Astron. Astrophys. 363, 917-925 (2000)

2. Identification and flux determination

The 175 µm map was obtained in February 1997 with ISO's (Kessler et al. 1996) photometer ISOPHOT (Lemke et al. 1996) covering a field of oriented along the major axis at PA with a spatial resolution of about . The data reduction and calibration is described in Haas et al. (1998).

The discrete sources of the 175 µm map have first been identified visually, then two-dimensional elliptical Gaussians have been fitted over areas of . In total, 47 sources have been detected above a threshold of 4 sigma of the local background; they are listed in Table 1. Columns (2) and (3) give the 175 µm position of the sources with an accuracy of about . Column (4) gives an approximation of the knot's size derived from the average of the FWHMs along the two axes. The sources are catalogued in right ascension order. Fig. 1 shows the distribution of the knots in a map of M 31, and gives a corresponding finding map with the positions for all knots indicated.

Fig. 1. Top: map displaying the ring structure with the individual knots. Bottom: Finding map displaying the positions of the 47 identified knots.

Table 1. The catalogued 175 knots with their reference numbers in Column (1). The positions are given in Columns (2) and (3), an approximated diameter in Column (4). The flux densities at 175 have been derived from the ISO map, the corresponding ones at 60 and 100 from the high resolution IRAS maps. Column (8) indicates the classification by spectral appearance. The temperature of the cold component, the mass ratio, the total mass, the luminosity ratio and the total FIR luminosity have been derived by fitting modified Planck functions to the spectra. Column (14) gives the number of the corresponding entry in the 60 µm point source catalogue of Xu & Helou (1996) (with uncertain identifications set in brackets), while Column (15) lists the detections at other wavelengths (see references below).
*)References for Column(15):
(A) 5C3 survey of sources at 1421 MHz (Gillespie 1979); (B) 6 cm HEASARC-NORTH catalogue; (C) Supernova remnants at 6 cm (Dickel & D'Odorico 1984); (D) Dark clouds listed in Hodge (1980); (E) Radio sources from the 36W catalogue at 610 MHz, thought to be H II regions (Bystedt et al. 1984); (F) H II regions from an H survey of M 31 (Pellet et al. 1978) (G) CO measurements from Dame et al. (1993); (H) H I measurements from Brinks & Shane (1984).

Comparing these knots with the list of 60 µm point sources in M 31 (39 entries, Xu & Helou 1996) yields no complete equivalence (see Column (14) of Table 1). In general, more discrete sources are found at 175 µm, but there are also point sources at 60 µm, which are not prominent at 175 µm. Except for Xu & Helou's point source # 2, which is a background galaxy located outside the 175 µm map, all of these point sources are faint at 60 µm. However, at their position we do find some brightness enhancement in the 175 µm map but no source above the 4 range is resolvable. The same holds for the 175 µm sources, that are not included in the list of Xu & Helou. They do all show up in the 60 µm map, but are either too faint or too extended to have been included in Xu & Helou's list of point sources.

A close inspection of the areas around the knots shows that most of the 175 µm knots coincide well with their 60 µm counterparts (e.g. Fig. 2a, Knot 12, Knot 14), but small shifts between the 60 µm and 175 µm peaks are also observed (e.g. Fig. 2a, Knot 15, Fig. 2b, Knot 31, Fig. 2c, Knot 43). Fig. 2c also shows an example of a distinct source at 60 µm, which is not prominent at longer wavelengths but located in the dip between two 175 µm knots (42 and 43), whereas Fig. 2b shows Knot 41 at 175 µm, which is too faint to be recognized at 60 µm. These shifts and also the additional sources prominent at only one wavelength can be explained by a smooth cold dust distribution which is just heated at several points yielding the 60 µm peaks, and with the 175 µm sources as the remaining less heated parts. A situation like this would be expected in a complex of dust clouds and H II regions, where only the dust close to the ionized regions reaches high temperatures. However, these shifts between 60 µm and 175 µm peaks are rare and do not affect the basic conclusions drawn from the following investigations.

Fig. 2a-c. Detailed maps of the areas around some of the knots at different wavelengths. The 60 and 100 µm maps are from the IRAS HIRES archive, the 175 µm maps from Haas et al. (1998).

The total flux densities of the knots have been determined by aperture photometry with standard routines of MIDAS and DAOPHOT, whereby the local background within M 31 has been subtracted. The flux density is sensitive to the size of the chosen aperture as the sources are not completely separated but situated very close together and on top of the ringlike structure. It has been measured within an aperture of , the background in the surrounding wide ring. The aperture has been chosen as being large enough to measure a sufficient area of the knots without being too much affected by the varying background.

The flux densities of the sources are given in Column (5) of Table 1. Their values vary between 3 and 32 Jy. Note that at 175 µm, the brightest source (Knot # 23) is not at the position of the galaxy's centre. Instead it is situated about to the NW and is part of the innermost of the concentric rings. It will later be discussed in Sect. 11.5.

The flux densities at 12, 25, 60 and 100 have been determined from the high resolution IRAS maps smeared to the spatial resolution of as for the ISO 175 µm data. Special care has been taken to use the same aperture of for all passbands to allow reasonable comparison. The flux values of the longer, more relevant wavelengths are listed as Columns (6) and (7) in Table 1.

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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