Starburst Disk

Our observations reveal a small irregular disk containing heavy obscuration and a central starburst. The H$\alpha$ and [OIII] flux maps (Figs. 1 and 2) reveal little ionized gas in the extended disk of M82, outside of the central nuclear starburst region, as found by slit spectra ([O'Connell & Mangano 1978]). Small concentrations of line emission that do exist in the disk outside of the starburst, such as the regions 0.'5 east of the nucleus (see also [O'Connell & Mangano 1978]; Fig. 3), appear to be giant HII regions. The [NII]/H$\alpha$ ratio map (Fig. 4) indicates moderate values for these regions ($\sim$0.56), consistent with a cooler range of photoionized HII regions ($T_{\hbox{max}}\sim40,000$ K, [Evans & Dopita 1985]; [Veilleux & Osterbrock 1987]). Such regions exhibit low levels of [OIII] emission, consistent with our non-detection in Figure 2. Although the high extinction in the disk of M82 (3-27 mag; [O'Connell et al. 1995]; [McLeod et al. 1993]) serves to hide smaller HII regions from our optical observations, the paucity of large HII regions suggests that the level of star formation must be low outside of the central starburst. This conclusion has also been reached by multi-epoch radio supernovae studies (e.g., [Kronberg, Biermann, & Schwab 1985]; [Huang et al. 1994]). The weak nature of the [OIII] emission implies a high overall metallicity in M82, as has been suggested by previous observations (e.g., [Duffy et al. 1987]; [Gaffney & Lester 1992]), and as is anticipated due to chemical enrichment by the starburst (e.g., [Kobulnicky & Skillman 1996]).

The nuclear emission is dominated by two large saturated regions, each approximately 200 pc in diameter, and centered $\sim$125 pc from the 2 micron stellar nucleus (see Fig. 1). These regions correspond to knots A and C of [O'Connell & Mangano 1978] and are known to be extremely high surface brightness ``clusters of clusters'' of young ($T\lesssim50$ Myr) stars ([O'Connell et al. 1995]). Also identifiable in Figure 1 are knots D and E (also saturated), as well as knots F, G, and H. Knot B is extremely faint at H$\alpha$ wavelengths, especially when compared to broadband ([O'Connell & Mangano 1978]) and ultraviolet ([Hennessy 1996]) observations, suggesting a higher gas content in the inner regions of the galaxy. The outflow can be traced to knots A and C. This relationship is particularly well-demonstrated by the [NII]/H$\alpha$ map (Fig. 4) and the [OIII] flux map (Fig. 2).

 

Figure: Rotation curves from the major axis of M82, in order of increasing wavelength. Literature sources include: a.-b. [McKeith et al. 1993], c. this paper, d.-e. [Castles, McKeith, & Greve 1991], f. [McKeith et al. 1993], g. [Beck et al. 1978], h. [Loiseau et al. 1990]. A systemic velocity of 208.7 km s^-1 has been subtracted from the Fabry-Perot data.  

We have plotted the H$\alpha$ radial velocities along a straight line corresponding to the major axis of the galaxy in Figure 6 (panel c), along with a number of rotation curves from the literature at a range of wavelengths. The published systemic velocity of M82, 203 km s^-1 ([de Vaucouleurs et al. 1991]), corrected to a heliocentric value of 208.7 km s^-1, has been subtracted from the H$\alpha$ Fabry-Perot data. The rotation curve rises to approximately 100 km s^-1 within $\sim$9'' ($\sim$140 pc) of the nucleus and remains relatively flat to the edges of the observations. No substantial fall-off in the rotation curve is seen, due primarily to the limited radial extent of line emission from the disk. The observed rotation curve matches well those found at H$\alpha$ by [Heckathorn 1972] (his Fig. 12) and [McKeith et al. 1993] (panel d of Fig. 6), including the turn-over and asymptotic velocities and the nuclear velocity gradient ($\sim$11 km s^-1 arcsec^-1). Figure 6 also illustrates an increase in the central velocity gradient with wavelength. As has been pointed out by other authors (e.g., [McKeith et al. 1993]), this effect is indicative of the large extinction toward the nuclear regions of the galaxy.

