Astron. Astrophys. 332, 503-513 (1998)

Near-infrared imaging of the host galaxies of flat spectrum radio quasars ^*

Jari K. Kotilainen ^{1, 2}, Renato Falomo ³ and Riccardo Scarpa <sup>4

1</sup> International School for Advanced Studies (SISSA), via Beirut 2-4, I-34014 Trieste, Italy (jkotilai@sissa.it)
² Tuorla Observatory, University of Turku, Väisäläntie 20, FIN-21500 Piikkiö, Finland
³ Osservatorio Astronomico di Padova, vicolo dell'Osservatorio 5, I-35122 Padova, Italy (falomo@astrpd.pd.astro.it)
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA (scarpa@stsci.edu)

Received 23 June 1997 / Accepted 24 October 1997

Abstract

We present the results of a high resolution ( px^-1) near-infrared H band (1.65 µm) imaging survey of a complete sample of 20 flat spectrum radio quasars (FSRQ) extracted from the 2Jy catalogue of radio sources (Wall & Peacock 1985). The observed objects are intrinsically luminous with median M(B) = -25.5. The median redshift of the objects in the sample is z = 0.65. At this redshift, the H band observations probe the old stellar population of the hosts at rest frame wavelength of 1 µm.

We are able to detect the host galaxy clearly for six (30 %) FSRQs and marginally for six (30 %) other FSRQs, while the object remains unresolved for eight (40 %) cases. We find the galaxies hosting FSRQs to be very luminous (M(H) -27). Compared with the typical galaxy luminosity L* (M*(H) -25) they appear 2 mag brighter, although the undetected hosts may reduce this difference. They are also at least as bright as, and probably by 1 mag brighter than, the brightest cluster galaxies (M(H) -26). The luminosities of the FSRQ hosts are intermediate between host galaxies of low redshift radio-loud quasars and BL Lac objects (M(H) -26), and the hosts of high redshift radio-loud quasars (M(H) -29), in good agreement with current unified models for radio-loud AGN, taking into account stellar evolution in the elliptical host galaxies. Finally, we find an indicative trend between the host and nuclear luminosity for the most luminous FSRQs, supporting the suggestion based on studies of lower redshift AGN, that there is a minimum host galaxy luminosity which increases linearly with the quasar luminosity.

Key words: BL Lac objects: general galaxies: active galaxies: nuclei infrared: galaxies quasars: general

* Based on observations collected at the European Southern Observatory, La Silla, Chile.

Send offprint requests to: J.K. Kotilainen (SISSA address)

© European Southern Observatory (ESO) 1998

Online publication: March 23, 1998
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