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AKARI observations of interstellar dust grains in our Galaxy and nearby galaxies
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AKARI observations of interstellar dust grains in our Galaxy and nearby galaxies H. Kaneda a,n, D. Ishihara a, K. Kobata a, T. Kondo a, S. Oyabu a, R. Yamada a, M. Yamagishi a, T. Onaka b, T. Suzuki c a
Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan Department of Astronomy, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan c Netherlands Institute for Space Research, SRON, Utrecht, The Netherlands b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 October 2013 Received in revised form 30 December 2013 Accepted 20 January 2014
The infrared (IR) emission from interstellar dust grains is a powerful tool to trace star-formation activities in galaxies. Beyond such star-formation tracers, spectral information on polycyclic aromatic hydrocarbons (PAHs) and large grains, or even their photometric intensity ratios, has deep physical implications for understanding the properties of the interstellar medium. With the AKARI satellite launched in 2006, we have performed a systematic study of interstellar dust grains in various environments of galaxies including our Galaxy. Because of its unique capabilities, such as mid-/far-IR all-sky surveys and near-/far-IR spectroscopy, AKARI has provided new knowledge on the processing of dust, particularly carbonaceous grains including PAHs, in the interstellar space. For example, the near-IR spectroscopy has revealed structural changes of hydrocarbon grains in harsh environments of galaxies. In this paper, we focus on the properties of the PAH emission obtained by the AKARI mid-IR all-sky survey and near-IR spectroscopy. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Intersteller dust PAH Galaxy AKARI
1. Introduction In star-forming regions, large grains and polycyclic aromatic hydrocarbons (PAHs; i.e., smallest form of carbonaceous grains) absorb a significant fraction of stellar ultraviolet photons and re-radiate them in the infrared (IR). Hence the IR luminosities due to PAHs and large grains are both powerful tools to trace star-forming activities in galaxies or search for young stellar objects embedded in clouds. However they are not merely star-formation tracers. Spectral information on PAHs and large grains, as well as relative abundance of PAHs to large grains, would have much deeper physical implications for understanding the properties of the interstellar medium (ISM). With the Infrared Camera (IRC; Onaka et al., 2007) and the FarInfrared Surveyor (FIS; Kawada et al., 2007) on board AKARI, the first Japanese infrared astronomical satellite launched in 2006 (Murakami et al., 2007), we have performed a systematic study of interstellar dust grains in various environments of galaxies including our Galaxy. Because of its unique capabilities, such as all-sky coverage in the mid- and the far-IR combined with near- and far-IR spectroscopy, AKARI has provided new knowledge on the processing of dust, particularly carbonaceous grains including PAHs, in the interstellar space. For example, we obtained all-sky diffuse maps in the 9, 18, 65, 90, 140, and 160 μm photometric bands by the all-sky surveys.
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Corresponding author. Tel.: þ 81 52 7892452. E-mail address: [email protected] (H. Kaneda).
Among them, the 9 μm diffuse map is the world-first all-sky map of the PAH emission, while the other maps mostly trace warm and cool components of large grains. In addition to such photometric datasets, we obtained near-IR (2–5 μm) spectroscopic data for more than 10,000 targets by pointed observations (Ohyama et al., 2007), most of which were performed during the warm mission phase after the boil-off of liquid helium cryogen. We also obtained far-IR (70–170 μm) spectroscopic data using the imaging Fourier Transform Spectrometer of the FIS (Kawada et al., 2008). For example, the near- and the far-IR spectroscopy have revealed structural changes of hydrocarbon particles (e.g., Yamagishi et al., 2012) and formation of large graphite grains (Kaneda et al., 2012b), respectively, in harsh environments of galaxies. In this paper, we focus on the properties of the PAH emission obtained by the AKARI mid-IR all-sky survey and near-IR spectroscopy. We discuss (1) spatial variations in the photometric intensity of the mid-IR emission due to PAHs relative to that of the far-IR emission due to large grains, based on the all-sky diffuse maps, and (2) spectral variations in the ratio of the aliphatic to the aromatic feature, based on the near-IR spectra.
