Published in "The Interstellar Medium in Galaxies", eds. Harley A. Thronson, Jr. and J. Michale Shull, 1990

INTERSTELLAR GAS IN GALAXIES

Michael Rowan-Robinson


Astronomy Unit, Queen Mary College, Mile End Rd, London E1 4NS

Abstract. Current ideas about the nature of interstellar dust in galaxies are reviewed, with a strong emphasis on the nature of the very small grain component needed to explain the mid-infrared diffuse emission and unidentified infrared features. Models for the infrared spectra of galaxies are reviewed and the evidence that most of the radiation in star-forming regions is being absorbed by a high visible-uv optical depth of dust is summarized. The evidence for destruction of very small grains in regions of high radiation intensity is discussed.

A new model for interstellar grains in galaxies is presented, based on a revised version of the model of Rowan-Robinson (1986) and is compared to observed far infrared colour-colour diagrams and to far infrared spectra of galaxies which have been mapped at 800 µm by Hughes et al (1989). Work on far infrared and submillimeter mapping of galaxies is reviewed, as also is recent work on infrared emission from ellipticals and lenticulars. The determination of dust mass in galaxies is briefly discussed.

Table of Contents

INTRODUCTION

GRAIN MODELS

FIRST ATTEMPTS TO EXPLAIN THE INFRARED SPECTRA OF IRAS GALAXIES

THE DESTRUCTION OF VERY SMALL GRAINS

TOWARDS A NEW PICTURE OF INTERSTELLAR DUST IN GALAXIES

FAR INFRARED AND SUBMILLIMETRE MAPPING OF GALAXIES

DETERMINATION OF DUST MASS IN GALAXIES

DUST IN ELLIPTICALS AND LENTICULARS

REFERENCES

1. INTRODUCTION

My task in reviewing interstellar dust in galaxies is greatly simplified by the appearance of several excellent review articles on this area during the past year or so. Although each covers only a specific aspect of the subject, together they comprise a good introduction to our current knowledge.

A comprehensive review of infrared emission from our Galaxy, with much historical background, has been given by Cox and Mezger (1989). They emphasize that the results from IRAS have led to a major reappraisal of estimates of the fraction of the infrared emission from our Galaxy which comes from interstellar dust illuminated by the interstellar radiation field, as opposed to regions of massive star formation. The latter are now believed to contribute only about 10% of the total infrared emission from the Galaxy. Boulanger and Perault (1988) have given an authoritative discussion of the infrared emission observed by IRAS from the different components of diffuse emission from our Galaxy, and the correlations between them, which must be the starting point for any analysis of the interstellar dust in normal galaxies. A general review of the IRAS view of the extragalactic sky has been given by Soifer et al (1987). Telesco (1988) has reviewed enhanced star formation and infrared emission in the centers of galaxies, with a strong emphasis on imaging and spectroscopic data derived from ground-based studies. Helou (1989) has reviewed the far infrared emission from Galactic and extragalactic dust seen by IRAS, emphasizing the similarity in the range of far infrared colors seen in external galaxies and in reflection nebulae in our Galaxy. Roche (1988) has given an interesting summary of the results from near and middle infrared spectroscopy of galaxies. Puget and Leger (1989) have given a very thorough review of the evidence for small grains and large aromatic molecules in the interstellar medium of our own and other galaxies. Finally Draine (1989) has reviewed interstellar extinction in the infrared.

In this review I shall concentrate on those areas where major controversy exists and where significant progress may be expected in the next few years. The topics I have selected are grain models, first attempts to explain the infrared spectra of IRAS galaxies, the destruction of the very small grain component, a new picture of interstellar dust in galaxies, results from far infrared and submillimeter mapping of galaxies, determination of the dust mass in galaxies, and dust in ellipticals and lenticulars.

2. GRAIN MODELS

Classical grain models consisting of silicate and carbon grains of radius 0.01-0.1 µ, for example those of Mathis et al (1977), Draine and Lee (1984), Rowan-Robinson (1986), Tielens and Allamandola (1987), are successful in accounting for the observed visible and ultraviolet extinction curve and the emission longward of 60 µ. However there is not yet a concensus on the grain properties longward of 300 µ, as emphasized by Draine (1989). I will discuss this further in section 5 below. The observations which the classical grain model definitely can not account for are (i) excess diffuse emission from the Milky Way at 2-20 µ (Price 1981, Boulanger et al 1985), (ii) 2-20 µ emission from reflection nebulae with color temperature approximately independent of distance from the star (Sellgren 1984) and (iii) the broad features at 3.3, 6.2, 7.7, 8.6 and 11.3 µ seen ubiquitously in emission (Gillett et al 1973).

