Using the surface photometric analysis data for a well defined sample of 79
nearby radio galaxies (z < 0.12), it is shown that the level of radio emission
associated with an elliptical galaxy during the radio-loud phase is related to
the presence of large amounts of isophotal twist in its optical image. In
particular, radio galaxies with P_408 > 3 x 10^{25} W/Hz appear to show a
preference to be associated with elliptical hosts exhibiting an isophotal twist
in excess of ~20 degrees.Comment: 3 pages, 2 figures, in press in Astronomy and Astrophysic

A. R. Dhakulkar

Bender

Calvani

Colina

Disney

Falomo

Fanaroff

Fasano

Ferrarese

Galletta

Gebhardt

González-Serrano

Gopal-Krishna, 

Govoni

Heckman

Jones

Jorgenson

Ledlow

Magorrian

P. J. Wiita

Rees

S. Dhurde

Small

Smith

Sparks

Wilson

English

arXiv

Crossref

Radio emission and the optical isophotal twist of radio-loud ellipticals

Using the surface photometric analysis data for a well defined sample of 
79 nearby radio galaxies ($z \le 0.12$), it is shown that the level of 
radio emission
associated with an elliptical galaxy during the radio-loud phase is 
related to the presence of large amounts of isophotal twist in its
optical image. In particular, radio galaxies with $P_{\rm 408} > 3 \times 10^{25}$
W Hz-1 appear to show a preference to be associated with
elliptical hosts exhibiting an isophotal twist in excess of ∼20°. 

Gopal-Krishna

EDP Sciences OAI-PMH repository (1.2.0)

Using the surface photometric analysis data for a well defined sample of 79 nearby radio galaxies (z&#8804;0.12 ), it is shown that the level of radio emission associated with an elliptical galaxy during the radio-loud phase is related to the presence of large amounts of isophotal twist in its optical image. In particular, radio galaxies with P408 &gt;3 &#215;1025 W Hz-1 appear to show a preference to be associated with elliptical hosts exhibiting an isophotal twist in excess of ~20&#176; 

Gopal-Krishna,

Dhakulkar, A. R.

Wiita, P. J.

Dhurde, S.

