1,219 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
Analysis of the SFR - M* plane at z<3: single fitting versus multi-Gaussian decomposition
The analysis of galaxies on the star formation rate - stellar mass (SFR-M*)
plane is a powerful diagnostic for galaxy evolution at different cosmic times.
We consider a sample of 24463 galaxies from the CANDELS/GOODS-S survey to
conduct a detailed analysis of the SFR-M* relation at redshifts 0.53 over more than three dex in stellar mass. To obtain SFR estimates, we
utilise mid- and far-IR photometry when available, and rest-UV fluxes for all
the other galaxies. We perform our analysis in different redshift bins, with
two different methods: 1) a linear regression fitting of all star-forming
galaxies, defined as those with specific star formation rates , similarly to what is typically done in the
literature; 2) a multi-Gaussian decomposition to identify the galaxy main
sequence (MS), the starburst sequence and the quenched galaxy cloud. We find
that the MS slope becomes flatter when higher stellar mass cuts are adopted,
and that the apparent slope change observed at high masses depends on the SFR
estimation method. In addition, the multi-Gaussian decomposition reveals the
presence of a starburst population which increases towards low stellar masses
and high redshifts. We find that starbursts make up ~5% of all galaxies at
z=0.5-1.0, while they account for ~16% of galaxies at 23 with
log8.25-11.25. We conclude that the dissection of the SFR-M* in
multiple components over a wide range of stellar masses is necessary to
understand the importance of the different modes of star formation through
cosmic time.Comment: 15 pages, 12 figures, 1 table. Accepted for publication in A&A, after
addressing referee report. Main changes with respect to v1: two new
appendixes to investigate the impact of redshift outliers and to test a
two-Gaussian component fit to the sSFR distribution. No conclusion change
Gemini Planet Imager Observational Calibrations II: Detector Performance and Calibration
The Gemini Planet Imager is a newly commissioned facility instrument designed
to measure the near-infrared spectra of young extrasolar planets in the solar
neighborhood and obtain imaging polarimetry of circumstellar disks. GPI's
science instrument is an integral field spectrograph that utilizes a HAWAII-2RG
detector with a SIDECAR ASIC readout system. This paper describes the detector
characterization and calibrations performed by the GPI Data Reduction Pipeline
to compensate for effects including bad/hot/cold pixels, persistence,
non-linearity, vibration induced microphonics and correlated read noise.Comment: 11 pages, 6 figures. Proceedings of the SPIE, 9147-28
Survey design for Spectral Energy Distribution fitting: a Fisher Matrix approach
The spectral energy distribution (SED) of a galaxy contains information on
the galaxy's physical properties, and multi-wavelength observations are needed
in order to measure these properties via SED fitting. In planning these
surveys, optimization of the resources is essential. The Fisher Matrix
formalism can be used to quickly determine the best possible experimental setup
to achieve the desired constraints on the SED fitting parameters. However,
because it relies on the assumption of a Gaussian likelihood function, it is in
general less accurate than other slower techniques that reconstruct the
probability distribution function (PDF) from the direct comparison between
models and data. We compare the uncertainties on SED fitting parameters
predicted by the Fisher Matrix to the ones obtained using the more thorough PDF
fitting techniques. We use both simulated spectra and real data, and consider a
large variety of target galaxies differing in redshift, mass, age, star
formation history, dust content, and wavelength coverage. We find that the
uncertainties reported by the two methods agree within a factor of two in the
vast majority (~ 90%) of cases. If the age determination is uncertain, the
top-hat prior in age used in PDF fitting to prevent each galaxy from being
older than the Universe needs to be incorporated in the Fisher Matrix, at least
approximately, before the two methods can be properly compared. We conclude
that the Fisher Matrix is a useful tool for astronomical survey design.Comment: Accepted by ApJ; online Fisher Matrix tool available at
http://galfish.physics.rutgers.ed
Star formation and quenching among the most massive galaxies at z~1.7
We have conducted a detailed object-by-object study of a mass-complete
(M*>10^11 M_sun) sample of 56 galaxies at 1.4 < z < 2 in the GOODS-South field,
showing that an accurate de-blending in MIPS/24um images is essential to
properly assign to each galaxy its own star formation rate (SFR), whereas an
automatic procedure often fails. This applies especially to galaxies with SFRs
below the Main Sequence (MS) value, which may be in their quenching phase.
After that, the sample splits evenly between galaxies forming stars within a
factor of 4 of the MS rate (~45%), and sub-MS galaxies with SFRs ~10-1000 times
smaller (~55%). We did not find a well defined class of intermediate, transient
objects below the MS, suggesting that the conversion of a massive MS galaxy
into a quenched remnant may take a relatively short time (<1 Gyr), though a
larger sample should be analyzed in the same way to set precise limits on the
quenching timescale. X-ray detected AGNs represent a ~30% fraction of the
sample, and are found among both star-forming and quenched galaxies. The
morphological analysis revealed that ~50% of our massive objects are
bulge-dominated, and almost all MS galaxies with a relevant bulge component
host an AGN. We also found sub-MS SFRs in many bulge-dominated systems,
providing support to the notion that bulge growth, AGN activity and quenching
of star formation are closely related to each other.Comment: 27 pages, 19 figures, accepted for publication by MNRA
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