Early-Type Galaxies
Activity in Early-Type Galaxies
Line emission is a spectroscopical hint for activity in galaxies. If emission is present in early- type galaxies of the morphological Hubble type elliptical to lenticular (E0 - S0/Sa), it has generally the characteristics of Low-Ionisation Nuclear Emission-line Region Galaxies, the so called LINER class which is based upon emission line diagnostics defined by Veilleux, S. & Osterbrock, D.E., 1987. The optical emission line spectra of LINERs broadly resemble those of traditional active galactic nuclei (AGN) such as Seyfert nuclei, but they have characteristically lower ionisation levels. The physical nature of LINERs is not solved up to now because these galaxies do not appear as a homogenous class. This proves that various of emission types in samples of early-type galaxies are found. Relevant excitation mechanisms range from non-stellar photoionisation to thermal, stellar heating processes. In case of non-thermal, non-stellar photoionisation by a central ionising source LINERs represent the faint end of the local luminosity function of AGNs where the central engine is described by a massive black hole. Other possible mechanisms of activity in connection with the LINER phenomenon are shock heating or ionisation by stellar objects like massive hot O stars which was discussed since the 80ies, for example by Fosbury R.A.E. et al. 1978 or Filippenko, A. V. & Terlevich, R. 1992. And ,of course, there are composite AGN excitation models like combination of stellar and shock heating. If LINERs are excited by young main sequence stars, star formation amount for most of the detected line emission, but this is in considerably contrast to the general picture of minimal star formation in early-type galaxies which are dominated by old stellar populations.
Dust and Ionized Gas in Early-Type Galaxies
Early-type galaxies and specially elliptical galaxies were usually known
to
be gas and dust free, but in the past ten years several surveys have been done
using CCD subtraction techniques in order to detect extended gaseous disks and
filaments of dust.
Optical emission lines in particular trace the
warm (T~104K) ionised and excited component of the circumnuclear (r < 1kpc)
interstellar medium (ISM). The warm gas appears in a disk- like filamentary
structure often with decoupled kinematics with the stellar body of the galaxy
suggesting an external origin of most of the gaseous matter during interaction
events.This is equally true for dust found in early-type galaxies,
this dust appears to be kinematically and morphologically coupled with the
ionised gas. Actually, precisely the opposite is true for the stellar
component.
The nature, the origin and the future evolution of the
gas and dust in elliptical galaxies is still a highly controversial subject. A
first scenario is one in which the ISM is a normal, quiescent component of
these galaxies. An alternative scenario is that in which merging and galaxy-
interaction plays a dominant role in formation and distribution of the ISM.
The possible external origin of most of the ISM in early-type galaxies with
signatures of activity like LINERs is a good nutrient in the discussion that
there is a connection between AGN and merging.
Stellar Properties of Early-Type Galaxies
Early-type galaxies are dominated by old (G-and K-type giants)
stellar populations. New studies show that most elliptical galaxies have too
much blue light to be completely explained by old stellar populations.
Therefore, early-types with their strong Balmer break at 4000A due to late
spectral stellar objects- could contain at least a few percent of young and/or
intermediate-age stellar populations. The contribution of these younger
stellar populations is difficult to determine because of the
age-metallicity-degeneracy. The main obstacle to a quantitative understanding
of the ages of old stellar populations is the fact that age and metallicity
have a similar effect on the spectral energy distribution of old stellar
populations. Fortunatly, a few indices break this degeneracy by being either
more abundance sensitive (Fe4668, Fe5015, Fe 5709, Fe5782) or more age
sensitive (G4300, H beta, and presumably higher order Balmer lines)
than usual.
Another crucial question not yet solved is to know whether
the stellar population in the nuclear regions of AGN differ from that of
non-active galaxies of the same Hubble type. Recent studies show that LINERs
are dominated by metal-rich old stellar populations, with little star formation
still going on and this maybe explains that young, massive stars are a possible
engine in a fraction of (weak) LINERs.
Maybe we can win a better
understanding for stellar composition in early-type galaxies if we look to the
UV range. The UV-upturn (UVXS) at lambda < 2000A is a possible indicator
for star formation of young stars but this UVXS can be explained by an hot and
old stellar population, too.
One of the possible explanations of young
stars in early-type galaxies is induced star formation as a consequence of
interacting events with other galaxies in the galaxy environment. Therefore, it
is possible that merging and other interaction influences the nuclear activity
in galaxies in different ways.
Literature:
Activity in Early-Type Galaxies:
Dust and Ionized Gas in Early-Type Galaxies:
Stellar Properties of Early-Type Galaxies: