MACROMOLECULES, VERY SMALL GRAINS, AND LARGE DUST PARTICLES IN THE. WHIRLPOOL GALAXY M51 AND ITS COMPANION: A UNIFIED VIEW.
THE ASTROPHYSICAL JOURNAL, 486 : L95–L98, 1997 September 10 © 1997. The American Astronomical Society. All rights reserved. Printed in U.S. A.
MACROMOLECULES, VERY SMALL GRAINS, AND LARGE DUST PARTICLES IN THE WHIRLPOOL GALAXY M51 AND ITS COMPANION: A UNIFIED VIEW DAVID L. BLOCK,1 BRUCE G. ELMEGREEN,2 ALAN STOCKTON,3
AND
MARC SAUVAGE4
Received 1997 May 13; accepted 1997 June 26
ABSTRACT A combined image using optical and mid-infrared ISOCAM data of the galaxy M51 and its companion reveals a unified view of the distribution of dust grains of all sizes. The image shows the large grains in extinction at blue light and the small grains in emission at 15 mm. Much of the emission from small grains coincides with extinction from large grains, indicating that dense cold clouds are surrounded by a warmer ultraviolet-exposed envelope; other emission from small grains has no obvious extinction counterpart. The diffuse gas in M51 is particularly striking: shell-like structures are common, the interarm clouds have spiral shapes, and the inner spiral dust lanes are remarkably symmetric. A circular shape to the gas spiral in the center of M51 suggests that there is a barrier for a wave mode. The dust lanes in the arms show sharp inner edges from shock fronts, dense, regularly spaced clumps with star formation, and feathered outer edges from disruption by star formation. This appearance strongly suggests that star formation is triggered in spiral arms by the gravitational collapse of shocked gas. The companion galaxy is barred and has a circumnuclear ring of dust that is similar to starburst rings at the inner Lindblad resonances of other barred galaxies; it has a radius of approximately 120, corresponding to about 500 pc. Subject headings: galaxies: spiral — ISM: structure — stars: formation — techniques: image processing of diffuse and dense gas in this grand-design galaxy and its companion.
1. INTRODUCTION
Theoretical models that quantify the physical properties of various dust grain populations in interstellar space now appear to be within reach (D´esert, Boulanger, & Puget 1990; Greenberg & Li 1996; Li & Greenberg 1997; Mathis 1996). The distribution and spatial extent of this dust is not easily observed, however. The largest dust grains, which dominate the extinction of optical light, are typically too cold (T 1 16 K in the diffuse interstellar medium [ISM] down to 16.5 K at a molecular cloud center; see Greenberg & Li 1996) to be observed in emission shortward of 100 mm, so they were systematically missed (Sauvage & Thuan 1994) by the Infrared Astronomical Satellite (IRAS) and other early surveys. Conversely, the smallest dust grains, which are relatively hot (30 –1000 K) and easily observed by IRAS, contribute little to the extinction in optical bands. In this Letter, we demonstrate that ground-based, digital imaging at optical wavelengths can be combined with midinfrared imaging from ISOCAM (Cesarsky et al. 1996), the camera on board the Infrared Space Observatory (ISO), to make maps that are sensitive to dust grains of all sizes. This technique offers a unique opportunity to map the distribution of large (cold) dust grains and very tiny grains/macromolecules at the same time, with unprecedented spatial resolution: 1 order of magnitude better than that possible with the largest submillimeter/millimeter telescopes and 2 orders of magnitude better than with IRAS. We apply the technique to M51 and discuss the implications for understanding the distribution
2. HOT DUST AT 15 MICRONS
Interstellar dust forms when refractory elements in space condense into solid particles in the envelopes of cool stars, novae, dense clouds, and other places. The particles range in size from 0.5 mm (based on near-infrared albedo studies by Block 1996 and references therein) down to 0.001 mm or smaller, at which point they are really macromolecules. Almost 30 years ago, Greenberg (1968) and Greenberg & Hong (1974) suggested that very small grains would be subject to temperature fluctuations as background photons get absorbed (see review by Duley 1996). The temperature spike associated with the absorption of a 10 eV photon may approach 103 K, as shown by Greenberg (1968) for 5 Å particles. Thereafter, the grain cools on a timescale of about 1 s to temperatures that may even be below 2.7 K (Duley 1996). Based on these earlier ideas, Sellgren, Werner, & Dinerstein (1983) suggested that temperature spiking in very small grains is responsible for the widespread diffuse Galactic emission detected by IRAS at 12 microns (see review in Greenberg & Li 1996). One type of macromolecule proposed for such interstellar grains is a PAH (e.g. De ´sert et al. 1990; Sellgren 1996 and references therein). These molecules also provide an interpretation for the emission bands at 3.3, 6.2, 7.7, 8.6, and 11.3 mm, which are now ubiquitously detected by ISO. Grain temperature spiking implies that a 15 mm image of a galaxy will reveal dust clouds containing tiny grains that are exposed to background UV light. Such clouds are very different in nature from the cold and dense dust clouds that dominate extinction at optical wavelengths (Block et al. 1994, 1997). Transparent (or diffuse) clouds contain the full range of grain sizes, whereas the interiors of dense, opaque clouds have probably lost their smallest grains as a result of accretion onto
1 Department of Computational and Applied Mathematics, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa. 2 IBM Research Division, T. J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598. 3 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822. 4 CEA/DSM/DAPNIA/Service d’Astrophysique, CE Saclay, F-91191 Gifsur-Yvette, France.
