1,502 research outputs found
A Strong-Lens Survey in AEGIS: the influence of large scale structure
We report on the results of a visual search for galaxy-scale strong
gravitational lenses over 650 arcmin^2 of HST/ACS imaging in the DEEP2-EGS
field. In addition to a previously-known Einstein Cross (the "Cross," HST
J141735+52264, with z_lens=0.8106 and a published z_source=3.40), we identify
two new strong galaxy-galaxy lenses with multiple extended arcs. The first, HST
J141820+52361 (the ``Dewdrop''; z_lens=0.5798, lenses two distinct extended
sources into two pairs of arcs z_source=0.while), 9818 the second, HST
J141833+52435 (the ``Anchor''; z_lens=0.4625), produces a single pair of arcs
(source redshift not yet known). All three definite lenses are fit well by
simple singular isothermal ellipsoid models including external shear. Using the
three-dimensional line-of-sight (LOS) information on galaxies from the DEEP2
data, we calculate the convergence and shear contributions, assuming singular
isothermal sphere halos truncated at 200 h^-1 kpc. These are also compared
against three-dimensional local-density estimates. We find that even strong
lenses in demonstrably underdense local environments may be considerably
affected by LOS contributions, which in turn, may be underestimates of the
effect of large scale structure.Comment: ApJ Letters, submitted. Part of the AEGIS ApJL Special Issue. 4
Figures, 1 Table. For a version with full-resolution figures, please see
http://www.slac.stanford.edu/~pjm/HAGGLeS/astroph/legs.pd
The role of AGN in the colour transformation of galaxies at redshifts z~1
We explore the role of AGN in establishing and/or maintaining the bimodal
colour distribution of galaxies by quenching their star-formation and hence,
causing their transition from the blue to the red cloud. Important tests for
this scenario include (i) the X-ray properties of galaxies in the transition
zone between the two clouds and (ii) the incidence of AGN in post-starbursts,
i.e. systems observed shortly after (<1Gyr) the termination of their
star-formation. We perform these tests by combining deep Chandra observations
with multiwavelength data from the AEGIS survey. Stacking the X-ray photons at
the positions of galaxies (0.4<z<0.9) not individually detected at X-ray
wavelengths suggests a population of obscured AGN among sources in the
transition zone and in the red cloud. Their mean X-ray and mid-IR properties
are consistent with moderately obscured low-luminosity AGN, Compton thick
sources or a mix of both. Morphologies show that major mergers are unlikely to
drive the evolution of this population but minor interactions may play a role.
The incidence of obscured AGN in the red cloud (both direct detections and
stacking results) suggests that BH accretion outlives the termination of the
star-formation. This is also supported by our finding that post-starburst
galaxies at z~0.8 and AGN are associated, in agreement with recent results at
low-z. A large fraction of post-starbursts and red cloud galaxies show evidence
for at least moderate levels of AGN obscuration. This implies that if AGN
outflows cause the colour transformation of galaxies, then some nuclear gas and
dust clouds either remain unaffected or relax to the central galaxy regions
after the quenching their star-formation.Comment: Accepted for publication in MNRA
Present day challenges in understanding the geomagnetic hazard to national power grids
Power grids and pipeline networks at all latitudes are known to be at risk from the natural hazard of geomagnetically induced currents. At a recent workshop in South Africa, UK and South African scientists and engineers discussed the current understanding of this hazard, as it affects major power systems in Europe and Africa. They also summarised, to better inform the public and industry, what can be said with some certainty about the hazard and what research is yet required to develop useful tools for geomagnetic hazard mitigation
AEGIS: Enhancement of Dust-enshrouded Star Formation in Close Galaxy Pairs and Merging Galaxies up to z ~ 1
Using data from the DEEP2 Galaxy Redshift Survey and HST/ACS imaging in the
Extended Groth Strip, we select nearly 100 interacting galaxy systems including
kinematic close pairs and morphologically identified merging galaxies. Spitzer
MIPS 24 micron fluxes of these systems reflect the current dusty star formation
activity, and at a fixed stellar mass (M_{*}) the median infrared luminosity
(L_{IR}) among merging galaxies and close pairs of blue galaxies is twice (1.9
+/- 0.4) that of control pairs drawn from isolated blue galaxies. Enhancement
declines with galaxy separation, being strongest in close pairs and mergers and
weaker in wide pairs compared to the control sample. At z ~ 0.9, 7.1% +/- 4.3%
of massive interacting galaxies (M_{*} > 2*10^{10} M_{solar}) are found to be
ULIRGs, compared to 2.6% +/- 0.7% in the control sample. The large spread of IR
luminosity to stellar mass ratio among interacting galaxies suggests that this
enhancement may depend on the merger stage as well as other as yet unidentified
factors (e.g., galaxy structure, mass ratio, orbital characteristics, presence
of AGN or bar). The contribution of interacting systems to the total IR
luminosity density is moderate (<= 36 %).Comment: 12 pages, 2 figures, 1 table, minor changes to match the proof
version, accepted for publication in the ApJL AEGIS Special Issu
Galaxy morphology and star formation in the Illustris Simulation at z = 0
We study how optical galaxy morphology depends on mass and star formation
rate (SFR) in the Illustris Simulation. To do so, we measure automated galaxy
structures in 10808 simulated galaxies at z=0 with stellar masses 10^9.7 <
M_*/M_sun < 10^12.3. We add observational realism to idealized synthetic images
and measure non-parametric statistics in rest-frame optical and near-IR images
from four directions. We find that Illustris creates a morphologically diverse
galaxy population, occupying the observed bulge strength locus and reproducing
median morphology trends versus stellar mass, SFR, and compactness. Morphology
correlates realistically with rotation, following classification schemes put
forth by kinematic surveys. Type fractions as a function of environment agree
roughly with data. These results imply that connections among mass, star
formation, and galaxy structure arise naturally from models matching global
star formation and halo occupation functions when simulated with accurate
methods. This raises a question of how to construct experiments on galaxy
surveys to better distinguish between models. We predict that at fixed halo
mass near 10^12 M_sun, disc-dominated galaxies have higher stellar mass than
bulge-dominated ones, a possible consequence of the Illustris feedback model.