2. All-sky survey in the mid-IR PAH emission Fig. 1a displays a diffuse map of the Galactic plane in the AKARI 9 μm band. It should be noted that the AKARI 9 μm band (the reference wavelength and the band width of 9.0 μm and 6.7–11.6 μm respectively; Onaka et al., 2007) efficiently covers
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Please cite this article as: Kaneda, H., et al., AKARI observations of interstellar dust grains in our Galaxy and nearby galaxies. Planetary and Space Science (2014), http://dx.doi.org/10.1016/j.pss.2014.01.017i
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Fig. 1. (a) AKARI 9 μm-band all-sky diffuse map in the galactic coordinates, shown together with the observational positions of AKARI near-IR (2–5 μm) spectroscopy. (b) A correlation plot between the AKARI 9 μm and 140 μm band intensities for the all-sky images regridded to a spatial scale of 60 arc m 60 arc m.
the major PAH emission features at wavelengths of 6.3, 7.7, 8.6, and 11.3 μm, as compared to the all-sky maps in the WISE or IRAS bands at similar wavelengths (Ishihara et al., 2010a). Fig. 1b shows a correlation plot between the AKARI 9 μm and 140 μm band intensities for the all-sky images regridded to a spatial scale of 60 arc m 60 arc m. The plot exhibits a tight correlation over a range of 4 orders of magnitude with the linear-correlation coefficient of 0.94. This correlation demonstrates that PAHs and large grains are mixed well in the ISM, as pointed out by many authors (e.g., Onaka et al., 1996; Kaneda et al., 2012a). Utilizing the AKARI all-sky point-source catalogs, we derived the spatial distributions of carbon-rich (C-rich) and oxygen-rich (O-rich) asymptotic giant branch (AGB) stars, based on the color– color diagrams of the 9 and 18 μm band fluxes with the 2MASS J, H, and K band fluxes (Ishihara et al., 2011). As a result, we find that the O-rich AGBs are more concentrated toward the Galactic center, while the C-rich AGBs are rather uniformly distributed throughout the Galactic plane. As can be seen in Fig. 1b, interstellar PAHs and far-IR dust grains are similar in the spatial distribution on both global and local scales, which do not follow well the distribution of either C-rich or O-rich stars. It is generally thought that silicate grains, a major far-IR dust component, are supplied into the interstellar space by O-rich stars, while carbonaceous grains including PAHs are produced by C-rich stars (e.g., Dwek, 1998). Thus our results show that PAHs and large grains are well mixed in the ISM whereas their suppliers have different spatial distributions. It is also worth to note that variation of the 3.4 μm aliphatic hydrocarbon absorption feature seems to follow that of the 9.7 μm
amorphous silicate absorption feature; their optical depths relative to the visual extinction in the Galactic center are both about twice as large as those in the local diffuse interstellar medium (Gao et al., 2010). This is an open issue to be addressed in future works. Hence the AKARI 9 μm map reveals that PAHs are widely distributed throughout the Galactic plane, similar to large grains. Yet the maps also exhibit significant variations in the relation between the PAH and the far-IR dust emission, depending on local interstellar conditions. For example, in shocked regions associated with supernova remnants (SNRs), PAH emission is extremely suppressed as compared to far-IR dust emission (e.g., Ishihara et al., 2010b), which is attributed to a large difference in the lifetime against strong shocks between PAHs and large grains (Micelotta et al., 2010). In post-shock hot plasmas, lifetimes of PAHs are two to three orders of magnitude shorter than those for equivalent dust grains of roughly the same size, because the sputtering yields of 3-dimensional grains (i.e. the number of sputtered atoms per incident high-energy particle) are much smaller than unity, while the dissociation yields of 2-dimensional PAHs are close to unity. Other than SNRs, Kaneda et al. (2012a) found that the ratios of the PAH to far-IR dust emission show a significant depression (by a factor of 5) near the foot points of the molecular loops revealed by the NANTEN 12CO ðJ ¼ 1 0Þ observations in the Galactic center