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larger grains. This implies that only the outer envelopes of dense clouds are likely to be rich in tiny grains (Bernard, Boulanger, & Puget 1993), so these clouds should exhibit little 15 mm emission per unit visual extinction relative to diffuse clouds. A combined image made by dividing the 15 mm image by the optical image therefore shows bright features for the dense cloud interiors, the temperature-spiking grains in the dense cloud envelopes, and the temperature-spiking grains in diffuse clouds. A comparison of this ratio image to a B 2 K or other image that shows primarily extinction will then reveal any differences between the spatial distributions of these two types of clouds. 3. A B 2 15 mm IMAGE OF M51
Figure 1 (Plate L13) shows a B 2 15 mm image of the double galaxy M51/NGC 5195. Here, B represents the magnitude scale in the blue passband, where magnitude is 22.5 log IB for intensity IB; the image subtracted from B is the magnitude scale for the 15 mm image, again evaluated as 22.5 log I15. Thus, the difference in magnitudes, plotted in Figure 1, is 2.5 log (I15/IB ), which is proportional to the log of the intensity ratio. In this image, bright represents cold dust in extinction plus temperature-spiking dust in emission. The 15 mm ISOCAM image was secured by Sauvage et al. (1996) using the LW3 filter at l 5 12–18 mm (Cesarsky et al. 1996). The ground-based B CCD image was secured at Kitt Peak by T. Boroson. The relative image scales and orientations were determined by measuring the positions of the nuclei of M51 and NGC 5195 in the two images. The ISOCAM 15 mm LW3 image was rebinned to the B image scale and then rotated and aligned so that the galaxy nuclei in the two images were coincident. The point-spread function was determined from stars in the B image and from ISOCAM calibration observations. The B image was convolved with the ISOCAM point-spread function, and the LW3 image was convolved with the B point-spread function, resulting in two images of identical angular resolution (70). Then the ratio of the two images was obtained. The remarkable aspect of Figure 1 is that all of the bright structure is from dust. Moreover, it shows virtually all of the dust, not just the hot or cold components alone, and, because dust and gas are generally well mixed in galaxies, it shows nearly the entire gas component at high angular resolution. Our 70 resolution in Figure 1 corresponds to 300 pc, assuming a distance to M51/NGC 5195 of 8.7 Mpc (Aaronson & Mould 1983). The molecular clouds that have been observed with millimeter-wave interferometers at 90 (Rand & Kulkarni 1990) and 3"5 (Rand 1993) resolution and with single-dish millimeterwave telescopes at 120 resolution (Garcia-Burillo, Guelin, & Cernicharo 1993) are visible in Figure 1 as dense knots in the spiral arm dust lanes. Atomic hydrogen that was observed at 50 to 340 resolution with radio interferometers at 21 cm (Rots et al. 1990) is visible as low-level emission around and between the molecular clouds and as clouds in the interarm regions. The only dust that is likely to be missing from the figure is cold dust on the far side of the galaxy, which would be invisible at optical wavelengths because of foreground stars, cold dust that is obscured by bright foreground H II regions, or dust in regions where the stellar radiation field is too low to significantly heat the grains or produce detectable extinction (i.e., in the outer disk).
