Astron. Astrophys. 329, 399-408 (1998)

3. Results

Table 1 shows the flux densities measured for our targets. The radio galaxy MG 1019+0535 is the only source detected at submillimetre wavelengths at greater than the 4- significance level. In this section we discuss the main implications of our observations for each individual source. The dust masses are estimated using the formula:

where S is the flux density, and are, respectively, the observed and rest-frame frequencies, is the luminosity distance, B is the black-body Planck function, is the dust temperature and cm² g^-1 is the adopted mass absorption coefficient. In order to obtain limits on , it is necessary to assume a temperature for the grains. We assume a representative temperature =60 K, consistent with the typical temperatures estimated for high-redshift active galaxies. For a discussion about the uncertainties of dust masses and temperatures, see Hughes et al. (1997). It should be emphasised here that, besides the uncertainties on and , the absolute dust masses are also strongly dependent on the choice of and ; in fact, they change by a factor of 4 for values of ranging from 50 to 100 km s^-1 Mpc^-1, and by a factor of around 2 if is changed from to .

For each source we have searched NED¹ in order to derive the spectral energy distributions (SEDs) over a broad range of frequencies. If the error of the flux density is unknown, we assume an uncertainty of 5%. Whenever the significance of the flux density is , we provide an upper limit at the 3- level. Whenever more than one flux density value is available at the same frequency, we plot all the available values. Figs. 1 and 2 and Table 3 show the SEDs derived for our targets.

Table 3. Flux densities.

Fig. 1. Spectral energy distributions of radio galaxies. The flux densities and the relative references are listed in Table 3.

Fig. 2. Spectral energy distributions of radio-loud quasars. The flux densities and the relative references are listed in Table 3.

3.1. Notes on individual sources

TX 0211-122

The optical emission line ratios of this radio galaxy are highly anomalous, with the flux of the Ly line relatively weak, and that of the N V 1240 line relatively strong compared to those of C IV 1549, He II 1640 and C III ] 1909 (van Ojik et al. 1994). This was interpreted as being due to a strong starburst and a large amount of dust. According to the 8.2-GHz flux density and spectral index (Röttgering et al. 1994; Carilli et al. 1997), the expected synchrotron flux density at 1.3 mm is only 0.3 mJy. Our SEST observation yields a 3- upper limit of 12 mJy which limits the total amount of dust, according to Eq. (1), to M . Although this limit is not very stringent, it does suggest that the total amount of dust cannot be much larger than that of the most dusty known active galaxies: for instance, the quasar BRI 1202-0725 has M (Isaak et al. 1994; Hughes et al. 1997). Finally, we recall that the amount of molecular hydrogen estimated from observations of CO is M ; not much greater than that of nearby gas-rich starburst galaxies (van Ojik et al. 1997).

MRC 0943-242

The detection of a halo of neutral hydrogen linked with the host galaxy of MRC 0943-242 and the dust which might be associated with the neutral ISM (Röttgering et al. 1995) prompted the SEST observation of this source. We recall that the limit on the molecular hydrogen mass is M (van Ojik 1995). According to the 8.2-GHz flux density and spectral index (Röttgering et al. 1994; Carilli et al. 1997), the expected synchrotron flux density at 1.3 mm is only 0.2 mJy. Our 1.3-mm 3- upper limit of 9.9 mJy limits the mass of dust to M ). Because of the large uncertainty in the total gas content (H+H₂) (Röttgering et al. 1995; van Ojik et al. 1997), it is not possible to meaningfully constrain the dust/gas mass ratio in this galaxy.

3.2. MG 1019+0535

This radio galaxy has spectroscopic properties similar to TX 0211-122, again indicating the possible presence of dust (Dey et al. 1995). Our IRAM observations provided a suggestive detection at 1.25 mm, and our JCMT observations yielded a marginal detection at 800 m. The IRAM observations were split into two nights but in this case, unlike 1243+036, MG 1019+0535 gave consistently positive signal. Although the result is formally significant, we consider that our 1.25-mm data provide only a tentative detection because of the very weak flux density. We note, however, that previous IRAM detections at around this level have since proved to be trustworthy - that of 8C 1435+635, for example (Ivison 1995; Ivison et al. 1998).

