Schmidt's classic definition is that quasars are star-like objects of large redshift. More quantitatively, quasars are generally considered to have z > 0.1 and MB < -23 (H0 = 50) mag (see Schmidt and Green 1983). Traditionally, i.e. at resolutions of 1 - 2 arcsec, they were considered as being star-like, a description that is intertwined with the redshift and absolute magnitude limits just given. We now know that better spatial resolution often yields evidence of a host galaxy. Other key properties of quasars are that they have broad emission lines 2 in their spectra and that they can emit continuum radiation across the electromagnetic spectrum from -rays to radio waves, with ultraviolet and X-ray emission usually being very prominent. Also, quasars show variability on time scales of days to years.
How well can we explain all these properties? Although it is generally believed that the picture of an accretion disk surrounding a black hole is correct, agreement between current models and observations is distressingly poor in many cases.
Operationally, it is important to be aware of the effect of the apparent size and luminosity limit in the definition of quasars on modern surveys. For example, as the angular resolution of surveys improves to 1 arcsec or better and the depth of surveys increases either because of the use of larger aperture telescopes or longer exposures, the host galaxies of quasars will be increasingly visible. In such cases, strict imposition of the "star-like" criterion for quasars will exclude bona-fide AGNs.
Similarly, we now know that high-luminosity Seyfert galaxies can overlap in absolute luminosity with low-luminosity quasars, and analyses of surveys must allow for this. For example, deep surveys for quasars with good angular resolution will find both quasars and AGNs. It is important for the determination of the evolution of the entire AGN population that surveys consider what classes of objects they are including and perhaps rejecting.
2 For this article, BL Lac objects will be considered as a separate class, although they are members of the AGN family. Back.