49 research outputs found
Focal Plate Structure Alignment of the Dark Energy Spectroscopic Instrument
The Dark Energy Spectroscopic Instrument (DESI) is under construction to
measure the expansion history of the universe using the Baryon Acoustic
Oscillation (BAO) technique. The spectra of 35 million galaxies and quasars
over will be measured during the life of the experiment.
A new prime focus corrector for the KPNO Mayall telescope will deliver light to
5000 robotically positioned optic fibres. The fibres in turn feed ten broadband
spectrographs. Proper alignment of focal plate structure, mainly consisting of
a focal plate ring (FPR) and ten focal plate petals (FPP), is crucial in
ensuring minimal loss of light in the focal plane. A coordinate measurement
machine (CMM) metrology-based approach to alignment requires comprehensive
characterisation of critical dimensions of the petals and the ring, all of
which were 100% inspected. The metrology data not only served for quality
assurance (QA), but also, with careful modelling of geometric transformations,
informed the initial choice of integration accessories such as gauge blocks,
pads, and shims. The integrated focal plate structure was inspected again on a
CMM, and each petal was adjusted according to the updated focal plate metrology
data until all datums were extremely close to nominal positions and optical
throughput nearly reached the theoretically best possible value. This paper
presents our metrology and alignment methodology and complete results for
twelve official DESI petals. The as-aligned, total RMS optical throughput for
6168 positioner holes of twelve production petals was indirectly measured to be
, well above the 99.5% project requirement. The successful
alignment fully demonstrated the wealth of data, reproducibility, and
micron-level precision made available by our CMM metrology-based approach.Comment: 17 pages, 10 figures, 3 table
Dark Energy Spectroscopic Instrument (DESI) Fiber Positioner Production
The Dark Energy Spectroscopic Instrument (DESI) is under construction to
measure the expansion history of the Universe using the Baryon Acoustic
Oscillation technique. The spectra of 35 million galaxies and quasars over
14000 sq deg will be measured during the life of the experiment. A new prime
focus corrector for the KPNO Mayall telescope will deliver light to 5000 fiber
optic positioners. The fibers in turn feed ten broad-band spectrographs. We
will describe the production and manufacturing processes developed for the 5000
fiber positioner robots mounted on the focal plane of the Mayall telescope.Comment: SPIE 201
The DESI One-Percent Survey: Evidence for Assembly Bias from Low-Redshift Counts-in-Cylinders Measurements
We explore the galaxy-halo connection information that is available in
low-redshift samples from the early data release of the Dark Energy
Spectroscopic Instrument (DESI). We model the halo occupation distribution
(HOD) from z=0.1-0.3 using Survey Validation 3 (SV3; a.k.a., the One-Percent
Survey) data of the DESI Bright Galaxy Survey (BGS). In addition to more
commonly used metrics, we incorporate counts-in-cylinders (CiC) measurements,
which drastically tighten HOD constraints. Our analysis is aided by the Python
package, galtab, which enables the rapid, precise prediction of CiC for any HOD
model available in halotools. This methodology allows our Markov chains to
converge with much fewer trial points, and enables even more drastic speedups
due to its GPU portability. Our HOD fits constrain characteristic halo masses
tightly and provide statistical evidence for assembly bias, especially at lower
luminosity thresholds: the HOD of central galaxies in samples with
limiting absolute magnitude and samples is
positively correlated with halo concentration with a significance of 99.9% and
99.5%, respectively. Our models also favor positive central assembly bias for
the brighter sample at (94.8% significance), but
there is no significant evidence for assembly bias with the same luminosity
threshold at . We provide our constraints for each threshold
sample's characteristic halo masses, assembly bias, and other HOD parameters.
