614 research outputs found
A new extensive catalog of optically variable AGN in the GOODS Fields and a new statistical approach to variability selection
Variability is a property shared by practically all AGN. This makes
variability selection a possible technique for identifying AGN. Given that
variability selection makes no prior assumption about spectral properties, it
is a powerful technique for detecting both low-luminosity AGN in which the host
galaxy emission is dominating and AGN with unusual spectral properties. In this
paper, we will discuss and test different statistical methods for the detection
of variability in sparsely sampled data that allow full control over the false
positive rates. We will apply these methods to the GOODS North and South fields
and present a catalog of variable sources in the z band in both GOODS fields.
Out of 11931 objects checked, we find 155 variable sources at a significance
level of 99.9%, corresponding to about 1.3% of all objects. After rejection of
stars and supernovae, 139 variability selected AGN remain. Their magnitudes
reach down as faint as 25.5 mag in z. Spectroscopic redshifts are available for
22 of the variability selected AGN, ranging from 0.046 to 3.7. The absolute
magnitudes in the rest-frame z-band range from ~ -18 to -24, reaching
substantially fainter than the typical luminosities probed by traditional X-ray
and spectroscopic AGN selection in these fields. Therefore, this is a powerful
technique for future exploration of the evolution of the faint end of the AGN
luminosity function up to high redshifts.Comment: Accepted for publication in The Astrophysical Journal (version 2:
minor changes to text after receiving comments
A New Stellar Atmosphere Grid and Comparisons with HST/STIS Calspec Flux Distributions
The Space Telescope Imaging Spectrograph (STIS) has measured the spectral
energy distributions (SEDs) for several stars of types O, B, A, F, and G. These
absolute fluxes from the CALSPEC database are fit with a new spectral grid
computed from the ATLAS-APOGEE ATLAS9 model atmosphere database using a
chi-square minimization technique in four parameters. The quality of the fits
are compared for complete LTE grids by Castelli & Kurucz (CK04) and our new
comprehensive LTE grid (BOSZ). For the cooler stars, the fits with the MARCS
LTE grid are also evaluated, while the hottest stars are also fit with the NLTE
Lanz & Hubeny OB star grids. Unfortunately, these NLTE models do not transition
smoothly in the infrared to agree with our new BOSZ LTE grid at the NLTE lower
limit of Teff =15,000K.
The new BOSZ grid is available via the Space Telescope Institute MAST archive
and has a much finer sampled IR wavelength scale than CK04, which will
facilitate the modeling of stars observed by the James Webb Space Telescope
(JWST). Our result for the angular diameter of Sirius agrees with the ground-
based interferometric value.Comment: 11 figure
Observational evidence of a slow downfall of star formation efficiency in massive galaxies during the last 10 Gyr
In this paper we study the causes of the reported mass-dependence of the
slope of SFR-M* relation, the so-called "Main Sequence" of star-forming
galaxies, and discuss its implication on the physical processes that shaped the
star formation history of massive galaxies over cosmic time. We use the CANDELS
near-IR imaging from the Hubble Space Telescope to perform the bulge-to-disk
decomposition of distant galaxies and measure for the first time the slope of
the SFR-Mdisk relation at z=1. We find that this relation follows very closely
the shape of the SFR-M* correlation, still with a pronounced flattening at the
high-mass end. This is clearly excluding, at least at z=1, the secular growth
of quiescent bulges in star-forming galaxies as the main driver for the change
of slope of the Main Sequence. Then, by stacking the Herschel data available in
the CANDELS field, we estimate the total gas mass and the star formation
efficiency at different positions on the SFR-M* relation. We find that the
relatively low SFRs observed in massive galaxies (M* > 5e10 Msun) are caused by
a decreased star formation efficiency, by up to a factor of 3 as compared to
lower stellar mass galaxies, and not by a reduced gas content. The trend at the
lowest masses is likely linked to the dominance of atomic over molecular gas.
