100 research outputs found
Darth Fader: Using wavelets to obtain accurate redshifts of spectra at very low signal-to-noise
We present the DARTH FADER algorithm, a new wavelet-based method for
estimating redshifts of galaxy spectra in spectral surveys that is particularly
adept in the very low SNR regime. We use a standard cross-correlation method to
estimate the redshifts of galaxies, using a template set built using a PCA
analysis on a set of simulated, noise-free spectra. Darth Fader employs wavelet
filtering to both estimate the continuum & to extract prominent line features
in each galaxy spectrum. A simple selection criterion based on the number of
features present in the spectrum is then used to clean the catalogue: galaxies
with fewer than six total features are removed as we are unlikely to obtain a
reliable redshift estimate. Applying our wavelet-based cleaning algorithm to a
simulated testing set, we successfully build a clean catalogue including
extremely low signal-to-noise data (SNR=2.0), for which we are able to obtain a
5.1% catastrophic failure rate in the redshift estimates (compared with 34.5%
prior to cleaning). We also show that for a catalogue with uniformly mixed SNRs
between 1.0 & 20.0, with realistic pixel-dependent noise, it is possible to
obtain redshifts with a catastrophic failure rate of 3.3% after cleaning (as
compared to 22.7% before cleaning). Whilst we do not test this algorithm
exhaustively on real data, we present a proof of concept of the applicability
of this method to real data, showing that the wavelet filtering techniques
perform well when applied to some typical spectra from the SDSS archive. The
Darth Fader algorithm provides a robust method for extracting spectral features
from very noisy spectra. The resulting clean catalogue gives an extremely low
rate of catastrophic failures, even when the spectra have a very low SNR. For
very large sky surveys, this technique may offer a significant boost in the
number of faint galaxies with accurately determined redshifts.Comment: 22 pages, 15 figures. Accepted for publication in Astronomy &
Astrophysic
Catastrophic Photo-z Errors and the Dark Energy Parameter Estimates with Cosmic Shear
We study the impact of catastrophic errors occurring in the photometric
redshifts of galaxies on cosmological parameter estimates with cosmic shear
tomography. We consider a fiducial survey with 9-filter set and perform photo-z
measurement simulations. It is found that a fraction of 1% galaxies at
z_{spec}~0.4 is misidentified to be at z_{phot}~3.5. We then employ both chi^2
fitting method and the extension of Fisher matrix formalism to evaluate the
bias on the equation of state parameters of dark energy, w_0 and w_a, induced
by those catastrophic outliers. By comparing the results from both methods, we
verify that the estimation of w_0 and w_a from the fiducial 5-bin tomographic
analyses can be significantly biased. To minimize the impact of this bias, two
strategies can be followed: (A) the cosmic shear analysis is restricted to
0.5<z<2.5 where catastrophic redshift errors are expected to be insignificant;
(B) a spectroscopic survey is conducted for galaxies with 3<z_{phot}<4. We find
that the number of spectroscopic redshifts needed scales as N_{spec} \propto
f_{cata}\times A where f_{cata}=1% is the fraction of catastrophic redshift
errors (assuming a 9-filter photometric survey) and A is the survey area. For
A=1000 {deg}^2, we find that N_{spec}>320 and 860 respectively in order to
reduce the joint bias in (w_0,w_a) to be smaller than 2\sigma and 1\sigma. This
spectroscopic survey (option B) will improve the Figure of Merit of option A by
a factor \times 1.5 thus making such a survey strongly desirable.Comment: 25 pages, 9 figures. Revised version, as accepted for publication in
Ap
Designing Future Dark Energy Space Missions: II. Photometric Redshift of Space Weak Lensing Optimized Survey
Accurate weak-lensing analysis requires not only accurate measurement of
galaxy shapes but also precise and unbiased measurement of galaxy redshifts.
The photometric redshift technique appears as the only possibility to determine
the redshift of the background galaxies used in the weak-lensing analysis.