Current models for these three phenomena all involve the non-equilibrium response of very small particles to absorption of an ultraviolet photon (Greenberg 1968, Duley 1973, Allen and Robinson 1975, Purcell 1976, Andriesse 1978, Sellgren 1984, Draine and Anderson 1985). The main contenders are:

(A) Polycyclic aromatic hydrocarbons (PAH), which can be thought of as hydrogenated graphite platelets consisting of about 50 atoms (Platt 1956, Donn 1968, Leger and Puget 1984, Allamandola et al 1985, Puget and Leger 1989). To account for the full range of observed phenomena, Puget and Leger (1989) have to include also a very small carbonaceous grain (VSG) component with radii in the range 0.0015-0.01 µ. Fig 1a shows how some particular examples of PAHs can give at least qualitative agreement in the wavelengths of (most of) the 3-12 µ broad-band features (Puget and Leger 1989). Fig 1b shows Puget and Legets fit to the interstellar extinction curve in the visible and ultraviolet.

Figure 1a
Figure 1b

Figure 1. (a) Emission spectra of several PAHs calculated from their laboratory absorption spectra, compared with observations of the reflection nebula NGC2023. (b) Fit to the interstellar extinction curve (Puget and Leger 1989)

(B) Hydrogenated amorphous carbon (HAC), which can be thought of as poorly connected PAH islands in a larger structure (Duley and Williams 1981, 1988a, b, Duley 1987, Jones et al 1987, Williams 1989). They attribute the 0.22 µ feature to small silicate particles. Fig 2 shows their fit to the visible and ultraviolet interstellar extinction curve. Broad-band emission in the 0.6-0.9 µm region is attributed to luminescence from a diamond-like component in the HAC (Duley and Williams 1988b).

Figure 2

Figure 2. Fit by Jones et al (1987) to the interstellar extinction curve in the visible and ultraviolet.

(C) Quenched carbonaceous composite (QCC) has been proposed by Sakato et al (1983, 1984). This material is made in the laboratory in a process intended to simulate the expanding atmospheres of carbon stars.

(D) Amorphous aggregates of small particles of silicates, amorphous carbon and graphite (Mathis and Whiffen 1989). These authors show that the optical properties of an aggregate can be significantly different from a simple sum of the ingredients in the aggregate. Fig 3a illustrates the appearance of the Mathis and Whiffen composite grains, Fig 3b shows their fit to the visible and ultraviolet extinction curve and Fig 3c shows the properties of their grains at 1-1000 µ.

Figure 3a
Figure 3b
Figure 3c

Figure 3. Schematic picture of the Mathis & Whiffen (1989) grain model. (b) Their fit to the ultraviolet and visual interstellar extinction curve. (c) The same for infrared wavelengths. (filled circles: calculated, open circles: observations).

It is clear that in the aggregate models (B-D), the very small grain component must retain its thermodynamic identity in order to explain the phenomena (i-iii) above. From the point of view of understanding infrared emission from dust, it may therefore be academic whether the very small grain component is integrated into a larger structure or not, since this integration must be so weak as to leave the specific properties of the component intact. Puget and Leger (1989) in fact query whether aggregate grain models can localize the energy of an incident photon for the several seconds required for infrared emission.

Draine (1988) has reviewed the variety of models which have been put forward specifically to explain the 0.2175 µ feature. The models which he considers are graphite, nongraphitic carbonaceous solids, OH- on small silicate grains, PAH, small MgO or CaO particles, dessicated microorganisms, radiation-damaged SiO2, charge transfer on Si, Fe or Mg, and finally the absorption edge in silicate grains. He concludes that only two are consistent with all the available observations, graphite or OH- on small silicate grains, and he notes that the latter hypothesis is less well developed than the graphite hypothesis.

In section 5 below I shall try to pull together some of these ingredients into a simple but comprehensive picture for interstellar dust in galaxies.

3. FIRST ATTEMPTS TO EXPLAIN THE INFRARED SPECTRA OF IRAS GALAXIES

Models for IRAS galaxy spectra have been reviewed by Rowan-Robinson (1987a, b) and Helou (1989). The first model proposed was a simple 2-component model consisting of warm (50 K) dust in molecular clouds/HII regions and cool (20 K) dust in the interstellar medium heated by the interstellar radiation field (de Jong et al 1984). This model has been developed further by de Jong and Brink (1987) and has been criticized by Eales and Devereux (1990). The model is rather similar to that proposed by Cox and Mezger over a number of years (see Cox and Mezger 1989).

Helou (1986) proposed an extension of this model in which the warm component becomes a one-parameter family, with the heating intensity as the parameter. As the intensity increases from that found in the solar neighborhood to the much higher value found in star-forming regions, the dust temperature increases from 20 to 50 K. More recently, Helou (1989) emphasizes the similar range of IRAS colors found in galaxies and in Galactic sources. Fig 4a shows log{ S(60) / S(100)} versus log{ S(12)IS(25)} for IRAS galaxies and Fig 4b shows the same diagram for Galactic sources. The sequence of colors found in the reflection nebulosity surrounding xi Per by Boulanger et al (1988) with increasing distance from the star is also shown. This appears to be telling support for Helous hypothesis that the variation of color is simply due to variation of the heating intensity experienced by the grains.

