Astron. Astrophys. 351, 495-505 (1999)

3. Results

The ISO photometry is presented in Tables 3 and 4.

Table 3. Far infrared photometry of our ISO sample of active galaxies. A dash indicates that no observation was performed at that filter.

Table 4. Far infrared photometry of our ISO sample of inactive spirals. A dash indicates that no observation was performed at that filter.

All measurements led to detections and have a high () signal-to-noise ratio. We find good agreement with IRAS fluxes. The decomposition of the spectral energy distributions and the optical images of the sources are shown in Figs. 1 to 15.

Fig. 1. Top: Optical image from the Digitized Sky Survey of the active galaxy Mkn323. North is up and East is left. The orientation of the C200 detector is indicated. Bottom: The spectral energy distribution of Mkn323 between 40 and 1500 µm. ISOPHOT () and IRAS data () are color corrected. The ISOPHOT error bars reflect the absolute photometric calibration uncertainty, the purely statistical errors of the data are negligible. Photometry at 1.3 mm by Chini et al. (1992b). We show the best fit by a modified black-body spectrum (solid curve), it is labeled by its dust temperature.

Fig. 2. Same as Fig. 1 for Mkn332.

Fig. 3. Same as Fig. 1 for Mkn534.

Fig. 4. Same as Fig. 1 for Mkn538.

Fig. 5. Same as Fig. 1 for Mkn555.

Fig. 6. Same as Fig. 1 for Mkn799.

Fig. 7. Same as Fig. 1 for Mkn928.

Fig. 8. Top: The source VV5a of the interacting system Arp 86 contaminates the ISOPHOT photometry of Mkn1134. Bottom: The spectral energy distribution of Mkn1134 and VV5a between 40 and 1500 µm. ISOPHOT fluxes of Mkn1134 () are connected by dots, those for VV5a () by the dashed line. Error bars represent calibration uncertainties. The solid curve is a modified black-body spectrum of 28 K shown to illustrate.

Fig. 9. Same as Fig. 1 for the inactive spiral NGC 6156. Photometry at 1.3 mm by Chini et al. (1995), see discussion in Sect. 2.3. The best fit model (solid curve) is separated into a cold (dotted) and a very cold dust (dashed) component.

Fig. 10. Same as Fig. 9 for NGC 6918.

Fig. 11. Same as Fig. 9 for NGC 7083. The C200 area of both observing epochs are shown.

Fig. 12. Same as Fig. 9 for NGC 5719. Photometry at 1.3 ,mm by Chini et al. (1996), see discussion in Sect. 2.3.

Fig. 13. Same as Fig. 9 for M+04-48-2.

Fig. 14. Same as Fig. 9 for UGC 2936.

Fig. 15. Same as Fig. 9 for UGC 2982.

For the active galaxies, the FIR and millimeter observations can be explained by a single modified black-body spectrum. The temperatures range from 27 K to 36 K with an average of K, in accord with the value of K of Chini et al. (1992b). If one presupposes that Chini et al. (1992b) miss in their beam half of the total 1.3 mm flux, the spectra would nevertheless be fit by a single dust component of similar temperature.

On the other hand, to interpret the spectra of the inactive spirals one needs two modified black-body components: one with an average temperature of K, the other has K. This estimate for the coldest dust component is by several degrees lower than previous ones which used IRAS and submm data (for instance, Devereux & Young 1990, Masi et al. 1995, Laureijs et al. 1996). However, evidence is accumulating that very cold dust is present in individual galactic clouds (Lehtinen et al. 1998, Ristorcelli et al. 1998, Hotzel et al. 1999) and abundant over whole galaxies (Alton et al. 1998, Haas 1998).

Table 5 summarizes several parameters of the galaxies. The gas mass is determined from the integrated 1.3 mm flux via

D is the distance, is the mass absorption coefficient and denotes the Planck function at the temperature for the active galaxies and for the inactive spirals. We use 0.003 cm per gram interstellar matter. The luminosities result from an integration between the 12 µm IRAS flux and the integrated 1.3 mm photometry.

Table 5. Physical parameters of the galaxies. The spectra of the active galaxies can be decomposed in a single modified black-body, whereas for the inactive spirals an additional very cold component is found; we give best fit temperatures for both components together with the statistical errors. The IR luminosity, , goes from 12 to 1300 µm.
Notes:
1) distances based on H₀ = 75 km s^-1 Mpc^-1
2) distance uncertain
3) velocity unknown; we adopt mean distance of Chini et al. (1996) sample
4) not constrained by a 1.3 mm observation

Based on IRAS and 1.3 mm data, Chini et al. (1995) estimated that the ratio of IR luminosity to gas mass, , is 20 times greater for active galaxies than for inactive spirals. The existence of a very cold component however increases the gas mass of inactive galaxies and thus changes this difference. We find for the active galaxies / and for the inactive spirals / .

3.1. Remarks on individual galaxies

Our fit for Mkn323 (Fig. 2) and Mkn538 (Fig. 4) is at 1.3 mm below the upper limit within a beam as quoted by Chini et al. (1992b). If this limit represents the total flux from the galaxy, we need two components to fit the spectrum: the colder one has 22.9 K and 18 K, respectively. The estimated dust mass would rise accordingly. For the other active galaxies, both a one and a two component fit give similar results.

The center of the galaxy Mkn332 is 1´ West of the central detector array (Fig. 2). Nevertheless, this does not noticably influence the precission of the photometry.

Mkn1134 (Fig. 8) is a member of the interacting pair of galaxies, NGC7753 and NGC7752, and not resolved by IRAS (Gardner 1995). For Mkn1134 (=UGC12779=VV 5b) we took correction factors from Table 2, but added only pixel 1, 4, 7, 5 and 8 of the C100 and array elements 1, 2 of the C200 detector. (Notation of pixel numbering as in PIA.) For the second component (VV 5a), we co-added pixels 2, 3, 6 and 9 of the C100 and took pixel 3 of the C200 array. The 1.3 mm photometry by Chini et al. (1992b) refers unfortunately to the IRAS position and thus does not include the central region of the galaxy.

We performed two ISOPHOT multi-filter observing templates separated in time by about half a year on NGC7083 (Fig. 11). The relative photometry between both observing sequences agrees to better than 5% demonstrating the stability of the ISOPHOT subsystem.

About 63" South-West of M04-48-2 (Fig. 13) is a second source (UGC11570), which slightly contaminates our FIR photometry. In IRAS plates, the confusion at 60 µm is strong, but weak at 100 µm. Therefore, we may neglect the second source for our C200 photometry.

© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999
helpdesk.link@springer.de