KISO H IMAGING OBSERVATIONS OF NEARBY GALAXIES

THE STAR FORMATION EFFICIENCY OF NGC 628

An isolated face-on spiral galaxy NGC 628 was observed with two narrow-band () interference filters centered on and off the H line. The seeing size was 3.5 arcseconds. Standard bias-subtraction, flat-fielding, and sky-subtraction was applied. We subtracted continuum light by scaling the flux of about 10 Galactic stars to obtain the H image (figure 1), which is very similar to the image obtained by Belley & Roy (1992).

The CO (J = 1-0) map of the central 4 arcminute square region (figure 1) was obtained with the NRO 45 m telescope (HPBW = 14 arcseconds). The grid was taken along the major and minor axes of the galaxy with 11 arcsecond spacing.

  
Figure 1: dK image (see text, left panel) and CO (J = 1-0) map (right panel) of NGC 628 with linear scale intensity. Contours indicate logarithmic H intensity with an interval of 0.6. The central 4 arcmin (which corresponds to 8 kpc) is shown. North is top and east is to the left.

  
Figure 2: Spectra of NGC 628 at it's nuclear region (thick line) and at an HII region (thin line). The spectrum at the HII region is shifted vertically for clarity.

The K'-band image was obtained with an infrared camera OASIS attached to the 188 cm reflector at the Okayama Astronomical Observatory (OAO). The seeing was 2 arcseconds. We subtracted a rotationally averaged component to extract arms and any possible embedded bar component. The resultant image, which we refer to as the dK image, is shown in figure 1. We obtained spectra for the nuclear region and an HII region at OAO (figure 2).

In the dK image, two spiral arms are clearly identified while no bar is discernible. Regions with strong H and CO emission are predominantly located along these spiral arms. Most of the bright spots in the dK image are identified with H emission (figure 1).

If the SFE is uniform over the entire galaxy, a tight correlation is expected between H and CO intensities. Unexpectedly, the peaks of H and CO emission are poorly correlated within 1 kpc. We find many giant HII regions in the outer part of the arms, but even these regions are hardly identified with giant molecular clouds. We find no giant HII regions at two giant molecular clouds located close to the center. The extinction in the HII region of this galaxy is small (Belley & Roy, 1992). The poor correlation between identified H and CO peaks may suggest that the SFE is not uniform in this galaxy and is regulated not only by the mass of molecular clouds but also by other physical parameters, such as the velocity dispersion of the molecular clouds. Alternatively, H and CO emission may represent two different evolutionary stages of star formation. The poor correlation observed is contrary to the case of M51, in which most of the giant HII regions are identified with giant molecular clouds (Kuno et al. 1995).

We detect no nuclear H emission. A slight overestimate (3-5 %) of the scaling factor in the subtraction of continuum light might lead to an underestimate of the nuclear H emission by a about 20%. However, the spectrum at the nuclear region (figure 2, thick line) indicates that H is actually seen in absorption here. Since the absorption at H is typical for spectra of late-B and A type stars (Gunn & Stryker, 1983), the absorption at the nuclear region of the galaxy suggests that these stars are the high-mass end of the present-day mass function and the nuclear region is in the post starburst phase.

© Copyright Astronomical Society of Australia 1997