102 research outputs found
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
Degradation analysis in the estimation of photometric redshifts from non-representative training sets
We perform an analysis of photometric redshifts estimated by using a
non-representative training sets in magnitude space. We use the ANNz2 and GPz
algorithms to estimate the photometric redshift both in simulations as well as
in real data from the Sloan Digital Sky Survey (DR12). We show that for the
representative case, the results obtained by using both algorithms have the
same quality, either using magnitudes or colours as input. In order to reduce
the errors when estimating the redshifts with a non-representative training
set, we perform the training in colour space. We estimate the quality of our
results by using a mock catalogue which is split samples cuts in the -band
between . We obtain slightly better results with GPz on single
point z-phot estimates in the complete training set case, however the
photometric redshifts estimated with ANNz2 algorithm allows us to obtain mildly
better results in deeper -band cuts when estimating the full redshift
distribution of the sample in the incomplete training set case. By using a
cumulative distribution function and a Monte-Carlo process, we manage to define
a photometric estimator which fits well the spectroscopic distribution of
galaxies in the mock testing set, but with a larger scatter. To complete this
work, we perform an analysis of the impact on the detection of clusters via
density of galaxies in a field by using the photometric redshifts obtained with
a non-representative training set.Comment: 19 pages, 9 figures. Accepted for publication in MNRA
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&
Degradation analysis in the estimation of photometric redshifts from non-representative training sets
We perform an analysis of photometric redshifts estimated by using a non-representative training sets in magnitude space. We use the ANNz2 and GPz algorithms to estimate the photometric redshift both in simulations and in real data from the Sloan Digital Sky Survey (DR12). We show that for the representative case, the results obtained by using both algorithms have the same quality, using either magnitudes or colours as input. In order to reduce the errors when estimating the redshifts with a non-representative training set, we perform the training in colour space. We estimate the quality of our results by using a mock catalogue which is split samples cuts in the r band between 19.4 < r < 20.8. We obtain slightly better results with GPz on single point z-phot estimates in the complete training set case, however the photometric redshifts estimated with ANNz2 algorithm allows us to obtain mildly better results in deeper r-band cuts when estimating the full redshift distribution of the sample in the incomplete training set case. By using a cumulative distribution function and a Monte Carlo process, we manage to define a photometric estimator which fits well the spectroscopic distribution of galaxies in the mock testing set, but with a larger scatter. To complete this work, we perform an analysis of the impact on the detection of clusters via density of galaxies in a field by using the photometric redshifts obtained with a non-representative training set
Comparison of the properties of two fossil groups of galaxies with the normal group NGC 6034 based on multiband imaging and optical spectroscopy
We collected multiband imaging and spectroscopy for two fossil groups (RX
J1119.7+2126 and 1RXS J235814.4+150524) and one normal group (NGC 6034). We
computed photometric redshifts in the central zones of each group, combining
previous data with the SDSS five-band data. For each group we investigated the
red sequence (RS) of the color-magnitude relation and computed the luminosity
functions, stellar population ages and distributions of the group members.
Spectroscopy allowed us to investigate the large-scale surroundings of these
groups and the substructure levels in 1RXS J235814.4+150524 and NGC 6034. The
large-scale environment of 1RXS J235814.4+150524 is poor, though its galaxy
density map shows a clear signature of the surrounding cosmic web. RX
J1119.7+2126 appears to be very isolated, while the cosmic environment of NGC
6034 is very rich. At the group scale, 1RXS J235814.4+150524 shows no
substructure. Galaxies with recent stellar populations seem preferentially
located in the group outskirts. A RS is discernable for all three groups in a
color-magnitude diagram. The luminosity functions based on photometric redshift
selection and on statistical background subtraction have comparable shapes, and
agree with the few points obtained from spectroscopic redshifts. These
luminosity functions show the expected dip between first and second brightest
galaxies for the fossil groups only. Their shape is also regular and relatively
flat at faint magnitudes down to the completeness level for RX J1119.7+2126 and
NGC 6034, while there is a clear lack of faint galaxies for 1RXS
J235814.4+150524. RX J1119.7+2126 is definitely classified as a fossil group;
1RXS J235814.4+150524 also has properties very close to those of a fossil
group, while we confirm that NGC 6034 is a normal group.Comment: Accepted in A&A, english-improved, 5 jpeg figures, and shortened
abstrac
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
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&
The MUSIC of CLASH: predictions on the concentration-mass relation
We present the results of a numerical study based on the analysis of the
MUSIC-2 simulations, aimed at estimating the expected concentration-mass
relation for the CLASH cluster sample. We study nearly 1400 halos simulated at
high spatial and mass resolution, which were projected along many
lines-of-sight each. We study the shape of both their density and
surface-density profiles and fit them with a variety of radial functions,
including the Navarro-Frenk-White, the generalised Navarro-Frenk-White, and the
Einasto density profiles. We derive concentrations and masses from these fits
and investigate their distributions as a function of redshift and halo
relaxation. We use the X-ray image simulator X-MAS to produce simulated Chandra
observations of the halos and we use them to identify objects resembling the
X-ray morphologies and masses of the clusters in the CLASH X-ray selected
sample. We also derive a concentration-mass relation for strong-lensing
clusters. We find that the sample of simulated halos which resemble the X-ray
morphology of the CLASH clusters is composed mainly by relaxed halos, but it
also contains a significant fraction of un-relaxed systems. For such a sample
we measure an average 2D concentration which is ~11% higher than found for the
full sample of simulated halos. After accounting for projection and selection
effects, the average NFW concentrations of CLASH clusters are expected to be
intermediate between those predicted in 3D for relaxed and super-relaxed halos.
Matching the simulations to the individual CLASH clusters on the basis of the
X-ray morphology, we expect that the NFW concentrations recovered from the
lensing analysis of the CLASH clusters are in the range [3-6], with an average
value of 3.87 and a standard deviation of 0.61. Simulated halos with X-ray
morphologies similar to those of the CLASH clusters are affected by a modest
orientation bias.Comment: 21 pages, 16 figures, 3 tables, submitted to Ap
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
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