95 research outputs found
Reconstructing Galaxy Spectral Energy Distributions from Broadband Photometry
We present a novel approach to photometric redshifts, one that merges the
advantages of both the template fitting and empirical fitting algorithms,
without any of their disadvantages. This technique derives a set of templates,
describing the spectral energy distributions of galaxies, from a catalog with
both multicolor photometry and spectroscopic redshifts. The algorithm is
essentially using the shapes of the templates as the fitting parameters. From
simulated multicolor data we show that for a small training set of galaxies we
can reconstruct robustly the underlying spectral energy distributions even in
the presence of substantial errors in the photometric observations. We apply
these techniques to the multicolor and spectroscopic observations of the Hubble
Deep Field building a set of template spectra that reproduced the observed
galaxy colors to better than 10%. Finally we demonstrate that these improved
spectral energy distributions lead to a photometric-redshift relation for the
Hubble Deep Field that is more accurate than standard template-based
approaches.Comment: 23 pages, 8 figures, LaTeX AASTeX, accepted for publication in A
A multiscale approach to environment and its influence on the colour distribution of galaxies
We present a multiscale approach to measurements of galaxy density, applied
to a volume-limited sample constructed from SDSS DR5. We populate a rich
parameter space by obtaining independent measurements of density on different
scales for each galaxy, avoiding the implicit assumptions involved, e.g., in
the construction of group catalogues. As the first application of this method,
we study how the bimodality in galaxy colour distribution (u-r) depends on
multiscale density. The u-r galaxy colour distribution is described as the sum
of two gaussians (red and blue) with five parameters: the fraction of red
galaxies (f_r) and the position and width of the red and blue peaks (mu_r,
mu_b, sigma_r and sigma_b). Galaxies mostly react to their smallest scale (<
0.5 Mpc) environments: in denser environments red galaxies are more common
(larger f_r), redder (larger mu_r) and with a narrower distribution (smaller
sigma_r), while blue galaxies are redder (larger mu_b) but with a broader
distribution (larger sigma_b). There are residual correlations of f_r and mu_b
with 0.5 - 1 Mpc scale density, which imply that total or partial truncation of
star formation can relate to a galaxy's environment on these scales. Beyond 1
Mpc (0.5 Mpc for mu_r) there are no positive correlations with density. However
f_r (mu_r) anti-correlates with density on >2 (1) Mpc scales at fixed density
on smaller scales. We examine these trends qualitatively in the context of the
halo model, utilizing the properties of haloes within which the galaxies are
embedded, derived by Yang et al, 2007 and applied to a group catalogue. This
yields an excellent description of the trends with multiscale density,
including the anti-correlations on large scales, which map the region of
accretion onto massive haloes. Thus we conclude that galaxies become red only
once they have been accreted onto haloes of a certain mass.Comment: 22 pages, 14 figures. Accepted for publication in MNRAS
Applying Machine Learning to Catalogue Matching in Astrophysics
We present the results of applying automated machine learning techniques to
the problem of matching different object catalogues in astrophysics. In this
study we take two partially matched catalogues where one of the two catalogues
has a large positional uncertainty. The two catalogues we used here were taken
from the HI Parkes All Sky Survey (HIPASS), and SuperCOSMOS optical survey.
Previous work had matched 44% (1887 objects) of HIPASS to the SuperCOSMOS
catalogue.
A supervised learning algorithm was then applied to construct a model of the
matched portion of our catalogue. Validation of the model shows that we
achieved a good classification performance (99.12% correct).
Applying this model, to the unmatched portion of the catalogue found 1209 new
matches. This increases the catalogue size from 1887 matched objects to 3096.
The combination of these procedures yields a catalogue that is 72% matched.Comment: 8 Pages, 5 Figure
Probabilistic Association of Transients to their Hosts (PATH)
We introduce a new method to estimate the probability that an extragalactic
transient source is associated with a candidate host galaxy. This approach
relies solely on simple observables: sky coordinates and their uncertainties,
galaxy fluxes and angular sizes. The formalism invokes Bayes' rule to calculate
the posterior probability P(O_i|x) from the galaxy prior P(O), observables x,
and an assumed model for the true distribution of transients in/around their
host galaxies. Using simulated transients placed in the well-studied COSMOS
field, we consider several agnostic and physically motivated priors and offset
distributions to explore the method sensitivity. We then apply the methodology
to the set of 13~fast radio bursts (FRBs) localized with an uncertainty of
several arcseconds. Our methodology finds nine of these are securely associated
to a single host galaxy, P(O_i|x)>0.95. We examine the observed and intrinsic
properties of these secure FRB hosts, recovering similar distributions as
previous works. Furthermore, we find a strong correlation between the apparent
magnitude of the securely identified host galaxies and the estimated cosmic
dispersion measures of the corresponding FRBs, which results from the Macquart
relation. Future work with FRBs will leverage this relation and other measures
from the secure hosts as priors for future associations. The methodology is
generic to transient type, localization error, and image quality. We encourage
its application to other transients where host galaxy associations are critical
to the science, e.g. gravitational wave events, gamma-ray bursts, and
supernovae. We have encoded the technique in Python on GitHub:
