Astron. Astrophys. 323, 323-336 (1997)

4. Morphology of the individual galaxies

Optical B band and 1.49 GHz radio contour overlays for six of the eight galaxies have been shown in DBDKS. In this Section we discuss the morphology of the galaxies based on these images of DBDKS, on the additional overlays shown here, and on a literature search using NED ⁴

color images were created by taking the logarithm of the ratio of the B - and R - images, with appropriate calibration constants added. The images are corrected for the galactic color excess, as discussed in the section on photometry. The greylevels of the images are therefore representative of calibrated surface color values. The images were smoothed with a Gaussian kernel of FWHM and have an approximate cutoff at a surface brightness of 23 mag/(arcsec)² in B and R band. 'Blue' regions ( small) are light and redder regions are dark.

If available, H images taken with the Kitt Peak 0.9m telescope are shown. The count rates of the H images taken with the 0.6m at Capilla Peak are very low, on the order of 10-20 counts per pixel at the maximum of the H emission, which accounts for the clearly visible background noise. For some of the galaxies, overlays of optical images with new X band (8.44 GHz) radio maps are shown; for L band (1.49 GHz) radio maps, the reader is referred to DBDKS.

4.1. Haro 15

Mazzarella et al. (1991) classify Haro 15 as a "strongly interacting separated galaxy or a single highly perturbed system which may be an advanced merger". The secondary nucleus has stronger H emission (Fig. 2a) than the main nucleus. This is also reflected in U band radio measurements, which at 14.9 GHz are mostly thermal emission, where the secondary nucleus is brighter than the central nucleus. The radio emission from it has a much flatter spectral index than the central nucleus and probably a larger fraction of thermal emission, possibly indicating a very young star forming region. This is also supported by the blue color () of the secondary nucleus within the fairly uniform colors of the disk (Fig. 2b).

Fig. 2. a Haro 15. H brightness (greyscale) versus B band (contours). b Haro 15. colors (greyscale) versus B band surface brightness (contours). In all frames with colors in Figs. 2-8, the calibration of the colors in millimag is indicated in the greyscale on the right side.

4.2. II Zw 40

II Zw 40 has long been known as a prototypical H II galaxy or BCDG, i.e. a Blue Compact Dwarf Galaxy (Sargent and Searle, 1970). The nucleus of II Zw 40 is dominated by one large H II region, with a diameter of about 0.5 kpc, and the radio emission is centered on the nucleus at all frequencies. As mentioned previously, II Zw 40's H emission is so strong, that it contributes 40% to the R Band flux at the nucleus. This indicates a starburst of extraordinary strength, as is also evident from the colors of the nucleus, which are much bluer than those of the southern and south-eastern extensions (Fig. 3). Although only one large, featureless H II region is seen, H I observations (Brinks and Klein, 1988) revealed two kinematically separate systems, interpreted as two interacting H I clouds with approximately equal mass, of which only the northern cloud has an optical counterpart in II Zw 40. This indicates an H I mass an order of magnitude larger than the visible mass. Based on the H I mass of the northern cloud, Sage et al. (1992) find an unusual star formation efficiency ⁵ for II Zw 40, which might be the result of the rare event of a merger between two gas-rich dwarf galaxies, being observed at the peak of its star forming episode. They call II Zw 40 "one of the most extreme galaxies known". This result is also supported by FIR observations (Joy & Lester 1988).

Fig. 3. II Zw 40. colors (greyscale) versus 8.44 GHz X Band (contours). The Peak flux density is 5.32 mJy/Beam, with a beam FWHM of . The contour lines are at fractions of -0.1, 0.1, 0.3, 0.5, 0.7, 0.9 of the peak flux density (dotted contours are 'negative flux density' artefacts from processing).

4.3. Haro 1

Haro 1 is described (Schneider and Salpeter, 1992) as a paired galaxy with little or no tidal disturbance. Its companion is UGC 3937, with a redshift difference of km s^-1 and a linear separation of 350 kpc. Van der Burg (1985) reports on a possible detection of a third companion in H I at , . The B -band image (Fig. 4a), reveals a large irregular nucleus, whereas in H , at least 6 distinct clumps are discernible. The X -band map (Fig. 4b) shows the radio emission to be peaked at the second and third strongest H II regions near the center of the disk and the eastern and southern H II regions can be seen as local maxima of the radio emission. The galaxy's star forming regions appear to suffer high extinction, as its color of is very uniform and gives no suggestion of concentrated starforming activity.

Fig. 4. a Haro 1. H brightness (greyscale) versus B band (contours). b Haro 1. H brightness (greyscale) versus 8.44 GHz X band (contours).The Peak flux density is 1.13 mJy/Beam, with a beam FWHM of . The contour lines are at fractions of -0.2, 0.2, 0.4, 0.6, 0.8 of the peak flux density.

