分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We report the first results of a high-redshift ($z$ >~ 5) quasar survey using the Dark Energy Spectroscopic Instrument (DESI). As a DESI secondary target program, this survey is designed to carry out a systematic search and investigation of quasars at $z$ >~ 5, up to redshift 6.8. The target selection is based on the DESI Legacy Imaging Surveys (the Legacy Surveys) DR9 photometry, combined with the Pan-STARRS1 data and $J$-band photometry from public surveys. A first quasar sample has been constructed from the DESI Survey Validation 3 (SV3) and first-year observations until May 2022. This sample includes more than 400 new quasars at redshift 4.7 = 5, more than one third of existing quasars previously published at this redshift. The observations so far result in an average success rate of 23% at $z$ > 4.7. The current spectral dataset has already allowed analysis of interesting individual objects (e.g., quasars with damped Ly$\alpha$ absorbers and broad absorption line features), and statistical analysis will follow the survey's completion. A set of science projects will be carried out leveraging this program, including quasar luminosity function, quasar clustering, intergalactic medium, quasar spectral properties, intervening absorbers, and properties of early supermassive black holes. Additionally, a sample of 38 new quasars at $z$ ~ 3.8-5.7 discovered from a pilot survey in the DESI SV1 is also published in this paper.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present DESI observations of the inner halo of M31, which reveal the kinematics of a recent merger - a galactic immigration event - in exquisite detail. Of the 11,416 sources studied in 3.75 hour of on-sky exposure time, 7,438 are M31 sources with well measured radial velocities. The observations reveal intricate coherent kinematic structure in the positions and velocities of individual stars: streams, wedges, and chevrons. While hints of coherent structures have been previously detected in M31, this is the first time they have been seen with such detail and clarity in a galaxy beyond the Milky Way. We find clear kinematic evidence for shell structures in the Giant Stellar Stream, the Northeast Shelf and Western Shelf regions. The kinematics are remarkably similar to the predictions of dynamical models constructed to explain the spatial morphology of the inner halo. The results are consistent with the interpretation that much of the substructure in the inner halo of M31 is produced by a single galactic immigration event 1 - 2 Gyr ago. Significant numbers of metal-rich stars ([Fe/H]$>-0.5$) are present in all of the detected substructures, suggesting that the immigrating galaxy had an extended star formation history. We also investigate the ability of the shells and Giant Stellar Stream to constrain the gravitational potential of M31, and estimate the mass within a projected radius of 125 kpc to be ${\rm log_{10}}\, M_{\rm NFW}(<125\,{\rm kpc})/M_\odot = 11.80_{-0.10}^{+0.12}$. The results herald a new era in our ability to study stars on a galactic scale and the immigration histories of galaxies.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We describe the Milky Way Survey (MWS) that will be undertaken with the Dark Energy Spectroscopic Instrument (DESI) on the Mayall 4m telescope at the Kitt Peak National Observatory. Over the next 5 yr DESI MWS will observe approximately seven million stars at Galactic latitudes |b|>20 degrees, with an inclusive target selection scheme focused on the thick disk and stellar halo. MWS will also include several high-completeness samples of rare stellar types, including white dwarfs, low-mass stars within 100pc of the Sun, and horizontal branch stars. We summarize the potential of DESI to advance understanding of Galactic structure and stellar evolution. We introduce the final definitions of the main MWS target classes and estimate the number of stars in each class that will be observed. We describe our pipelines for deriving radial velocities, atmospheric parameters, and chemical abundances. We use ~500,000 spectra of unique stellar targets from the DESI Survey Validation program (SV) to demonstrate that our pipelines can measure radial velocities to ~1 km/s and [Fe/H] accurate to ~0.2 dex for typical stars in our main sample. We find the stellar parameter distributions from ~100 sq. deg of SV observations with >90% completeness on our main sample are in good agreement with expectations from mock catalogs and previous surveys.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We introduce the DESI LOW-Z Secondary Target Survey, which combines the wide