While Illustris galaxies at M_* ~ 10^11 M_sun have a reasonable size
distribution, those at M_* ~ 10^10 M_sun have half-light radii larger than
observed by a factor of two. Furthermore, at M_* ~ 10^10.5-10^11 M_sun, a
relevant fraction of Illustris galaxies have distinct "ring-like" features,
such that the bright pixels have an unusually wide spatial extent
AEGIS: The Diversity of Bright Near-IR Selected Distant Red Galaxies
We use deep and wide near infrared (NIR) imaging from the Palomar telescope combined with DEEP2 spectroscopy and Hubble Space Telescope (HST) and Chandra Space Telescope imaging to investigate the nature of galaxies that are red in NIR colors. We locate these `distant red galaxies' (DRGs) through the color cut (J-K)_{vega} > 2.3 over 0.7 deg^{2}, where we find 1010 DRG candidates down to K_s = 20.5. We combine 95 high quality spectroscopic redshifts with photometric redshifts from BRIJK photometry to determine the redshift and stellar mass distributions for these systems, and morphological/structural and X-ray properties for 107 DRGs in the Extended Groth Strip. We find that many bright (J-K)_{vega}>2.3 galaxies with K_s2 systems massive with M_*>10^{11} M_solar. HST imaging shows that the structural properties and morphologies of DRGs are also diverse, with the majority elliptical/compact (57%), and the remainder edge-on spirals (7%), and peculiar galaxies (29%). The DRGs at z < 1.4 with high quality spectroscopic redshifts are generally compact, with small half light radii, and span a range in rest-frame optical properties. The spectral energy distributions for these objects differ from higher redshift DRGs: they are bluer by one magnitude in observed (I-J) color. A pure IR color selection of high redshift populations is not sufficient to identify unique populations, and other colors, or spectroscopic redshifts are needed to produce homogeneous samples
The Morphology of Passively Evolving Galaxies at Z approximately 2 from HST/WFC3 Deep Imaging in the Hubble Ultradeep Field
We present near-IR images of six passive galaxies (SSFR< 10(exp -2)/ Gyr) at redshift 1.3 < z < 2.4 with stellar mass M approximately 10(exp 11) solar M, selected from the Great Observatories Origins Deep Survey (GOODS), obtained with the Hubble Space Telescope (HST) and the WFC3/IR camera. These images provide the deepest and highest angular resolution view of the optical rest-frame morphology of such systems to date. We find that the light profile of these galaxies is generally regular and well described by a Sersic model with index typical of today's spheroids. We confirm the existence of compact and massive early-type galaxies at z approximately 2: four out of six galaxies have r(sub e) approximately 1 kpc or less. The images reach limiting surface brightness mu approximates 26.5 mag/square arcsec in the F160W bandpass; yet there is no evidence of a faint halo in the galaxies of our sample, even in their stacked image. We also find very weak "morphological k-correction" in the galaxies between the rest-frame UV (from the ACS z-band), and the rest-frame optical (WFC3 H-band): the visual classification, Sersic indices and physical sizes of these galaxies are independent or only mildly dependent on the wavelength, within the errors. The presence of an active nucleus is suspected in two out of six galaxies (33%), opening the intriguing possibility that a large, presently unaccounted population of AGN is hosted in these galaxies, possibly responsible for the cessation of star formation
Star Formation in AEGIS Field Galaxies since z=1.1 : The Dominance of Gradually Declining Star Formation, and the Main Sequence of Star-Forming Galaxies
We analyze star formation (SF) as a function of stellar mass (M*) and
redshift z in the All Wavelength Extended Groth Strip International Survey
(AEGIS). For 2905 field galaxies, complete to 10^10(10^10.8) Msun at z<0.7(1),
with Keck spectroscopic redshifts out to z=1.1, we compile SF rates (SFR) from
emission lines, GALEX, and Spitzer MIPS 24 micron photometry, optical-NIR M*
measurements, and HST morphologies. Galaxies with reliable signs of SF form a
distinct "main sequence (MS)", with a limited range of SFR at a given M* and z
(1 sigma < +-0.3 dex), and log(SFR) approximately proportional to log(M*). The
range of log(SFR) remains constant to z>1, while the MS as a whole moves to
higher SFR as z increases. The range of SFR along the MS constrains the
amplitude of episodic variations of SF, and the effect of mergers on SFR.
