter-arm regions relative to spiral arms support an internal age bimodality.
Although regions of higher stellar-mass surface-density, mu*, are typically
older, mu* alone does not determine the stellar population age and a bimodal
distribution is found at any fixed mu*. We identify an "old ridge" of regions
of age ~9 Gyr, independent of mu*, and a "young sequence" of regions with age
increasing with mu* from 1-1.5 Gyr to 4-5 Gyr. We interpret the former as
regions containing only old stars, and the latter as regions where the relative
contamination of old stellar populations by young stars decreases as mu*
increases. The reason why this bimodal age distribution is not inconsistent
with the unimodal shape of the cosmic-averaged star-formation history is that
i) the dominating contribution by young stars biases the age low with respect
to the average epoch of star formation, and ii) the use of a single average age
per region is unable to represent the full time-extent of the star-formation
history of "young-sequence" regions.Comment: 17 pages, 11 figures, MNRAS accepte
Imaging of the protoelliptical NGC 1700 and its globular cluster system
An excellent candidate for a young elliptical, or `protoelliptical' galaxy is
NGC 1700. Here we present new B, V and I band imaging using the Keck telescope
and reanalyse existing V and I band images from the Hubble Space Telescope.
After subtracting a model of the galaxy from the Keck images NGC 1700 reveals
two symmetric tidal tail-like structures. If this interpretation is correct, it
suggests a past merger event involving two spiral galaxies. These tails are
largely responsible for the `boxiness' of the galaxy isophotes observed at a
radius of about 13 kpc. We also show that the B-I colour distribution of the
globular cluster system is bimodal. The mean colour of the blue population is
consistent with those of old Galactic globular clusters. Relative to this old,
metal poor population, we find that the red population is younger and more
metal rich. This young population has a similar age and metallicity as that
inferred for the central stars, suggesting that they are both associated with
an episode of star formation triggered by the merger that may have formed the
galaxy. Although possessing large errors, we find that the majority of the age
estimates of NGC 1700 are reasonably consistent and we adopt a `best estimate'
for the age of 3.0 +/-1.0 Gyr. This relatively young age places NGC 1700 within
the age range where there is a notable lack of obvious candidates for
protoellipticals. The total globular cluster specific frequency is rather low
for a typical elliptical, even after taking into account fading of the galaxy
over the next 10 Gyr. We speculate that NGC 1700 will eventually form a
relatively `globular cluster poor' elliptical galaxy.Comment: 15 pages, 15 figures, accepted for publication in MNRA
The Globular Cluster Population of NGC 7457: Clues to the Evolution of Field S0 Galaxies
In this paper we present the results of a wide-field imaging study of the
globular cluster (GC) system of the field S0 galaxy NGC 7457. To derive the
global properties of the GC system, we obtained deep BVR images with the WIYN
3.5 m telescope and Minimosaic Imager and studied the GC population of NGC 7457
to a projected radius of approximately 30 kpc. Our ground-based data were
combined with archival and published Hubble Space Telescope data to probe the
properties of the GC system close to the galaxy center and reduce contamination
in the GC candidate sample from foreground stars and background galaxies. We
performed surface photometry of NGC 7457 and compared the galaxy's surface
brightness profile with the surface density profile of the GC system. The
profiles have similar shapes in the inner 1 arcminute (3.9 kpc), but the GC
system profile appears to flatten relative to the galaxy light at larger radii.