area capabilities of the Dark Energy Spectroscopic Instrument (DESI) with an
efficient, low-redshift target selection method. Our selection consists of a
set of color and surface brightness cuts, combined with modern machine learning
methods, to optimally target low-redshift dwarf galaxies (z 95% complete in target
selection at z < 0.03 between 19
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We describe the spectroscopic data processing pipeline of the Dark Energy Spectroscopic Instrument (DESI), which is conducting a redshift survey of about 40 million galaxies and quasars using a purpose-built instrument on the 4-m Mayall Telescope at Kitt Peak National Observatory. The main goal of DESI is to measure with unprecedented precision the expansion history of the Universe with the Baryon Acoustic Oscillation technique and the growth rate of structure with Redshift Space Distortions. Ten spectrographs with three cameras each disperse the light from 5000 fibers onto 30 CCDs, covering the near UV to near infrared (3600 to 9800 Angstrom) with a spectral resolution ranging from 2000 to 5000. The DESI data pipeline generates wavelength- and flux-calibrated spectra of all the targets, along with spectroscopic classifications and redshift measurements. Fully processed data from each night are typically available to the DESI collaboration the following morning. We give details about the pipeline's algorithms, and provide performance results on the stability of the optics, the quality of the sky background subtraction, and the precision and accuracy of the instrumental calibration. This pipeline has been used to process the DESI Survey Validation data set, and has exceeded the project's requirements for redshift performance, with high efficiency and a purity greater than 99 percent for all target classes.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: A new class of white dwarfs, dubbed DAHe, that present Zeeman-split Balmer lines in emission has recently emerged. However, the physical origin of these emission lines remains unclear. We present here a sample of 21 newly identified DAHe systems and determine magnetic field strengths and (for a subset) periods which span the ranges of ~ 6.5 -- 147 MG and ~ 0.4 -- 36 h respectively. All but four of these systems were identified from the Dark Energy Spectroscopic Instrument (DESI) survey sample of more than 47000 white dwarf candidates observed during its first year of observations. We present detailed analysis of the new DAHe WDJ161634.36+541011.51 with a spin period of 95.3 min, which exhibits an anti-correlation between broadband flux and Balmer line strength that is typically observed for this class of systems. All DAHe systems cluster closely on the Gaia Hertzsprung-Russell diagram where they represent ~ 1 per cent of white dwarfs within that region. This grouping further solidifies their unexplained emergence at relatively late cooling times and we discuss this in context of current formation theories. Nine of the new DAHe systems are identifiable from SDSS spectra of white dwarfs that had been previously classified as featureless DC-type systems. We suggest high S/N, unbiased observations of DCs as a possible route for discovering additional DAHe systems.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The shear measurement from DECaLS (Dark Energy Camera Legacy Survey) provides an excellent opportunity for galaxy-galaxy lensing study with DESI (Dark Energy Spectroscopic Instrument) galaxies, given the large ($\sim 9000$ deg$^2$) sky overlap. We explore this potential by combining the DESI 1\% survey and DECaLS DR8. With $\sim 106$ deg$^2$ sky overlap, we achieve significant detection of galaxy-galaxy lensing for BGS and LRG as lenses. Scaled to the full BGS sample, we expect the statistical errors to improve from $18(12)\%$ to a promising level of $2(1.3)\%$ at $\theta>8^{'}(<8^{'})$. This brings stronger requirements for future systematics control. To fully realize such potential, we need to control the residual multiplicative shear bias $|m|<0.01$ and the bias in the mean redshift $|\Delta z|<0.015$. We also expect significant detection of galaxy-galaxy lensing with DESI LRG/ELG full samples as lenses, and cosmic magnification of ELG through cross-correlation with low-redshift DECaLS shear. {If such systematical error control can be achieved,} we find the advantages of DECaLS, comparing with KiDS (Kilo Degree Survey) and HSC (Hyper-Suprime Cam), are at low redshift, large-scale, and in measuring the shear-ratio (to $\sigma_R\sim 0.04$) and cosmic magnification.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The Dark Energy Spectroscopic Instrument (DESI) is carrying out a 5-year