Figure 4a
Figure 4b

Figure 4. IRAS color-color diagrams for (a) galaxies, (b) Galactic star-forming regions (Helou 1989). In (b) the crosses denote data for xi Per.

Rowan-Robinson and Crawford (1986, 1989) have also used the analogy with Galactic sources to derive a rather different model for IRAS galaxy spectra. They propose that the galaxy spectra are a mixture of three components, the general disc emission of the galaxy consisting of reradiation of the interstellar radiation field absorbed by interstellar grains (Fig 5a), a component present in Seyferts peaking at 25 µm due to dust in the narrow-line region, and a starburst component with a spectrum similar to that for Galactic compact HII regions. Their models for the latter (Crawford and Rowan-Robinson 1986) are optically thick at visible and ultraviolet wavelengths, with AV approx 20 (they are optically thin in the far infrared, of course). Fig 5b compares their starburst model spectrum with the Telesco et al (1984) spectrum of the NGC1068 starburst component and with the average spectrum for Galactic compact HII regions/regions of massive star formation derived by Rowan-Robinson (1979). Confirmation of the fact that most of the massive star formation in galaxy starbursts takes place at high visible-uv optical depth comes from a comparison of the 60 µ luminosity of a large sample of IRAS galaxies with their Halpha luminosity (Leech et al 1988, Fig 5c). Ratios of these luminosities range from 200-4000, compared with 30-100 for the nearby normal galaxies studied by Persson and Helou (1987). The Halpha / Hbeta ratios for these IRAS galaxies indicate values for AV of only a few, so the bulk of the far infrared radiation must come from stars whose visible light is heavily extinguished, while the Halpha radiation must come from near the surface of the star-forming volume (Leech et al 1989). Further evidence for high visual extinction comes from the Brackett-alpha and -gamma observations of Kawara et al (1989) for a sample of starburst galaxies. From these they infer values for AV in the range 7-33. These values are in agreement with those inferred from the depth of the 10 µm silicate feature in these galaxies.

Figure 5a
Figure 5b

Figure 5. Models by Rowan-Robinson & Crawford (1989) for (a) the cirrus and (b) the starburst components in galaxy spectra. The broken curve in Fig (b) shows the effect of changing the wavelength at which the grain absorption efficiency steepens to 80 µm.

Figure 5c

Figure 5c. H-alpha luminosity versus infrared luminosity for sample of IRAS galaxies (Leech et al 1988). The broken lines correspond to L(60µm) / L(H-alpha) = 400 and 4000.

Once we are dealing with dust clouds with AV >> 1, then the illumination geometry becomes of critical importance for models of the infrared spectra. Evolved HII regions in our Galaxy show strong deviations from spherical geometry, often displaying a blister geometry, although it is possible that for young compact HII regions spherical symmetry is a reasonable approximation (Rowan-Robinson 1982, Crawford and Rowan-Robinson 1986). Efstathiou and Rowan-Robinson (1990) have developed an accurate radiative transfer code for axially symmetric dust clouds. Fig 6 illustrates the crucial importance of the aspect angle when viewing a non spherically-symmetric system. Leisawitz (1990) has also studied the role of non-spherical geometry in star-forming regions.

Figure 6

Figure 6. Sequence of flared disc models for the narrow-line region of NGC4151, as a function of the viewing angle, from face-on (top) to edge-on (bottom) (Efstathiou & Rowan-Robinson 1990).

An improved model for IRAS galaxy spectra, which is essentially a fusion of the approaches of Helou and of Rowan-Robinson and Crawford, will be described in section 5.

4. THE DESTRUCTION OF VERY SMALL GRAINS

In the past two years several lines of evidence have begun to point towards the destruction of very small grains in regions of very high uv radiation intensity. The most direct evidence comes from infrared spectroscopy. Roche (1988) and Desert and Dennefeld (1988) have shown that the broad 3-12 µ features attributed to very small grains are absent in the spectra of many Seyfert galaxies (Fig 7a). Destruction of very small grains is also presumably the reason that Rowan-Robinson and Crawford (1989) found that the disc component was very weak or absent in many Seyferts (Fig 7b).

Figure 7a

Figure 7. (a) 8-13µm and 17-22µm spectra of six galaxy nuclei. Note that the unidentified ir features are completely absent from the Seyferts NGC4151 and IC4329A.

Figure 7b

Figure 7. (b) Ratio of infrared luminosity in starburst component to optical luminosity, versus ratio of infrared luminosity in cirrus component to optical luminosity for IRAS galaxies (Rowan-Robinson & Crawford 1989). The Seyferts (filled circles) are deficient in the cirrus component.

