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Title: Weakly barred early-type ringed galaxies. III. The remarkable outer-ringed S0+ galaxy NGC 7020

Abstract

The southern S0+ galaxy NGC 7020 presents an unusual morphology: it includes a very regular outer ring which is completely detached and which envelops an inner ring/lens zone with an exotic hexagonal shape. The ring has a high contrast compared with those usually observed in barred galaxies, yet NGC 7020 is not obviously barred. In this paper, the structure of this galaxy is studied by means of UBVRI CCD surface photometry. The photometry reveals a complex system and shows that most of the recent star formation in the galaxy has taken place in the outer ring. Two bright knots are found on the major axis of the hexagonal zone that appear to be true enhancements of old stars rather than young associations. Between these knots and the bulge there are dips in the surface brightness and a clear zone of rectangular isophotes. 56 refs.

Authors:
 [1]
  1. (Alabama Univ., Tuscaloosa (USA))
Publication Date:
OSTI Identifier:
6697053
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; (USA); Journal Volume: 356
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; GALAXIES; MORPHOLOGY; BRIGHTNESS; CHARGE-COUPLED DEVICES; GALACTIC EVOLUTION; PHOTOMETRY; SPIRAL CONFIGURATION; STAR EVOLUTION; CONFIGURATION; OPTICAL PROPERTIES; PHYSICAL PROPERTIES; SEMICONDUCTOR DEVICES 640105* -- Astrophysics & Cosmology-- Galaxies

Citation Formats

Buta, R. Weakly barred early-type ringed galaxies. III. The remarkable outer-ringed S0+ galaxy NGC 7020. United States: N. p., 1990. Web. doi:10.1086/168819.
Buta, R. Weakly barred early-type ringed galaxies. III. The remarkable outer-ringed S0+ galaxy NGC 7020. United States. doi:10.1086/168819.
Buta, R. 1990. "Weakly barred early-type ringed galaxies. III. The remarkable outer-ringed S0+ galaxy NGC 7020". United States. doi:10.1086/168819.
@article{osti_6697053,
title = {Weakly barred early-type ringed galaxies. III. The remarkable outer-ringed S0+ galaxy NGC 7020},
author = {Buta, R.},
abstractNote = {The southern S0+ galaxy NGC 7020 presents an unusual morphology: it includes a very regular outer ring which is completely detached and which envelops an inner ring/lens zone with an exotic hexagonal shape. The ring has a high contrast compared with those usually observed in barred galaxies, yet NGC 7020 is not obviously barred. In this paper, the structure of this galaxy is studied by means of UBVRI CCD surface photometry. The photometry reveals a complex system and shows that most of the recent star formation in the galaxy has taken place in the outer ring. Two bright knots are found on the major axis of the hexagonal zone that appear to be true enhancements of old stars rather than young associations. Between these knots and the bulge there are dips in the surface brightness and a clear zone of rectangular isophotes. 56 refs.},
doi = {10.1086/168819},
journal = {Astrophysical Journal; (USA)},
number = ,
volume = 356,
place = {United States},
year = 1990,
month = 6
}
  • CCD surface photometry of the southern ringed S0(+) galaxy NGC 7187, a particularly good example of a system with two bright clear rings with significantly different apparent shapes and almost no sign of a bar, is presented. The galaxy has below average luminosity and the rings are small compared to those observed in more typical (R)SB(r) type galaxies, even though the ratio of their sizes is similar to those in such galaxies. Bulge isophotes reveal the existence of two nearly orthogonal ovals, one having the same shape and position angle as isophotes of the outer disk. The central oval showsmore » significant m = 4 deviations from elliptical isophote shapes and could be a nuclear bar. The rings are slightly blue enhancements compared to their surroundings. The mean colors of both rings do not imply exceptionally high star formation at the present time. 44 refs.« less
  • UBVRI surface photometry of NGC 7702, obtained with a CCD detector on the 3.9-m Anglo-Australian Telescope and with photographic plates on the 4-m telescope at CTIO, is reported. The data are presented in tables, graphs, and contour maps and characterized in detail. NGC 7702 is found to be a true S0(+) galaxy with a bright high-contrast inner ring and a faint low-contrast outer ring; the inner ring is significantly elongated relative to typical SB inner rings and has colors which suggest a burst of star formation less than 2 Gyr ago. A small oval revealed by the bulge isophotes inmore » the inner 4 arcsec radius is attributed to a nuclear bar. 52 refs.« less
  • Four extreme examples illustrate the diverging characteristics of bright galactic rings. The galaxies are studied by means of multicolor CCD surface photometry. The ringed Seyfert galaxy NGC 3081 is an S0/a system possessing an unusually high contrast inner ring enveloping a weak bar. The structure of this galaxy is described using BVR(C)I(C) surface photometry. The observations reveal a complex system which shares many properties in common with NGC 1433, a prototype SB(r) type spiral with a strong bar. Significant azimuthal variations in the color indices occur around the ring which are probably due to the elongated shape. It is suggestedmore » that while many properties of the three rings in this galaxy can be explained in terms of orbit resonance theory, the extreme shape of the inner ring and the weakness of the bar are contradictions to the theory that need to be addressed. It is suggested that NGC 3081 is a possible example of an object where secular dissolution of the bar is occurring. 60 refs.« less
  • We present azimuthally averaged radial profiles of R-band surface brightness for a complete sample of 47 early-type, unbarred galaxies, as a complement to our previous study of early-type barred galaxies. Following very careful sky subtraction, the profiles can typically be determined down to brightness levels well below 27 mag arcsec{sup -2} and in the best cases below 28 mag arcsec{sup -2}. We classified the profiles according to the scheme used previously for the barred sample: Type I profiles are single unbroken exponential radial declines in brightness; Type II profiles ('truncations') have an inner shallow slope (usually exponential) which changes atmore » a well-defined break radius to a steeper exponential; and Type III profiles ('antitruncations') have an inner exponential that is steeper, giving way to a shallower outer (usually exponential) decline. By combining these profiles with previous studies, we can make the first clear statements about the trends of outer-disk-profile types along the Hubble sequence (including both barred and unbarred galaxies), and their global frequencies. We find that Type I profiles are most frequent in early-type disks, decreasing from one-third of all S0-Sa disks to barely 10% of the latest-type spirals. Conversely, Type II profiles (truncations) increase in frequency with Hubble type, from only {approx}25% of S0 galaxies to {approx}80% of Sd-Sm spirals. Overall, the fractions of Type I, II, and III profiles for all disk galaxies (Hubble types S0-Sm) are 21%, 50%, and 38%, respectively; this includes galaxies ({approx}8% of the total) with composite Type II+III profiles (counted twice). Finally, we note the presence of bars in 10 galaxies previously classified (optically) as 'unbarred'. This suggests that {approx}20% of optically unbarred galaxies are actually barred; the bars in such cases can be weak, obscured by dust, or so large as to be mistaken for the main disk of the galaxy.« less