Quasar observations
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High-Redshift Quasar Observations and Surveys
Recent years have seen significant progress in the discovery and study of high-redshift quasars, which are crucial for understanding the early universe, supermassive black hole growth, and the intergalactic medium. Large surveys like the Dark Energy Spectroscopic Instrument (DESI) and the Sloan Digital Sky Survey (SDSS) have systematically identified hundreds of new quasars at redshifts z ≳ 5, expanding the known population and enabling detailed statistical and individual analyses. For example, DESI has discovered over 400 new quasars at 4.7 ≤ z < 6.6, with more than 220 at z ≥ 5, providing a substantial increase in the sample size for high-redshift studies . Similarly, the SDSS-IV/eBOSS quasar catalog now contains over 750,000 spectroscopically confirmed quasars, including more than 225,000 new discoveries, with high completeness and low contamination rates .
Techniques for Quasar Identification and Confirmation
Quasar candidates are typically selected using multi-band photometry from surveys such as the DESI Legacy Imaging Surveys, Pan-STARRS1, VISTA, and WISE, often combined with spectral energy distribution (SED) model fitting. This approach allows for efficient identification of high-redshift quasars and minimizes contamination from stars and other sources. Spectroscopic confirmation, using facilities like the NTT, Gemini South, and Keck telescopes, is essential for accurate redshift determination and characterization of quasar properties Yang2023Reed2017Bañados2022.
Physical Properties and Variability of Quasars
Quasar observations reveal a wide range of physical properties, including emission line strengths, absorption features, and variability. Optical monitoring programs have shown that all quasars exhibit some level of variability, with some sources displaying violent optical outbursts and others showing more moderate changes. However, well-defined periodicities are generally not observed . Spectroscopic studies of low-redshift, radio-loud quasars provide uniform datasets for analyzing emission lines such as Hα and [Ne v], contributing to our understanding of quasar diversity .
Insights from High-Redshift Quasar Spectroscopy
Near-infrared spectroscopy of quasars at z > 6.3 has enabled measurements of black hole masses, accretion rates, and chemical abundances. These studies indicate that early quasars host massive black holes (up to several billion solar masses) and exhibit high accretion rates, often near or above the Eddington limit. The lack of strong redshift evolution in emission line ratios suggests that these environments are already metal-enriched at very early cosmic times . Additionally, high-resolution observations with ALMA have resolved the dust and [C ii] emission in quasar host galaxies, revealing complex morphologies and evidence for ongoing galaxy mergers .
Multiwavelength and Gravitational Lensing Observations
Quasar catalogs now include multiwavelength data from X-ray, infrared, and radio surveys, providing a more complete picture of quasar properties across the electromagnetic spectrum . X-ray observations of extremely luminous quasars at high redshift, such as CFHQS J142952+544717 at z = 6.18, reveal soft spectra and persistent high luminosity, possibly linked to inverse Compton scattering in relativistic jets . Gravitational lensing surveys using Gaia and Pan-STARRS have identified dozens of lensed quasars, which are valuable for studying quasar structure, cosmology, and the distribution of dark matter .
Conclusion
Quasar observations, especially at high redshift, are rapidly advancing due to large-scale surveys, improved selection techniques, and multiwavelength follow-up. These efforts are uncovering new populations of quasars, refining our understanding of their physical properties, and providing key insights into the early universe, black hole growth, and galaxy evolution Yang2023Lyke2020Reed2017+5 MORE.
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