Reasonably direct evidence for the destruction of very small grains in a high radiation intensity comes from the decline in the ratio of S(12) / S(100) near hot stars. Ryter et al (1987) showed this effect for sigma Sco and Boulanger et al (1988) showed it for xi Per.

Telesco et al (1989) argue that a similar effect is seen in the center of M82. Fig 8a shows the increase in S(25) / S(12) with increasing uv intensity found by Telesco et al for M82 superposed on the curve derived from Boulanger et al's observations of xi Per. However the spectrum of the emission from outside the nucleus of M82 (and of the integrated emission from the galaxy) is very similar to that for the NGC1068 starburst, and for compact Galactic HII regions, shown in Fig 5b, and one would normally assume that the bulk of this emission arises in regions where the visible and ultraviolet optical depth is >> 1. The 10 µm emission from such a cloud does not arise from very small grains. Fig 8b shows the integrated spectrum of M82 compared to the optically thick starburst model of Rowan-Robinson and Crawford (1989): the agreement is good. Also shown is the shape of the spectrum of the central region of M82, derived from the colors measured by Telesco et al (1989). The change in spectrum towards the center of M82 is essentially a shift of the emission peak from 80 µm to 60 µm, presumably due to the increase in intensity of the radiation from the starburst towards to nucleus. It seems unlikely that we are seeing emission from optically thin dust (the ratio of Brackett-alpha to -gamma gives a value for AV of 14 for M82 (Kawara et al 1989) ) and hence the analogy with xi Per appears to be spurious.

Figure 8a

Figure 8a. Variation of 25/12 µm color ratio with intensity of radiation field in center of M82 (filled circles, Telesco et al 1989)) compared with relation found in xi Per by Boulanger et al (1988).

Figure 8b

Figure 8b. Top: Integrated spectrum of M82 (filled circles) compared with starburst model (data from Telesco 1988, Smith et al 1990a). The crosses (arbitrary vertical scale) show the relative shape of the spectrum of the core of M82. Bottom: Integrated spectrum of the SMC compared with cirrus model (X = 30) in which abundance of 5 Å grains has been reduced by 2/3rds.

Similarly unconvincing evidence comes from the far infrared colors of galaxies (Pajot et al 1986, Gosh & Drapatz 1987, Helou 1989). Here again the problem is confusion with the role of the optically thick starburst component, for which, in the model of Rowan-Robinson and Crawford (1989), S(12) / S(60) = 0.04, but radiative transfer effects in normal 0.01-0.1 µm dust rather than small grain depletion is the cause. Fig 4b above showed Helou's (1989) compilation of the IRAS colors of compact Galactic HII regions and of galaxies superposed on the range of colors seen in xi Per by Boulanger et al (1988). The agreement is good, but in my view this is fortuitous in the case of Galactic HII regions and galaxies dominated by starbursts since in most cases the optical depth in these sources is high and the analogy with xi Per therefore of doubtful significance. If the 60/25 µm color ratio, ignored by Helou, is also considered, the agreement with xi Per is less impressive. However the case of the Small Magellanic Cloud (Schwering 1988) is convincing because the spectrum of this galaxy does indeed look like cirrus in which the smallest grain component is depleted (see Fig 8b).

In an interesting development, Leene and Cox (1987) have found that the 0.22 µ feature is also suppressed in regions of high radiation intensity, which suggests that this feature is associated with the very small carbonaceous grains responsible for the broad features and diffuse emission at 2-20 µ.

5. TOWARDS A NEW PICTURE OF INTERSTELLAR DUST IN GALAXIES

If we concentrate first on the 'cirrus' component in galaxies, the reradiation by interstellar dust of the energy absorbed from the interstellar radiation field, then it is clear that a satisfactory model involves a number of ingredients. Firstly a multiple (or aggregate ?) grain model is required to account for the interstellar extinction curve and it must incorporate very small grains and/or PAH. Secondly the model must allow for the fact that there is a range of heating intensities within galaxies and from galaxy to galaxy. For our Galaxy and a few other nearby galaxies we may hope to study how the observed spectrum varies with heating intensity. For more distant galaxies for which we have only the integrated spectrum we have to make do, for the moment, with a characteristic heating intensity. Let me define X = I / Iisrf, where I is the intensity in the region under consideration and Iisrf is the intensity of the interstellar radiation field in the solar neighborhood, which I assume to be as characterized by Mathis et al (1983). Finally we may have to allow for the fact that for X > some critical value, the very small grain component starts to be destroyed.