region (Fukui et al., 2006). Because the CO observations indicated that a violent motion and shock heating of gas took place in the loops (Torii et al., 2010), the relative decrease in the PAH emission suggests the destruction of PAHs by shocks at the foot points of the molecular loops. External galaxies provide much wider ranges of physical conditions for the ISM than our Galaxy. Among them, nearby edge-on starburst galaxies with prominent galactic superwinds are important targets to understand the processing of dust in high energetic phenomena. For example, in M 82 (Kaneda et al., 2010) and NGC 253 (Kaneda et al., 2009), we find that copious amounts of large grains and PAHs are flowing out of the galaxies by galactic superwinds, both of which are likely being shattered and destroyed in the galactic haloes. Fig. 2a and b shows correlation plots between the AKARI 9 μm and 140 μm band intensities for M 82 and NGC 253, respectively, where the different symbols and colors are used to discriminate the center, disk, northern and southern halo regions. In each panel, the solid line represents the relationship obtained for our Galaxy (i.e., Fig. 1b). As can be seen in the figure, they exhibit global relations quite similar to our Galaxy, despite the fact that the environments are much harsher in these galaxies. In the halo regions, it appears that the 9 μm to 140 μm ratios are systematically shifted toward lower values. Moreover the data points for NGC 253 exhibit an apparently larger scatter than those for M 82, which may reflect that NGC 253 is a starburst galaxy in a later evolutionary stage than M 82, causing difference in the degree of the dust processing. In the center of M 82, the 9 μm to 140 μm ratios are well above the other regions, which is likely to be caused by a large increase in interstellar radiation field intensity (see Fig. 13 in Draine and Li, 2007). In interpreting the above results, we have to take into account the photo-dissociation of PAHs exposed to a strong ultraviolet (UV) radiation field (Boulanger et al., 1988; Bendo et al., 2008), although it is much less effective than destruction by shocks; the lifetimes of PAHs against the photo-dissociation are 105 years in massive starforming regions, while they are 108 years for the diffuse ISM (Allain et al., 1996). Even for a constant abundance ratio of PAHs to large grains, the 9 μm to 140 μm intensity ratio will change with other parameters such as the UV radiation field and dust extinction (Kaneda et al., 2012a). The ratios increase with the UV radiation field as long as it is stronger than that in the solar neighborhood (Draine and Li, 2007). Larger interstellar extinction in the mid-IR than in the far-IR can systematically lower the ratios of 9 μm to 140 μm intensities in dense gas regions. The dominance
Please cite this article as: Kaneda, H., et al., AKARI observations of interstellar dust grains in our Galaxy and nearby galaxies. Planetary and Space Science (2014), http://dx.doi.org/10.1016/j.pss.2014.01.017i
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Fig. 2. (a) A correlation plot between the AKARI 9 μm and 140 μm band intensities for M 82, where the images in both bands are regridded to a spatial scale of 90 arc s 90 arc s. (b) Same as panel (a), but for NGC 253. The plus (black), diamond (green), triangle (blue), and square (magenta) symbols correspond to data points for the galactic center, disk, northern halo, and southern halo regions, respectively. In each panel, the solid line represents the relationship for our Galaxy. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)
of neutral PAHs over ionized ones in very dense gas regions can also lower the ratios, because neutral PAHs emit much less in the 6–9 μm region than ionized PAHs (Szczepanski and Vala, 1993; Joblin et al., 1994; Hudgins and Allamandola, 1995). Condensation of PAHs in grain ice mantles would also be an important factor in lowering the relative intensity of the PAH emission features in dense gas regions. In short, even for a constant relative abundance of PAHs, the 9 μm to 140 μm intensity ratio can decline in UV-poor, dense gas regions, the situation of which does not seem to be applicable to the cases presented above.
