Is Dust Extinction the Cause of Hollow-disk Profiles in Spiral Galaxies?
Session 77 -- Spirals II
Display presentation, Wednesday, 11, 1995, 9:20am - 6:30pm

## [77.04] Is Dust Extinction the Cause of Hollow-disk Profiles in Spiral Galaxies?

H.A. Thronson, D. Barnaby, P.E. Johnson, E.J. Spillar (WIRO)

Barnaby, Thronson, \& Estep (1994) presented a bulge/disk decomposition of the edge-on, Sbc-galaxy NGC 4013. From this decomposition, we derive a bulge-to-disk brightness ratio (B/D ) equal to 0.03, which means that NGC 4013 exhibits one of the smallest B/D s among the Sbc class of galaxies. However, the disk radial profile of this galaxy resembles a so-called hollow-disk'' profile. That is, the inward extrapolation of the model disk-light profile becomes brighter than that of the observed profile. This suggests that a hollow-disk model, a model in which the disk profile is truncated inside an inner radius, can be fit to the surface photometry. Starlight inside the inner-cutoff radius is then part of the bulge distribution. Thus, the hollow-disk model has the effect of apparently increasing the bulge surface brightness at the expense of the disk surface brightness, thereby increasing $B/D$.

In this paper, we present our hollow-disk decomposition for NGC 4013. From the decomposition, we find that the disk exponential scale length is 2.2 kpc and the disk exponential scale height is 0.22 kpc. We also derive a bulge core radius of 0.285 kpc and a bulge axial ratio of 0.75. In addition, we find that $B/D = 0.43$, consistent with the Sbc classification of NGC 4013.

Finally, we obtain a good fit (reduced $\chi^{2} = 0.025$) between the model and data with an inner-cutoff radius equal to 0.83 kpc. What makes this last parameter interesting is that we make our observations in the near-infrared $H$-band (1.68 $\mu$m), a wavelength at which starlight can penetrate about $6^{\rm m}$\hspace{0.25em} of visual extinction. For comparison, the disk inner-cutoff radius derived from visual observations of NGC 4013 equals 2.3 kpc. This decrease in size of the hollow region with increasing wavelength is expected if extinction of starlight by dust around the bulge is the cause of the hollow-disk appearance in this spiral galaxy.

This work is supported by NSF grant AST-9117096; the Theodore Dunham, Jr. Grant of the Fund for Astrophysical Research, and by a Grant-in-Aid of Research from the National Academy of Sciences through Sigma Xi, The Scientific Research Society. \\

\noindent Barnaby, D., Thronson, H. A., \& Estep G. M. 1994, BAAS, 24, 2, 830