Models which satisfy the first two of these requirements were presented by Draine and Anderson (1985). Bernard and Desert (1990) have given some details of work which satisfies all three requirements. Here I give some results from an extension of my earlier interstellar grain model (Rowan-Robinson 1986), which is intended to be the simplest possible model that fits all the present observational data. The model retains the 6 grain types of the earlier work, with some modifications: (i) 0.1 µ amorphous carbon grains, their optical properties derived from circumstellar dust shells around carbon stars. The absorption efficiency of these has been reduced by a factor of 1.5 at wavelengths > 0.4 µ to improve the fit to the interstellar extinction curve at 5-9 µ, while retaining the same total extinction at wavelengths < 1 µm. This also has the effect of increasing the visible and ultraviolet albedo to a more acceptable value of 0.7. (ii) 0.1 µ amorphous silicate grains, their optical properties derived from circumstellar dust shells around M stars. (It is worth noting that 50% of the mass of carbon and 80% of the mass of silicon in interstellar grains is in the form of these larger amorphous grains. We see them being manufactured in situ. We know that this is where the bulk of interstellar grains were last made.) (iii) 0.03 µ graphite grains, (iv) 0.03 µ silicate grains, (v) 0.01 µ graphite grains, (vi) 0.01 µ silicate grains, all four types with properties as given by Draine and Lee (1984). These components are required to explain the interstellar extinction curve in the ultraviolet and the 0.22 µ feature. The main difference from the earlier model is that the mass in 0.01 µ graphite grains is now redistributed between 0.01 µ grains, 0.002 µ (20 Å) grains and 0.0005 µ (5 Å) grains. The absorption and scattering properties of these latter two species are assumed to be the same as the 0.01 µ grains at wavelengths > 0.1 µ, but because they are so small they will not be in equilibrium with the incident radiation field. Instead we have to assume that they have a certain probability p(T) dT of having a temperature between T and T + dT. The emission spectrum from these grains then has to be calculated from

IV = integ QV BV(T) p(T) dT .   (1)

The calculation of p(T) is a complex matter but has been carried out by Draine and Anderson (1985) for the grain properties of Draine and Lee (1984) adopted here (see also Guhathakurta and Draine 1989). Their results can be approximated analytically as

p(T) = k T-b     for T1 leq T leq T2 ,   (2)

where b = 2.75, and k = 6.68, T1 = 2.7 K, T2 = 500 K, for the a = 20 Å grains, and k = 0.168, T1 = 2.7 K, T2 = 80 K, for the a = 5 Å grains. Here I am assuming that the very small grains emit the bulk of their radiation as a continuum. Roche (1988) estimates that galaxies emit 1% of their energy in the form of unidentified features and as only 10% of the energy of galaxies is emitted at 2-20 µ, we can infer that only about 10% of the radiation from very small grains emerges as the unidentified features. It will be relatively simple to incorporate these features into the calculation in future.

In the earlier calculation (Rowan-Robinson 1986), I considered values of the wavelength at which the absorption efficiency of the 0.1 µ grains steepened from Qnu propto nu to Qnu propto nu2 of 100, 316 and 1000 µ. In the present model I take this wavelength to be 80 µ, which is still consistent with the IRAS data for circumstellar dust shells (with the possible exception of IRC+10216, Rowan-Robinson et al 1986) and gives an acceptable fit to the data for high latitude dust clouds in our Galaxy. The possibility that Qnu propto nu to a wavelength significantly larger than 100 µm is now completely ruled out by observations in our Galaxy and other galaxies. Fig 9a shows the fit to the interstellar extinction curve at visible and ultraviolet wavelengths. Fig 9b shows the overall fit at 0.1-1000 µ. Fig 10 shows the predicted emission spectra for interstellar dust in the infrared for a range of heating intensities (the temperatures of the different grain components are given in Table 1). For X geq 30, the effect of 90% depletion of the 5 Å grains is also illustrated. Fig 11a shows the predicted emissivity for grains immersed in the local interstellar radiation field compared with observations of high latitude clouds. The agreement with observations is excellent both in the shape of the spectrum and in the absolute value of the emissivity. The 12-100 µm emissivity of the isolated cloud observed by Herter et al (1990) also agrees with that predicted in Fig 11a.

Figure 9a
Figure 9b

Figure 9. Fit to interstellar extinction curve for model described in section 5 (a) at visible and ultraviolet wavelengths (upper curve: Rowan-Robinson 1986, lower curve: revised model (differs only near 0.4 µm) (b) in the infrared (solid and broken curves: Rowan-Robinson 1986, dotted curve: revised model). References for observations are given in Rowan-Robinson (1986).

TABLE 1: PARAMETERS FOR GRAIN MODEL
type B0 abundance % cosmic grain temperatures
(× 10-4) X = 1 X = 10 X = 500
0.1µ amor Si 1.36 0.257 77 14.5 21.4 42.6
0.03µ Si 1.10 0.063 19 16.8 24.5 47.0
0.01µ Si 0.52 0.010 3 17.5 25.4 48.5
 
0.1µ amor C 0.45 1.02 20 15.3 22.6 46.0
0.03µ C 0.65 0.45 9 19.3 28.7 60.3
0.01µ C 1.0 0.24 4.7 19.7 29.0 61.7
0.002µ C 0.8 0.039 0.8
0.0005µ C 0.8 0.010 0.2

Figure 10

Figure 10. Predicted infrared emissivity of interstellar grains (ergs/cm2/s/mag) as a function of the intensity of the radiation field. For X geq 30, the effect of a 90% destruction of the smallest grains (5 Å) is shown as broken curves.