We argue that this stellar-mass-dependent SFE can explain the varying slope of
the Main Sequence since z=1.5, hence over 70% of the Hubble time. The drop of
SFE occurs at lower masses in the local Universe (M* > 2e10 Msun) and is not
present at z=2. Altogether this provides evidence for a slow downfall of the
star formation efficiency in massive Main Sequence galaxies. The resulting loss
of star formation is found to be rising starting from z=2 to reach a level
comparable to the mass growth of the quiescent population by z=1. We finally
discuss the possible physical origin of this phenomenon.Comment: 21 pages, 15 figures, accepted for publication in A&
MC: Subaru and Hubble Space Telescope Weak-Lensing Analysis of the Double Radio Relic Galaxy Cluster PLCK G287.0+32.9
The second most significant detection of the Planck Sunyaev Zel'dovich
survey, PLCK~G287.0+32.9 () boasts two similarly bright radio relics
and a radio halo. One radio relic is located kpc northwest of the
X-ray peak and the other Mpc to the southeast. This large difference
suggests that a complex merging scenario is required. A key missing puzzle for
the merging scenario reconstruction is the underlying dark matter distribution
in high resolution. We present a joint Subaru Telescope and {\it Hubble Space
Telescope} weak-lensing analysis of the cluster. Our analysis shows that the
mass distribution features four significant substructures. Of the
substructures, a primary cluster of mass
$M_{200\text{c}}=1.59^{+0.25}_{-0.22}\times 10^{15} \ h^{-1}_{70} \
\text{M}_{\odot}M_{200\text{c}}=1.16^{+0.15}_{-0.13}\times 10^{14} \ h^{-1}_{70} \
\text{M}_{\odot}\sim 400\sim 2M_{200\text{c}}=1.68^{+0.22}_{-0.20}\times
10^{14} \ h^{-1}_{70} \ \text{M}_{\odot}M_{200\text{c}}=1.87^{+0.24}_{-0.22}\times 10^{14} \ h^{-1}_{70} \
\text{M}_{\odot}$, is northwest of the X-ray peak and beyond the NW radio
relic.Comment: 19 pages, 14 figures; Accepted to Ap
CIR v Niko : a question of economic reality
This article analyses the facts and judgment in CIR v Niko, involving the
transfer of business assets from a sole trader to a company, the shares of
which were substantially owned by the same sole trader. This case changed
the inherently fl awed, but prevailing practice at that stage of regarding a
lump-sum payment from a lock-stock-and-barrel sale of a business as a
receipt of a capital nature, to a receipt that needed to be allocated to the
various assets included in the sale, and therefore potentially the receipt
would be partly of a capital and partly of a revenue nature. Although the
conclusion relating to lock-stock-and-barrel sales in general was sound,
the submission made in this article is that, in the particular circumstances
of the case, the economic reality of the transaction was not considered
– virtually no economic gain was realised by J. Niko, the seller and sole
owner of the business to a company of which he was also the substantial
shareholder. Two subsequent court decisions, which similarly ignored
the economic reality of the transactions in the context of a group of
companies, followed this judgment. In this article, the problematic nature
of the decisions that ignored the economic reality of the transactions
is demonstrated with reference to accepted canons of a good taxation
system. The article also explains the partial legislative relief that has
subsequently been granted for transfers of assets from a person to a
company and for transfers within a group of companies, but concludes
that there is a need for full recognition of a group of companies as an
economic entity for tax purposes.http://www.unisa.ac.za/default.asp?Cmd=ViewContent&ContentID=22335am201
Kpc-scale Properties of Emission-line Galaxies
We perform a detailed study of the resolved properties of emission-line
galaxies at kpc-scale to investigate how small-scale and global properties of
galaxies are related. 119 galaxies with high-resolution Keck/DEIMOS spectra are
selected to cover a wide range in morphologies over the redshift range
0.2<z<1.3. Using the HST/ACS and HST/WFC3 imaging data taken as a part of the
CANDELS project, for each galaxy we perform SED fitting per resolution element,
producing resolved rest-frame U-V color, stellar mass, star formation rate, age
and extinction maps. We develop a technique to identify blue and red "regions"
within individual galaxies, using their rest-frame color maps. As expected, for
any given galaxy, the red regions are found to have higher stellar mass surface
densities and older ages compared to the blue regions. Furthermore, we quantify
the spatial distribution of red and blue regions with respect to both redshift
