168 research outputs found
A MMT/Hectospec Redshift Survey of 24 Micron Sources in the Spitzer First Look Survey
We present a spectroscopic survey using the MMT/Hectospec fiber spectrograph
of 24 micron sources selected with the Spitzer Space Telescope in the Spitzer
First Look Survey. We report 1296 new redshifts for 24 micron sources,
including 599 with f(24micron) > 1 mJy. Combined with 291 additional redshifts
for sources from the Sloan Digital Sky Survey (SDSS), our observing program was
highly efficient and is ~90% complete for i' 1 mJy,
and is 35% complete for i' < 20.5 mag and 0.3 mJy < f(24micron) < 1 mJy. Our
Hectospec survey includes 1078 and 168 objects spectroscopically classified as
galaxies and QSOs, respectively. Combining the Hectospec and SDSS samples, we
find 24 micron-selected galaxies to z < 0.98 and QSOs to z < 3.6, with mean
redshifts of = 0.27 and =1.1. As part of this publication, we
include the redshift catalogs and the reduced spectra; these are also available
online (http://mips.as.arizona.edu/~papovich/fls) and through the NASA/IPAC
Infrared Science Archive (http://irsa.ipac.caltech.edu).Comment: Accepted for publication in the Astronomical Journal, AASTEX format,
23 pages, 7 figures (some in color). This replacement is the accepted
version, and includes minor changes from previous version. Data tables and
spectra available at http://mips.as.arizona.edu/~papovich/fls or at
http://irsa.ipac.caltech.ed
Automated reliability assessment for spectroscopic redshift measurements
We present a new approach to automate the spectroscopic redshift reliability
assessment based on machine learning (ML) and characteristics of the redshift
probability density function (PDF).
We propose to rephrase the spectroscopic redshift estimation into a Bayesian
framework, in order to incorporate all sources of information and uncertainties
related to the redshift estimation process, and produce a redshift posterior
PDF that will be the starting-point for ML algorithms to provide an automated
assessment of a redshift reliability.
As a use case, public data from the VIMOS VLT Deep Survey is exploited to
present and test this new methodology. We first tried to reproduce the existing
reliability flags using supervised classification to describe different types
of redshift PDFs, but due to the subjective definition of these flags, soon
opted for a new homogeneous partitioning of the data into distinct clusters via
unsupervised classification. After assessing the accuracy of the new clusters
via resubstitution and test predictions, unlabelled data from preliminary mock
simulations for the Euclid space mission are projected into this mapping to
predict their redshift reliability labels.Comment: Submitted on 02 June 2017 (v1). Revised on 08 September 2017 (v2).
Latest version 28 September 2017 (this version v3
Average Infrared Galaxy Spectra From Spitzer Flux Limited Samples
The mid-infrared spectroscopic analysis of a flux-limited sample of galaxies
with fv(24um) > 10 mJy is presented. Sources observed are taken from the
Spitzer First Look Survey (FLS) catalog and from the NOAO Deep Wide-Field
Survey region in Bootes (NDWFS). The spectroscopic sample includes 60 of the
100 sources in these combined catalogs having fv(24um) > 10 mJy. New spectra
from the Spitzer Infrared Spectrograph are presented for 25 FLS sources and for
11 Bootes AGN; these are combined with 24 Bootes starburst galaxies previously
published to determine the distribution of mid-infrared spectral
characteristics for the total 10 mJy sample. Sources have 0.01 < z < 2.4 and
41.8 < log vLv(15um) < 46.2 (ergs/s). Average spectra are determined as a
function of luminosity; lower luminosity sources (log vLv(15um) < 44.0) are
dominated by PAH features and higher luminosity sources (log vLv(15um) > 44.0)
are dominated by silicate absorption or emission. We find that a rest frame
equivalent width of 0.4um for the 6.2um PAH emission feature provides a well
defined division between lower luminosity, "pure" starbursts and higher
luminosity AGN or composite sources. Using the average spectra, fluxes fv(24um)
which would be observed with the Spitzer MIPS are predicted as a function of
redshift for sources with luminosities that correspond to the average spectra.