Fig 11b shows the corresponding prediction and observations for the central regions of the Galaxy (I < 30°), where the intensity of the radiation field corresponds to X = 5: the fit is also satisfactory. Fig 12 shows the predicted IRAS colour-colour diagrams for X = 1-500 (color-corrected as in Appendix A of Rowan-Robinson and Crawford 1989) compared with observations. Figs 12a and b show the data for the unresolved IRAS galaxies studied by Rowan-Robinson and Crawford (1989). Figs 12c and d show data for resolved IRAS galaxies mapped by Rice et al (1988) and Young et al (1989). Galaxies with log{S(60) / S(25)} leq 0.5 need the additional Seyfert component peaking at 25 µm. For the resolved galaxies, most of which can be explained as pure disc (cirrus) emission, it can be seen that a range of heating intensities are present, from X = 1 for NGC205 to X = 30 for M33. The vast majority of the galaxies whose colors are shown in Figs 12 can be understood as a mixture of Seyfert (S) + starburst (B) + one of the cirrus models (curved lines). There are 3 classes of exception to this. (a) Two galaxies, NGC1569 and Arp 220 appear to lie on the locus of a highly extinguished starburst model. (b) Several galaxies, notably the SMC, lie to the right of the cirrus curve in the 25-60-100 µm colour-colour diagram and above and to the right of the cirrus curve in the 12-25-60 µm diagram, consistent with the effect of destruction of very small grains at high heating intensities. However not all galaxies with high heating intensity show evidence for very small grain destruction. M33, which like the SMC has a spectrum consistent with X = 30, appears to have a normal abundance of very small grains. It is also worth noting that there appear to be no galaxies in which the abundance of 5 Å grains is reduced by more than a factor of 10 compared to the solar neighborhood. The possibility that reduction in the carbon abundance in galaxies (but not the silicon) is the cause of the anomalous colors needs to be explored, especially for the SMC. (c) Several galaxies, for example M31, have 25-60-100 µm colors consistent with cirrus but have very low values of S(25)/S(12), implying excess radiation at 12 µm. Possible explanations of this are a strong contribution from circumstellar dust shells (Soifer et al 1986, Rowan-Robinson and Chester 1987: though for M31 the spatial distribution of the 12 µm radiation does not differ from that at longer wavelengths) or an unusually strong contribution from PAH/very small grains. 8-13 µm spectroscopy of these galaxies would be very valuable.

Figure 11a
Figure 11b

Figure 11. Predicted emissivity, compared with observations (a) towards the Galactic pole (X = 1, data from Boulanger & Perault 1988, Halpern et al 1988, Fabbri et al 1988, assumed E(B - V) = 0.05), (b) towards the central regions of the Galaxy (X = 5, data from Beichman 1987 and refs therein, assumed E(B - V) = 6.1). The broken curve shows the effect of assuming the wavelength at which the absorption efficiency of the grains steepens is > 1mm.

Figure 12a
Figure 12b
Figure 12c
Figure 12d

Figure 12. Predicted IRAS colour-colour diagrams for dust model of section 5. (a) and (b) Data for unresolved IRAS galaxies with good quality fluxes in all 4 bands from Rowan-Robinson and Crawford (1989). S, B denote the location of Seyfert and starburst components. The curved lines denote the cirrus models of section 5, labeled with the value of X. (c) and (d). Data for resolved IRAS galaxies with coadded fluxes in all four bands from Rice et al (1988, crosses) and Young et al (1989, filled circles). Only galaxies with fluxes brighter than 0.4 Jy in all four bands were included. The broken line in Fig 12c shows the effect of reddening (AV = 40) on the starburst component.

Discrepant colors occasionally result from poorly determined fluxes, especially at 25 µm where not all IRAS detectors were functioning. For this reason galaxies with fluxes less than 0.4 Jy in any band were omitted from Fig 12. However in general experience suggests that IRAS colors are accurate to 0.1 in log10 and that any discrepancy greater than this has a real cause.

6. FAR INFRARED AND SUBMILLIMETRE MAPPING OF GALAXIES

Prior to the launch of IRAS rather little information on the spatial extent of far infrared emission in galaxies was available. Some of the earlier work has been reviewed by Telesco (1988). One of the most significant pre-IRAS studies was by Smith (1982), who produced a 170 µ map of the disk of M51, which showed that the bulk of the far infrared emission in M51 is produced by dust associated with the diffuse gas in the disk of the galaxy.