The GC system of NGC 7457 is noticeably elliptical in our images; we measure an
ellipticity of 0.66 +/- 0.14 for the GC distribution, which is consistent with
our measured ellipticity of the galaxy light. We integrated the radial surface
density profile of the GC system to derive a total number of GCs N_GC = 210 +/-
30. The GC specific frequency normalized by the galaxy luminosity and mass are
S_N = 3.1 +/- 0.7 and T = 4.8 +/- 1.1, respectively. Comparing the derived GC
system properties and other empirical data for NGC 7457 to S0 formation
scenarios suggests that this field S0 galaxy may have formed in an unequal-mass
merger.Comment: 40 pages, 10 figures, accepted for publication in The Astrophysical
Journa
The ACS Fornax Cluster Survey. II. The Central Brightness Profiles of Early-Type Galaxies: A Characteristic Radius on Nuclear Scales and the Transition from Central Luminosity Deficit to Excess
We analyse HST surface brightness profiles for 143 early-type galaxies in the
Virgo and Fornax Clusters. Sersic models provide accurate descriptions of the
global profiles with a notable exception: the observed profiles deviate
systematically inside a characteristic "break" radius of R_b ~ 0.02R_e where
R_e is the effective radius of the galaxy. The sense of the deviation is such
that bright galaxies (M_B < -20) typically show central light deficits with
respect to the inward extrapolation of the Sersic model, while the great
majority of low- and intermediate-luminosity galaxies (-19.5 < M_B < -15) show
central light excesses; galaxies occupying a narrow range of intermediate
luminosities (-20 < M_B < -19.5) are usually well fitted by Sersic models over
all radii. The slopes of the central surface brightness profiles, when measured
at fixed fractions of R_e, vary smoothly as a function of galaxy luminosity in
a manner that depends sensitively on the choice of measurement radius. We show
that a recent claim of strong bimodality in slope is likely an artifact of the
galaxy selection function used in that study. To provide a more robust
characterization of the inner regions of galaxies, we introduce a parameter
that describes the central luminosity deficit or excess relative to the inward
extrapolation of the outer Sersic model. We find that this parameter varies
smoothly over the range of ~ 720 in blue luminosity spanned by the Virgo and
Fornax sample galaxies, with no evidence for a dichotomy. We argue that the
central light excesses (nuclei) in M_B > -19 galaxies may be the analogs of the
dense central cores that are predicted by some numerical simulations to form
via gas inflows. (ABRIDGED)Comment: To appear in the Astrophysical Journal, December 2007. Full
resolution paper available at
http://www.cfa.harvard.edu/~ajordan/ACSFCS/Home.htm
The DEEP Groth Strip Galaxy Redshift Survey. III. Redshift Catalog and Properties of Galaxies
The Deep Extragalactic Evolutionary Probe (DEEP) is a series of spectroscopic
surveys of faint galaxies, targeted at the properties and clustering of
galaxies at redshifts z ~ 1. We present the redshift catalog of the DEEP 1 GSS
pilot phase of this project, a Keck/LRIS survey in the HST/WFPC2 Groth Survey
Strip. The redshift catalog and data, including reduced spectra, are publicly
available through a Web-accessible database. The catalog contains 658 secure
galaxy redshifts with a median z=0.65, and shows large-scale structure walls to
z = 1. We find a bimodal distribution in the galaxy color-magnitude diagram
which persists to z = 1. A similar color division has been seen locally by the
SDSS and to z ~ 1 by COMBO-17. For red galaxies, we find a reddening of only
0.11 mag from z ~ 0.8 to now, about half the color evolution measured by
COMBO-17. We measure structural properties of the galaxies from the HST
imaging, and find that the color division corresponds generally to a structural
division. Most red galaxies, ~ 75%, are centrally concentrated, with a red
bulge or spheroid, while blue galaxies usually have exponential profiles.
However, there are two subclasses of red galaxies that are not bulge-dominated:
edge-on disks and a second category which we term diffuse red galaxies
(DIFRGs). The distant edge-on disks are similar in appearance and frequency to
those at low redshift, but analogs of DIFRGs are rare among local red galaxies.
DIFRGs have significant emission lines, indicating that they are reddened
mainly by dust rather than age. The DIFRGs in our sample are all at z>0.64,
suggesting that DIFRGs are more prevalent at high redshifts; they may be
related to the dusty or irregular extremely red objects (EROs) beyond z>1.2
that have been found in deep K-selected surveys. (abridged)Comment: ApJ in press. 24 pages, 17 figures (12 color). The DEEP public
database is available at http://saci.ucolick.org
Evolution of blue E/S0 galaxies from z~1: merger remnants or disk rebuilding galaxies?