AGN identical to those in this 10 mJy sample could be seen to z = 3 with
fv(24um) > 1 mJy, but starbursts fall to fv(24um) < 1 mJy by z ~ 0.5. This
indicates that substantial luminosity evolution of starbursts is required to
explain the numerous starbursts found in other IRS results having fv(24um) > 1
mJy and z ~ 2.Comment: Accepted for publication in The Astrophysical Journa
Redshift Distribution of Extragalactic 24 micron Sources
We present the redshift distribution of a complete, unbiased sample of 24
micron sources down to fnu(24 micron) = 300 uJy (5-sigma). The sample consists
of 591 sources detected in the Bootes field of the NOAO Deep Wide-Field Survey.
We have obtained optical spectroscopic redshifts for 421 sources (71%). These
have a redshift distribution peaking at z~0.3, with a possible additional peak
at z~0.9, and objects detected out to z=4.5. The spectra of the remaining 170
(29%) exhibit no strong emission lines from which to determine a redshift. We
develop an algorithm to estimate the redshift distribution of these sources,
based on the assumption that they have emission lines but that these lines are
not observable due to the limited wavelength coverage of our spectroscopic
observations. The redshift distribution derived from all 591 sources exhibits
an additional peak of extremely luminous (L(8-1000 micron) > 3 x 10^{12} Lsun)
objects at z~2, consisting primarily of sources without observable emission
lines. We use optical line diagnostics and IRAC colors to estimate that 55% of
the sources within this peak are AGN-dominated. We compare our results to
published models of the evolution of infrared luminous galaxies. The models
which best reproduce our observations predict a large population of
star-formation dominated ULIRGs at z > 1.5 rather than the AGN-dominated
sources we observe.Comment: 21 pages, 11 figures; accepted for publication in Ap
The Mid-IR and X-ray Selected QSO Luminosity Function
We present the J-band luminosity function of 1838 mid-infrared and X-ray
selected AGNs in the redshift range 0<z<5.85. These luminosity functions are
constructed by combining the deep multi-wavelength broad-band observations from
the UV to the mid-IR of the NDWFS Bootes field with the X-ray observations of
the XBootes survey and the spectroscopic observations of the same field by
AGES. Our sample is primarily composed of IRAC-selected AGNs, targeted using
modifications of the Stern et al.(2005) criteria, complemented by MIPS 24
microns and X-ray selected AGNs to alleviate the biases of IRAC mid-IR
selection against z~4.5 quasars and AGNs faint with respect to their hosts.
This sample provides an accurate link between low and high redshift AGN
luminosity functions and does not suffer from the usual incompleteness of
optical samples at z~3. We find that the space density of the brightest quasars
strongly decreases from z=3 to z=0, while the space density of faint quasars is
at least flat, and possibly increasing, over the same redshift range. At z>3 we
observe a decrease in the space density of quasars of all brightnesses. We
model the luminosity function by a double power-law and find that its evolution
cannot be described by either pure luminosity or pure density evolution, but
must be a combination of both. Our best-fit model has bright and faint
power-law indices consistent with the low redshift measurements based on the
2QZ and 2SLAQ surveys and it generally agrees with the number of bright quasars
predicted by other LFs at all redshifts. If we construct the QSO luminosity
function using only the IRAC-selected AGNs, we find that the biases inherent to
this selection method significantly modify the behavior of phi*(z) only for z<1
and have no significant impact upon the characteristic magnitude M*_J(z).Comment: Corrected minor typo in equations (4) and (6). Accepted for
publication in The Astrophysical Journal. 56 pages + 6 tables + 16 figure
Modeling the color evolution of luminous red galaxies - improvements with empirical stellar spectra
Predicting the colors of Luminous Red Galaxies (LRGs) in the Sloan Digital
Sky Survey (SDSS) has been a long-standing problem. The g,r,i colors of LRGs
are inconsistent with stellar population models over the redshift range