IRAS extended data is still under active study by several groups. Detailed maps have been produced of M31 (Habing et al 1986, Soifer et al 1987 and Walterbos and Schwering 1987), M33 (Rice et al 1990) and of the Magellanic Clouds (Schwering 1988). Rice et al (1988) have published coadded IRAS maps for all galaxies with optical extent greater than 8'. Higher resolution images may be expected for many of these galaxies from the use of maximum entropy and other deconvolution techniques now under active study at IPAC and elsewhere (e.g. Canterna et al 1990).

Subsequent studies have for the most part concentrated on wavelengths longer than 100 µ. Stark et al (1989) have mapped 4 Virgo spirals at 160 and 350 µ and shown that that there is no evidence for a grain component whose emission peaks beyond 200 µ, a prediction of grain models with emissivity Qnu propto nu at wavelengths > 100 µ. Eales and Wynn-Williams (1989) have measured 350 and 450 µ fluxes at locations centered on several galaxies with a 100 beam. 160 µm maps have been published of NGC4449 (Thronson et al 1987), NGC4214 (Tnronson et al 1988), NGC 4485 and 4490 () and NGC1569 and 3593 (Hunter et al 1989). have published maps of IC10 at 95 and 160 µ and given fluxes for several other galaxies. Eckart et al (1990) have mapped Centaurus A at 50 and 100 µ, and have separated the cirrus and starburst components. have mapped M82 at 450 µm, have mapped M83 at 100 µm and Engargiola and Harper (1990) have mapped NGC6946 at 100, 160 and 250 µ. Hughes et al (1989) have mapped 8 IRAS galaxies at 800 µm with JCMT and given some 450 and 1100 µm data for some of them. The importance of the longer wavelengths is that the most reliable estimates of dust mass can be obtained at these wavelengths. Fig 13 shows far infrared spectra of selected galaxies from this latter study, with theoretical fits derived from the models described in section 5. Fig 14a-d show colour-colour diagrams derived from the work of Thronson et al (1990), Hughes et al (1989) and Eales and Wynn-Williams (1989) compared with the predictions of the models of section 5.

Figure 13a
Figure 13b

Figure 13. Far infrared and submillimeter spectra of galaxies mapped by at 800 µm, compared with models of section 5. For NGC 2076 a pure cirrus model, with depletion of the smallest grains, is satisfactory. For the other galaxies both cirrus and starburst components are required (the dotted curve indicates the total predicted flux). Parameters for the models are given in Table 2.

Figure 14a
Figure 14bc
Figure 14d

Figure 14. Far infrared colour-colur diagrams, compared with predictions of interstellar grain model of section 5. (a) 100/60 versus 100/800 (data from Hughes et al 1989), (b) 100/60 and (c) 450/350 versus 100/350 (data from Eales et al 1990). (d) 160/100 versus 100/60 (data from Thronson et al 1990: solid curves, nun Bnu (T) fits; broken curve, cirrus model of section 5).

7. DETERMINATION OF DUST MASS IN GALAXIES

Hildebrand (1983) gave a prescription for deriving dust masses in galaxies from far infrared data which has been widely used. Young et al (1989) have used the Hildebrand prescription to conclude that the average gas-to-dust ratio in galaxies is 1200. Draine (1990) has given a discussion of the derivation of dust masses which emphasizes some of the difficulties. He emphasizes that here is considerable disagreement about the grain opacity at long wavelengths, though this disagreement is somewhat exaggerated by illustrating the most extreme of the models discussed by Rowan-Robinson (1986) in which Qnu propto nu all the way to 1 mm. Draine shows that if only IRAS observations are available of galaxies, then the derivation of dust mass is very uncertain, since several rather different models could in principle be fitted to the same observations. However provided a significant proportion of the 12-100 µm emission from a galaxy is due to cirrus, and fluxes are available in all four IRAS bands, a good separation into cirrus and starburst components can be made, and reasonable estimates of dust mass derived. Observations at long wavelength (> 300 µm) are very valuable in tying down the value of X , the radiation field intensity, and are essential if the 12-100 µm spectrum is dominated by a starburst.

Table 2 gives dust masses derived from the study of Hughes et al (1989) based on the grain model of Rowan-Robinson (1989) described in section 5 above. Comparison of the dust mass in the cirrus component with the neutral hydrogen masses given by Young et al (1989) shows normal gas-to-dust ratios for this model in most cases. However since in many cases the neutral hydrogen in a galaxy extends well beyond the optical image, whereas the bulk of the infrared emission is generally located within the optical image, there may be a tendency to underestimate the total dust mass from far infrared observations. Dust in the outer parts of a galaxy, illuminated with a starlight intensity much lower than in the central regions, may contribute only a very small fraction of the total infrared flux. Sensitive observations at long wavelengths with large beam-throws will be needed to characterize such dust.