Studying outliers from the bimodal distribution of galaxies in the color-mass
space, such as morphological early-type galaxies residing in the blue cloud,
can help to better understand the physical mechanisms that lead galaxy
migrations in this space. In this paper we study the evolution of the
properties of 210 M*/Msol>10^10 blue E/S0s between z~1.4 and z~0.2 in the
COSMOS field with confirmed spectroscopic redshifts from the zCOSMOS 10k
release. We first observe that the threshold mass, defined at z=0 in previous
studies as the mass below which the population of blue early-type galaxies
starts to be abundant relative to passive E/S0s, evolves from log(M*/Msol)~10.1
at z~0.3 to log(M*/Msol)~10.9 at z~1. Second, there seems to be a turn-over
mass in the nature of blue E/S0 galaxies. Above log(M*/Msol)~10.8 blue E/S0
resemble to merger remnants probably migrating to the red-sequence in a
time-scale of ~3 Gyr. Below this mass, they seem to be closer to normal
late-type galaxies as if they were the result of minor mergers which triggered
the central star-formation and built a central bulge component or were
(re)building a disk from the surrounding gas, suggesting that they are moving
back or staying in the blue-cloud. This turn-over mass does not seem to evolve
significantly from z~1 in contrast with the threshold mass and therefore does
not seem to be linked with the relative abundance of blue E/S0s.Comment: accepted for publication in A&
Deep GALEX Imaging of the HST/COSMOS Field: A First Look at the Morphology of z~0.7 Star-forming Galaxies
We present a study of the morphological nature of redshift z~0.7 star-forming
galaxies using a combination of HST/ACS, GALEX and ground-based images of the
COSMOS field. Our sample consists of 8,146 galaxies, 5,777 of which are
detected in the GALEX near-ultraviolet band down to a limiting magnitude of
25.5 (AB). We make use of the UV to estimate star formation rates, correcting
for the effect of dust using the UV-slope, and compute, from the ACS F814W
images, the C,A,S,G,M20 morphological parameters for all objects in our sample.
We observe a morphological bimodality in the galaxy population and show that it
has a strong correspondence with the FUV - g color bimodality. We conclude that
UV-optical color predominantly evolves concurrently with morphology. We observe
many of the most star-forming galaxies to have morphologies approaching that of
early-type galaxies, and interpret this as evidence that strong starburst
events are linked to bulge growth and constitute a process through which
galaxies can be brought from the blue to the red sequence while simultaneously
modifying their morphology accordingly. We conclude that the red sequence has
continued growing at z~<0.7. We also observe z~0.7 galaxies to have physical
properties similar to that of local galaxies, except for higher star formation
rates. Whence we infer that the dimming of star-forming galaxies is responsible
for most of the evolution in the star formation rate density of the Universe
since that redshift, although our data are also consistent with a mild number
evolution. [abridged]Comment: 29 pages including 22 figures. Accepted for publication in ApJS
COSMOS Special Issue. A copy of the paper with high resolution figures is
available at http://www.astro.columbia.edu/~michel/galex_cosmos_paper.pd
Machine Assisted Analysis of Vowel Length Contrasts in Wolof
Growing digital archives and improving algorithms for automatic analysis of
text and speech create new research opportunities for fundamental research in
phonetics. Such empirical approaches allow statistical evaluation of a much
larger set of hypothesis about phonetic variation and its conditioning factors
(among them geographical / dialectal variants). This paper illustrates this
vision and proposes to challenge automatic methods for the analysis of a not
easily observable phenomenon: vowel length contrast. We focus on Wolof, an
under-resourced language from Sub-Saharan Africa. In particular, we propose
multiple features to make a fine evaluation of the degree of length contrast
under different factors such as: read vs semi spontaneous speech ; standard vs
dialectal Wolof. Our measures made fully automatically on more than 20k vowel
tokens show that our proposed features can highlight different degrees of
contrast for each vowel considered. We notably show that contrast is weaker in
semi-spontaneous speech and in a non standard semi-spontaneous dialect.Comment: Accepted to Interspeech 201
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