Are radio galaxies and quiescent galaxies different? Results from the analysis of HST brightness profiles

Free access article

Issue		A&A Volume 439, Number 2, August IV 2005


Page(s)		487 - 496
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20042529

A&A 439, 487-496 (2005)
DOI: 10.1051/0004-6361:20042529

Are radio galaxies and quiescent galaxies different? Results from the analysis of HST brightness profiles

H. R. de Ruiter^{1, 2}, P. Parma², A. Capetti³, R. Fanti^{4, 2}, R. Morganti⁵ and L. Santantonio⁶

¹ INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
² INAF - Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
³ INAF - Osservatorio Astronomico di Torino, Strada Osservatorio 25, 10025 Pino Torinese, Italy
⁴ Istituto di Fisica, Università degli Studi di Bologna, via Irnerio 46, 40126 Bologna, Italy
⁵ Netherlands Foundation for Research in Astronomy, Postbus 2, 7990 AA, Dwingeloo, The Netherlands
⁶ Università degli Studi di Torino, via Giuria 1, 10125 Torino, Italy

(Received 14 December 2004 / Accepted 12 April 2005 )

Abstract
We present a study of the optical brightness profiles of early type galaxies, using a number of samples of radio galaxies and optically selected elliptical galaxies. For the radio galaxy samples - B2 of Fanaroff-Riley type I and 3C of Fanaroff-Riley type II - we determined a number of parameters that describe a "Nuker-law" profile, which were compared with those already known for the optically selected objects.

We find that radio active galaxies are always of the "core" type (i.e. an inner Nuker law slope $\gamma < 0.3$ ). However, there are core-type galaxies which harbor no significant radio source and which are indistinguishable from the radio active galaxies. We do not find any radio detected galaxy with a power law profile ( $\gamma > 0.5$ ). This difference is not due to any effect with absolute magnitude, since in a region of overlap in magnitude the dichotomy between radio active and radio quiescent galaxies remains. We speculate that core-type objects represent the galaxies that have been, are, or may become, radio active at some stage in their lives; active and non-active core-type galaxies are therefore identical in all respects except their eventual radio-activity: on HST scales we do not find any relationship between boxiness and radio-activity.

There is a fundamental plane, defined by the parameters of the core (break radius $r_{\rm b}$ and break brightness $\mu_{\rm b}$ ), which is seen in the strong correlation between $r_{\rm b}$ and $\mu_{\rm b}$ . The break radius is also linearly proportional to the optical Luminosity in the I band. Moreover, for the few galaxies with an independently measured black hole mass, the break radius turns out to be tightly correlated with $M_{{\rm BH}}$ . The black hole mass correlates even better with the combination of fundamental plane parameters $r_{\rm b}$ and $\mu_{\rm b}$ , which represents the central velocity dispersion.

Key words: galaxies: active -- galaxies: elliptical and lenticulars, cD -- galaxies: nuclei

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