76 research outputs found
On the Evolution of the Velocity-Mass-Size Relations of Disk-Dominated Galaxies over the Past 10 Billion Years
We study the evolution of the scaling relations between maximum circular
velocity, stellar mass and optical half-light radius of star-forming
disk-dominated galaxies in the context of LCDM-based galaxy formation models.
Using data from the literature combined with new data from the DEEP2 and AEGIS
surveys we show that there is a consistent observational and theoretical
picture for the evolution of these scaling relations from z\sim 2 to z=0. The
evolution of the observed stellar scaling relations is weaker than that of the
virial scaling relations of dark matter haloes, which can be reproduced, both
qualitatively and quantitatively, with a simple, cosmologically-motivated model
for disk evolution inside growing NFW dark matter haloes. In this model optical
half-light radii are smaller, both at fixed stellar mass and maximum circular
velocity, at higher redshifts. This model also predicts that the scaling
relations between baryonic quantities evolve even more weakly than the
corresponding stellar relations. We emphasize, though, that this weak evolution
does not imply that individual galaxies evolve weakly. On the contrary,
individual galaxies grow strongly in mass, size and velocity, but in such a way
that they move largely along the scaling relations. Finally, recent
observations have claimed surprisingly large sizes for a number of star-forming
disk galaxies at z \sim 2, which has caused some authors to suggest that high
redshift disk galaxies have abnormally high spin parameters. However, we argue
that the disk scale lengths in question have been systematically overestimated
by a factor \sim 2, and that there is an offset of a factor \sim 1.4 between
H\alpha sizes and optical sizes. Taking these effects into account, there is no
indication that star forming galaxies at high redshifts (z\sim 2) have
abnormally high spin parameters.Comment: 19 pages, 10 figures, accepted to MNRAS, minor changes to previous
versio
Disruption of Star Clusters in the Interacting Antennae Galaxies
We reexamine the age distribution of star clusters in the Antennae in the
context of N-body+hydrodynamical simulations of these interacting galaxies. All
of the simulations that account for the observed morphology and other
properties of the Antennae have star formation rates that vary relatively
slowly with time, by factors of only 1.3 - 2.5 in the past 10^8 yr. In
contrast, the observed age distribution of the clusters declines approximately
as a power law, dN/dt \propto t^{gamma} with gamma = -1.0, for ages 10^6 yr \la
t \la 10^9 yr. These two facts can only be reconciled if the clusters are
disrupted progressively for at least 10^8 yr and possibly 10^9 yr. When we
combine the simulated formation rates with a power-law model, f_surv \propto
t^{delta}, for the fraction of clusters that survive to each age t, we match
the observed age distribution with exponents in the range -0.9 \la delta \la
-0.6 (with a slightly different delta for each simulation). The similarity
between delta and gamma indicates that dN/dt is shaped mainly by the disruption
of clusters rather than variations in their formation rate. Thus, the situation
in the interacting Antennae resembles that in relatively quiescent galaxies
such as the Milky Way and the Magellanic Clouds.Comment: 9 pages, 5 figures, 1 table, accepted for publication in the
Astrophysical Journal, including revisions after referee repor
Oxford SWIFT IFS and multi-wavelength observations of the Eagle galaxy at z=0.77
The `Eagle' galaxy at a redshift of 0.77 is studied with the Oxford Short
Wavelength Integral Field Spectrograph (SWIFT) and multi-wavelength data from
the All-wavelength Extended Groth strip International Survey (AEGIS). It was
chosen from AEGIS because of the bright and extended emission in its slit
spectrum. Three dimensional kinematic maps of the Eagle reveal a gradient in
velocity dispersion which spans 35-75 +/- 10 km/s and a rotation velocity of 25
+/- 5 km/s uncorrected for inclination. Hubble Space Telescope images suggest
it is close to face-on. In comparison with galaxies from AEGIS at similar
redshifts, the Eagle is extremely bright and blue in the rest-frame optical,
highly star-forming, dominated by unobscured star-formation, and has a low
metallicity for its size. This is consistent with its selection. The Eagle is
likely undergoing a major merger and is caught in the early stage of a
star-burst when it has not yet experienced metal enrichment or formed the mass
of dust typically found in star-forming galaxies.Comment: accepted for publication in MNRA
The Epoch of Disk Settling: z~1 to Now
We present evidence from a sample of 544 galaxies from the DEEP2 Survey for
evolution of the internal kinematics of blue galaxies with stellar masses
ranging 8.0 < log M* (M_Sun) < 10.7 over 0.2<z<1.2. DEEP2 provides galaxy
spectra and Hubble imaging from which we measure emission-line kinematics and
galaxy inclinations, respectively. Our large sample allows us to overcome