4.4. II Zw 70

II Zw 70 has been included in the "Atlas and Catalogue of Interacting Galaxies" by Vorontsov-Vel'yaminov (1959, 1977). Together with II Zw 71 (UGC 5962 VV 324a) at a projected distance of 46 kpc, Dahiri (1985) classifies them as "two Sa spirals, far apart, with signs of interaction". Lake & Schommer (1984) derive a ratio of total-to- visible mass for the II Zw 70/71 system of from orbital analysis. This is somewhat higher than that derived from H I observations by Balkowski et al. (1978) of 5 and 14 for II Zw 70 and II Zw 71. Besides being classified as a "genuine BCDG" by Skillman and Klein (1988), II Zw 70 appears to be a distorted spiral (Fig. 5) with the nucleus being its distinctively bluest region. The same authors also speculate on the presence of a radio supernova, based on the very steep nonthermal spectral index. Unfortunately, no useful radio maps of this galaxy could be obtained in the course of this work, but the position derived from 6 cm VLA C-array observations by Wynn-Williams and Becklin (1986) agrees well with the nucleus as seen in optical broad- band and H emission.

Fig. 5. II Zw 70. colors (greyscale) versus B band (contours).

4.5. Mkn 297

Mkn 297 is an unusual galaxy in several respects. Its optical morphology, especially in H , is dominated by a large number of clumpy features (see Fig. 5 in DBDKS) and contains a possible radio supernova (Lonsdale et al., 1992; DBDKS and references therein). Saakyan and Khachikyan (1975) included Mkn 297 in their list of super associations of H II regions. Vorontsov-Velyaminov (1977) classifies it among tight interacting systems, as "coalescent galaxies and a chain of 3 dwarfs". Meahara et al. (1988) define 20 clumps in Mkn 297, on which they provide detailed photometry. Alloin and Duflot (1979) interpret Mkn 297 as the collision of two late spirals, and Taniguchi & Noguchi (1991) report on numerical N -body simulations of the collision of two disk galaxies; the results of their simulations are overlaid onto an optical image of Mkn 297. The interpretation of Mkn 297 as the merger of two disk galaxies is inconsistent with the findings by Burenkov (1988), who concludes from spectroscopic observations that the abundance gradients of nitrogen and oxygen are only consistent with one spiral galaxy; the second galaxy is probably an irregular.

4.6. Mkn 314

Mkn 314 ( NGC7468 UGC 12329 CGCG 453-052 CGCG 2300.5 1620) is considered a possible polar ring galaxy by Whitmore et al. (1990), with the polar rings being the outer extensions at the northern and southern end, visible in the contour lines of the B -band image (Fig. 6). The galaxy has at least two optical nuclei distributed along a line in the NNE-SSW direction of which only the northernmost one has significant H emission.

Fig. 6. Mkn 314. H brightness (greyscale) versus B band (contours).

4.7. Mkn 527

Mkn 527 is a spiral galaxy, classified as SAB(r)a; with the bars (Fig. 7) extending from the nucleus in the SE and NW directions, and an outer ring in the plane of the disk. As a member of the Pegasus cluster it appears in several publications related to cluster galaxies (e.g. Cornell et. al 1987, Giovanelli & Haynes, 1985). Its H emission is centered on the nucleus. Nuclear starforming activity is not reflected in the surface colors, in which Mkn 527 appears like a normal spiral galaxy.

Fig. 7. Mkn 527. H brightness (greyscale) versus B band (contours).

4.8. III Zw 102

Whitmore et al. (1990) consider III Zw 102 as 'related to polar-ring galaxies'; with its dust lanes being similar to polar rings without the luminosity. Various catalogs and publications classify III Zw 102 among elliptical, Sa or S0 galaxies; most mention its peculiarity. Brosch and Loinger (1991) note that III Zw 102's common classification among BCDGs is apparently the result of its selection among the 115 BCDGs of Thuan & Martin (1981), as its diameter of 13 kpc hardly classifies it as a dwarf. Brosch and Loinger (1991) perform extensive modeling of III Zw 102's internal extinction. Although the brightness profile of the underlying disk agrees well with a fit to the de Vaucouleurs r law which indicates elliptical structure, they are unable to decide on its morphology. The colors of the disk are probably dominated by variations in the dust extinction (Fig. 8a). The elongated reddish extinction feature in the NE to SW direction near the center also appears prominently in the models of III Zw 102's extinction by Brosch and Loinger. The radio-emission (Fig. 8b) traces the H emission quite well, implying that at least in the area of the H emission, optical extinction cannot be very large. The radio emission does not reflect the extent of the dust envelope (Fig. 1f in DBDKS).

Fig. 8. a III Zw 102. colors (greyscale) versus B band (contours). b III Zw 102. H brightness (greyscale) versus 8.44 GHz X Band (contours). The Peak flux density is 1.16 mJy/Beam, with a beam FWHM of . The contour lines are at increments of 10% the peak flux density.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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