86 research outputs found
Bulges of disk galaxies at intermediate redshifts.II. Nuclear, disk and global colours in the Groth Strip
We analyse colours of the nuclear regions of intermediate redshift disk
galaxies, with the aim of obtaining empirical information of relative ages of
bulges and disks at 0.1 < z < 1.3. We work with an apparent-diameter limited
parent sample of 248 galaxies from the HST Groth Strip Survey. We apply a
conservative criterion to identify bulges and potential precursors of
present-day bulges based on nuclear surface brightness excess above the
exponential profile of the outer parts. We measure bulge colours on wedge
profiles opening on the semi-minor axis, and compare them to disk, and global
galaxy colours. For 60% of galaxies with bulges, the rest-frame nuclear colour
distribution shows a red sequence that is well fit by passive evolution models
of various ages, while the remainder 40% scatters towards bluer colours. In
contrast, galaxies without central brightness excess show typical colours of
star forming population and lack a red sequence. We also see that, as in the
local Universe, most of the minor axis colour profiles are negative (bluer
outward), and fairly gentle, indicating that nuclear colours are not distinctly
different from disk colours. This is corroborated when comparing nuclear,
global and disk colours: these show strong correlations, for any value of the
central brightness prominence of the bulge. Comparison with synthetic models of
red sequence bulge colours suggests that such red bulges have stopped forming
stars at an epoch earlier than ~ 1 Gyr before the observation. The correlation
between nuclear and disk colours and the small colour gradients hints at an
intertwined star formation history for bulges and disks: probably, most of our
red bulges formed in a process in which truncation of star formation in the
bulge did not destroy the disk.Comment: 13 pages, 9 figures, 5 tables. Accepted for publication in A&
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
The morphologies and masses of extremely red galaxies in the Groth Strip survey
We present a new cataloge of EROs from the Groth strip and study the relation
between their morphology and mass. We find 102 EROs (F814W-K=>4, K<=21.0), over
a survey area of 155 arcmin^2. The photometric data include U,B,F606W,F814W,J,K
bands. Morphologies are based on a by eye classification and we distinguish
between 3 basic classes: compact objects, targets with a disc and/or a bulge
component and irregular or merger candidates. The majority of our targets has
either a very compact morphology (33+-6%), or show more or less distinct disc
components (41+-6%). 14+-4% are merger or irregulars and 7 objects could not be
classified. We also study the dependence of structural parameters on
morphological appearance. EROs that are either compact or show a distinct bulge
component have smaller median effective radii (1.22+-0.14 kpc and 3.31+-0.53
kpc) than disc dominated (5.50+-0.51 kpc) or possible irregular galaxies or
merger candidates (4.92+-0.14 kpc). The Sersic index changes from 2.30+-0.34
and 3.24+-0.55, to 1.03+-0.24 and 1.54+-0.40 respectively.
Most the EROs in our sample have redshifts between z=1 and z=2; however,
compact EROs in our sample are found at redshifts as low as z=0.4 and as high
as z=2.8; the latter qualify as well as DRGs. Disc-like EROs are also found up
to z=2.8; however those with a bulge-disc structure are only seen at z<1.5. For
each of these EROs we determined the stellar mass and mean population age by
fitting synthetic Bruzual (2007) spectra to the SED. Mass estimates were
obtained by assuming an exponentially declining star formation rate. Total
stellar masses are in the range 9.1<log(M/M_sun)<11.6. We cannot detect
significant differences between the stellar mass distribution of the
morphological classes. EROs with masses of log(M/M_sun)>11.0 dominantly show
compact morphologies, but also include a significant number of sources with a
disc morphology.Comment: 21 pages, 17 figures, accepted for publication in MNRA
On the scarcity of Magellanic Cloud-like satellites