scatter intrinsic to galaxy properties in order to examine trends in
kinematics. We find that at a fixed stellar mass galaxies systematically
decrease in disordered motions and increase in rotation velocity and potential
well depth with time. Massive galaxies are the most well-ordered at all times
examined, with higher rotation velocities and less disordered motions than less
massive galaxies. We quantify disordered motions with an integrated gas
velocity dispersion corrected for beam smearing (sigma_g). It is unlike the
typical pressure-supported velocity dispersion measured for early type galaxies
and galaxy bulges. Because both seeing and the width of our spectral slits
comprise a significant fraction of the galaxy sizes, sigma_g integrates over
velocity gradients on large scales which can correspond to non-ordered gas
kinematics. We compile measurements of galaxy kinematics from the literature
over 1.2<z<3.8 and do not find any trends with redshift, likely for the most
part because these datasets are biased toward the most highly star-forming
systems. In summary, over the last ~8 billion years since z=1.2, blue galaxies
evolve from disordered to ordered systems as they settle to become the
rotation-dominated disk galaxies observed in the Universe today, with the most
massive galaxies being the most evolved at any time.Comment: submitted to ApJ and responded to referee repor
The Radius of Baryonic Collapse in Disc Galaxy Formation
In the standard picture of disc galaxy formation, baryons and dark matter
receive the same tidal torques, and therefore approximately the same initial
specific angular momentum. However, observations indicate that disc galaxies
typically have only about half as much specific angular momentum as their dark
matter haloes. We argue this does not necessarily imply that baryons lose this
much specific angular momentum as they form galaxies. It may instead indicate
that galaxies are most directly related to the inner regions of their host
haloes, as may be expected in a scenario where baryons in the inner parts of
haloes collapse first. A limiting case is examined under the idealised
assumption of perfect angular momentum conservation. Namely, we determine the
density contrast Delta, with respect to the critical density of the Universe,
by which dark matter haloes need to be defined in order to have the same
average specific angular momentum as the galaxies they host. Under the
assumption that galaxies are related to haloes via their characteristic
rotation velocities, the necessary Delta is ~600. This Delta corresponds to an
average halo radius and mass which are ~60% and ~75%, respectively, of the
virial values (i.e., for Delta = 200). We refer to this radius as the radius of
baryonic collapse R_BC, since if specific angular momentum is conserved
perfectly, baryons would come from within it. It is not likely a simple step
function due to the complex gastrophysics involved, therefore we regard it as
an effective radius. In summary, the difference between the predicted initial
and the observed final specific angular momentum of galaxies, which is
conventionally attributed solely to angular momentum loss, can more naturally
be explained by a preference for collapse of baryons within R_BC, with possibly
some later angular momentum transfer.Comment: MNRAS accepted, 7 page
The DEEP2 Galaxy Redshift Survey: Design, Observations, Data Reduction, and Redshifts
We describe the design and data sample from the DEEP2 Galaxy Redshift Survey,
the densest and largest precision-redshift survey of galaxies at z ~ 1
completed to date. The survey has conducted a comprehensive census of massive
galaxies, their properties, environments, and large-scale structure down to
absolute magnitude M_B = -20 at z ~ 1 via ~90 nights of observation on the
DEIMOS spectrograph at Keck Observatory. DEEP2 covers an area of 2.8 deg^2
divided into four separate fields, observed to a limiting apparent magnitude of
R_AB=24.1. Objects with z < 0.7 are rejected based on BRI photometry in three
of the four DEEP2 fields, allowing galaxies with z > 0.7 to be targeted ~2.5
times more efficiently than in a purely magnitude-limited sample. Approximately
sixty percent of eligible targets are chosen for spectroscopy, yielding nearly
53,000 spectra and more than 38,000 reliable redshift measurements. Most of the
targets which fail to yield secure redshifts are blue objects that lie beyond z
~ 1.45. The DEIMOS 1200-line/mm grating used for the survey delivers high
spectral resolution (R~6000), accurate and secure redshifts, and unique
internal kinematic information. Extensive ancillary data are available in the
DEEP2 fields, particularly in the Extended Groth Strip, which has evolved into
one of the richest multiwavelength regions on the sky. DEEP2 surpasses other
deep precision-redshift surveys at z ~ 1 in terms of galaxy numbers, redshift
accuracy, sample number density, and amount of spectral information. We also
provide an overview of the scientific highlights of the DEEP2 survey thus far.