3. Spectroscopy of the near-IR PAH emission In Fig. 1a, we plot the positions of the AKARI near-IR spectroscopic observations. The properties of PAHs are probed by the spectroscopy of the 3.3 μm feature and the 3.4–3.5 μm features. Both of them are attributed to the C–H stretching vibration. The former is due to aromatic (sp2) hydrocarbons, while the latter is attributed to aliphatic (sp3) hydrocarbons (e.g., Duley and Williams, 1981; Wagner et al., 2000). To be exact, the 3.4–3.5 μm feature intensities give upper limits on the aliphatic fractions in hydrocarbon grains (Li and Draine, 2012; Yang et al., 2013). The
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aromatic feature at 3.3 μm is sensitive to smallest PAHs (Schutte et al., 1993), as compared to the other PAH features at longer wavelengths. For about 200 star-forming galaxies in a redshift range of 0.01–0.1, we systematically investigated a global relation between the PAH 3.3 μm luminosity, L3:3 , and the total IR (8–1000 μm) luminosity, LIR (Yamada et al., 2013). We classified the samples into IR galaxies (IRGs; LIR o 1011 L ), luminous IR galaxies (LIRGs: LIR 1011 1012 L ) and ultra-luminous IR galaxies (ULIRGs: LIR 41012 L ). M82 is categorized into IRGs. We confirm that many of the IRGs and LIRGs follow the relationship, L3:3 =LIR C10 3 , which is a ratio typical of starburst galaxies (Mouri et al., 1990). However we also find that the L3:3 =LIR ratio considerably decreases toward the luminous end in the ULIRG population. We conclude that local ULIRGs intrinsically possess smaller amounts of PAHs relative to large grains, as a result of PAH processing through recent galaxy mergers. Some fraction of PAHs may have been destroyed once by a shock induced during a merging process, whereas large grains survive. Hence our result is consistent with the observational fact that local ULIRGs are merging galaxies (e.g., Clements et al., 1996). Considering a starburst age typical of local ULIRGs ( 10–100 Myr; Genzel et al., 1998) as well as a lifetime of intermediate-mass stars ( 100–1000 Myr) responsible for the production of PAHs at their late stages, it is unlikely that PAHs have been reproduced and replenished by the stars that were newly born after the merger. Therefore the observed PAHs are likely to be merger remnants in local ULIRGs. The intensity ratios of the aliphatic to the aromatic feature are known to show regional variations in the ISM. A huge dataset of AKARI near-IR spectroscopy reveals that they indeed change very much, depending on the interstellar conditions, which implies structural changes of hydrocarbon grains. Fig. 3 displays examples of the spectra with relatively normal aliphatic/aromatic ratios, while Fig. 4 shows those with unusually high aliphatic/aromatic ratios. As for Galactic sources, comparing the spectra in Figs. 3a and 4a, we find that the spectrum for the foot point of the molecular loop in the Galactic center region has spectral properties notably different from a typical Galactic diffuse spectrum at 3.2–3.6 μm; the properties in Fig. 4a are characterized by the faint PAH 3.3 μm emission and the broad excess above the linear baseline (Kaneda et al., 2012a). The broad excess may be explained by a combination of a series of the features at 3.47, 3.51, and 3.56 μm accompanying the 3.4 μm aliphatic feature (e.g., Sloan et al., 1997). Assuming that all the features are due to aliphatic C–H, the spectrum suggests that hydrogenated amorphous carbon grains may have been produced near the foot point of the molecular loop by shattering of larger carbonaceous grains (Jones et al., 1996), while pre-existing small PAHs may have been destroyed there. These hydrocarbon grains are likely to be H-rich and aliphatic-rich, and have not yet been evolved into H-poor, aromatic-rich materials (Jones et al., 2013); the aromatization requires subsequent UV radiation and/or thermal annealing. For M 82, we clearly detect the aromatic 3.3 μm emission and the aliphatic 3.4–3.6 μm features even in the galactic halo regions, which are located at a distance of 2 kpc away from the galactic center (Yamagishi et al., 2012). We thus confirm the presence of PAHs even in the harsh environment of the M82 halo. We find that the aliphatic 3.4–3.5 μm features are unusually abundant in the halo spectra, comparing Fig. 4b and c with Fig. 3b and c. The spectra of 34 regions in M 82 reveal that the aliphatic/aromatic ratio significantly increases with the distance from the galactic center (Yamagishi et al., 2012), which again indicates the dominance of aliphatic structures over aromatic ones by shattering of hydrogenated amorphous carbon grains in the galactic superwind. As for the mid-IR PAH emission, we found that there is an excellent correlation between the PAH and Hα distributions
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Fig. 3. AKARI near-IR spectra with relatively normal aliphatic/aromatic ratios: (a) Galactic diffuse emission observed at the galactic longitude of 311:51, (b) center and (c) disk in M 82, (d) starburst ring in NGC 1097, (e) J1224360þ 392258 (IRG), and (f) J1456077 þ833122 (LIRG). All the spectra are shown in the observed, but not rest-frame wavelengths. The emission at 4.05 μm is the hydrogen Brα line.