0.1<z<0.7. The g-r colors in the models are on average redder than the data
while the r-i colors in the models are bluer towards low redshift. Beyond
redshift 0.4, the predicted r-i color becomes instead too red, while the
predicted g-r agrees with the data. We provide a solution to this problem,
through a combination of new astrophysics and a fundamental change to the
stellar population modeling. We find that the use of the empirical library of
Pickles (1998) instead of theoretical spectra modifies the predicted colors
exactly in the way suggested by the data. The reason is a lower flux in the
empirical libraries, with respect to the theoretical ones, in the wavelength
range 5500-6500 AA. The discrepancy increases with decreasing effective
temperature independently of gravity. This result has general implications for
a variety of studies from globular clusters to high-redshift galaxies. The
astrophysical part of our solution regards the composition of the stellar
populations of these massive Luminous Red Galaxies. We find that on top of the
previous effect one needs to consider a model in which ~3% of the stellar mass
is in old metal-poor stars. Other solutions such as substantial blue Horizontal
Branch at high metallicity or young stellar populations can be ruled out by the
data. Our new model provides a better fit to the g-r and r-i colors of LRGs and
gives new insight into the formation histories of these most massive galaxies.
Our model will also improve the k- and evolutionary corrections for LRGs which
are critical for fully exploiting present and future galaxy surveys.Comment: Submitted to ApJ Letters. High resolution version available at
http://www.maraston.eu/Maraston_etal_2008.pd
The AGN Contribution to the Mid-IR Emission of Luminous Infrared Galaxies
We determine the contribution of AGN to the mid-IR emission of luminous
infrared galaxies (LIRGs) at z>0.6 by measuring the mid-IR dust continuum slope
of 20,039 mid-IR sources. The 24 micron sources are selected from a
Spitzer/MIPS survey of the NOAO Deep Wide-Field Survey Bo\"otes field and have
corresponding 8 micron data from the IRAC Shallow Survey. There is a clear
bimodal distribution in the 24 micron to 8 micron flux ratio. The X-ray
detected sources fall within the peak corresponding to a flat spectrum in
nufnu, implying that it is populated by AGN-dominated LIRGs, whereas the peak
corresponding to a higher 24 micron to 8 micron flux ratio is likely due to
LIRGs whose infrared emission is powered by starbursts. The 24 micron emission
is increasingly dominated by AGN at higher 24 micron flux densities (f_24): the
AGN fraction of the z>0.6 sources increases from ~9% at f_24 ~ 0.35 mJy to
74+/-20% at f_24 ~ 3 mJy in good agreement with model predictions. Deep 24
micron, small area surveys, like GOODS, will be strongly dominated by starburst
galaxies. AGN are responsible for ~ 3-7% of the total 24 micron background.Comment: 6 pages, accepted for publication in Ap
Mid-Infrared Galaxy Luminosity Functions from the AGN and Galaxy Evolution Survey
We present galaxy luminosity functions at 3.6, 4.5, 5.8, and 8.0 micron
measured by combining photometry from the IRAC Shallow Survey with redshifts
from the AGN and Galaxy Evolution Survey of the NOAO Deep Wide-Field Survey
Bootes field. The well-defined IRAC samples contain 3800-5800 galaxies for the
3.6-8.0 micron bands with spectroscopic redshifts and z < 0.6. We obtained
relatively complete luminosity functions in the local redshift bin of z < 0.2
for all four IRAC channels that are well fit by Schechter functions. We found
significant evolution in the luminosity functions for all four IRAC channels
that can be fit as an evolution in M* with redshift, \Delta M* = Qz. While we
measured Q=1.2\pm0.4 and 1.1\pm0.4 in the 3.6 and 4.5 micron bands consistent
with the predictions from a passively evolving population, we obtained
Q=1.8\pm1.1 in the 8.0 micron band consistent with other evolving star
formation rate estimates. We compared our LFs with the predictions of
semi-analytical galaxy formation and found the best agreement at 3.6 and 4.5
micron, rough agreement at 8.0 micron, and a large mismatch at 5.8 micron.