We have used H alpha narrow-band imaging to search for star-forming satellite
galaxies around 143 luminous spiral galaxies, with the goal of quantifying the
frequency of occurrence of satellites resembling the Magellanic Clouds, around
galaxies comparable to the Milky Way. For two-thirds of the central galaxies,
no star-forming satellites are found, down to luminosities and star-formation
rates well below those of the Magellanic Clouds. A total of 62 satellites is
found, associated with 47 of the central galaxies searched. The R-band
magnitude difference between central galaxies and their satellites has a median
value of 4.6 mag, and a maximum of 10.2 mag. The mean projected separation of
the satellites from their central galaxies is 81 kpc, or 98 kpc for systems
beyond 30 Mpc. Thus star-forming satellites are quite rare, and the Milky Way
is unusual both for the luminosity and the proximity of its two brightest
satellites. We also find that the Clouds themselves are unusual in that they
appear to form a bound binary pair; such close satellite pairs, of any
luminosity, are also extremely rare in our survey.Comment: 11 pages, 10 figures, accepted for publication in MNRA
Galactic bulges from Hubble Space Telescope NICMOS observations: the lack of r^{1/4} bulges
We use HST near-infrared imaging to explore the shapes of the surface
brightness profiles of bulges of S0-Sbc galaxies at high resolution. Modeling
extends to the outer bulge via bulge-disk decompositions of combined HST -
ground based profiles. Compact, central unresolved components similar to those
reported by others are found in ~84% of the sample. We also detect a moderate
frequency (~34%) of nuclear components with exponential profiles which may be
disks or bars. Adopting the S\'ersic r^{1/n} functional form for the bulge,
none of the bulges have an r^{1/4} behaviour; derived S\'ersic shape-indices
are = 1.7 \pm 0.7. For the same sample, fits to NIR ground-based profiles
yield S\'ersic indices up to n = 4-6. The high- of ground-based profiles are
a result of nuclear point sources blending with the bulge extended light due to
seeing. The low S\'ersic indices are not expected from merger violent
relaxation, and argue against significant merger growth for most bulges.Comment: 4 pages, 3 figures, accepted for the Astrophysical Journal Letter
The Effects of Gas on Morphological Transformation in Mergers: Implications for Bulge and Disk Demographics
Transformation of disks into spheroids via mergers is a well-accepted element
of galaxy formation models. However, recent simulations have shown that bulge
formation is suppressed in increasingly gas-rich mergers. We investigate the
global implications of these results in a cosmological framework, using
independent approaches: empirical halo-occupation models (where galaxies are
populated in halos according to observations) and semi-analytic models. In
both, ignoring the effects of gas in mergers leads to the over-production of
spheroids: low and intermediate-mass galaxies are predicted to be
bulge-dominated (B/T~0.5 at <10^10 M_sun), with almost no bulgeless systems),
even if they have avoided major mergers. Including the different physical
behavior of gas in mergers immediately leads to a dramatic change: bulge
formation is suppressed in low-mass galaxies, observed to be gas-rich (giving
B/T~0.1 at <10^10 M_sun, with a number of bulgeless galaxies in good agreement
with observations). Simulations and analytic models which neglect the
similarity-breaking behavior of gas have difficulty reproducing the strong
observed morphology-mass relation. However, the observed dependence of gas
fractions on mass, combined with suppression of bulge formation in gas-rich
mergers, naturally leads to the observed trends. Discrepancies between
observations and models that ignore the role of gas increase with redshift; in
models that treat gas properly, galaxies are predicted to be less
bulge-dominated at high redshifts, in agreement with the observations. We
discuss implications for the global bulge mass density and future observational
tests.Comment: 14 pages, 11 figures, accepted to MNRAS (matched published version).