Publications of the IAS Fellows

A&A 410, 139–141 (2003)
DOI: 10.1051/0004-6361:20031329
c© ESO 2003
Astronomy
&Astrophysics
Research Note
Radio emission and the optical isophotal twist of radio-loud
ellipticals
Gopal-Krishna1, A. R. Dhakulkar2, P. J. Wiita3, and S. Dhurde2
1 National Centre for Radio Astrophysics, TIFR, Pune University Campus, Pune 411007, India
2 Department of Physics, University of Pune, Pune 411007, India
3 Dept. of Physics and Astronomy, MSC 8R0314, Georgia State University, Atlanta, GA, 30303-3088, USA
Received 23 April 2003 / Accepted 22 August 2003
Abstract. Using the surface photometric analysis data for a well defined sample of 79 nearby radio galaxies (z  0.12), it is
shown that the level of radio emission associated with an elliptical galaxy during the radio-loud phase is related to the presence
of large amounts of isophotal twist in its optical image. In particular, radio galaxies with P408 > 3  1025 W Hz−1 appear to
show a preference to be associated with elliptical hosts exhibiting an isophotal twist in excess of 20.
Key words. galaxies: active – galaxies: elliptical and lenticular – galaxies: formation – galaxies: interactions –
galaxies: photometry – radio continuum: galaxies
1. Introduction
Whereas supermassive black holes are now known to reside
in the nuclei of many elliptical galaxies (e.g. Magorrian et al.
1998; Gebhardt et al. 2000; Ferrarese & Merritt 2000), only a
tiny fraction of them hosts powerful (twin-lobed) radio sources.
What triggers the nuclear activity and how it is related to the ac-
tual formation and evolution of the host galaxy are widely de-
bated issues. It is well established that powerful radio emission
is almost exclusively associated with the spheroidal component
of galaxies. However, at any given epoch, an overwhelming
majority of luminous spheroidal galaxies sampled is found to
be radio-quiet (e.g., Wisotzki et al. 2001). One oft discussed
possibility, based on direct observations, is that galaxy merg-
ers or interactions could be playing an important role in ra-
dio activity, as indicated by the frequent presence of optical
isophotal distortions, shells and dust lanes seen in radio-loud
ellipticals, particularly in the more powerful Fanaroff-Riley
(1974) Class II (FR II) radio galaxies (RGs) (e.g., Heckman
et al. 1986). It has also been proposed that the formation of the
weaker FR I RGs may involve merging of early-type galaxies
deficient in dense ISM, as the FR I RGs are often located in
richer environments (Colina & Pe´rez-Fournon 1990; Colina &
de Juan 1995).
Another set of potentially important clues reported on the
basis of morphological analyses of the optical images are: (i) a
Send offprint requests to: Gopal-Krishna,
e-mail: krishna@ncra.tifr.res.in
marked tendency for the radio-loud ellipticals to be rounder
than normal ellipticals (Disney et al. 1984; Calvani et al. 1989);
(ii) a strong correlation between boxiness of the elliptical host
and its radio emission (Bender et al. 1987, 1989). Both of these
issues are addressed in a more recent study based on a sur-
face photometric analysis of a well defined sample of 79 ra-
dio galaxies at z  0.12 (Govoni et al. 2000, hereafter GFFS).
Their analysis, however, did not confirm the suggestion that
radio-loud ellipticals are rounder in shape, in agreement with
the conclusions also reached by Smith & Heckman (1989)
for powerful RGs, by Ledlow & Owen (1995) for (mostly)
FR I RGs in Abell clusters, and by Falomo et al. (2000) for
the hosts of BL Lacertae objects. Secondly, GFFS find no
clear correlation between radio emission and the presence of
boxy isophotes in the host, in agreement with Ledlow & Owen
(1995) (see also, Jorgenson et al. 1995; Gonza´lez-Serrano et al.
1993). Another potentially interesting result emerging from
their study concerns the presence of non-concentric disposition
of the isophotes, which is believed to be a strong measure of
galaxy interaction. In this respect, they find no significant dif-
ference between the FR I and FR II members of their sample
of radio galaxies.
Larger isophotal twist is another potential indicator of
strong galaxy interaction or merger events, although a small
amount of isophotal twist could arise simply from the triaxial-
ity of the galaxy (e.g., Kormendy 1982). The twist is quantified
in terms of the total position angle variation, ∆PA, of the major
axis of the stellar component over the entire range in surface
Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20031329
140 Gopal-Krishna et al.: Isophotal twists of radio-loud ellipticals
brightness. However, isolated ellipticals are found to show a
