Pictures of quasars
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Pictures of Quasars: Insights from Recent Research
Hubble Space Telescope Quasar Host Galaxy Images
The Hubble Space Telescope (HST) has been instrumental in capturing detailed images of quasar host galaxies. Studies have utilized HST's capabilities to observe luminous type-2 quasars, revealing significant details about their host galaxies and the impact of quasar winds on galaxy evolution. For instance, observations in the rest-frame yellow-band (~5000 Angstroms) of 20 luminous quasar host galaxies at redshifts between 0.2 and 0.6 show that these galaxies are primarily bulge-dominated and exhibit high star-formation rates, often triggered by recent minor mergers. Additionally, HST images have shown bright merger signatures in about half of these galaxies, a higher fraction compared to inactive ellipticals.
Gravitationally Lensed Quasars
Gravitational lensing has provided unique opportunities to study quasars at high redshifts. The discovery of COOL J0335−1927, a quasar at z = 3.27, lensed into three images by a galaxy cluster, exemplifies the power of lensing in quasar studies. This quasar, with a maximum image separation of 23.3 arcseconds, is the highest redshift wide-separation lensed quasar known to date. Such lensed quasars allow for detailed studies of quasar properties and the intervening absorbers visible in their spectra.
High-Redshift Quasar Surveys
Surveys targeting high-redshift quasars have expanded our understanding of the early universe. A recent survey reported the discovery of J0025−0145, an intermediately lensed quasar at z = 5.07, and J2329−0522, a kiloparsec-scale close quasar pair at z = 4.85. These discoveries highlight the effectiveness of using color and shape-based selection methods in public imaging surveys to identify high-redshift quasar candidates. The HST images of these quasars provide insights into their host galaxies and the effects of lensing magnification.
Quasar Host Galaxy Morphology
Deep imaging studies have revealed that the host galaxies of quasars are often massive ellipticals, especially for those with high nuclear luminosities. For example, a comprehensive HST imaging study of radio-quiet quasars (RQQs), radio-loud quasars (RLQs), and radio galaxies (RGs) found that spheroidal hosts become more prevalent with increasing nuclear luminosity. This study also demonstrated that the basic properties of these hosts are indistinguishable from those of quiescent, evolved, low-redshift ellipticals of comparable mass. This finding suggests that the morphology of the host galaxy does not determine radio-loudness and supports the relationship between black hole and spheroid mass.
Type II Quasars and Host Galaxy Interactions
Type II quasars, characterized by their obscured centers, provide a unique opportunity to study their host galaxies without the interference of bright nuclei. HST images of nine type II quasars at redshifts between 0.2 and 0.4 reveal that most of these host galaxies are elliptical with de Vaucouleurs light profiles. These galaxies are more luminous and bluer than typical galaxies at the same redshift, indicating active star formation and significant dust obscuration. The images also show that scattered light from the nucleus contributes significantly to the broadband continuum emission, especially in the UV.
Conclusion
Recent research leveraging advanced imaging techniques, particularly with the Hubble Space Telescope, has significantly enhanced our understanding of quasars and their host galaxies. From detailed morphological studies to the discovery of high-redshift lensed quasars, these observations provide critical insights into the nature of quasars and their role in galaxy evolution. The ongoing exploration of quasar host galaxies continues to reveal the complex interactions and characteristics that define these fascinating astronomical objects.
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