Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

Spectroscopy and HST images of ultra-faint radio sources ( Jy)

F. Hammer
DAEC, Observatoire de Paris-Meudon, 92195 Meudon, France

D. Schade
Department of Astronomy, University of Toronto, Toronto, Canada

D. Crampton
DAO, N.R.C.C., Victoria, Canada

O. Le Fèvre
DAEC, Observatoire de Paris-Meudon, 92195 Meudon, France

S. Lilly
Department of Astronomy, University of Toronto, Toronto, Canada

Visiting Astronomer, Canada-France-Hawaii Telescope, which is operated by NRC of Canada, CNRS of France and the University of Hawaii.



Up to 50 of the Jy radio sources are associated to elliptical galaxies, lying generally at z> 0.7. Other contributors to the Jy radio source population are AGN-like spectrum galaxies and post-starburst galaxies experiencing major interactions. In the field, a large fraction of luminous and early type galaxies at z> 0.7 and of post-starburst galaxies have radio emission at Jy levels. The abundant population of starburst galaxies at z> 0.5 is a very small contributor to the radio emission. Assuming that radio emission in ellipticals is actually linked with low-powered AGNs lying in their cores, provides that the radio emission at Jy is related more to active nuclei than to star formation.

Keywords: galaxies,radiogalaxies,cosmology


Deep radio counts show a surprisingly steep slope close to the Euclidean value, below a few 0.1 mJy (at 1.4 to 8 GHz, Fomalont et al. 1991, hereafter FWKK). This suggests an evolution of faint radio source properties. At sub mJy levels, it is acknowledged that starburst galaxies are major contributors to the radio source counts (Benn et al. 1993). Towards the Jy levels (S 16Jy at 5 GHz), the projected density of radiosources reaches the number density of B= 21.5 field galaxies. The fraction of flat spectrum radio sources also increases continuously towards extremely low radio fluxes (FWKK). The understanding of Jy may provide new and interesting contraints on the galaxy evolution as well as on the relative contribution of AGNs in deep radio source counts.

During the preliminary deep imaging phase of our large spectroscopic survey of faint field galaxies (CFRS), one of our fields (10 arcmin 10 arcmin) was chosen to coincide with the FWKK radiosource field, including 36 S 16Jy radiosources of their complete sample. All sources but two have been identified to V < 25 and/or I 24, and/or K 21.

In section 2 are described the results of the spectroscopic classification of optical counterparts of Jy radio sources (see Hammer et al. 1995 for details) and HST images of nearly half the complete sample are presented in section 3.

Optical Counterparts and Spectroscopy

Spectroscopy of the 25 Radio Counterparts

Among objects bright enough for spectroscopy, we find five early type galaxies (26%), six ``spirals'' (32%), five emission line galaxies (26%), one QSO and one M star but we failed to identify three objects.

The early type galaxies are rather luminous ( -21.9, 50, 0.5), and have redshift ranging from 0.7 to 1, the latter being the limit of our completness for our spectroscopy. Their spectra show a red continuum with a significant 4000Å\ break and very faint or no [OII] emission. They all have inverted radio spectra ( = -0.40.3). Inverted-spectrum radio emission has been observed in nearby ellipticals (Wrobel & Heeschen 1984) and may indicate the presence of a low-power AGN (Rees 1984), although other alternatives are possible. We suggest that the radio emission from our high redshift early type galaxies has the same origin, although their radio power is ten times higher than the one of nearby galaxies.

The ``spirals'' are rather luminous galaxies ( -21.6) and lie at moderately high redshift (0.37 < z < 0.81). They all show moderate [OII] emission (average W =12Å at rest), and relatively strong Balmer continuum and absorption lines (equivalent width ranging from 3 to 5Å), indicating the presence of A and F stars and suggesting that strong star formation occurred in these objects 1Gyr ago. We classify them as post-starbursting galaxies or ``S+A''. They all have moderately steep spectra ( = 0.400.18), noticeably flatter than the mJy starburst galaxies, which might indicate an increasing contribution of thermal radiation from the IGM (Condon 1992).