It is important to stress that there are major uncertainties in the interpretation of the millimetric observations of this galaxy. Optical imaging shows the presence of two objects separated by about 1.5 - object A, identified as the counterpart of the radio source at , and object B (Dey et al. 1995). The nature of B is unclear: it may be physically related to A, or be a foreground galaxy at . The problem is that the beam widths of our 1.25-mm and 800- m observations include both objects. However, if the two objects are unrelated, the depression of the Ly line favours component A being the dusty object and the source of the observed flux density at 1.3 mm.

In order to better constrain the SED of this galaxy, we used data from IRAS. However, since MG 1019+0535 is not detected, upper limits have been estimated at 12, 25, 60 and 100 m by searching a 1 square degree field centred on MG 1019+0535 for sources from the IRAS Faint Source Catalogue, adopting the faintest in each band as the upper limit (0.11, 0.17, 0.20 and 0.47 Jy, respectively). This crude method relies on the fact that if the FSC 's sophisticated search routines cannot find a point source, then the source must be below the 3- threshold. The method is less prone than some to providing misleadingly low limits (Ivison 1995). The implication of this result is dicussed in detail in Sect. 4.

MRC 1043-291

This radio-loud quasar has radio flux densities of 1.09 and 0.68 Jy at 408 MHz and 5 GHz, respectively (Kapahi et al. 1997). Therefore, if we adopt a spectral index (defined as ), we derive an expected 1.3-mm synchrotron flux density of around 326 mJy. Our SEST observation provides a flux density around an order of magnitude lower than expected, suggesting that the radio spectrum steepens rapidly at high frequencies.

1243+036 (= 4C 03.24)

This is the radio galaxy with the highest redshift in our observed sample. One spectacular feature is the presence of a Ly halo (with a luminosity of erg s^-1) which extends over 20 ( kpc) (van Ojik et al. 1996). The Ly images, coupled with high-resolution spectra, indicate that the radio jet is interacting vigorously with the gas in the inner region. Perhaps most surprising is the low-surface-brightness outer region of the Ly halo. Deep spectroscopy shows that it is relatively quiescent ( km s^-1 FWHM), but that there is a velocity gradient of 450 km s^-1 over the extent of the emission ( kpc). Because the halo extends beyond the radio source, it is probable that its kinematics must predate the radio source. The ordered motion may be a large-scale rotation caused by the accretion of gas from the environment of the radio galaxy or by a merger.

The extrapolation of the radio flux density at 8.3 GHz (Röttgering et al. 1994; van Ojik et al. 1996) provides expected synchrotron flux densities of 0.2 and 0.4 mJy, respectively, at 800 m and 1.3 mm. We observed this source, both with the IRAM telescope and the JCMT, but we obtained only non-significant detections at the 2-3- level (see Table 1). The IRAM observations were performed on two different nights. Although the combined observations of the two nights provide a formal 3- detection, we found that this result is not reliable because the source was detected only during the first night. In fact, deeper observations with the IRAM telescope failed to detect the galaxy and provided a 3- upper limit mJy (R. Chini, private communication).

However, we can see that our 3- upper limits ( 9.3 mJy at 800 m; 2.6 mJy at 1.3 mm) provide a relevant result. In fact, the inferred total dust masses are and M using the 800- m and 1.3-mm upper limits, respectively. The most stringent limit on the dust mass (provided by the JCMT observation) implies that the amount of dust in 1243+036 is lower than that inferred for those high-z radio galaxies and quasars so far detected (see Hughes et al. 1997 and references therein). The upper limit on can be lowered still further if we use the limit provided by Chini at 1.3 mm, which gives M . It is also important to notice that CO observations of this galaxy have provided a stringent limit on the amount of molecular hydrogen M (van Ojik 1995).

PKS 1251-407

To date, this is the furthest known radio-loud quasar (Shaver et al. 1996). Our SEST observation provides a tentative (2.6- ) detection. Fig. 2 shows the SED of this quasar. The 1.3-mm upper limit hints that the synchrotron spectrum steepens at high frequencies, as do the other two radio-loud quasars, MRC 1043-291 and PKS 1354-107.

PKS 1354-107

The optical counterpart of this radio source was identified by Shaver et al. (P.A. Shaver, J.V. Wall, K.I. Kellermann, C.A. Jackson, M.R.S. Hawkins, private communication) with a quasar at . The available radio flux densities (see Fig. 2) suggest the presence of a very flat spectrum from 2.7 to 8.4 GHz. Our SEST observation provides a 3- upper limit of 15.3 mJy, implying a sharp steepening of the synchrotron spectrum at high frequencies.

© European Southern Observatory (ESO) 1998

Online publication: December 8, 1997
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