extended in the halo (Kaneda et al., 2010). The spectro-polarimetry showed that Hα is significantly (5–15%) polarized (Yoshida et al., 2011), implying that Hα photons from the galactic disk are scattered by dust grains in the halo. Thus the excellent PAH–Hα correlation can be explained if the grains are being fragmented to produce PAHs by the galactic superwinds (Jones et al., 1996, 2013). Moreover the estimated dust flow velocity was found to decrease with the height from the disk (Yoshida et al., 2011), suggesting that the grains thus processed may be falling back toward the disk. We also detect strong aliphatic emission from the central region of the barred spiral galaxy NGC 1097 (Kondo et al., 2012). The galaxy is categorized as Seyfert 1 with the starburst ring of 2 kpc in diameter and the inner bar structure of 1 kpc in length connecting the ring and the nucleus (Hsieh et al., 2008). Hence NGC 1097 is an ideal laboratory to study the ISM in galaxies showing both active galactic nucleus (AGN) and circumnuclear starburst activities. Figs. 3d and 4d present the spectra for the starburst ring and inner bar regions in NGC 1097, respectively, from which we find that the spectrum taken from the inner bar exhibits a relatively high aliphatic/aromatic ratio. By spectral mapping in the 3.3 μm feature and the 3.4–3.5 μm features, we find that the distribution of the aliphatic relative to the aromatic feature spatially corresponds to the inner bar connecting the ring and the nucleus (Kondo et al., 2012). Thus the local enhancements of the aliphatic features are, again, consistent with the picture that small hydrocarbon grains, not much aromatized, may be newly formed through shattering of carbonaceous grains in the inner bar (Jones et al., 1996, 2013), which might provide observational
evidence that the gas and dust in the bar is in a turbulent motion, likely fueling the central AGN from the starburst ring. Among the afore-mentioned AKARI near-IR spectral sample of star-forming galaxies, Fig. 3e and f presents examples of the spectra of IRGs and LIRGs, respectively, with relatively normal aliphatic/aromatic ratios, while Fig. 4e and f shows those with unusually high aliphatic/aromatic ratios. A significant fraction of the LIRG and ULIRG samples is found to exhibit similarly high aliphatic/aromatic ratios. However we do not find any clear dependence of the aliphatic/aromatic ratio on LIR or the galaxy population. The variation of the ratio can be explained by difference in the scale of the past merger, i.e., minor or major, or difference in the stage of the current merging process. Since most of the sample galaxies are not spatially resolved, it is notable that not only particular regions in galaxies but also galaxies as a whole exhibit such unusually high aliphatic/aromatic ratios. We estimate the ratios of the integrated intensities of the aliphatic to the aromatic feature to be 0.1–0.4 for the spectra in Fig. 3 and 0.6–3 for the spectra in Fig. 4. Using A3:4 =A3:3 C 1:76 (Yang et al., 2013), where A3:3 and A3:4 are the band strengths of the 3.4 μm aliphatic and 3.3 μm aromatic C–H bonds, along with the assumption that one aliphatic and one aromatic C atom correspond to 2.5 aliphatic and 0.75 aromatic C–H bonds, respectively (Yang et al., 2013), we derive the fraction of C atoms in aliphatic form to be 2–7% for the spectra in Fig. 3 and 10–50% for the spectra in Fig. 4. Thus a significant fraction of C atoms are in aliphatic form for the latter (Fig. 4), while they are predominantly aromatic for the former targets (Fig. 3). It should be noted,
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Fig. 4. AKARI near-IR spectra with unusually high aliphatic/aromatic ratios: (a) the foot point of one of the molecular loops in the Galactic center region, (b) southern and (c) northern haloes in M 82, (d) inner bar in NGC 1097, (e) J0055449-502749 (IRG), and (f) NGC 3110 (LIRG). All the spectra are shown in the observed, but not in rest-frame wavelengths. In panel (a), instrumental artifacts are masked out.