Typical galaxies spend ~67(95)% of their lifetime since z=1 within a factor of
<~ 2(4) of their average SFR at a given M* and z. The dominant mode of the
evolution of SF since z~1 is apparently a gradual decline of the average SFR in
most individual galaxies, not a decreasing frequency of starburst episodes, or
a decreasing factor by which SFR are enhanced in starbursts. LIRGs at z~1 seem
to mostly reflect the high SFR typical for massive galaxies at that epoch. The
smooth MS may reflect that the same set of few physical processes governs star
formation prior to additional quenching processes. A gradual process like gas
exhaustion may play a dominant role.Comment: 5 pages, 1 figure, emulateapj; ApJ Letters, accepted; AEGIS special
issue; proof-level corrections added; title change
MASSIV: Mass Assembly Survey with SINFONI in VVDS. V. The major merger rate of star-forming galaxies at 0.9 < z < 1.8 from IFS-based close pairs
We aim to measure the major merger rate of star-forming galaxies at 0.9 < z
<1.8, using close pairs identified from integral field spectroscopy (IFS). We
use the velocity field maps obtained with SINFONI/VLT on the MASSIV sample,
selected from the star-forming population in the VVDS. We identify physical
pairs of galaxies from the measurement of the relative velocity and the
projected separation (r_p) of the galaxies in the pair. Using the well
constrained selection function of the MASSIV sample we derive the gas-rich
major merger fraction (luminosity ratio mu = L_2/L_1 >= 1/4), and, using merger
time scales from cosmological simulations, the gas-rich major merger rate at a
mean redshift up to z = 1.54. We find a high gas-rich major merger fraction of
20.8+15.2-6.8 %, 20.1+8.0-5.1 % and 22.0+13.7-7.3 % for close pairs with r_p <=
20h^-1 kpc in redshift ranges z = [0.94, 1.06], [1.2, 1.5) and [1.5, 1.8),
respectively. This translates into a gas-rich major merger rate of
0.116+0.084-0.038 Gyr^-1, 0.147+0.058-0.037 Gyr^-1 and 0.127+0.079-0.042 Gyr^-1
at z = 1.03, 1.32 and 1.54, respectively. Combining our results with previous
studies at z < 1, the gas-rich major merger rate evolves as (1+z)^n, with n =
3.95 +- 0.12, up to z = 1.5. From these results we infer that ~35% of the
star-forming galaxies with stellar masses M = 10^10 - 10^10.5 M_Sun have
undergone a major merger since z ~ 1.5. We develop a simple model which shows
that, assuming that all gas-rich major mergers lead to early-type galaxies, the
combined effect of gas-rich and dry mergers is able to explain most of the
evolution in the number density of massive early-type galaxies since z ~ 1.5,
with our measured gas-rich merger rate accounting for about two-thirds of this
evolution.Comment: Published in Astronomy and Astrophysics, 24 pages, 30 figures, 2
tables. Appendix with the residual images from GALFIT added. Minor changes
with respect to the initial versio
A robust morphological classification of high-redshift galaxies using support vector machines on seeing limited images. II. Quantifying morphological k-correction in the COSMOS field at 1<z<2: Ks band vs. I band
We quantify the effects of \emph{morphological k-correction} at by
comparing morphologies measured in the K and I-bands in the COSMOS area.
Ks-band data have indeed the advantage of probing old stellar populations for
, enabling a determination of galaxy morphological types unaffected by
recent star formation. In paper I we presented a new non-parametric method to
quantify morphologies of galaxies on seeing limited images based on support
vector machines. Here we use this method to classify
selected galaxies in the COSMOS area observed with WIRCam at CFHT. The obtained
classification is used to investigate the redshift distributions and number
counts per morphological type up to and to compare to the results
obtained with HST/ACS in the I-band on the same objects from other works. We
associate to every galaxy with and a probability between 0 and
1 of being late-type or early-type. The classification is found to be reliable
up to . The mean probability is . It decreases with redshift
and with size, especially for the early-type population but remains above
. The classification is globally in good agreement with the one
obtained using HST/ACS for . Above , the I-band classification
tends to find less early-type galaxies than the Ks-band one by a factor
1.5 which might be a consequence of morphological k-correction effects.
We argue therefore that studies based on I-band HST/ACS classifications at
could be underestimating the elliptical population. [abridged]Comment: accepted for publication in A&A, updated with referee comments, 12
pages, 10 figure
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