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

4. The case of MG 1019+0535: a dusty radio galaxy

Although the JCMT observation provides only a marginal detection, its combination with the IRAM detection of an excess over the synchrotron spectrum strongly suggests the presence of thermal dust emission from this galaxy (Fig. 3).

Fig. 3. Spectral energy distribution of MG 1019+0535. The lines drawn in the rest-frame far-IR are modified 35- and 180-K blackbodies, with a =+2 frequency dependence for the dust-grain emissivity, representing the most extreme dust temperatures compatible with our data. The 180-K blackbody is optically thick at 200 m. Key: circles - VLA, JCMT, IRAM and IRAS measurements described in the text; diamonds - measurements from Dey et al. (1995); squares - measurements from Griffith et al. (1995), Becker, White & Edwards (1991), Wright & Otrupcek (1990) and White & Becker (1992).

Hughes et al. (1997) have demonstrated that the uncertainties in calculating the mass of dust responsible for the optically thin, thermal, submillimetre emission are: (i) our limited knowledge of the rest-frame mass absorption coefficient, , and how this quantity varies with frequency; (ii) the dust temperature (), and, finally, (iii) the unknown values of and . Unfortunately, these problems are coupled. For example, Hughes et al. (1993) noted that there is a trade off between and the critical frequency at which the dust becomes optically thick, .

Typically, each of these uncertainties can account for changes of up to a factor in derived dust masses (see Hughes et al. 1997 for more details). However, a conservative value for the dust mass can be obtained if the paramaters we use to estimate the dust mass are taken such that the dust mass is minimised. We further note that (since for high-z objects the submillimetre observations sample the Rayleigh-Jeans region) the slope of the dust spectrum is not a function of temperature.

Our measurements of MG 1019+0535 at 240 and 384 GHz suggest that the submillimetre spectral index (, where ) is large and positive (). We recall that the maximum allowed spectral index for self-absorption is +2.5. Our result therefore rules out the possibility that the emission is due to self-absorbed synchrotron radiation (Chini et al. 1989). However, given the uncertainty of the 384-GHz flux density, data at more frequencies are needed to better constrain .

Strong support for the thermal nature of the submillimetre emission is provided by our deep measurements at 22 and 43 GHz using the VLA. These show that the steepening centimetre spectral index (Figs. 1 and 3) becomes still more negative as it approaches the millimetre domaine; the predicted contribution at 240 GHz from the dominant centimetre component lies several orders of magnitude below the measured 240-GHz flux density.

At first sight this indicates that the frequency dependence of the dust grain emissivity (or the emissivity index, ) is , which encompasses the range normally quoted for interstellar grains () as well as some less physical values (). However, the redshift is high and the rest-frame frequency of the observed 374-GHz emission is close to the turnover of the dust spectrum, so we do in fact require a high value of to fit both the 240- and 384-GHz data. For , we find that . The lowest temperatures (35 K) are found for an optically thin solution; the highest temperature (180 K) is permitted when we allow the dust to become optically thick (say at THz or 200 m) and to be constrained by the IRAS upper limits.

Although it is clear that our observations do not constrain stringently the dust temperature, we can make use of the usual theory (Hughes et al. 1997) to estimate the mass of dust responsible for the emission detected by IRAM and JCMT. For , .

It is reassuring that K is viable since this is the temperature of the dust measured in the radio galaxy, 8C 1435+635 (Ivison et al. 1998). For K, adopting the same dust parameters as Ivison et al., we derive M which, when compared with the dust mass estimate of M for 8C 1435+635, suggests that the dust mass in powerful radio galaxies does not change significantly between and 2.76 (though we note that the rest-frame 6-cm luminosity of 8C 1435+635 is around 5 times that of MG 1019+0535 and that observations of complete samples of radio galaxies spanning a range of redshifts and radio luminosities will be required to trace their evolution in detail).

4.1. Modelling the UV-to-FIR SED

The interpretation of the observed spectral energy distributions (SEDs), is not straightforward, since a non-thermal contribution cannot be neglected and the commonly used population synthesis models do not allow for dust extinction. Using the same approach followed by Mazzei & De Zotti (1996), based on chemo-photometric population synthesis models incorporating extinction and re-emission by dust and accounting for non-thermal emission, we have attempted to analyse the SED of MG 1019+0535. We recall here that Dey et al. (1995) estimate an upper limit to the internal reddening, mag.

Our IRAM (and, marginally, JCMT) observations strongly favour the presence of dust. Although our spectral coverage is rather poor, we attempt to fit the overall SED of this galaxy, from the optical to 1.25 mm in the observed frame, with the aim of constraining the evolutionary properties of MG 1019+0535. For this study, we assume that the source of the submillimetre continuum radiation is component A (as suggested by its depleted Ly emission), and we adopt its optical and near-IR fluxes accordingly (Dey et al. 1995).

We have computed several models with Salpeter's initial mass function (IMF) and different lower mass limits, , as described in Mazzei & De Zotti (1996) (and references therein). For a given model we derive the age of the system which matches the data, with different amounts of non-thermal AGN emission. We find an interesting result: these data are well matched by models which always correspond to a 0.8-1-Gyr-old host galaxy, accounting for a non-thermal contribution ranging from 50 to 90% of the total flux density at 0.6 m (see Fig. 4). According to this result, MG 1019+0535 cannot be considered a "primaeval" galaxy candidate because the bulk of its stellar population is significantly evolved. For km s^-1 Mpc^-1, the formation redshift, , of MG 1019+0535 is between 10 and 4 if and if . In the following we will refer to the first value of .

Fig. 4. The fits to the SED of MG 1019+0535 as obtained with the Mazzei & De Zotti (1996) model. Thick lines correspond to the overall match of the SED of MG 1019+0535 for models including a non thermal contribution, stellar emission and dust effects (see text); a shows the results for two models corresponding to a host galaxy 1-Gyr old, short-dashed () and long-dashed lines (), with a non-thermal contribution at m of 70 and respectively (thin line); the overall match for a host galaxy 0.8-Gyr old (dot-dashed line, ) with a non-thermal contribution of at the same wavelength is also shown; in b are the results for the same models raising the non-thermal contribution at the same wavelength to ; short-dashed curves require a dust temperature of 46 K instead of 60 K.

The expected bolometric luminosity is always larger than ; in particular, this rises by a factor 2.5 if . This corresponds to a residual gas fraction of 2%, i.e. to a total gas mass of about M , which is well below the Evans et al. (1996) upper limit, with , a total barionic mass, , of around M , and a star-formation rate of 800 M yr^-1. In this scenario, the hot stars are almost completely obscured by dust which, heated by their radiation field, transfers their bolometric luminosity to the far-IR wavelength regime. Models with lower require larger and higher star-formation rates. We derive for a residual gas fraction as large as 30% - the largest allowed by models - and .

The available data can be fully accounted for by opaque models like those already used by Mazzei & De Zotti (1994) to fit the spectrum of the ultraluminous galaxy IRAS . However, there is still considerable latitude for modelling. Crucial constraints may be provided by ground-based submillimetre measurements and by observations with the Infrared Space Observatory (ISO) ; these measurements will help to define the shape of the far-IR SED, so settling the dust temperature, and the role of PAHs in the near-IR spectral range.

© European Southern Observatory (ESO) 1998

Online publication: December 8, 1997
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