These constraints are expected to be significantly tightened with future DESI
data, which will span an area 100 times larger than that of SV3
The DESI One-percent Survey: Evidence for Assembly Bias from Low-redshift Counts-in-cylinders Measurements
We explore the galaxy-halo connection information that is available in low-redshift samples from the early data release of the Dark Energy Spectroscopic Instrument (DESI). We model the halo occupation distribution (HOD) from z = 0.1 to 0.3 using Survey Validation 3 (SV3; a.k.a., the One-Percent Survey) data of the DESI Bright Galaxy Survey. In addition to more commonly used metrics, we incorporate counts-in-cylinders (CiC) measurements, which drastically tighten HOD constraints. Our analysis is aided by the Python package, galtab, which enables the rapid, precise prediction of CiC for any HOD model available in halotools. This methodology allows our Markov chains to converge with much fewer trial points, and enables even more drastic speedups due to its GPU portability. Our HOD fits constrain characteristic halo masses tightly and provide statistical evidence for assembly bias, especially at lower luminosity thresholds: the HOD of central galaxies in z ∼ 0.15 samples with limiting absolute magnitude M r < −20.0 and M r < −20.5 samples is positively correlated with halo concentration with a significance of 99.9% and 99.5%, respectively. Our models also favor positive central assembly bias for the brighter M r < −21.0 sample at z ∼ 0.25 (94.8% significance), but there is no significant evidence for assembly bias with the same luminosity threshold at z ∼ 0.15. We provide our constraints for each threshold sample’s characteristic halo masses, assembly bias, and other HOD parameters. These constraints are expected to be significantly tightened with future DESI data, which will span an area 100 times larger than that of SV3
DESI mock challenge: constructing DESI galaxy catalogues based on FastPM simulations
Together with larger spectroscopic surveys such as the Dark Energy Spectroscopic Instrument (DESI), the precision of large scale structure studies and thus the constraints on the cosmological parameters are rapidly improving. Therefore, one must buildrealistic simulations and robust covariance matrices. We build galaxy catalogues by applying a halo occupation distribution(HOD) model upon the FASTPM simulations, such that the resulting galaxy clustering reproduces high-resolution N-bodysimulations. While the resolution and halo finder are different from the reference simulations, we reproduce the reference galaxytwo-point clustering measurements – monopole and quadrupole – to a precision required by the DESI Year 1 emission line galaxysample down to non-linear scales, i.e. k 10 Mpc h−1. Furthermore, we compute covariance matrices basedon the resulting FASTPM galaxy clustering – monopole and quadrupole. We study for the first time the effect of fitting on Fourierconjugate (e.g. power spectrum) on the covariance matrix of the Fourier counterpart (e.g. correlation function). We estimate theuncertainties of the two parameters of a simple clustering model and observe a maximum variation of 20 per cent for the differentcovariance matrices. Nevertheless, for most studied scales the scatter is between 2 and 10 per cent. Consequently, using thecurrent pipeline we can precisely reproduce the clustering of N-body simulations and the resulting covariance matrices providerobust uncertainty estimations against HOD fitting scenarios. We expect our methodology will be useful for the coming DESIdata analyses and their extension for other studies
The DESI One-Percent survey: constructing galaxy-halo connections for ELGs and LRGs using auto and cross correlations
In the current Dark Energy Spectroscopic Instrument (DESI) survey, emission
line galaxies (ELGs) and luminous red galaxies (LRGs) are essential for mapping
the dark matter distribution at . We measure the auto and cross
correlation functions of ELGs and LRGs at from the DESI
One-Percent survey. Following Gao et al. (2022), we construct the galaxy-halo
connections for ELGs and LRGs simultaneously. With the stellar-halo mass
relation (SHMR) for the whole galaxy population (i.e. normal galaxies), LRGs
can be selected directly by stellar mass, while ELGs can also be selected
randomly based on the observed number density of each stellar mass, once the
probability of a satellite galaxy becoming an ELG is
determined. We demonstrate that the observed small scale clustering prefers a
halo mass-dependent model rather than a constant. With this
model, we can well reproduce the auto correlations of LRGs and the cross
correlations between LRGs and ELGs at
. We can also reproduce the auto correlations of ELGs at
( ) in
real (redshift) space. Although our model has only seven parameters, we show
that it can be extended to higher redshifts and reproduces the observed auto
correlations of ELGs in the whole range of , which enables us to
generate a lightcone ELG mock for DESI. With the above model, we further derive
halo occupation distributions (HODs) for ELGs which can be used to produce ELG
mocks in coarse simulations without resolving subhalos.Comment: 27 pages, 16 figures, accepted by Ap
Mitigating the noise of DESI mocks using analytic control variates
In order to address fundamental questions related to the expansion history of
the Universe and its primordial nature with the next generation of galaxy
experiments, we need to model reliably large-scale structure observables such
as the correlation function and the power spectrum. Cosmological -body
simulations provide a reference through which we can test our models, but their
output suffers from sample variance on large scales. Fortunately, this is the
regime where accurate analytic approximations exist. To reduce the variance,
which is key to making optimal use of these simulations, we can leverage the
accuracy and precision of such analytic descriptions using Control Variates
(CV). We apply two control variate formulations to mock catalogs generated in
anticipation of upcoming data from the Dark Energy Spectroscopic Instrument
(DESI) to test the robustness of its analysis pipeline. Our CV-reduced
measurements, of the power spectrum and correlation function, both pre- and
post-reconstruction, offer a factor of 5-10 improvement in the measurement
error compared with the raw measurements from the DESI mock catalogs. We
explore the relevant properties of the galaxy samples that dictate this
reduction and comment on the improvements we find on some of the derived
quantities relevant to Baryon Acoustic Oscillation (BAO) analysis. We also
provide an optimized package for computing the power spectra and other
two-point statistics of an arbitrary galaxy catalog as well as a pipeline for
obtaining CV-reduced measurements on any of the AbacusSummit cubic box outputs.