similar distribution of twist angle as a randomly selected sam-
ple of ellipticals, suggesting that by and large, twisting is prob-
ably an intrinsic property (Fasano & Bonoli 1989; also, Falomo
2003).
2. Is the radio output of radio-loud ellipticals
related to isophotal twist?
Recently, Govoni et al. (2000) have reported a two-dimensional
surface photometry analysis for 79 ellipticals with z < 0.12, ex-
tracted from two complete samples of radio galaxies. Among
other parameters, they have published for each elliptical the
value of its ellipticity,  = 1 − b/a, measured at its effective
radius. Also, for 78 out of the 79 ellipticals, they have reported
the isophotal twist angle, ∆PA. This is the maximum twist ob-
served over the range of isophotes. (Note that they have ex-
cluded both the isophotes at radii <500 as they are usually af-
fected by seeing related smearing as well as the values of PAs
having errors larger than 8.) These superior data support the
earlier claim that, statistically, rounder radio galaxies tend to
have larger isophotal twists (Galletta 1980; Fasano & Bonoli
1989). As seen from the  versus ∆PA diagram (Fig. 16 of
GFFS), essentially all the elongated ellipticals ( > 0.3) have
∆PA < 10. In contrast, for less elongated ellipticals ( < 0.3),
the amount of isophotal twist appears to be independent of el-
lipticity (although few galaxies have ∆PA > 40). The origin
of this trend is unclear.
The median value ∆PA for the GFFS sample is found to be
10, similar to ∆PA  16 found for a sample of isolated ellip-
ticals using an identical method (Fasano & Bonoli 1989). Also,
at least for z < 0.2, there is no significant statistical difference
between the isophotal twist for radio galaxies and radio-quiet
ellipticals (Falomo et al. 2000). Thus, it would appear that radio
emission has no obvious dependence on isophotal twisting. In
contrast, Colina & de Juan (1995) found a significantly larger
twisting for FR I radio galaxies, with a median twist angle of
14, compared to a sample of isolated normal ellipticals stud-
ied by Sparks et al. (1991) for which the median twist angle is
only 4.
To follow up these rather conflicting claims, we have ex-
amined the distribution of ∆PA against P408 for the GFFS
sample (Fig. 1). The values of these two parameters are di-
rectly taken from GFFS, so we follow them in taking H0 =
50 km s−1 Mpc−1 and q0 = 0. For two of the 78 radio galaxies,
the values of P408 are not tabulated by GFFS and we have de-
termined these by extrapolating from the values of P2700 given
in GFFS and P843 reported by Jones & McAdam (1992).
From Fig. 1, one notices an upper envelope of the data
points, which rises with the radio luminosity until P408 
1026 W Hz−1. In particular, larger twist angles (∆PA > 20)
are found almost exclusively above P408 ’ 3  1025, which is
close to the well known average transition luminosity between
the low and high power radio sources (Fanaroff& Riley 1974).
We note, however, that there is a considerable overlap between
the morphologically determined FR I and FR II sources in the
luminosity range considered here (e.g. Ledlow & Owen 1995).
In fact, as can be seen from Fig. 1, there is no obvious relation
Fig. 1. Isophotal twist angles against radio power for this sample. FR I
sources are denoted by circles, FR II’s by triangles, and intermediate
or unclear sources by squares.
23 24 25 26 27 28
   0
   5
  10
  15
  20
   0
   5
  10
Fig. 2. Distributions of large (∆PA > 20) and small (∆PA < 20)
isophotal twists against radio power.
between twist angle and FR classification; in order to avoid
introducing possible bias into the present analysis, we have
simply adopted the FR classification given by GFFS. Govoni
et al. (2000) themselves note that the mean twist for FR I’s is
13  12 while for FR II’s it is nearly identical: 15  12.
Another version of Fig. 1 is displayed in Fig. 2 where the
histograms of P408 are presented for two ranges of the twist
angle separated at ∆PA = 20. A Kolmogorov-Smirnov test
shows that the two distributions are statistically different at the
97% confidence level. Thus, there seems to be a relation be-
tween isophotal twist and radio luminosity for massive ellip-
ticals in their active phase, and the two may share a common
origin.
Here it may be recalled that in any flux limited sample,
such as the present one (GFFS), luminosity is strongly corre-
lated with redshift. It is therefore conceivable that the primary
correlation is between the parameters ∆PA and z. However,
this is highly unlikely, since that would imply an unphysi-
cally steep cosmological evolution of ∆PA, given that the en-