Using the photometric redshift quality, simple shape measurement requirements,
and a proper sky model, we explore what could be an optimal weak-lensing dark
energy mission based on FoM calculation. We found that photometric redshifts
reach their best accuracy for the bulk of the faint galaxy population when
filters have a resolution R~3.2. We show that an optimal mission would survey
the sky through 8 filters using 2 cameras (visible and near infrared). Assuming
a 5-year mission duration, a mirror size of 1.5m, a 0.5deg2 FOV with a visible
pixel scale of 0.15", we found that a homogeneous survey reaching IAB=25.6
(10sigma) with a sky coverage of ~11000deg2 maximizes the Weak Lensing FoM. The
effective number density of galaxies then used for WL is ~45gal/arcmin2, at
least a factor of two better than ground based survey. This work demonstrates
that a full account of the observational strategy is required to properly
optimize the instrument parameters to maximize the FoM of the future
weak-lensing space dark energy mission.Comment: 25 pages, 39 figures, accepted in A&
Evidence for Ubiquitous, High-EW Nebular Emission in z~7 Galaxies: Towards a Clean Measurement of the Specific Star Formation Rate using a Sample of Bright, Magnified Galaxies
Growing observational evidence now indicates that nebular line emission has a
significant impact on the rest-frame optical fluxes of z~5-7 galaxies observed
with Spitzer. This line emission makes z~5-7 galaxies appear more massive, with
lower specific star formation rates. However, corrections for this line
emission have been very difficult to perform reliably due to huge uncertainties
on the overall strength of such emission at z>~5.5. Here, we present the most
direct observational evidence yet for ubiquitous high-EW [OIII]+Hbeta line
emission in Lyman-break galaxies at z~7, while also presenting a strategy for
an improved measurement of the sSFR at z~7. We accomplish this through the
selection of bright galaxies in the narrow redshift window z~6.6-7.0 where the
IRAC 4.5 micron flux provides a clean measurement of the stellar continuum
light. Observed 4.5 micron fluxes in this window contrast with the 3.6 micron
fluxes which are contaminated by the prominent [OIII]+Hbeta lines. To ensure a
high S/N for our IRAC flux measurements, we consider only the brightest
(H_{160}<26 mag) magnified galaxies we have identified in CLASH and other
programs targeting galaxy clusters. Remarkably, the mean rest-frame optical
color for our bright seven-source sample is very blue, [3.6]-[4.5]=-0.9+/-0.3.
Such blue colors cannot be explained by the stellar continuum light and require
that the rest-frame EW of [OIII]+Hbeta be greater than 637 Angstroms for the
average source. The bluest four sources from our seven-source sample require an
even more extreme EW of 1582 Angstroms. Our derived lower limit for the mean
[OIII]+Hbeta EW could underestimate the true EW by ~2x based on a simple
modeling of the redshift distribution of our sources. We can also set a robust
lower limit of >~4 Gyr^-1 on the specific star formation rates based on the
mean SED for our seven-source sample. (abridged)Comment: 9 pages, 6 figures, 1 table, submitted to the Astrophysical Journa
Designing Future Dark Energy Space Mission: I. Building Realistic Galaxy Spectro-Photometric Catalogs and their first applications
Future dark energy space missions such as JDEM and EUCLID are being designed
to survey the galaxy population to trace the geometry of the universe and the
growth of structure, which both depend on the cosmological model. To reach the
goal of high precision cosmology they need to evaluate the capabilities of
different instrument designs based on realistic mock catalog. The aim of this
paper is to construct realistic and flexible mock catalogs based on our
knowledge of galaxy population from current deep surveys. We explore two
categories of mock catalog : (i) based on luminosity functions fit of
observations (GOODS, UDF,COSMOS,VVDS) using the Le Phare software (ii) based on
the observed COSMOS galaxy distribution which benefits from all the properties
of the data-rich COSMOS survey. For these two catalogs, we have produced
simulated number counts in several bands, color diagrams and redshift
distribution for validation against real observational data. We also derive
some basic requirements to help designing future Dark Energy mission in terms
of number of galaxies available for the weak-lensing analysis as a function of
the PSF size and depth of the survey. We also compute the spectroscopic success
rate for future spectroscopic redshift surveys (i) aiming at measuring BAO in
the case of the wide field spectroscopic redshift survey, and (ii) for the
photometric redshift calibration survey which is required to achieve weak
lensing tomography with great accuracy. They will be publicly accessible at
http://lamwws.oamp.fr/cosmowiki/RealisticSpectroPhotCat, or by request to the
first author of this paper
CLASH: A Census of Magnified Star-Forming Galaxies at z ~ 6-8
We utilize 16 band Hubble Space Telescope (HST) observations of 18 lensing
clusters obtained as part of the Cluster Lensing And Supernova survey with
Hubble (CLASH) Multi-Cycle Treasury program to search for galaxies.
