2,285 research outputs found
Spectroscopic evolution of dusty starburst galaxies
By using a one-zone chemical and spectrophotometric evolution model of a disk
galaxy undergoing a dusty starburst, we investigate, numerically, the optical
spectroscopic properties in order to explore galaxy evolution in distant
clusters. We adopt an assumption that the degree of dust extinction
(represented by ) depends on the ages of starburst populations in such a
way that younger stars have larger (originally referred to as selective
dust extinction by Poggianti & Wu 2000). In particular, we investigate how the
time evolution of the equivalent widths of [OII]3727 and H is
controlled by the adopted age dependence. This leads to three main results: (1)
If a young stellar population (with the age of yr) is more
heavily obscured by dust than an old one ( yr), the galaxy can show
an ``e(a)'' spectrum characterized by strong H absorption and
relatively modest [OII] emission. (2) A dusty starburst galaxy with an e(a)
spectrum can evolve into a poststarburst galaxy with an a+k (or k+a) spectrum
0.2 Gyr after the starburst and then into a passive one with a k-type spectrum
1 Gyr after the starburst. This result clearly demonstrates an evolutionary
link between galaxies with different spectral classes (i.e., e(b), e(a), a+k,
k+a, and k). (3) A dusty starburst galaxy can show an a+k or k+a spectrum even
in the dusty starburst phase if the age-dependence of dust extinction is rather
weak, i.e., if young starburst populations with different ages (
yr) are uniformly obscured by dust.Comment: 27 pages 12 figures,2001,ApJ,in pres
Luminosity Functions of Elliptical Galaxies at z < 1.2
The luminosity functions of E/S0 galaxies are constructed in 3 different
redshift bins (0.2 < z < 0.55, 0.55 < z < 0.8, 0.8 < z < 1.2), using the data
from the Hubble Space Telescope Medium Deep Survey (HST MDS) and other HST
surveys. These independent luminosity functions show the brightening in the
luminosity of E/S0s by about 0.5~1.0 magnitude at z~1, and no sign of
significant number evolution.
This is the first direct measurement of the luminosity evolution of E/S0
galaxies, and our results support the hypothesis of a high redshift of
formation (z > 1) for elliptical galaxies, together with weak evolution of the
major merger rate at z < 1.Comment: To be published in ApJ Letters, 4 pages, AAS Latex, 4 figures, and 2
table
GMRT Detection of HI 21 cm-line Absorption from the Peculiar Galaxy in Abell 2125
Using the recently completed Giant Meterwave Radio Telescope, we have
detected the HI 21 cm-line absorption from the peculiar galaxy C153 in the
galaxy cluster Abell 2125. The HI absorption is at a redshift of 0.2533, with a
peak optical depth of 0.36. The full width at half minimum of the absorption
line is 100 km/s. The estimated column density of atomic Hydrogen is
0.7e22(Ts/100K) per sq. cm. The HI absorption is redshifted by ~ 400 km/s
compared to the [OIII] emission line from this system. We attribute this to an
in-falling cold gas, or to an out-flowing ionised gas, or to a combination of
both as a consequence of tidal interactions of C153 with either a cluster
galaxy or the cluster potential.Comment: 9 pages, 2 figures, uses jaa.sty (included
Time Evolution of Galaxy Formation and Bias in Cosmological Simulations
The clustering of galaxies relative to the mass distribution declines with
time because: first, nonlinear peaks become less rare events; second, the
densest regions stop forming new galaxies because gas there becomes too hot to
cool and collapse; third, after galaxies form, they are gravitationally
``debiased'' because their velocity field is the same as the dark matter. To
show these effects, we perform a hydrodynamic cosmological simulation and
examine the density field of recently formed galaxies as a function of
redshift. We find the bias b_* of recently formed galaxies (the ratio of the
rms fluctuations of these galaxies and mass), evolves from 4.5 at z=3 to around
1 at z=0, on 8 h^{-1} Mpc comoving scales. The correlation coefficient r_*
between recently formed galaxies and mass evolves from 0.9 at z=3 to 0.25 at
z=0. As gas in the universe heats up and prevents star formation, star-forming
galaxies become poorer tracers of the mass density field. After galaxies form,
the linear continuity equation is a good approximation to the gravitational
debiasing, even on nonlinear scales. The most interesting observational
consequence of the simulations is that the linear regression of the
star-formation density field on the galaxy density field evolves from about 0.9