A routine to return the galaxy merger rates discussed here is available at
http://www.cfa.harvard.edu/~phopkins/Site/mergercalc.htm
Early type galaxies have been the predominant morphological class for massive galaxies since only z~1
Present-day massive galaxies are composed mostly of early-type objects. It is
unknown whether this was also the case at higher redshifts. In a hierarchical
assembling scenario the morphological content of the massive population is
expected to change with time from disk-like objects in the early Universe to
spheroid-like galaxies at present. In this paper we have probed this
theoretical expectation by compiling a large sample of massive
(M_{stellar}>10^{11} h_{70}^{-2} M_{Sun}$) galaxies in the redshift interval 0
< z < 3. Our sample of 1082 objects comprises 207 local galaxies selected from
SDSS plus 875 objects observed with the HST belonging to the POWIR/DEEP2 and
GNS surveys. 639 of our objects have spectroscopic redshifts. Our morphological
classification is performed as close as possible to the optical restframe
according to the photometric bands available in our observations both
quantitatively (using the Sersic index as a morphological proxy) and
qualitative (by visual inspection). Using both techniques we find an enormous
change on the dominant morphological class with cosmic time. The fraction of
early-type galaxies among the massive galaxy population has changed from
~20-30% at z~3 to ~70% at z=0. Early type galaxies have been the predominant
morphological class for massive galaxies since only z~1.Comment: 24 pages, 11 figures, 7 tables, MNRAS accepted, acknowledgement adde
The Rise and Fall of Passive Disk Galaxies: Morphological Evolution Along the Red Sequence Revealed by COSMOS
The increasing abundance of passive "red-sequence" galaxies since z=1-2 is
mirrored by a coincident rise in the number of galaxies with spheroidal
morphologies. In this paper, however, we show that in detail the correspondence
between galaxy morphology and color is not perfect, providing insight into the
physical origin of this evolution. Using the COSMOS survey, we study a
significant population of red sequence galaxies with disk-like morphologies.
These passive disks typically have Sa-Sb morphological types with large bulges,
but they are not confined to dense environments. They represent nearly one-half
of all red-sequence galaxies and dominate at lower masses (log Mstar < 10)
where they are increasingly disk-dominated. As a function of time, the
abundance of passive disks with log Mstar < 11 increases, but not as fast as
red-sequence spheroidals in the same mass range. At higher mass, the passive
disk population has declined since z~1, likely because they transform into
spheroidals. We estimate that as much as 60% of galaxies transitioning onto the
red sequence evolve through a passive disk phase. The origin of passive disks
therefore has broad implications for understanding how star formation shuts
down. Because passive disks tend to be more bulge-dominated than their
star-forming counterparts, a simple fading of blue disks does not fully explain
their origin. We explore several more sophisticated explanations, including
environmental effects, internal stabilization, and disk regrowth during
gas-rich mergers. While previous work has sought to explain color and
morphological transformations with a single process, these observations open
the way to new insight by highlighting the fact that galaxy evolution may
actually proceed through several separate stages.Comment: 16 pages, Accepted version to appear in Ap
Spectro-photometric close pairs in GOODS-S: major and minor companions of intermediate-mass galaxies
(Abriged) Our goal here is to provide merger frequencies that encompass both
major and minor mergers, derived from close pair statistics. We use B-band
luminosity- and mass-limited samples from an Spitzer/IRAC-selected catalogue of
GOODS-S. We present a new methodology for computing the number of close
companions, Nc, when spectroscopic redshift information is partial. We select
as close companions those galaxies separated by 6h^-1 kpc < rp < 21h^-1 kpc in
the sky plane and with a difference Delta_v <= 500 km s^-1 in redshift space.
We provide Nc for four different B-band-selected samples. Nc increases with
luminosity, and its evolution with redshift is faster in more luminous samples.
We provide Nc of M_star >= 10^10 M_Sun galaxies, finding that the number
including minor companions (mass ratio >= 1/10) is roughly two times the number
of major companions alone (mass ratio >= 1/3) in the range 0.2 <= z < 1.1. We
compare the major merger rate derived by close pairs with the one computed by
morphological criteria, finding that both approaches provide similar merger
rates for field galaxies when the progenitor bias is taken into account.
Finally, we estimate that the total (major+minor) merger rate is ~1.7 times the
major merger rate. Only 30% to 50% of the M_star >= 10^10 M_Sun early-type
(E/S0/Sa) galaxies that appear z=1 and z=0 may have undergone a major or a
minor merger. Half of the red sequence growth since z=1 is therefore unrelated
to mergers.Comment: Accepted in A&A. 14 pages, 6 figures, 8 tables. We have tested the
method with a local, volume-limited spectroscopic sample
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