however, that a significant fraction of aromatic or aliphatic C atoms can be locked inside of grains, which cannot be probed by the 3.3 and 3.4–3.5 μm emission features. As pointed out by Yang et al. (2013), in benign, UV-poor environments, fragile species such as aliphatic hydrocarbon chains tend to attach to an aromatic skeleton as side groups. Therefore we can expect that PAHs are rich with aliphatic chains in quiescent ISM conditions. Our AKARI results on interstellar PAHs in galaxies, however, appear to favor rather opposite situations for the presence of aliphatic-rich PAHs, i.e. they are detected in harsh interstellar environments, the presence of which can be explained by shock fragmentation of grains with carbonaceous mantles (Jones et al., 2013).
4. Summary With the AKARI's unique capabilities of mid-/far-IR all-sky surveys and near-IR spectroscopy, we have revealed various phenomena about interstellar dust grains in galaxies including our Galaxy. Based on the all-sky maps, we find that the intensity ratios of PAHs to far-IR large grains systematically decrease in harsh environments, which can be used as probes to study the conditions of interstellar shocks on both global and local scales. With the near-IR spectroscopy we find that the ratios of the aliphatic to aromatic feature strength considerably change, depending on the interstellar conditions; all the cases presented above indicate structural changes of hydrocarbon grains by shocks in harsh interstellar environments.
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AKARI observations of interstellar dust grains in our Galaxy and nearby galaxies H. Kaneda a,n, D. Ishihara a, K. Kobata a, T. Kondo a, S. Oyabu a, R. Yamada a, M. Yamagishi a, T. Onaka b, T. Suzuki c a
Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan Department of Astronomy, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan c Netherlands Institute for Space Research, SRON, Utrecht, The Netherlands b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 October 2013 Received in revised form 30 December 2013 Accepted 20 January 2014
The infrared (IR) emission from interstellar dust grains is a powerful tool to trace star-formation activities in galaxies. Beyond such star-formation tracers, spectral information on polycyclic aromatic hydrocarbons (PAHs) and large grains, or even their photometric intensity ratios, has deep physical implications for understanding the properties of the interstellar medium. With the AKARI satellite launched in 2006, we have performed a systematic study of interstellar dust grains in various environments of galaxies including our Galaxy. Because of its unique capabilities, such as mid-/far-IR all-sky surveys and near-/far-IR spectroscopy, AKARI has provided new knowledge on the processing of dust, particularly carbonaceous grains including PAHs, in the interstellar space. For example, the near-IR spectroscopy has revealed structural changes of hydrocarbon grains in harsh environments of galaxies. In this paper, we focus on the properties of the PAH emission obtained by the AKARI mid-IR all-sky survey and near-IR spectroscopy. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Intersteller dust PAH Galaxy AKARI
1. Introduction In star-forming regions, large grains and polycyclic aromatic hydrocarbons (PAHs; i.e., smallest form of carbonaceous grains) absorb a significant fraction of stellar ultraviolet photons and re-radiate them in the infrared (IR). Hence the IR luminosities due to PAHs and large grains are both powerful tools to trace star-forming activities in galaxies or search for young stellar objects embedded in clouds. However they are not merely star-formation tracers. Spectral information on PAHs and large grains, as well as relative abundance of PAHs to large grains, would have much deeper physical implications for understanding the properties of the interstellar medium (ISM). With the Infrared Camera (IRC; Onaka et al., 2007) and the FarInfrared Surveyor (FIS; Kawada et al., 2007) on board AKARI, the first Japanese infrared astronomical satellite launched in 2006 (Murakami et al., 2007), we have performed a systematic study of interstellar dust grains in various environments of galaxies including our Galaxy. Because of its unique capabilities, such as all-sky coverage in the mid- and the far-IR combined with near- and far-IR spectroscopy, AKARI has provided new knowledge on the processing of dust, particularly carbonaceous grains including PAHs, in the interstellar space. For example, we obtained all-sky diffuse maps in the 9, 18, 65, 90, 140, and 160 μm photometric bands by the all-sky surveys.