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The structure in the spiral arm dust lanes and diffuse ISM of M51 is striking. The two inner dust lanes are continuous and extremely regular; they curve into a circle in the inner region rather than extend to the nucleus. This curvature suggests that there is a wave barrier in the region of the bulge, as expected theoretically. It is possible that the main spiral in M51 is a wave mode, which requires such a barrier (Bertin et al. 1989). Presumably, the mode was initiated or rejuvenated by the companion (Byrd & Salo 1995) and is now evolving independently. Inside the central circle there are weaker spirals in near-infrared images of the old stellar population (Zaritsky, Rix, & Rieke 1993) and there is a molecular structure like a bar (Rand & Kulkarni 1990); this structure is not visible here, perhaps because the nuclear region is washed out in our B image by a bright overlying bulge. The spiral arms in M51 are very symmetric: the two main arms terminate at the same middisk radius, diametrically across from the nucleus. Optical studies place the wave’s corotation at the middisk radius, where the dust spirals in this figure terminate (Tully 1974; Elmegreen, Elmegreen, & Montenegro 1992). Such a location for corotation is consistent with the origin of the dust lanes as spiral arm shock fronts: the shock strength diminishes near corotation, so the dust lanes become less well defined there. Irregular dust lanes continue through and beyond corotation, however, with large bifurcations or broadenings. Such structure is not predicted by simple shock models. The dust lanes of the main spiral arms are clump y. The clumps are not comet shaped with tails pointing inward to the inner disk, as would be expected for interarm clouds that were compressed and swept back by a spiral wave shock (Woodward 1976). Instead, the clumps are round and strung out along the arms in a semi-regular fashion. This is most likely the result of a gravitational condensation in the dense shocked gas (Elmegreen 1979). The outer edges of the dust lanes are more irregular, with protrusions extending slightly downstream at the locations of the knots. Evidently, we view here the main effect of density waves on interstellar gas and star formation: the density waves compress the gas and dust, giving the dust lanes a sharp inner edge at the shock front (e.g., Roberts 1969; Greenberg 1970; Greenberg & Li 1995). The shocked material then collapses along the length of the arm, forming the main clumps. These clumps contain molecules and form stars (as evident from CO surveys and optical images), and the associated H II regions cause the gas to burst out of the dust lane and drift downstream (as evident from the downstream protrusions). The H II regions that dominate the appearance of this galaxy at optical wavelengths are not obviously displaced to the outside of the dust arms (see Fig. 1b in Sauvage et al. 1996). This implies observationally that most of the gas phases involved with star formation are mixed at our resolution of 70 5 300 pc. It implies physically that gravitational collapse and star formation occur quickly once the gas enters the spiral arms of this galaxy; most of the condensed material is still close to the density wave shock, as predicted theoretically for strong shocks (Elmegreen 1994a). Figure 1 shows giant bubbles of dust strung out along the northern arm. A similar string of bubbles is seen optically in the southern arm of the spiral galaxy M83. Such bubbles presumably arise from high pressures associated with star formation in the arms. One bubble, approximately 1.0 kpc in diameter, is relatively dark in the center, indicating that most of the gas has been cleared away. Some bubbles are bright and
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clumped on the periphery where there is triggered star formation evident from the distribution of H II regions in optical photographs. The flow-through time for the gas in the spiral arms is very long at these large radii, so star-forming regions in the arm can become highly evolved before the arm passes by. This gives the massive stars enough time to inflate giant bubbles with their ionized gas, winds, and supernovae, and these bubbles have enough time to form new stars in the dense gas. The bubbles are nearly circular in this image (they could be chimneys seen end-on) because spiral arms have relatively low shear compared to the interarm region or to the average rate of shear at the same radius in the disk (Elmegreen 1994a). They are regularly spaced because most of the star formation in this arm has this regular distribution. Interarm dust and its associated gas are unevenly distributed throughout the disk. Interarm clouds are clearly present. These clouds are interesting as precursors to density wave– triggered star formation. When the spiral wave passes, they will get pushed together in the shock to make future dust lanes. Figure 1 suggests that many interarm clouds have spiral shapes, as if they were remnants from former dust lanes. Presumably, they were broken off from the main dust ridges by star formation in the arms and by the higher galactic tidal force in the interarm region (Elmegreen 1992). This spiral shape helps explain why interarm clouds do not shred or distort the dust lanes upon impact: the impacting material is long and thin when it hits the arm, not a collection of random dense bullets. Thus, the interarm gas joins the arms smoothly, without distortion and without the formation of cometary clouds in the arm. The dust components responsible for extinction in the B band and emission at 15 mm are distinct, so we might have expected their distributions to differ as well. However, the structure in Figure 1 is essentially the same as the dust structure in pure extinction maps (as from the B image alone) or in pure emission maps (as at 15 mm). That is, most of the structures correspond to both extinction and emission clouds at the same place. This correlation between extinction and emission has important consequences for our understanding of galactic dust, because it implies that cold large grains and hot tiny grains are spatially correlated at our angular resolution. Some of the hot tiny grains are probably on the periphery of the cold molecular clouds, as mentioned above, and some are in diffuse clouds that must also be close to the molecular clouds, in giant cloud complexes. Since this correlation between optical extinction and 15 mm emission occurs over the whole galaxy, interstellar cloud complexes must have similar properties at all radii.
