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198 9Apj. . .341. .6583 the Astrophysical Journal, 341:658-678 .6583 The Astrophysical Journal, 341:658-678,1989 June 15 © 1989. The American Astronomical Society. All rights reserved. Printed in U.S.A. .341. 9ApJ. 198 MULTICOLOR SURFACE PHOTOMETRY OF POWERFUL RADIO GALAXIES. II. MORPHOLOGY AND STELLAR CONTENT Eric P. Smith1,2 Laboratory for Astronomy and Solar Physics, Goddard Space Flight Center AND T. M. Heckman1 University of Maryland Astronomy Program Received 1988 August 29; accepted 1988 December 2 ABSTRACT We present results on the morphology, photometric structure, and colors of 72 powerful radio galaxies (Pi7s > 5 x 1024 W Hz-1), and discuss the implications these findings have for theories concerning the origin of activity in galaxies. Sample selection, observational procedures, and data reduction techniques for this study are detailed in a recent paper by Smith and Heckman. We report three major conclusions regarding the nature of powerful radio galaxies (PRGs). The first is that galaxy interactions/mergers play an important role in the PRG phenomenon. We find that over 50% of the sample galaxies display optical morphological deviations from elliptical symmetry at high levels of surface - 2 brightness (iiv < 25 mag arsec ). More specifically, we find that about half of the PRGs with strong optical emission line spectra (SE PRGs)—but only 7% of the PRGs with weak or no emission lines (WE/ABS PRGs)—exhibit peculiar optical morphologies (tails, fans, bridges, shells, and dust lanes). Narrow or sharp features such as these may be explained by galaxy interactions involving at least one gas-rich, dynamically cold galaxy (i.e., a disk galaxy). We find that about 20% of our galaxies have a second nucleus less than 10 kpc in projection from the main nucleus (30% of the WE/ABS PRGs and only 10% of the SE PRGs). The fraction of PRGs in a “common envelope” with neighboring galaxies is even larger (40% for the WE/ABS PRGs and 20% for the SE galaxies). Photometric investigations of the sample indicate that the surface bright- ness profiles for the WE/ABS galaxies are typically shallower in slope than normal radio-quiet elliptical gal- axies, but similar to brightest cluster galaxies. Surface brightness profiles for the SE galaxies are more diverse in form. The second major conclusion is that the SE PRGs have unusually blue average colors relative to giant elliptical galaxies [by ~0.2 mag in (B— K)]. It is important to note that these blue colors are spatially extended and not merely due to light from a bright nucleus or extended emission-line gas (both of these con- tributions having been explicitly removed), and as such they must reflect the stellar content of the PRG. Also the SE PRGs often exhibit strong spatial variations in color. We hypothesize that the unusual colors and color variations are related to merger-induced star formation. In contrast, the WE/ABS PRGs have normal colors and color gradients for giant elliptical galaxies. Third, we note the strong similarity between the WE/ABS PRGs and brightest cluster galaxies. They have similar absolute magnitudes, incidence rates of double/multiple nuclei, colors, and surface brightness profiles. Subject headings: galaxies: photometry — galaxies: stellar content — galaxies: structure — radio sources: galaxies I. INTRODUCTION out to large redshifts (e.g., Chambers, Miley, and van Breugel This is the second in a series of papers dedicated to the 1988) and therefore are important probes of early galaxy evolu- optical (broad-band) investigation of the class of galaxies tion (Spinrad and Djorgovski 1987). Currently it is believed associated with powerful radio sources (which we define as that galaxy-galaxy interactions may play a major role in initi- 24 -1 having radio power at 178 MHz, P178 > 5 x 10 W Hz for ating galaxy activity. A link between galaxy interactions and -1 -1 a cosmology of H0 = 100 km s Mpc and q0 = 0). These activity for PRGs would dovetail with evidence for similar galaxies, the so-called powerful radio galaxies (PRGs), rep- phenomena occurring in other classes of active galaxies (cf. resent a unique laboratory for the study of activity in galaxies. Hutchings and Campbell 1983; Kennicutt et al 1987). Most nearby radio galaxies (which are generally low in radio They exhibit the full range of galaxy activity, yet remain close 25 -1 enough for detailed optical scrutiny. They are also observable power; P178 < 10 W Hz ) are rather normal giant ellip- ticals. The morphology of the galaxies associated with more 1 Visiting Astronomer at the Kitt Peak National Observatory and Cerro powerful radio sources has been less well determined except in Tololo Inter-American Observatory, National Optical Astronomy Observa- several nearby cases (e.g., Fornax A, Cen A). Matthews, tories, which is operated by Associated Universities for Research in Morgan, and Schmidt (1964) conducted a study of the more Astronomy, Inc., under contract with the National Science Foundation. powerful radio galaxies, and typically classified such PRGs as 2 National Research