We report the discovery of 204, 45, and 13 Lyman-break galaxy candidates at
, , and , respectively, identified from purely
photometric redshift selections. This large sample, representing nearly an
order of magnitude increase in the number of magnified star-forming galaxies at
presented to date, is unique in that we have observations in four
WFC3/UVIS UV, seven ACS/WFC optical, and all five WFC3/IR broadband filters,
which enable very accurate photometric redshift selections. We construct
detailed lensing models for 17 of the 18 clusters to estimate object
magnifications and to identify two new multiply lensed
candidates. The median magnifications over the 17 clusters are 4, 4, and 5 for
the , , and samples, respectively, over an average
area of 4.5 arcmin per cluster. We compare our observed number counts with
expectations based on convolving "blank" field UV luminosity functions through
our cluster lens models and find rough agreement down to mag, where we
begin to suffer significant incompleteness. In all three redshift bins, we find
a higher number density at brighter observed magnitudes than the field
predictions, empirically demonstrating for the first time the enhanced
efficiency of lensing clusters over field surveys. Our number counts also are
in general agreement with the lensed expectations from the cluster models,
especially at , where we have the best statistics.Comment: Accepted for publication in the Astrophysical Journal, 25 pages, 13
figures, 7 table
CLASH: Extending galaxy strong lensing to small physical scales with distant sources highly-magnified by galaxy cluster members
We present a strong lensing system in which a double source is imaged 5 times
by 2 early-type galaxies. We take advantage in this target of the multi-band
photometry obtained as part of the CLASH program, complemented by the
spectroscopic data of the VLT/VIMOS and FORS2 follow-up campaign. We use a
photometric redshift of 3.7 for the source and confirm spectroscopically the
membership of the 2 lenses to the galaxy cluster MACS J1206.2-0847 at redshift
0.44. We exploit the excellent angular resolution of the HST/ACS images to
model the 2 lenses in terms of singular isothermal sphere profiles and derive
robust effective velocity dispersions of (97 +/- 3) and (240 +/- 6) km/s. The
total mass distribution of the cluster is also well characterized by using only
the local information contained in this lensing system, that is located at a
projected distance of more than 300 kpc from the cluster luminosity center.
According to our best-fitting lensing and composite stellar population models,
the source is magnified by a total factor of 50 and has a luminous mass of
about (1.0 +/- 0.5) x 10^{9} M_{Sun}. By combining the total and luminous mass
estimates of the 2 lenses, we measure luminous over total mass fractions
projected within the effective radii of 0.51 +/- 0.21 and 0.80 +/- 0.32. With
these lenses we can extend the analysis of the mass properties of lens
early-type galaxies by factors that are about 2 and 3 times smaller than
previously done with regard to, respectively, velocity dispersion and luminous
mass. The comparison of the total and luminous quantities of our lenses with
those of astrophysical objects with different physical scales reveals the
potential of studies of this kind for investigating the internal structure of
galaxies. These studies, made possible thanks to the CLASH survey, will allow
us to go beyond the current limits posed by the available lens samples in the
field.Comment: 20 pages, 10 figures, accepted for publication in the Astrophysical
Journa
CLASH: Photometric redshifts with 16 HST bands in galaxy cluster fields
The Cluster Lensing And Supernovae survey with Hubble (CLASH) is an Hubble
Space Telescope (HST) Multi-Cycle Treasury program observing 25 massive galaxy
clusters. CLASH observations are carried out in 16 bands from UV to NIR to
derive accurate and reliable estimates of photometric redshifts. We present the
CLASH photometric redshifts and study the photometric redshift accuracy of the
arcs in more detail for the case of MACS1206.2-0847. We use the publicly
available Le Phare and BPZ photometric redshift codes on 17 CLASH galaxy
clusters. Using Le Phare code for objects with StoN>=10, we reach a precision
of 3%(1+z) for the strong lensing arcs, which is reduced to 2.4%(1+z) after
removing outliers. For galaxies in the cluster field the corresponding values
are 4%(1+z) and 3%(1+z). Using mock galaxy catalogues, we show that 3%(1+z)
precision is what one would expect from the CLASH photometry when taking into
account extinction from dust, emission lines and the finite range of SEDs
included in the photo-z template library. We study photo-z results for
different aperture photometry and find that the SExtractor isophotal photometry
works best. Le Phare and BPZ give similar photo-z results for the strong
lensing arcs as well as galaxies of the cluster field. Results are improved
when optimizing the photometric aperture shape showing an optimal aperture size
around 1" radius giving results which are equivalent to isophotal photometry.
