The host galaxy/AGN connection in nearby early-type galaxies

Free access article

Issue		A&A Volume 447, Number 1, February III 2006


Page(s)		97 - 112
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20054031

A&A 447, 97-112 (2006)
DOI: 10.1051/0004-6361:20054031

Is there a miniature radio-galaxy in every "core" galaxy?

B. Balmaverde¹ and A. Capetti²

¹ Universitá di Torino, Via Giuria 1, 10125, Torino, Italy
e-mail: balmaverde@ph.unito.it
² INAF - Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
e-mail: capetti@to.astro.it

(Received 11 August 2005 / Accepted 20 September 2005)

Abstract
This is the second of a series of three papers exploring the connection between the multiwavelength properties of AGN in nearby early-type galaxies and the characteristics of their hosts. We selected two samples with 5 GHz VLA radio flux measurements down to 1 mJy, reaching levels of radio luminosity as low as 10³⁶ erg s^-1. In Paper I we presented a study of the surface brightness profiles for the 65 objects with available archival HST images out of the 116 radio-detected galaxies. We classified early-type galaxies into "core" and "power-law" galaxies, discriminating on the basis of the slope of their nuclear brightness profiles, following the Nukers scheme. Here we focus on the 29 core galaxies (hereafter CoreG).

We used HST and Chandra data to isolate their optical and X-ray nuclear emission. The CoreG invariably host radio-loud nuclei, with an average radio-loudness parameter of Log $R = L_{5~\rm {GHz}} / L_{B} \sim 3.6$ . The optical and X-ray nuclear luminosities correlate with the radio-core power, smoothly extending the analogous correlations already found for low luminosity radio-galaxies (LLRG) toward even lower power, by a factor of $\sim$ 1000, covering a combined range of 6 orders of magnitude. This supports the interpretation of a common non-thermal origin of the nuclear emission also for CoreG. The luminosities of the nuclear sources, most likely dominated by jet emission, set firm upper limits, as low as $L/L_{\rm Edd} \sim 10^{-9}$ in both the optical and X-ray band, on any emission from the accretion process.

The similarity of CoreG and LLRG when considering the distributions host galaxies luminosities and black hole masses, as well as of the surface brightness profiles, indicates that they are drawn from the same population of early-type galaxies. LLRG represent only the tip of the iceberg associated with (relatively) high activity levels, with CoreG forming the bulk of the population.

We do not find any relationship between radio-power and black hole mass. A minimum black hole mass of $M_{\rm BH} = 10^8 ~M_{\odot}$ is apparently associated with the radio-loud nuclei in both CoreG and LLRG, but this effect must be tested on a sample of less luminous galaxies, likely to host smaller black holes.

In the unifying model for BL Lacs and radio-galaxies, CoreG likely represent the counterparts of the large population of low luminosity BL Lac now emerging from the surveys at low radio flux limits. This suggests the presence of relativistic jets also in these quasi-quiescent early-type "core" galaxies.

Key words: galaxies: active -- galaxies: bulges -- galaxies: nuclei -- galaxies: elliptical and lenticular, cD -- galaxies: jets -- galaxies: BL Lacertae objects: general

SIMBAD Objects

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.