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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 Halpha (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 Halpha and [NII]lambdalambda6548, 6584 by means of observations with an imaging Fabry-Perot interferometer and long-slit spectra (which also include [SII]lambdalambda6717, 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 Halpha (Fig. 4a). The ionized mass of the active jets is 2.4 x 106 Msun (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]lambda6583 / Halpha flux ratio is high in the SE jet and low, as is typical of normal HII regions, in the spiral arms. The Halpha velocity field (Fig. 4c) shows clear kinematic perturbations from the ambient velocity field along each jet. Lastly, Halpha 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

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) Halpha luminosity distribution in units of Msun pc-2 (1 cm-3/ne). The contours show the distribution of soft X-rays; note the coincidence with the SE jet. (b) [NII]lambda6583 / Halpha flux ratio. Note the abnormally high values associated with the SE jet (the [NII]lambda6583 line was not fully scanned in the NW jet). (c) Halpha velocity field, with 68 km s-1 FWHM resolution. (d) Halpha velocity dispersion, corrected in quadrature for instrumental resolution. The large widths along the SE jets arise from the putative helical gas motions (see original reference for a superior reproduction).

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