6.1. Background
NGC 4258 is a spiral galaxy (type SABbc) which
contains a well-known pair of
"anomalous arms", visible in optical emission lines and radio continuum
(e.g.,
Martin et al. 1989).
These arms curve in the same sense as the normal spiral
arms delineated
by HII regions, but differ from them both morphologically (the
"anomalous arms" are
much smoother) and in the gaseous excitation. The nucleus of NGC 4258 exhibits
weak activity in the form of a faint, broad component to
H
(Stauffer 1982)
and luminous water vapor masers
(Claussen, Heiligman
& Lo 1984).
The Einstein HRI image
(Fabbiano, Kim &
Trinchieri 1991)
shows X-ray emission from the SE anomalous arm.
The two anomalous arms are generally interpreted as oppositely directed "jets"
(e.g.,
Ford et al. 1986)
that emerge from the nuclear region as linear features, and
then bend at 2-5 kpc radius in a trailing sense with respect to galactic
rotation. They are undoubtedly the most spectacular examples of
emission-line jets known in a spiral
galaxy, being prominent due to both the proximity of NGC 4258 (D = 7 Mpc, so 1 arc
sec = 35 pc) and the likelihood that the nuclear ejection axis lies
close to the plane of the galaxy disk (as discussed below). Recently, we
(Cecil, Wilson &
Tully 1992,
hereafter CWT) have completed a kinematic study of NGC 4258 in
H and
[NII]
6548, 6584 by
means of observations with an imaging Fabry-Perot interferometer and
long-slit spectra
(which also include [SII]6717, 6734) taken along the jets.
Fig. 4 shows maps of various parameters derived
from the Fabry-Perot observations.
Both the jets, extending to SE and NW of the nucleus, and the "normal"
spiral arms, extending to N and S, are prominent in
H
(Fig. 4a). The ionized mass
of the active jets is 2.4 x 106
M
(1
cm-3/ne). X-ray emission (contours in
Fig. 4a) is
found along the SE jet and in a localized region in the "crook" of the NW
jet. Fig. 4b shows that
the [NII]6583 /
H flux ratio is high in the SE
jet and low, as is
typical of normal HII regions, in the spiral arms.
The H velocity field
(Fig. 4c) shows
clear kinematic
perturbations from the ambient velocity field along each jet. Lastly,
H is considerably
broader along the jets than in the normal HII regions
(Fig. 4d). In the
following discussion, I summarize our results on each jet.
|
Figure 4. (from
Cecil, Wilson &
Tully 1992).
Maps of NGC 4258. N is up and E is at
left; ticks at 1.2 kpc (36 arc sec) increments; cross at the
nucleus. (a) H |