Since active galactic nuclei (AGN) comprise only a small fraction of all galaxies, it is more relevant for our general understanding of galactic processes to observe normal galaxies. With the availability of imaged X-ray observations, galaxies not necessarily dominated by a point source in their nucleus have now been studied in detail. This review will give an overview of these results, with an emphasis on galaxies exhibiting moderate or low amounts of activity in their nuclei.
In this paper, a low-luminosity AGN (LLAGN) is considered to be a galaxy with
Seyfert-like optical spectra and an X-ray luminosity less than
1042 ergs s-1.
A starburst galaxy is a galaxy in which evidence of enhanced
star-formation rates are observed, particularly in kpc-sized nuclear regions.
Most often this is observed by the presence of HII-region like optical
emission lines, however other signs include high IR luminosities (i.e.,
LIR > 1010
L
;
c.f., Telesco 1988).
In LINER galaxies the
optical emission lines are observed with line diagnostic ratios that
differ from both starburst and LLAGN ratios. The most likely scenarios for the
ionization flux in LINERs are a LLAGN (albeit with different physical
properties than LLAGN with Seyfert-like spectra), shocks
(Dopita et al. 1996)
and hot stars
(c.f., Shields 1992).
Interestingly, 15% of LINERs exhibit broad
H
emission
(Ho, Filippenko,
Sargent, & Chen 1997).
These ``LINER 1'' galaxies are almost certainly LLAGN, and
the fact that the ratio of LINER 1 to LINER 2 galaxies is similar
to the fraction of Seyfert 1/Seyfert 2 galaxies is suggestive that most LINERs
are indeed LLAGN.