and stellar mass, finding that the stronger concentration of red regions toward
the centers of galaxies is not a significant function of either redshift or
stellar mass. We find that the "main sequence" of star forming galaxies exists
among both red and blue regions inside galaxies, with the median of blue
regions forming a tighter relation with a slope of 1.1+/-0.1 and a scatter of
~0.2 dex compared to red regions with a slope of 1.3+/-0.1 and a scatter of
~0.6 dex. The blue regions show higher specific Star Formation Rates (sSFR)
than their red counterparts with the sSFR decreasing since z~1, driver
primarily by the stellar mass surface densities rather than the SFRs at a giver
resolution element.Comment: 17 pages, 17 figures, Submitted to the Ap
Pixel-z: Studying Substructure and Stellar Populations in Galaxies out to z~3 using Pixel Colors I. Systematics
We perform a pixel-by-pixel analysis of 467 galaxies in the GOODS-VIMOS
survey to study systematic effects in extracting properties of stellar
populations (age, dust, metallicity and SFR) from pixel colors using the
pixel-z method. The systematics studied include the effect of the input stellar
population synthesis model, passband limitations and differences between
individual SED fits to pixels and global SED-fitting to a galaxy's colors. We
find that with optical-only colors, the systematic errors due to differences
among the models are well constrained. The largest impact on the age and SFR
e-folding time estimates in the pixels arises from differences between the
Maraston models and the Bruzual&Charlot models, when optical colors are used.
This results in systematic differences larger than the 2{\sigma} uncertainties
in over 10 percent of all pixels in the galaxy sample. The effect of
restricting the available passbands is more severe. In 26 percent of pixels in
the full sample, passband limitations result in systematic biases in the age
estimates which are larger than the 2{\sigma} uncertainties. Systematic effects
from model differences are reexamined using Near-IR colors for a subsample of
46 galaxies in the GOODS-NICMOS survey. For z > 1, the observed optical/NIR
colors span the rest frame UV-optical SED, and the use of different models does
not significantly bias the estimates of the stellar population parameters
compared to using optical-only colors. We then illustrate how pixel-z can be
applied robustly to make detailed studies of substructure in high redshift
galaxies such as (a) radial gradients of age, SFR, sSFR and dust and (b) the
distribution of these properties within subcomponents such as spiral arms and
clumps. Finally, we show preliminary results from the CANDELS survey
illustrating how the new HST/WFC3 data can be exploited to probe substructure
in z~1-3 galaxies.Comment: 37 pages, 21 figures, submitted to Ap
Modeling and Correcting the Time-Dependent ACS PSF
The ability to accurately measure the shapes of faint objects in images taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST) depends upon detailed knowledge of the Point Spread Function (PSF). We show that thermal fluctuations cause the PSF of the ACS Wide Field Camera (WFC) to vary over time. We describe a modified version of the TinyTim PSF modeling software to create artificial grids of stars across the ACS field of view at a range of telescope focus values. These models closely resemble the stars in real ACS images. Using 10 bright stars in a real image, we have been able to measure HST s apparent focus at the time of the exposure. TinyTim can then be used to model the PSF at any position on the ACS field of view. This obviates the need for images of dense stellar fields at different focus values, or interpolation between the few observed stars. We show that residual differences between our TinyTim models and real data are likely due to the effects of Charge Transfer Efficiency (CTE) degradation. Furthermore, we discuss stochastic noise that is added to the shape of point sources when distortion is removed, and we present MultiDrizzle parameters that are optimal for weak lensing science. Specifically, we find that reducing the MultiDrizzle output pixel scale and choosing a Gaussian kernel significantly stabilizes the resulting PSF after image combination, while still eliminating cosmic rays/bad pixels, and correcting the large geometric distortion in the ACS. We discuss future plans, which include more detailed study of the effects of CTE degradation on object shapes and releasing our TinyTim models to the astronomical community
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