tire sample is defined over a very narrow range in redshift (z 
0.12). Explicitly, a plot of twist angle against redshift shows
that sources with ∆PA > 20 are essentially uniformly dis-
tributed in z. Alternatively, the radio power could be correlated
Gopal-Krishna et al.: Isophotal twists of radio-loud ellipticals 141
primarily with the rounder shape of the host (Sect. 1), given
the ∆PA −  dependence mentioned above. However, as also
concluded by Govoni et al. (2000), we find no significant cor-
relation of radio power with ellipticity of the host to be ev-
ident in the GFFS sample. Thus, it appears more likely that,
during the radio-loud phase, powerful radio sources are pref-
erentially associated with ellipticals exhibiting relatively large
isophotal twist. At the same time, a large twist cannot be a suf-
ficient condition for generating and sustaining strong nuclear
activity, since similarly large twists are also seen in radio-quiet
ellipticals (Falomo et al. 2000; Fasano & Bonoli 1989).
In summary, the relation between radio power and the
isophotal twist reported here suggests that during the radio-
loud phase of an elliptical galaxy the conditions giving rise to
a more powerful radio source are reflected in the occurrence of
larger isophotal twist of the stellar component. At present, no
satisfactory theoretical explanation is available for this trend. It
is certainly conceivable that a recent merger or strong tidal in-
teraction could both twist the isophotes and drive more gas into
the central engine, thereby triggering the radio activity, but no
convincing results on the mechanisms and relative timescales
of these phenomena are yet available. We also note that there is
some support for the scenario in which radio loudness is linked
to the spin of the nuclear black hole (Wilson & Colbert 1995;
also, Small & Blandford 1992; Rees 1978). Thus it is of con-
siderable interest to investigate any possible link between the
isophotal twist of the stellar body and the process of spinning
up the supermassive black hole at the nucleus. It is clearly also
important at this stage to verify the empirical trend emerging
in the present analysis, using an independent sample of radio
galaxies.
Acknowledgements. We thank Prof. V. Kulkarni for helpful comments
and the anonymous referee for suggestions that improved the clar-
ity of the manuscript. P.J.W. is grateful for hospitality at Princeton
University Observatory and for support from RPE funds at GSU.
References
Bender, R., Do¨bereiner, S., & Mo¨llenhoff, C. 1987, A&A, 177, L53
Bender, R., Surma, P., Do¨bereiner, S., Mo¨llenhoff, C., & Madejsky, R.
1989, A&A, 271, 35
Calvani, M., Fasano, G., & Franceschini, A. 1989, AJ, 97, 1319
Colina, L., & de Juan, L. 1995, ApJ, 448, 548
Colina, L., & Pe´rez-Fournon, I. 1990, ApJ, 349, 77
Disney, M. J., Sparks, W. B., & Wall, J. V. 1984, MNRAS, 206, 899
Falomo, R. 2003, in High Energy Blazar Astronomy, ed. A. Sillanpaa,
L. Takalo, & E. Valtaoja, ASP Conf. Ser. (San Francisco: ASP),
in press [astro-ph/0212164]
Falomo, R., Scarpa, R., Treves, A., & Urry, C. M. 2000, ApJ, 542, 731
Fanaroff, B. L., & Riley, J. M. 1974, MNRAS, 167, 31
Fasano, G., & Bonoli, C. 1989, A&AS, 79, 291
Ferrarese, L., & Merritt, D. 2000, ApJ, 539, L9
Galletta, G. 1980, A&A, 81, 179
Gebhardt, K., Bender, R., Bower, G., et al. 2000, ApJ, 539, L13
Gonza´lez-Serrano, J. I., Carballo, R., & Pe´rez-Fournon, I. 1993, AJ,
105, 1710
Govoni, F., Falomo, R., Fasano, G., & Scarpa, R. 2000, A&A, 353,
507 (GFFS)
Heckman, T., Smith, E. P., Baum, S. A., et al. 1986, ApJ, 311, 526
Jones, P. A., & McAdam, W. B. 1992, ApJS, 80, 137
Jorgenson, I., Franx, M., & Kjaergaard, P. 1995, MNRAS, 273, 1097
Kormendy, J. 1982, in Morphology and Dynamics of Galaxies, ed.
L. Martinet, & M. Mayor, 12th Advanced Course of the Swiss
Society of Astronomy and Astrophysics, 113
Ledlow, M. J., & Owen, F. N. 1995, AJ, 110, 1959
Magorrian, J., Tremaine, S., Richstone, D., et al. 1998, AJ, 115, 2285
Rees, M. J. 1978, Nature, 275, 516
Small, T. A., & Blandford, R. D. 1992, MNRAS, 259, 725
Smith, E. P., & Heckman, T. M. 1989, ApJ, 341, 658
Sparks, W. B., Wall, J. V., Jorden, P. R., Thorne, D. J., & van Breda, I.
1991, ApJS, 76, 471
Wilson, A. S., & Colbert, E. J. M. 1995, ApJ, 438, 62
Wisotzki, L., Kuhlbrodt, B., & Jahnke, K. 2001, in QSO Hosts and
their Environments, ed. I. Marquez, J. Masegosa, A., Del Olmo
et al. (Dordrecht: Kluwer), 83


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Radio emission and the optical isophotal twist of radio-loud ellipticals

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