at z=1 to 0.35 at z=0. These effects also provide a possible explanation for
the Butcher-Oemler effect, the excess of blue galaxies in clusters at redshift
z ~ 0.5. Finally, we examine cluster mass-to-light ratio estimates of Omega,
finding that while Omega(z) increases with z, one's estimate Omega_est(z)
decreases. (Abridged)Comment: 31 pages of text and figures; submitted to Ap
The mass-metallicity gradient relation of early-type galaxies
We present a newly observed relation between galaxy mass and radial
metallicity gradients of early-type galaxies. Our sample of 51 early-type
galaxies encompasses a comprehensive mass range from dwarf to brightest cluster
galaxies. The metallicity gradients are measured out to one effective radius by
comparing nearly all of the Lick absorption-line indices to recent models of
single stellar populations. The relation shows very different behaviour at low
and high masses, with a sharp transition being seen at a mass of ~ 3.5 x 10^10
M_sun (velocity dispersion of ~140 km/s, M_B ~ -19). Low-mass galaxies form a
tight relation with mass, such that metallicity gradients become shallower with
decreasing mass and positive at the very low-mass end. Above the mass
transition point several massive galaxies have steeper gradients, but a clear
downturn is visible marked by a broad scatter. The results are interpreted in
comparison with competing model predictions. We find that an early star-forming
collapse could have acted as the main mechanism for the formation of low-mass
galaxies, with star formation efficiency increasing with galactic mass. The
high-mass downturn could be a consequence of merging and the observed larger
scatter a natural result of different merger properties. These results suggest
that galaxies above the mass threshold of ~ 3.5 x 10^10 M_sun might have formed
initially by mergers of gas-rich disc galaxies and then subsequently evolved
via dry merger events. The varying efficiency of the dissipative merger-induced
starburst and feedback processes have shaped the radial metallicity gradients
in these high-mass systems.Comment: 5 pageg, 3 figures, accepted by ApJ Lette
A comparison of the galaxy populations in the Coma and distant clusters: the evolution of k+a galaxies and the role of the intracluster medium
The spectroscopic properties of galaxies in the Coma cluster are compared
with those of galaxies in rich clusters at , to investigate the
evolution of the star formation history in clusters. Luminous galaxies with
and post-starburst/post-starforming (k+a) spectra which
constitute a significant fraction of galaxies in distant cluster samples are
absent in Coma, where spectacular cases of k+a spectra are found instead at
and represent a significant proportion of the cluster dwarf galaxy
population. A simple inspection of their positions on the sky indicates that
this type of galaxy does not show a preferential location within the cluster,
but the bluest and strongest-lined group of k+a's lies in projection towards
the central 1.4 Mpc of Coma and have radial velocities significantly higher
than the cluster mean. We find a striking correlation between the positions of
these young and strong post-starburst galaxies and substructure in the hot
intracluster medium (ICM) identified from {\it XMM-Newton} data, with these
galaxies lying close to the edges of two infalling substructures. This result
strongly suggests that the interaction with the dense ICM could be responsible
for the quenching of the star formation (thus creating the k+a spectrum), and
possibly, for any previous starburst. The evolution with redshift of the
luminosity distribution of k+a galaxies can be explained by a ``downsizing
effect'', with the maximum luminosity/mass of actively star-forming galaxies
infalling onto clusters decreasing at lower redshift. We discuss the possible
physical origin of this downsizing effect and the implications of our results
for current scenarios of environmental effects on the star formation in
galaxies.Comment: 21 pages, 7 figures, to appear in ApJ, version after referee's
change
Mid-Infrared Emission from E+A Galaxies in the Coma Cluster
We have used ISO to observe at 12m seven E+A galaxies plus an additional
emission line galaxy, all in the Coma cluster. E+A galaxies lacking narrow
emission lines have 2.2m to 12m flux density ratios or limits similar
to old stellar populations (typical of early-type galaxies). Only galaxies with
emission lines have enhanced 12m flux density. Excess 12m emission is
therefore correlated with the presence of on-going star formation or an active
galactic nucleus (AGN).