The emission line galaxies have relatively moderate luminosities (L<L*), while the emission line ratio of all of them but the most distant one are typical of AGNs (from current diagnostic diagrams, see Rola 1995). The bluest emission-line galaxies have inverted spectra, also with the exception of the most distant one ( = 0.7), which could be classified as a starburst. This supports the hypothesis that AGNs are present in their cores too, producing both the radio emission and the emission lines.

Faintest Counterparts

There is considerable evidence that virtually all of the 11 remaining sources (fainter than I22.5) are likely to be at z > 1. Indeed all the ones for which we have K photometry have I-K color much redder than any object in the CFRS sample. Several of the faintest counterpart have inverted radio spectra and are suspected to be early type galaxies at z > 1.

HST Images of a Sub-sample of 19 Jy Radio Source Counterparts

HST images have been provided from Cycle 4 observations of CFRS galaxies (see Schade et al. 1995) as well as from the Groth-Westphal strip (Groth et al. 1995) which overlaps the FWKK radio field. Nineteen radio galaxies have been identified in this sample, among them 16 belong to the complete radio sample defined by FWKK (see Table 1).

Table 1 compares the spectral properties derived by Hammer et al. (1995) to the morphological properties, namely the fraction of bulge luminosity to the total luminosity (B/T, see the contribution of Schade et al., same volume). It shows a very good agreement between the two classifications. Spectral early-type galaxies (with inverted radio spectra) are bulge-dominated objects, while galaxies spectroscopically classified as S+A (post-starburst, moderately steep radio spectra) show a lower contribution of the bulge or are irregulars. Among the emission line objects (AGN-like spectra, spectral class EM), we find one irregular, one spiral and one bulge-dominated object, the latter having the spectrum of a Seyfert 1 (14.0573). HST images also allow to study the environment of the radiosources. A rapid examination provides that(see Figure 1):
-2/3 of the ellipticals (E) show a very small companion within 25 kpc.
-3 of the 4 S+As (poststarbursts) are apparently experiencing a major interaction with a neighbour of almost the same brightness.
-emission line objects (EM) either show no obvious companions or have very irregular morphologies.

Table 1: HST observations of 16 sources of the complete sample

Figure: HST images of 14 optical counterparts of Jy radio sources. Early type galaxies (ellipticals) are shown in the two top lines, S+A galaxies in the third line from the top and AGN-like spectrum objects in the bottom line. Each stamp has a 5 size.

Discussion and Conclusion

Half of Jy radio sources are associated to elliptical galaxies (B/T > 0.45), while Schade et al. (1995) found 20 of ellipticals in the field at z0.6. This is mainly due to the emergence of radio emission with inverted spectra in distant and luminous ellipticals (z > 0.7), an emission which can be related to the presence of an active nucleus in their cores. Indeed, all the Jy radio sources but one are compact (size ). We have compared the number of detected radio sources in the complete sample of I galaxies in the same field (Lilly et al. 1995). We find that a considerable fraction of high-z ellipticals (40% at z > 0.7 and 66% if we only count the more luminous ones, ) have radio emission at Jy levels.

In the field, we also find that half the luminous post-starbursts () show radio emission at Jy levels. Conversely, in the very abundant population of emission line galaxies ( 70 at z > 0.5, Hammer et al. in preparation), we identify only one starburst emitting at radio wavelengths. Radio emission in post-starburst galaxies is more likely associated to the presence of major interactions that to star formation.

The strong decrease of the radio spectral index from sub-mJy to Jy counts appears to be due to a combination of three factors: (1) the emergence of an elliptical population at high redshifts with moderate radio emission; (2) an increasing fraction of narrow emission-line AGNs (Seyfert 2 and LINER); (3) a higher contribution of the thermal radiation to the radio emission from spirals, and the almost complete disappearance of starburst galaxies. The fact that radio sources have much flatter radio spectra at Jy levels compared to those above 0.1 mJy can thus be mostly attributed to the emergence of radio sources driven by low-power AGNs (> 50 per cent of the whole Jy population). Since the space density of AGN-driven sources apparently overtakes those powered by stellar emission, the contribution of AGN light to the faint source counts should be reevaluated.


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