The H$\alpha$ rotation curve also correlates well with that of the 250 pc nuclear ring seen in molecular emission lines (e.g., [Loiseau et al. 1990]). The double-lobed structure of the central H$\alpha$ emission is suggestive of an edge-on ring structure as well, interior to the molecular ring. The starburst could be identified with the ring of ionized gas and is probably propagating outward, fueled by the cold gas in the molecular ring ([Waller, Gurwell, & Tamura 1992]; cf. [Shen & Lo 1995]). This ring may also provide much of the extinction toward the nucleus of M82 (e.g., [Telesco & Gezari 1992]). The dynamic conditions of the central regions would have removed most of the obscuring material interior to the ring, as has been suggested in our Galaxy (e.g., [Becklin, Gatley, & Werner 1982]). The resolution of the central star clusters by recent HST observations ([O'Connell et al. 1995]) suggests that any such ring would be very clumpy however, and the two bright regions may indeed represent real spatial enhancements in the distribution of ionized gas and star formation. The relationship between this ring structure and the proposed bar in M82 ($r\sim500$ pc; [Telesco et al. 1991]; [Achtermann & Lacy 1995]) is not clear.

The [NII]/H$\alpha$ line ratio map (Fig. 4) shows a high ratio in the disk, especially at large radii, where the value is observed to exceed 1.0. The actual line ratios in the disk may be even slightly higher and possibly more uniform, due to dilution by the central starburst and the halo. Such high ratios are by no means rare in the nuclei of disk galaxies ([Keel 1983]), particularly LINERS (e.g., NGC 4319 [[Sulentic & Arp 1987]], NGC 5194 [[Ford et al. 1985]; [Goad & Gallagher 1985]]), and are understood to originate with shock excitation and/or photoionization by a power-law source ([Veilleux & Osterbrock 1987]). Although it is unlikely that M82 harbors an AGN (e.g., [Rieke et al. 1980]; [Colina & Pérez-Olea 1992]; [Muxlow et al. 1994]), a comparison with other galaxies which exhibit pervasive high [NII]/H$\alpha$ line ratios in their extended disks is instructive: studies of NGC 3079 ([Veilleux et al. 1994]) and especially NGC 1068 ([Bland-Hawthorn, Sokolowski, & Cecil 1991]) have found high [NII]/H$\alpha$ ratios of 0.6-1.3 across much of the inner disk. In NGC 1068, the disk HII regions are found to reside in regions of lower [NII]/H$\alpha$ ratio ($\sim$0.3-0.8), while the disk as a whole is permeated with gas exhibiting the higher ratios, similar to what we observe in M82. In order to boost this forbidden line to recombination line ratio, the ``heating per ionization'' must be high, for which most models require high energy photons, energetic electrons, or a dilute radiation bath ([Sokolowski 1992]).

In the case of M82, an attractive candidate to enhance the [NII]/H$\alpha$ ratio in the disk is the concept of ``mixing layers'' ([Slavin & Cox 1993]; [Voit, Donahue, & Slavin 1994]). The turbulence resulting from the interaction of hot supernovae remnants with the ambient ISM creates an intermediate temperature phase, which models suggest emits a radiation field somewhat harder than thermal bremsstrahlung. This process can produce [NII]/H$\alpha$ ratios in excess of 1.0 ([Slavin, Shull, & Begelman 1993]). By employing soft X-rays as the photoionization mechanism, mixing ratios have been used to model line emission ratios of up to 3-4 in cooling flows ([Donahue & Voit 1991]; [Crawford & Fabian 1992]). Considering the current optical appearance of the disk of M82, its interaction with the galaxy M81 approximately 10⁸ years ago, and the energetic activity associated with the nuclear starburst, a turbulent inner disk would not be unexpected.

However, as mentioned above, our observations do not detect significant numbers of star forming regions outside the starburst nucleus. Even if this is attributed to high levels of extinction in the disk, other wavebands confirm the low level of star formation. All of the radio supernovae discovered by [Kronberg, Biermann, & Schwab 1985] are within 300 pc of the galaxy's nucleus, well inside the region of highest [NII]/H$\alpha$ ratios. The diffuse X-ray flux in the disk has also been shown to decrease rapidly with radius, implying a reduced star formation rate outside the nuclear regions ([Bregman, Schulman, & Tomisaka 1995]). Other models for producing high [NII]/H$\alpha$ ratios in the disk must be considered, such as chemical enrichment and cosmic ray heating. A combination of shock and photoionization may also provide a solution (e.g., [Hunter & Gallagher 1990]), although detailed models are not yet available.