survey that aims to measure the redshifts of tens of millions of galaxies and
quasars, including 8 million luminous red galaxies (LRGs) in the redshift range
of $0.4
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Whiting 1 is a faint and young globular cluster in the halo of the Milky Way, and was suggested to have originated in the Sagittarius spherical dwarf galaxy (Sgr dSph). In this paper, we use the deep DESI Legacy Imaging Surveys to explore tentative spatial connection between Whiting 1 and the Sgr dSph. We redetermine the fundamental parameters of Whiting 1 and use the best-fitting isochrone (age $\tau$=6.5 Gyr, metalicity Z=0.005 and $\rm d_{\odot}$=26.9 kpc) to construct a theoretical matched filter for the extra-tidal features searching. Without any smooth technique to the matched filter density map, we detect a round-shape feature with possible leading and trailing tails on either side of the cluster. This raw image is not totally new compared to old discoveries, but confirms that no more large-scale features can be detected under a depth of r<=22.5 mag. In our results, the whole feature stretches 0.1-0.2 degree along the orbit of Whiting 1, which gives a much larger area than the cluster core. The tails on both sides of the cluster align along the orbital direction of the Sgr dSph as well as the cluster itself, which implies that these debris are probably stripped remnants of Whiting 1 by the Milky Way.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The shear measurement from DECaLS (Dark Energy Camera Legacy Survey) provides an excellent opportunity for galaxy-galaxy lensing study with DESI (Dark Energy Spectroscopic Instrument) galaxies, given the large ($\sim 9000$ deg$^2$) sky overlap. We explore this potential by combining the DESI 1\% survey and DECaLS DR8. With $\sim 106$ deg$^2$ sky overlap, we achieve significant detection of galaxy-galaxy lensing for BGS and LRG as lenses. Scaled to the full BGS sample, we expect the statistical errors to improve from $18(12)\%$ to a promising level of $2(1.3)\%$ at $\theta>8^{'}(<8^{'})$. This brings stronger requirements for future systematics control. To fully realize such potential, we need to control the residual multiplicative shear bias $|m|<0.01$ and the bias in the mean redshift $|\Delta z|<0.015$. We also expect significant detection of galaxy-galaxy lensing with DESI LRG/ELG full samples as lenses, and cosmic magnification of ELG through cross-correlation with low-redshift DECaLS shear. {If such systematical error control can be achieved,} we find the advantages of DECaLS, comparing with KiDS (Kilo Degree Survey) and HSC (Hyper-Suprime Cam), are at low redshift, large-scale, and in measuring the shear-ratio (to $\sigma_R\sim 0.04$) and cosmic magnification.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Photometric redshift (photo-z) is a fundamental parameter for multi-wavelength photometric surveys, while galaxy clusters are important cosmological probers and ideal objects for exploring the dense environmental impact on galaxy evolution. We extend our previous work on estimating photo-z and detecting galaxy clusters to the latest data releases of the Dark Energy Spectroscopic Instrument (DESI) imaging surveys, Dark Energy Survey (DES), and Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) imaging surveys and make corresponding catalogs publicly available for more extensive scientific applications. The photo-z catalogs include accurate measurements of photo-z and stellar mass for about 320, 293, and 134 million galaxies with $r<23$, $i<24$, and $i<25$ in DESI DR9, DES DR2, and HSC-SSP PDR3 data, respectively. The photo-z accuracy is about 0.017, 0.024, and 0.029 and the general redshift coverage is $z<1$, $z<1.2$, and $z<1.6$, respectively for those three surveys. The uncertainties of the logarithmic stellar mass that is inferred from stellar population synthesis fitting is about 0.2 dex. With the above photo-z catalogs, galaxy clusters are detected using a fast cluster-finding algorithm. A total of 532,810, 86,963, and 36,566 galaxy clusters with the number of members larger than 10 are discovered for DESI, DES, and HSC-SSP, respectively. Their photo-z accuracy is at the level of 0.01. The total mass of our clusters are also estimated by using the calibration relations between the optical richness and the mass measurement from X-ray and radio observations. The photo-z and cluster catalogs are available at ScienceDB (https://www.doi.org/10.11922/sciencedb.o00069.00003) and PaperData Repository (https://doi.org/10.12149/101089).