Tailored photometry of the arcs improve the photo-z results.Comment: Accepted in A&A on nov 201
The SDSS-IV extended Baryon Oscillation Spectroscopic Survey: selecting emission line galaxies using the Fisher discriminant
We present a new selection technique of producing spectroscopic target
catalogues for massive spectroscopic surveys for cosmology. This work was
conducted in the context of the extended Baryon Oscillation Spectroscopic
Survey (eBOSS), which will use ~200 000 emission line galaxies (ELGs) at
0.6<zspec<1.0 to obtain a precise baryon acoustic oscillation measurement. Our
proposed selection technique is based on optical and near-infrared broad-band
filter photometry. We used a training sample to define a quantity, the Fisher
discriminant (linear combination of colours), which correlates best with the
desired properties of the target: redshift and [OII] flux. The proposed
selections are simply done by applying a cut on magnitudes and this Fisher
discriminant. We used public data and dedicated SDSS spectroscopy to quantify
the redshift distribution and [OII] flux of our ELG target selections. We
demonstrate that two of our selections fulfil the initial eBOSS/ELG redshift
requirements: for a target density of 180 deg^2, ~70% of the selected objects
have 0.6<zspec<1.0 and only ~1% of those galaxies in the range 0.6<zspec<1.0
are expected to have a catastrophic zspec estimate. Additionally, the stacked
spectra and stacked deep images for those two selections show characteristic
features of star-forming galaxies. The proposed approach using the Fisher
discriminant could, however, be used to efficiently select other galaxy
populations, based on multi-band photometry, providing that spectroscopic
information is available. This technique could thus be useful for other future
massive spectroscopic surveys such as PFS, DESI, and 4MOST.Comment: Version published in A&
CLASH: z ~ 6 young galaxy candidate quintuply lensed by the frontier field cluster RXC J2248.7-4431
We present a quintuply lensed z ~ 6 candidate discovered in the field of the
galaxy cluster RXC J2248.7-4431 (z ~ 0.348) targeted within the Cluster Lensing
and Supernova survey with Hubble (CLASH) and selected in the deep HST Frontier
Fields survey. Thanks to the CLASH 16-band HST imaging, we identify the
quintuply lensed z ~ 6 candidate as an optical dropout in the inner region of
the cluster, the brightest image having magAB=24.81+-0.02 in the f105w filter.
We perform a detailed photometric analysis to verify its high-z and lensed
nature. We get as photometric redshift z_phot ~ 5.9, and given the extended
nature and NIR colours of the lensed images, we rule out low-z early type and
galactic star contaminants. We perform a strong lensing analysis of the
cluster, using 13 families of multiple lensed images identified in the HST
images. Our final best model predicts the high-z quintuply lensed system with a
position accuracy of 0.8''. The magnifications of the five images are between
2.2 and 8.3, which leads to a delensed UV luminosity of L_1600 ~ 0.5L*_1600 at
z=6. We also estimate the UV slope from the observed NIR colours, finding a
steep beta=-2.89+-0.38. We use singular and composite stellar population SEDs
to fit the photometry of the hiz candidate, and we conclude that it is a young
(age <300 Myr) galaxy with mass of M ~ 10^8Msol, subsolar metallicity
(Z<0.2Zsol) and low dust content (AV ~ 0.2-0.4).Comment: 21 pages, 13 figures, 6 tables, submitted to MNRAS on 11 Aug 2013,
accepted on 23 Nov 201
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