3 Discussion

Figure 1 shows greyscale I, J and K band plots of M 74 respectively. Figure 2 shows the equivalent contour plots. While it is not obvious in the greyscale plots, the I-band contour plot clearly shows an oval distortion in the contours at a radius of approximately 100 pixels (or 33.0 arcsec for a pixel scale of 0.33 arcsec/pixel) in the y-direction. The ellipticity of this contour is approximately $0.20\pm0.07$. The J-band and K-band images both show this elliptical contour as well, with ellipticities of $0.16\pm0.05$ and $0.17\pm0.05$ respectively. The position angle of this contour is approximately 45$^{\circ}$. At a larger radius, of approximately 40-44 arcsec, the J and K band images also show an elliptical contour at a position angle of $-55^{\circ}$. The ellipticities of this contour are $0.28\pm0.05$(in the J-band image) and $0.33\pm0.05$ (in the K-band image). As M 74 is a face-on galaxy, the ellipticities calculated here are real and not an inclination effect.

Figure 3 shows a plot of ellipticity of the contours shown in Fig. 2 versus minor-axis radius for both the I-band images (circles), the J-band image (triangles) and the K-band image (squares). Table 1 shows the same data. Both Fig. 3 and Table 1 show how the ellipticity of the I-band contours increases from the centre outwards and then decreases again at larger radii. The J and K band contours continue to increase, but their position angle changes abruptly from approximately 45$^{\circ}$ to $-55^{\circ}$ between a radius of 35 and 40 arcsec. This is not seen in the I band, probably due to the increase in dust extinction in this waveband. This suggests that M 74 may contain a bar within a bar, i.e. a smaller scale bar which is almost perpendicular to the large scale bar. This is a common feature often seen in high resolution images of barred galaxies with circumnuclear rings of star formation (e.g. Laine et al. 2002).

What we seem to have uncovered here is an oval distortion, rather than a strong bar. Is this oval distortion strong enough to create the circumnuclear star formation ring seen in M 74?

It has been shown in simulations of barred galaxies, that the process of funnelling gas towards the central regions of galaxies by a bar, eventually leads to the self-destruction of the bar (Friedli & Pfenniger 1991). Bar distortions are useful for driving gas towards an ILR, where the gas will then slowly accumulate. As this process continues, the gas can become gravitationally unstable and fall deeper into the potential well (see the double bar model in Friedli & Martinet 1993). This accumulation of material would eventually trigger a burst of star formation, thus leading to a metal rich, red inner stellar disk. The gas could fall further inwards and create a central mass concentration. If the mass of the central accretion becomes significant an extended ILR can be formed leading to dissolution of the bar (Friedli & Benz 1993; Friedli 1994). When the bar is dissolved signatures of its existence are often seen, e.g. boxy or peanut shaped bulges (e.g. Bureau & Freeman 1999) or a metal rich inner stellar disk, as seen in the Sombrero galaxy (Emsellem 1995; Emsellem et al. 1996). However, these signatures are usually only observed in edge-on galaxies. It is therefore difficult to tell if the oval distortion seen in M 74 is part of the progression of the dissolution of a bar.

Figure 3: Radius versus ellipticity of the isophotal contours of M 74. The circles represent the ellipticities of the isophotal contours in the the I band image, triangles the J band image and squares the K band image.

 

 

Table 1: Ellipticity versus radius in M 74.

Radius	Ellipticity
(arcsec)	I band	J band	K band
13.4	-	0.00$~\pm~$0.02	0.00$~\pm~$0.02
17.2	0.00$~\pm~$0.05	-	-
23.5	-	0.10$~\pm~$0.05	-
23.8	0.00$~\pm~$0.05	-	-
28.1	-	-	0.17$~\pm~$0.05
33.0	0.20$~\pm~$0.07	0.16$~\pm~$0.05	-
40.2	-	-	0.33$~\pm~$0.05
43.6	-	0.28$~\pm~$0.05	-
54.8	0.05$~\pm~$0.08	-	-

Weliachew et al. (1988) have suggested that even a weak bar, such as the bars they infer in NGC 6946 and Maffei 2, and the bar observed in M 74 here, can affect the radial distribution of the gas. Combes & Gerin (1985) simulated the behaviour of an ensemble of molecular clouds in a barred galaxy, using an N-body barred potential and a collisional scheme for gas clouds. They used different values for the angular velocity of the bar and showed that, when this the angular velocity of the bar is slow, spiral structure develops inside the corotation radius. Angular momentum is transferred inwards, and particles are trapped in a central orbit coincident with the ILR. Gaseous material can then accumulate at the ILR, forming a circular ring, if the bar angular velocity, $\Omega_{\rm p} \simeq (\Omega - \kappa/2)$, where $\Omega$ is the material velocity and $\kappa$ is the epicyclic frequency. The results of the Combes & Gerin (1985) simulations suggests that circumnuclear rings of star formation can exist, even in the presence of a weak bar potential. Thus, the oval distortion we have observed in M 74 here, may be responsible for the circumnuclear star formation activity observed by Wakker & Adler (1995).