Brightness distributions in compact and normal galaxies

Citation

Kormendy, John (1976) Brightness distributions in compact and normal galaxies. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/YZFX-X226. https://resolver.caltech.edu/CaltechETD:etd-09102008-104759

Abstract

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A study is made of three problems on luminosity distributions in galaxies: the nature of red compact galaxies, the systematic properties of spheroids, and the effect of decomposing observed profiles into spheroid and disk components.

Photographic brightness profiles are measured and discussed for sixteen compact and four normal galaxies, with the following results.

(i) Red compacts are qualitatively E and SO galaxies.

(ii) Their central and mean surface brightnesses are generally normal. Only II Zw 67 and VII Zw 421 are noticeably compact.

(iii) Profile components which Sandage, in the Hubble Atlas, called lenses occur with great frequency and strength. They are the "large, nearly uniform brightness halos" described by Zwicky. VII Zw 793 has an exceptionally strong lens, which must be a disk because it shows spiral structure.

(iv) II Zw 67 and VII Zw 421 seem to be prototypes of a class of very luminous, early-type objects with dominant spheroid-like lenses, and spectra characteristic of metal-poor stars.

(v) Lenses are a distinct and independent component in galaxy profiles. They seem to be morphologically intermediate between spheroids and disks.

A systematic study of galaxy spheroids is made by fitting the de Vaucouleurs r[superscript 1/4]-law to the profiles of sixteen compact and nineteen normal galaxies. The results are as follows.

(i) Ellipticals with massive neighbors have bright outer halos not possessed by more isolated objects. Possible tidal explanations are considered.

(ii) The brightness and radius parameters are related by B[subscript oV]= 3.02 log r[subscript o] + 19.74 B mag arcsec[superscript -2] ([...]). Here r[subscript o] varies from 1.0 to 14.1 kpc, and B[subscript oV] from 19.8 to 23.3 B[...]. Brighter galaxies tend to have larger r[subscript o] and fainter [subscript oV'] with considerable scatter.

(iii) Some compacts are truly compact in having abnormally small r[subscript o] and bright B[subscript oV] for their absolute magnitude; most are just intrinsically faint.

(iv) The B[subscript oV](log r[subscript o]) relation can be used to determine distances. We show that the Virgo cluster is closer than its redshift distance by δ(m-M) ≤ 0[...]07 ± 0[...]23, a null result supporting the uniform Hubble flow advocated by Sandage and Tammann.

Detailed comparisons are made of how profile models due to de Vaucouleurs, Hubble and King fit galaxies. We show that the above results are model independent.

Finally, exponential disk parameters are derived for seven S0 compacts and for NGC 7457. The central disk brightness B(0)[subscript c] is generally normal, but is unusually faint in two objects. Scale lengths are ~ 7 kpc, at the long end of the distribution found in normal galaxies. Spheroids clearly contribute significantly to the disk parameters. We therefore decompose the profiles into their underlying components. The disks, actually lenses in these objects, are shown to have a (possibly complete) inner brightness cutoff, where they enter the dominant part of the spheroid. Parameters of the underlying distributions are poorly determined, because there is considerable freedom in their relative strength. However, spheroids, and the above results on spheroids, are little affected by decomposition. In contrast, disks become significantly fainter.

Motivated by this, we use model galaxies to investigate the spheroid contribution to B(0)[subscript c] derived for a composite profile. Exponentials with B(0)[subscript c] = 20-25 B[...] are added to spheroids with a wide range of parameters. These reproduce very well the distribution of B(0)[subscript c] observed for normal galaxies. Thus the near universality of B(0)[superscript c] = 21.65 ± 0.30 B[...], observed by Freeman for 28 of 36 galaxies, is largely due to the contribution of the spheroid. Real disks can be much fainter. Measurements of galaxies with no spheroid are found to be consistent with this conclusion.

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