This paper is intended as a handbook for users of the DEEP2 Data Release 4,
which includes all DEEP2 spectra and redshifts, as well as for the
publicly-available DEEP2 DEIMOS data reduction pipelines. [Abridged]Comment: submitted to ApJS; data products available for download at
http://deep.berkeley.edu/DR4
The Tully-Fisher relations of early-type spiral and S0 galaxies
We demonstrate that the comparison of Tully-Fisher relations (TFRs) derived
from global HI line widths to TFRs derived from the circular velocity profiles
of dynamical models (or stellar kinematic observations corrected for asymmetric
drift) is vulnerable to systematic and uncertain biases introduced by the
different measures of rotation used. We therefore argue that to constrain the
relative locations of the TFRs of spiral and S0 galaxies, the same tracer and
measure must be used for both samples. Using detailed near-infrared imaging and
the circular velocities of axisymmetric Jeans models of 14 nearby edge-on Sa-Sb
spirals and 14 nearby edge-on S0s drawn from a range of environments, we find
that S0s lie on a TFR with the same slope as the spirals, but are on average
0.53+/-0.15 mag fainter at Ks-band at a given rotational velocity. This is a
significantly smaller offset than that measured in earlier studies of the S0
TFR, which we attribute to our elimination of the bias associated with using
different rotation measures and our use of earlier type spirals as a reference.
Since our measurement of the offset avoids systematic biases, it should be
preferred to previous estimates. A spiral stellar population in which star
formation is truncated would take ~1 Gyr to fade by 0.53 mag at Ks-band. If S0s
are the products of a simple truncation of star formation in spirals, then this
finding is difficult to reconcile with the observed evolution of the spiral/S0
fraction with redshift. Recent star formation could explain the observed lack
of fading in S0s, but the offset of the S0 TFR persists as a function of both
stellar and dynamical mass. We show that the offset of the S0 TFR could
therefore be explained by a systematic difference between the total mass
distributions of S0s and spirals, in the sense that S0s need to be smaller or
more concentrated than spirals.Comment: Accepted for publication in MNRAS; 17 pages; v2 incorporates minor
proof corrections and updated reference
Anatomy of a post-starburst minor merger: a multi-wavelength WFC3 study of NGC 4150
(Abridged) We present a spatially-resolved near-UV/optical study of NGC 4150,
using the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope.
Previous studies of this early-type galaxy (ETG) indicate that it has a large
reservoir of molecular gas, exhibits a kinematically decoupled core (likely
indication of recent merging) and strong, central H_B absorption (indicative of
young stars). The core of NGC 4150 shows ubiquitous near-UV emission and
remarkable dusty substructure. Our analysis shows this galaxy to lie in the
near-UV green valley, and its pixel-by-pixel photometry exhibits a narrow range
of near-UV/optical colours that are similar to those of nearby E+A
(post-starburst) galaxies. We parametrise the properties of the recent star
formation (age, mass fraction, metallicity and internal dust content) in the
NGC 4150 pixels by comparing the observed near-UV/optical photometry to stellar
models. The typical age of the recent star formation (RSF) is around 0.9 Gyrs,
consistent with the similarity of the near-UV colours to post-starburst
systems, while the morphological structure of the young component supports the
proposed merger scenario. The RSF metallicity, representative of the
metallicity of the gas fuelling star formation, is around 0.3 - 0.5 Zsun.
Assuming that this galaxy is a merger and that the gas is sourced mainly from
the infalling companion, these metallicities plausibly indicate the gas-phase
metallicity (GPM) of the accreted satellite. Comparison to the local mass-GPM
relation suggests (crudely) that the mass of the accreted system is around
3x10^8 Msun, making NGC 4150 a 1:20 minor merger. A summation of the pixel RSF
mass fractions indicates that the RSF contributes about 2-3 percent of the
stellar mass. This work reaffirms our hypothesis that minor mergers play a
significant role in the evolution of ETGs at late epochs.Comment: 28 pages, 2 tables, accepted for publication in Ap
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