S. van den Bergh

Picture processing analysis of the optical structure of NGC 5128 /Centaurus A/

The spatial luminosity and color structure of the peculiar elliptical radio galaxy NGC 5128 are studied using photoelectrically calibrated photometry from plates taken with the CTIO 4-m telescope under conditions of excellent seeing (approx.1 arcsec). Using picture processing systems at KPNO and at the NASA Johnson Space Center, 10/sup 6/-pixel maps were constructed of the UBV surface brightness and of the color indices: U-B, B-V, and Q= (U-B)-0.72(B-V) over a 15.5 x 15.5-arcmin region centered on the nucleus of NGC 5128. The color maps reveal the existence of a young stellar population situated in a disk within the elliptical component of NGC 5128 that was previously masked by the absorption lane across the galaxy. The projection of the disk in the plane of the sky, as defined by OB stars and HII regions, is an ellipse with the major axis in PA=122/sup 0/ +- 4/sup 0/. If circular, the inclination of the plane of the disk in the sky is i=72/sup 0/ +- 3/sup 0/. At an assumed distance of 6 Mpc for NGC 5128, the diameter of the disk is approx.13 kpc. Vigorous star formation is occurring, mostly in elongated structures throughout the disk, with the greatest activity alongmorexa0» most of its outer perimeter, where apparently the largest amounts of dust are present. The ages of blue associations in the disk vary from approx.2 x 10/sup 6/ to approx.4 x 10/sup 7/ yr, with most in the range (1--3) x 10/sup 7/ yr. The luminosity distribution of the elliptical component of NGC 5128 follows de Vaucouleurss law over the range 20< or approx. =sigma/sub B/< or approx. =25 mag arcsec/sup -2/ in a manner that is similar to normal E2 galaxies. Optical emission above that expected from the elliptical population was marginally detected in the inner northeast and southwest radio lobes. The maximim B surface brightness of this emission was sigma/sub B/=25.1 mag arcsec/sup -2/ in the strongest regions of the northeast radio lobe. This emission may be an optical counterpart of synchrotron emission in the radio lobes.«xa0less

UBV photometry in the dwarf irregular galaxy NGC 6822

We present photoelectric UBV photometry for a sequence of 63 stars with 9.7 =0.42 +- 0.05, resulting in a true distance modulus (m-M)/sub 0/=23.0 and a distance Rapprox. =400 kpc. A comparison with Kaysers photographic photometry is given in an appendix.

A SEARCH FOR OB ASSOCIATIONS NEAR SOUTHERN LONG-PERIOD CEPHEIDS. V. AQ PUPPIS AND V620 PUPPIS

A photometric UBV survey is presented for 610 stars in a region surrounding the Cepheid AQ Puppis and centered southwest of the variable, based upon photoelectric measures for 14 stars and calibrated iris photometry of photographic plates of the field for 596 stars. An analysis of reddening and distance for program stars indicates that the major dust complex in this direction is ~1.8xa0kpc distant, producing differential extinction described by a ratio of total-to-selective extinction of R = AV /E B – V = 3.10 ± 0.20. Zero-age main-sequence fitting for the main group of B-type stars along the line of sight yields a distance of 3.21 ± 0.19xa0kpc (V 0 – MV = 12.53 ± 0.13xa0s.e.). The 2997 Cepheid AQ Pup, of field reddening E B – V = 0.47 ± 0.07 (E B – V (B0) = 0.51 ± 0.07), appears to be associated with B-type stars lying within 5 of it as well as with a sparse group of stars, designated Turner 14, centered south of it at J2000.0 = 07:58:37, –29:25:00, with a mean reddening of E B – V = 0.81 ± 0.01. AQ Pup has an inferred luminosity as a cluster member of MV = –5.40 ± 0.25 and an evolutionary age of 3 × 107 yr. Its observed rate of period increase of +300.1 ± 1.2 s yr–1 is an order of magnitude larger than what is observed for Cepheids of comparable period in the third crossing of the instability strip, and may be indicative of a high rate of mass loss or a putative fifth crossing. Another sparse cluster, designated Turner 13, surrounds the newly recognized 259 Cepheid V620 Pup, of space reddening E B – V = 0.64 ± 0.02 (E B – V (B0) = 0.68 ± 0.02), distance 2.88 ± 0.11xa0kpc (V 0 – MV = 12.30 ± 0.08xa0s.e.), evolutionary age 108 yr, and an inferred luminosity as a likely cluster member of MV = –2.74 ± 0.11. V620 Pup is tentatively identified as a first crosser, pending additional observations.

Metal-rich globular cluster NGC 6528

On the basis of its color-magnitude diagram NGC 6528 is one of the two most metal-rich globular clusters that have so far been found in the Galaxy. Such a conclusion is consistent with Morgans metallic line-strength classification of the integrated spectrum of this cluster. The reddening of NGC 6528 is estimated to be E/sub B/-V=0.52 +- 0.07, from which R=6.1 kpc and Z=-0.4 kpc.

Quasar 3C351: VLA maps and a deep search for optical emission in the outer lobes

VLA radio maps of the quasar 3C351 (z=0.371) at approx.2 and 0.4 resolution (a) show interaction with a relatively dense intergalactic medium, (b) show that there is electron acceleration within at least one of the radio lobes, and (c) imply that the intergalactic gas density is different on one side of the source than on the other. Striking similarities are found between the northern radio lobe of 3C351 and one of the outer hotspots of Cygnus A, and possibly other similar systems, in that the outer, on-axis hotspot is resolved and cusp-shaped, and the secondary off-axis hotspot is more compact. A search for optical emission in the outer lobes shows no emission stronger than 22/sup m/ in the J band and approx.21/sup m/ in the F band. There is also no evidence at these limits for a cluster of galaxies near the radio source, as is suggested by our conclusion that it is interacting with a medium of typical intracluster density.

A search for optical counterparts to gamma-ray sources

It is shown that up to 8 of the 13 presently known ..gamma..-ray sources might plausibly be identified with optically visible supernova remnants. A preliminary relationship between ..gamma..-ray luminosity and supernova age is derived. Available data suggest that approx.1/4 of the integrated galactic ..gamma..-ray flux may be due to the remnants of young supernovae.

Interstellar extinction and the flattening of the galactic globular cluster system

It is shown that the apparent flattening of the galactic globular cluster system is strongly affected by absorption. With A/sub v/=1.5 mag kpc/sup -1/ the observed circular symmetry of cluster positions in the Y,Z plane can be accounted for if the outer halo is approximately spherical while the inner halo has an axial ratio b/a< or approx. =0.5

A possible correlation between the luminosities of supernovae of type II and the absolute magnitudes of their parent galaxies

Published observations of the magnitudes of supernovae at maximum light indicate that supernovae of Type II (SN II) are more luminous in bright galaxies than they are in faint ones. No such correlation is found for supernovae of Type I (SN I). The correlation between M (max) of SN II and parent galaxy luminosity might be due to a dependence of SN II luminosity on metallicity. 20 references.