Table 2. PARAMETERS FOR GALAXIES MAPPED BY HUGHES ET AL (1989)
galaxy distance ________ cirrus model ___________ starburst model
(Mpc) X depletion log Md(C) log M(HI) log Md(SB)
(H=50) of 5 Å grains
NGC520 45.4 30 90% 7.25 10.10 5.94
NGC1614 92.9 30 90% 7.57 9.88 6.83
NGC2076 48.4 10 50% 7.73
NGC2339 46.7 10 90% 7.50 10.05 5.75
NGC3690 62.1 1 - 8.34 <9.73 6.97
NGC4102 19.7 10 90% 6.97 9.02 5.60
NGC7469 102.0 30 90% 7.63 9.90 6.80

It is important when modeling the far infrared emission from dust in galaxies to take account of the fact that several grain components are present, at different temperatures. Calculations based on the assumption of a single composite grain model and a single temperature are unlikely to yield accurate results. However the cirrus models of Fig 10 can be approximately fitted at long wavelengths with a 2B (T) curve, with the values of T as given in Table 3 for different X. The validity of this fit is for lambda > 1700/T µm. Also given for these models are the values of log{S(100) / S(60)} and log {Md / S(100µm) D2}, log {Md / S(800µm) D2}. Note that whereas Md/S100 D2 approximately proportional to X, Md / S800 D2 approximately proportional to X0.3, so much more accurate dust masses can be obtained if long wavelength observations are available.

Table 3. CIRRUS MODEL PARAMETERS FOR DUST MASS DETERMINATION
X= 1 3 5 10 20 30 50 100 200 500 SB
log{S(100) / S(60)} 0.69 0.70 0.64 0.54 0.44 0.34 0.24 0.13 0.03 -0.11 0.0
T(nu 2Bnu) a 16 19 21 24 27 29 31 34 37 43 40
log(Md/s100 D2) b 4.08 3.42 3.16 2.83 2.60 2.37 2.17 1.93 1.67 1.44 1.33
log (Md/S800 D2) b 4.99 4.84 4.78 4.70 4.62 4.57 4.52 4.43 4.34 4.19
a valid for lambda > 1700/T µm
b solar masses/(Jy Mpc2)

8. DUST IN ELLIPTICALS AND LENTICULARS

There has been a growing realization that ellipticals and lenticulars have a significant interstellar medium, and that interesting amounts of star formation take place there. There has been a decade of work on HI emission from ellipticals and more recently CO observations in several cases (see e.g. the reviews by Wardle & Knapp (1986) and Schweizer (1987)). Although the majority of the galaxies detected by IRAS are spirals, quite a number of ellipticals and lenticulars were detected (e.g. Jura 1986, Jura et al 1987, Knapp et al 1989). Thronson and Bally (1987) have studied the IRAS colour-colour diagrams for these galaxies and conclude that they occupy the same region of the diagrams as spiral galaxies (and, for that matter, star forming regions in our Galaxy). About 2/3rds of the sample they studied have the colors characteristic of cirrus and 1/3rd those of dusty regions surrounding young stars. The star formation rate they derive (0.1-1 M0/year) is comparable to the mass-loss rate for evolved stars in these galaxies, but mergers and gas infall may also contribute significantly. Bally and Thronson (1989) studied the IRAS data for a sample of 74 S0 galaxies which had known single-dish radio fluxes. 30% were detected in all 4 IRAS bands and 80% were detected in at least one band. The galaxies divided into those which followed the infrared-radio relation for spirals, for which the radio emission is presumably due to normal star formation, and those with excess radio emission, presumably due to an active nucleus and jets or lobes. A small number showed a slight excess of infrared to radio. Similar conclusions were reached by Walsh et al (1989). Knapp et al (1989) report that 2/3rds of a sample of several hundred S0s are detected by IRAS at 60 and 100 µm.

Thronson et al (1989b) examined the IRAS data for 150 lenticular and elliptical 'shell' galaxies (Malin and Carter 1983), which are believed to be the result of low velocity mergers. Although some of the galaxies showed evidence for enhanced star formation, the majority did not and they concluded that either (1) the merging galaxies are almost always E or S0 with only modest amounts of interstellar gas, or (2) the time-scale for creation and maintenance of the shell is longer than the timescale for the starburst event, or (3) the formation of a shell structure requires a mass difference between the galaxies of a factor 10-100, so only a small fraction of the i.s.m. is heated or participates in star formation.

Walsh and Knapp (1990) find that the ellipticals detected by IRAS tend preferentially to be those with dust lanes visible in the optical. However the infrared properties are not strongly dependent on the visible dust content. They also find a slightly enhanced 100 µm detection rate for ellipticals with shells, boxy isophotes or inner discs, all of which are evidence of a recent merger, a result which is not necessarily inconsistent with that of Thronson et al (1990).

It is unfortunate that the infrared sources associated with early type galaxies are almost all rather weak, so that there is little immediate prospect of detection at wavelengths > 300 µm, and hence of accurate dust mass determinations.

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