Council Postdoctoral Fellow, Goddard Space Flight Center; Guest Observer at the Canada-France-Hawaii Telescope, which is “ D ” or “ cD,” suggesting a kinship to giant ellipticals operated by the National Research Council of Canada, the Centre Nationale (especially those found at the centers of rich clusters). These de la Recherche Scientifique de France, and the University of Hawaii. notions prevailed until quite recently, when improved detec- 658 © American Astronomical Society • Provided by the NASA Astrophysics Data System .6583 MULTICOLOR SURFACE PHOTOMETRY OF PRGs. II. 659 .341. tors, advanced image processing techniques, and observations . II. GALAXY MORPHOLOGY of larger and more diverse samples of PRGs demonstrated that a significant fraction (~30%-50%) had peculiar optical mor- a) Peculiarities 9ApJ. phologies (Heckman et al 1986, hereafter H86; Hutchings Paper I presented the isophotal contour maps of the galaxies 198 1987). in our sample. It is readily apparent from inspection of these In H86 we argued that PRGs fall into two broad categories. images that many of the galaxies display unusual Class A radio galaxies are defined to have strong optical emis- (nonelliptical) morphologies. Table 1 gives a listing of the gal- sion lines (“SE”). They also usually have “edge-brightened” axies, cataloging these distortions, the classification of which Fanaroff-Riley (Fanaroff and Riley 1974) type II radio mor- was done by one of us (E. S.) independent of any knowledge of phologies (“ FR II ”). We found that a large fraction ( ~ 50%) of emission-line gas morphology. There is, unfortunately, no these PRGs have peculiar morphologies. The class B radio good way of quantifying these large deviations from elliptical galaxies are defined to have weak emission lines or pure symmetry (as opposed to small ones which can be studied absorption line spectra (“WE/ABS”) and usually have FR I using the Fourier descriptors for the isophotes; see Paper I); radio morphologies. These galaxies rarely (~ 10%) exhibit the rather, a zoological approach must be taken. H86 have classi- morphological peculiarities found in class A PRGs. Not only fied some of the features found in optically distorted PRGs, the incidence rate, but also the form of the optical peculiarities, and we have adopted their terminology for our classification of is of principal interest in light of the extensive modeling of the peculiarities found in this sample. We find that 54% of our galaxy-galaxy interactions (e.g., Toomre and Toomre 1972; sample show morphological peculiarities at surface brightness Quinn 1984; Aguilar and White 1986, hereafter AW). Rem- levels brighter than 25 V mag arcsec-2. It must be emphasized nants with sharp or coherent features are produced when at that the fraction of galaxies with optically peculiar structure is least one of the participants contains a dynamically cold (disk) most likely only a lower limit, because there could be cases for structure. Galaxy merger simulations (AW) and observations which deviations from elliptical symmetry are not visible 1/4 (Schweizer 1982) have shown that an r law light distribution because of the galaxy’s distance [a result of the (1 + z)4 cosmo- is the natural result of violent relaxation in a merger (both logical dimming in surface brightness], these features possibly elliptical-elliptical and disk-disk). In light of this, it would being lost in the sky background. We note that similar frac- appear difficult to use the photometric structure (surface tions of those objects classed as SE (57%; see Table 1) and brightness profile) of a galaxy to understand its dynamical WE/ABS (52%) are morphologically disturbed. The forms history. However, if the galaxy light profile remains peculiar these deviations take for the two classes is, however, quite after the disappearance of the more “ spectacular ” optical dis- 8 different. tortions (a few times 10 yr), they also could provide clues to a To consider further the nature and significance of the mor- galaxy’s past. phological peculiarities, it is useful to subdivide them into In addition to the morphology and structure of PRGs, their several categories based on the physical intuition provided by stellar content is of considerable interest (particularly in light models of interacting galaxies. In the first category are features of the use of high-redshift radio galaxies as cosmological which are relatively narrow or “ sharp ” in appearance : “ tails ” probes). Interacting galaxies are known to have colors that are (features with a length-to-width ratio >3:1 which extend out bluer than those of noninteracting galaxies (Larson and from the main body of the galaxy), “ bridges ” (narrow emission Tinsley 1978), presumably because of enhanced star formation. linking two galaxies), “ shells ” (curvilinear features with a pre- Sandage (1972) demonstrated that many of the PRGs also dominantly azimuthal orientation), “fans” (similar to tails, but have colors that are bluer than those of normal giant elliptical less narrow), and dust lanes or patches (see Fig.
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