By comparing the current star formation rates with previous rates estimated
from the Balmer absorption features, we divide the galaxies into two groups:
those for which star formation has declined significantly following a dramatic
peak 1 Gyr ago; and those with a significant level of ongoing star
formation or/and an AGN. There is no strong difference in the spatial
distribution on the sky between these two groups. However, the first group has
systemic velocities above the mean cluster value and the second group below
that value. This suggests that the two groups differ kinematically.
Based on surveys of the Coma cluster in the radio, the IRAS sources, and
galaxies detected in H emission, we sum the far infrared luminosity
function of galaxies in the cluster. We find that star formation in late type
galaxies is probably the dominant component of the Coma cluster far infrared
luminosity. The presence of significant emission from intracluster dust is not
yet firmly established. The member galaxies also account for most of the far
infrared output from nearby rich clusters in general.Comment: AAS Latex, accepted for publication in Ap
Age, Metallicity and Star Formation History of Cluster Galaxies at z~0.3 F
We investigate the color-magnitude distribution in the rich cluster AC 118 at
z=0.31. The sample is selected by the photometric redshift technique, allowing
to study a wide range of properties of stellar populations, and is complete in
the K-band, allowing to study these properties up to a given galaxy mass. We
use galaxy templates based on population synthesis models to translate the
physical properties of the stellar populations - formation epoch, time-scale of
star formation, and metallicity - into observed magnitudes and colors. In this
way we show that a sharp luminosity-metallicity relation is inferred without
any assumption on the galaxy formation scenario (either monolithic or
hierarchical). Our data exclude significant differences in star formation
histories along the color-magnitude relation, and therefore confirm a pure
metallicity interpretation for its origin, with an early (z~5) formation epoch
for the bulk of stellar populations. The dispersion in the color-magnitude
diagram implies that fainter galaxies in our sample (K~18) ceased to form stars
as late as z~0.5, in agreement with the picture that these galaxies were
recently accreted into the cluster environment. The trend with redshift of the
total stellar mass shows that half of the luminous mass in AC 118 was already
formed at $z~2, but also that 20% of the stars formed at z<1.Comment: 16 pages, 10 figures. ApJ in pres
Deep near-infrared luminosity function of a cluster of galaxies at z=0.3
The deep near-infrared luminosity function of AC118, a cluster of galaxies at
z=0.3, is presented. AC118 is a bimodal cluster, as evidenced both by our
near-infrared images of lensed galaxies, by public X-ray Rosat images and by
the spatial distribution of bright galaxies. Taking advantage of the extension
and depth of our data, which sample an almost unexplored region in the depth
vs. observed area diagram, we derive the luminosity function (LF), down to the
dwarf regime (M*+5), computed in several cluster portions. The overall LF,
computed on a 2.66 Mpc2 areas (H_0=50 km/s/Mpc), has an intermediate slope
(alpha=-1.2). However, the LF parameters depend on the surveyed cluster region:
the central concentration has 2.6^{+5.1}_{-1.7} times more bright galaxies and
5.3^{+7.2}_{-2.3} times less dwarfs per typical galaxy than the outer region,
which includes galaxies at an average projected distance of ~580 kpc (errors
are quoted at the 99.9 % confidence level). The LF in the secondary AC118 clump
is intermediate between the central and outer one. In other words, the
near-infrared AC118 LF steepens going from high to low density regions. At an
average clustercentric distance of ~580 kpc, the AC118 LF is statistically
indistinguishable from the LF of field galaxies at similar redshift, thus
suggesting that the hostile cluster environment plays a minor role in shaping
the LF at large clustercentric distances, while it strongly affects the LF at
higher galaxy density.Comment: ApJ, in press. The whole paper with all high resolution images is
available at http://www.na.astro.it/~andreon/listapub.htm
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