Near-infrared imaging of the host galaxies of intermediate redshift steep spectrum radio quasars

Abstract

We present the results of near-infrared H-band (1.65 microns) imaging of 19 steep spectrum radio quasars (SSRQ) in the redshift range 0.5 < z < 1.0. This sample of SSRQs is matched with our previously studied complete sample of 20 flat spectrum radio quasars (FSRQ) with respect to redshift and optical and radio luminosity. We are able to clearly detect the host galaxy in 10 (53 %) SSRQs and marginally in 6 (32 %) others, while the host remains unresolved in 3 (16 %) SSRQs. The galaxies hosting the SSRQs are large (average bulge scale-length R(e) = 9.0+-1.7 kpc) and luminous (average M(H) = -27.2+-1.1). They are, therefore, about 2 mag more luminous than the typical galaxy luminosity L* (M*(H) = -25.0+-0.2), and about 1 mag more luminous than the brightest cluster galaxies (M(H) = -26.3+-0.3). The SSRQ hosts appear to have similar luminosity to those of the FSRQ hosts (M(H) = -27), and they fall between the luminosities of lower redshift (M(H) = -26) and higher redshift (M(H) = -29) radio-loud quasars. The average nucleus-to-galaxy luminosity ratio of SSRQs (LN/LG = 3.8+-3.2) is much smaller than that found for the FSRQs (LN/LG = 21+-11), in good agreement with the current unified models. We confirm for the most luminous SSRQs the trend noted for the FSRQs between the host and nuclear luminosity. This trend supports the suggestion based on studies of lower redshift AGN, that there is a minimum host galaxy luminosity which increases linearly with the quasar luminosity. Finally, FSRQs seem to reside in richer environments than SSRQs, as evidenced by a larger number of close companion galaxies.