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4. COMMENTS ON NGC 5195
The companion galaxy NGC 5195 is extreme in its starforming properties. Heckman (1980) found it to be a member of the LINER class, and 8 –13 mm spectral signatures typical of enhanced nuclear star formation were published by Roche & Aitken (1985). Sage & Wrobel (1989) found NGC 5195 to have the highest CO luminosity of all the high, L(FIR) S0 galaxies they studied with the NRAO 12 m antenna. The existence of a rather compact radio source at the nucleus has been reported (van der Hulst et al. 1988). Smith (1982) found an unusually warm dust temperature of 65 K but also noted that “NGC 5195 has by far the scarcest interstellar medium, amounting to the mass of Sgr B2, a single giant molecular cloud located near the nucleus of the Galaxy.” NGC 5195 has a D25 diameter of 5#37, equivalent to 13.5 kpc at our adopted distance of 8.7 Mpc. In this context, it is of great interest to note that the nucleus of NGC 5195 is actually surrounded by concentrations (blobs) of dust, in the shape of a ring. This is evident from a close examination of Figure 1, as enlarged in Figure 2 (Plate L14). The ring has a radius of about 120, and the elongation of the ring closely matches the elongation of the parent galaxy (the inclination is 43°). Assuming a distance of 8.7 Mpc, 10 5 42 pc, the radius of the ring is about 500 pc. This radius is typical for starburst rings at the inner Lindblad resonances of galaxies (Antonucci 1993). It has been suggested that the mid-infrared emission from NGC 5195 could come from dust heated by an evolved central starburst (Boulade et al. 1996). The ringlike structure that we detect in our B 2 15 mm map confirms the existence of such a remnant ISM, and the blobs in the ring point to the likely starburst mechanism, i.e., global selfgravitational collapse (Elmegreen 1994b), as in the main spiral arm dust lanes. The research of D. L. B. is supported in South Africa by the Anglo-American and de Beers Chairman’s Fund Educational Trust, and a note of great appreciation is expressed to M. C. Keeton and the Board of Trustees. D. L. B. also thanks the University of the Witwatersrand for their support. A. S. thanks the NSF for partial support under grant AST95-29078. The graphic enhancement for the figures was done by B. G. E. using the IBM Data Explorer. ISO is an ESA project with instruments funded by ESA member states (especially the PI countries: France, Germany, the Netherlands, and the United Kingdom) and with participation of ISAS and NASA. Helpful comments by M. Greenberg are appreciated.
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FIG. 1.—B 2 15 mm map of M51/NGC 5195. North is up. Features that are bright at 15 mm or dark in B are bright in this image. Brightness is plotted proportionally to the logarithm of the intensity of the source in order to reveal faint features. BLOCK et al. (see 486, L96)
PLATE L13
FIG. 2.—An enlargement of Fig. 1. A ring of dust surrounds the nucleus of the companion galaxy NGC 5195. The ring is not visible in optical images. The radius of the circumnuclear ring corresponds to approximately 500 pc. BLOCK et al. (see 486, L97)
PLATE L14