These models also predicted a comparable Q value to our luminosity functions at
8.0 micron, but predicted smaller values at 3.6 and 4.5 micron. We also
measured the luminosity functions separately for early and late-type galaxies.
While the luminosity functions of late-type galaxies resemble those for the
total population, the luminosity functions of early-type galaxies in the 3.6
and 4.5 micron bands indicate deviations from the passive evolution model,
especially from the measured flat luminosity density evolution. Combining our
estimates with other measurements in the literature, we found (53\pm18)% of the
present stellar mass of early-type galaxies has been assembled at z=0.7.Comment: 39 pages, 15 figures, submitted to ApJ (revised following the referee
report
Low Resolution Spectral Templates For AGNs and Galaxies From 0.03 -- 30 microns
We present a set of low resolution empirical SED templates for AGNs and
galaxies in the wavelength range from 0.03 to 30 microns based on the
multi-wavelength photometric observations of the NOAO Deep-Wide Field Survey
Bootes field and the spectroscopic observations of the AGN and Galaxy Evolution
Survey. Our training sample is comprised of 14448 galaxies in the redshift
range 0<~z<~1 and 5347 likely AGNs in the range 0<~z<~5.58. We use our
templates to determine photometric redshifts for galaxies and AGNs. While they
are relatively accurate for galaxies, their accuracies for AGNs are a strong
function of the luminosity ratio between the AGN and galaxy components.
Somewhat surprisingly, the relative luminosities of the AGN and its host are
well determined even when the photometric redshift is significantly in error.
We also use our templates to study the mid-IR AGN selection criteria developed
by Stern et al.(2005) and Lacy et al.(2004). We find that the Stern et
al.(2005) criteria suffers from significant incompleteness when there is a
strong host galaxy component and at z =~ 4.5, when the broad Halpha emission
line is redshifted into the [3.6] band, but that it is little contaminated by
low and intermediate redshift galaxies. The Lacy et al.(2004) criterion is not
affected by incompleteness at z =~ 4.5 and is somewhat less affected by strong
galaxy host components, but is heavily contaminated by low redshift star
forming galaxies. Finally, we use our templates to predict the color-color
distribution of sources in the upcoming WISE mission and define a color
criterion to select AGNs analogous to those developed for IRAC photometry. We
estimate that in between 640,000 and 1,700,000 AGNs will be identified by these
criteria, but will have serious completeness problems for z >~ 3.4.Comment: Accepted for publication in The Astrophysical Journal. 26 text pages
+ 3 tables + 20 figures, modified to include comments made by the referee.
Fortran codes, templates and electronic tables available at
http://www.astronomy.ohio-state.edu/~rjassef/lrt
The 1<z<5 Infrared Luminosity Function of Type I Quasars
We determine the rest-frame 8 micron luminosity function of type I quasars
over the redshift range 1<z<5. Our sample consists of 292 24 micron sources
brighter than 1 mJy selected from 7.17 square degrees of the Spitzer Space
Telescope MIPS survey of the NOAO Deep Wide-Field Survey Bootes field. The AGN
and Galaxy Evolution Survey (AGES) has measured redshifts for 270 of the R<21.7
sources and we estimate that the contamination of the remaining 22 sources by
stars and galaxies is low. We are able to select quasars missed by ultra-violet
excess quasar surveys, including reddened type I quasars and 2.2<z<3.0 quasars
with optical colors similar to main sequence stars. We find reddened type I
quasars comprise 20% of the type I quasar population. Nonetheless, the shape,
normalization, and evolution of the rest-frame 8 micron luminosity function is
comparable to that of quasars selected from optical surveys. The 8 micron
luminosity function of type I quasars is well approximated by a power-law with
index -2.75(+/-0.14). We directly measure the peak of the quasar space density
to be at z=2.6(+/-0.3).Comment: Accepted for publication in the ApJ, 19 pages, 12 figure
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