https://github.com/FRBs/astropath.Comment: In press, ApJ; comments still welcome; Visit
https://github.com/FRBs/astropath to use and build PAT
SPIDERS: Selection of spectroscopic targets using AGN candidates detected in all-sky X-ray surveys
SPIDERS (SPectroscopic IDentification of eROSITA Sources) is an SDSS-IV
survey running in parallel to the eBOSS cosmology project. SPIDERS will obtain
optical spectroscopy for large numbers of X-ray-selected AGN and galaxy cluster
members detected in wide area eROSITA, XMM-Newton and ROSAT surveys. We
describe the methods used to choose spectroscopic targets for two
sub-programmes of SPIDERS: X-ray selected AGN candidates detected in the ROSAT
All Sky and the XMM-Newton Slew surveys. We have exploited a Bayesian
cross-matching algorithm, guided by priors based on mid-IR colour-magnitude
information from the WISE survey, to select the most probable optical
counterpart to each X-ray detection. We empirically demonstrate the high
fidelity of our counterpart selection method using a reference sample of bright
well-localised X-ray sources collated from XMM-Newton, Chandra and Swift-XRT
serendipitous catalogues, and also by examining blank-sky locations. We
describe the down-selection steps which resulted in the final set of
SPIDERS-AGN targets put forward for spectroscopy within the eBOSS/TDSS/SPIDERS
survey, and present catalogues of these targets. We also present catalogues of
~12000 ROSAT and ~1500 XMM-Newton Slew survey sources which have existing
optical spectroscopy from SDSS-DR12, including the results of our visual
inspections. On completion of the SPIDERS program, we expect to have collected
homogeneous spectroscopic redshift information over a footprint of ~7500
deg for >85 percent of the ROSAT and XMM-Newton Slew survey sources having
optical counterparts in the magnitude range 17<r<22.5, producing a large and
highly complete sample of bright X-ray-selected AGN suitable for statistical
studies of AGN evolution and clustering.Comment: MNRAS, accepte
The clustering of Luminous Red Galaxies around MgII absorbers
We study the cross-correlation between 212 MgII quasar absorption systems and
\~20,000 Luminous Red Galaxies (LRGs) selected from the Sloan Digital Sky
Survey Data Release 1 in the redshift range 0.4<z<0.8. The MgII systems were
selected to have 2796 & 2803 rest-frame equivalent widths >=1.0 Angstrom and
identifications confirmed by the FeII 2600 or MgI 2852 lines. Over comoving
scales 0.05--13 h^-1 Mpc, the MgII--LRG cross-correlation has an amplitude
0.69+/-0.09 times that of the LRG--LRG auto-correlation. Since LRGs have
halo-masses greater than 3.5 x 10^12 solar masses for M_R<-21, this relative
amplitude implies that the absorber host-galaxies have halo-masses greater than
2--8 x 10^11 Msun. For 10^13 Msun LRGs, the absorber host-galaxies have
halo-masses 0.5--2.5 x 10^12 Msun. Our results appear consistent with those of
Steidel et al. (1994) who found that MgII absorbers with W_r>=0.3 Angstrom are
associated with ~0.7 L^*_B galaxies.Comment: 7 pages, 3 figs; Accepted for publication in MNRAS Letters; Extended
version with Appendix; Text version of MgII absorber catalogue (Table 1) can
be found at http://www.ast.cam.ac.uk/~mim/pub.html. Minor changes to match
the published tex
Spectral Energy Distributions and Multiwavelength Selection of Type 1 Quasars
We present an analysis of the mid-infrared (MIR) and optical properties of type 1 (broad-line) quasars detected by the Spitzer Space Telescope. The MIR color-redshift relation is characterized to z ~ 3, with predictions to z = 7. We demonstrate how combining MIR and optical colors can yield even more efficient selection of active galactic nuclei (AGNs) than MIR or optical colors alone. Composite spectral energy distributions (SEDs) are constructed for 259 quasars with both Sloan Digital Sky Survey and Spitzer photometry, supplemented by near-IR, GALEX, VLA, and ROSAT data, where available. We discuss how the spectral diversity of quasars influences the determination of bolometric luminosities and accretion rates; assuming the mean SED can lead to errors as large as 50% for individual quasars when inferring a bolometric luminosity from an optical luminosity. Finally, we show that careful consideration of the shape of the mean quasar SED and its redshift dependence leads to a lower estimate of the fraction of reddened/obscured AGNs missed by optical surveys as compared to estimates derived from a single mean MIR to optical flux ratio
Photometric redshifts from reconstructed QSO templates
From SDSS commissioning photometric and spectroscopic data, we investigate
the utility of photometric redshift techniques to the task of estimating QSO
redshifts. We consider empirical methods (e.g. nearest-neighbor searches and
polynomial fitting), standard spectral template fitting and hybrid approaches
(i.e. training spectral templates from spectroscopic and photometric
observations of QSOs). We find that in all cases, due to the presence of strong
emission-lines within the QSO spectra, the nearest-neighbor and template
fitting methods are superior to the polynomial fitting approach. Applying a
novel reconstruction technique, we can, from the SDSS multicolor photometry,
reconstruct a statistical representation of the underlying SEDs of the SDSS
QSOs. Although, the reconstructed templates are based on only broadband
photometry the common emission lines present within the QSO spectra can be
recovered in the resulting spectral energy distributions. The technique should
be useful in searching for spectral differences among QSOs at a given redshift,
in searching for spectral evolution of QSOs, in comparing photometric redshifts
for objects beyond the SDSS spectroscopic sample with those in the well
calibrated photometric redshifts for objects brighter than 20th magnitude and
in searching for systematic and time variable effects in the SDSS broad band
photometric and spectral photometric calibrations.Comment: 21 pages, 9 figures, LaTeX AASTeX, submitted to A
- …