We make our scripts, notebooks, and benchmark tests against existing software
publicly available and report a speed improvement of a factor of 10 for a
grid size of compared with .Comment: 15 pages, 9 figures, public package (for power spectrum and control
variates estimation
Changing-look Active Galactic Nuclei from the Dark Energy Spectroscopic Instrument. I. Sample from the Early Data
Changing-look active galactic nuclei (CL AGNs) can be generally confirmed by the emergence (turn-on) or disappearance (turn-off) of broad emission lines (BELs), associated with a transient timescale (about 100 ∼ 5000 days) that is much shorter than predicted by traditional accretion disk models. We carry out a systematic CL AGN search by crossmatching the spectra coming from the Dark Energy Spectroscopic Instrument and the Sloan Digital Sky Survey. Following previous studies, we identify CL AGNs based on Hα, Hβ, and Mg ii at z ≤ 0.75 and Mg ii, C iii], and C iv at z > 0.75. We present 56 CL AGNs based on visual inspection and three selection criteria, including 2 Hα, 34 Hβ, 9 Mg ii, 18 C iii], and 1 C iv CL AGN. Eight cases show simultaneous appearances/disappearances of two BELs. We also present 44 CL AGN candidates with significant flux variation of BELs, but remaining strong broad components. In the confirmed CL AGNs, 10 cases show additional CL candidate features for different lines. In this paper, we find: (1) a 24:32 ratio of turn-on to turn-off CL AGNs; (2) an upper-limit transition timescale ranging from 330 to 5762 days in the rest frame; and (3) the majority of CL AGNs follow the bluer-when-brighter trend. Our results greatly increase the current CL census (∼30%) and would be conducive to exploring the underlying physical mechanism
Validation of semi-analytical, semi-empirical covariance matrices for two-point correlation function for Early DESI data
We present an extended validation of semi-analytical, semi-empirical
covariance matrices for the two-point correlation function (2PCF) on simulated
catalogs representative of Luminous Red Galaxies (LRG) data collected during
the initial two months of operations of the Stage-IV ground-based Dark Energy
Spectroscopic Instrument (DESI). We run the pipeline on multiple extended
Zel'dovich (EZ) mock galaxy catalogs with the corresponding cuts applied and
compare the results with the mock sample covariance to assess the accuracy and
its fluctuations. We propose an extension of the previously developed formalism
for catalogs processed with standard reconstruction algorithms. We consider
methods for comparing covariance matrices in detail, highlighting their
interpretation and statistical properties caused by sample variance, in
particular, nontrivial expectation values of certain metrics even when the
external covariance estimate is perfect. With improved mocks and validation
techniques, we confirm a good agreement between our predictions and sample
covariance. This allows one to generate covariance matrices for comparable
datasets without the need to create numerous mock galaxy catalogs with matching
clustering, only requiring 2PCF measurements from the data itself. The code
used in this paper is publicly available at
https://github.com/oliverphilcox/RascalC.Comment: 19 pages, 1 figure. Code available at
https://github.com/oliverphilcox/RascalC, table and figure data available at
https://dx.doi.org/10.5281/zenodo.775063
Long-term follow-up observations of extreme coronal line emitting galaxies
We present new spectroscopic and photometric follow-up observations of the
known sample of extreme coronal line emitting galaxies (ECLEs) identified in
the Sloan Digital Sky Survey (SDSS). With these new data, observations of the
ECLE sample now span a period of two decades following their initial SDSS
detections. We confirm the nonrecurrence of the iron coronal line signatures in
five of the seven objects, further supporting their identification as the
transient light echoes of tidal disruption events (TDEs). Photometric
observations of these objects in optical bands show little overall evolution.
In contrast, mid-infrared (MIR) observations show ongoing long-term declines.
The remaining two objects had been classified as active galactic nuclei (AGN)
with unusually strong coronal lines rather than being TDE related, given the
persistence of the coronal lines in earlier follow-up spectra. We confirm this
classification, with our spectra continuing to show the presence of strong,
unchanged coronal-line features and AGN-like MIR colours and behaviour. We have
constructed spectral templates of both subtypes of ECLE to aid in
distinguishing the likely origin of newly discovered ECLEs. We highlight the
need for higher cadence, and more rapid, follow-up observations of such objects
to better constrain their properties and evolution. We also discuss the
relationships between ECLEs, TDEs, and other identified transients having
significant MIR variability.Comment: Submitted to MNRAS. 33 pages, 15 figure