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Corresponding author. Tel.: þ 81 52 7892452. E-mail address: [email protected] (H. Kaneda).
Among them, the 9 μm diffuse map is the world-first all-sky map of the PAH emission, while the other maps mostly trace warm and cool components of large grains. In addition to such photometric datasets, we obtained near-IR (2–5 μm) spectroscopic data for more than 10,000 targets by pointed observations (Ohyama et al., 2007), most of which were performed during the warm mission phase after the boil-off of liquid helium cryogen. We also obtained far-IR (70–170 μm) spectroscopic data using the imaging Fourier Transform Spectrometer of the FIS (Kawada et al., 2008). For example, the near- and the far-IR spectroscopy have revealed structural changes of hydrocarbon particles (e.g., Yamagishi et al., 2012) and formation of large graphite grains (Kaneda et al., 2012b), respectively, in harsh environments of galaxies. In this paper, we focus on the properties of the PAH emission obtained by the AKARI mid-IR all-sky survey and near-IR spectroscopy. We discuss (1) spatial variations in the photometric intensity of the mid-IR emission due to PAHs relative to that of the far-IR emission due to large grains, based on the all-sky diffuse maps, and (2) spectral variations in the ratio of the aliphatic to the aromatic feature, based on the near-IR spectra.
2. All-sky survey in the mid-IR PAH emission Fig. 1a displays a diffuse map of the Galactic plane in the AKARI 9 μm band. It should be noted that the AKARI 9 μm band (the reference wavelength and the band width of 9.0 μm and 6.7–11.6 μm respectively; Onaka et al., 2007) efficiently covers
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Fig. 1. (a) AKARI 9 μm-band all-sky diffuse map in the galactic coordinates, shown together with the observational positions of AKARI near-IR (2–5 μm) spectroscopy. (b) A correlation plot between the AKARI 9 μm and 140 μm band intensities for the all-sky images regridded to a spatial scale of 60 arc m 60 arc m.
the major PAH emission features at wavelengths of 6.3, 7.7, 8.6, and 11.3 μm, as compared to the all-sky maps in the WISE or IRAS bands at similar wavelengths (Ishihara et al., 2010a). Fig. 1b shows a correlation plot between the AKARI 9 μm and 140 μm band intensities for the all-sky images regridded to a spatial scale of 60 arc m 60 arc m. The plot exhibits a tight correlation over a range of 4 orders of magnitude with the linear-correlation coefficient of 0.94. This correlation demonstrates that PAHs and large grains are mixed well in the ISM, as pointed out by many authors (e.g., Onaka et al., 1996; Kaneda et al., 2012a). Utilizing the AKARI all-sky point-source catalogs, we derived the spatial distributions of carbon-rich (C-rich) and oxygen-rich (O-rich) asymptotic giant branch (AGB) stars, based on the color– color diagrams of the 9 and 18 μm band fluxes with the 2MASS J, H, and K band fluxes (Ishihara et al., 2011). As a result, we find that the O-rich AGBs are more concentrated toward the Galactic center, while the C-rich AGBs are rather uniformly distributed throughout the Galactic plane. As can be seen in Fig. 1b, interstellar PAHs and far-IR dust grains are similar in the spatial distribution on both global and local scales, which do not follow well the distribution of either C-rich or O-rich stars. It is generally thought that silicate grains, a major far-IR dust component, are supplied into the interstellar space by O-rich stars, while carbonaceous grains including PAHs are produced by C-rich stars (e.g., Dwek, 1998). Thus our results show that PAHs and large grains are well mixed in the ISM whereas their suppliers have different spatial distributions. It is also worth to note that variation of the 3.4 μm aliphatic hydrocarbon absorption feature seems to follow that of the 9.7 μm
amorphous silicate absorption feature; their optical depths relative to the visual extinction in the Galactic center are both about twice as large as those in the local diffuse interstellar medium (Gao et al., 2010). This is an open issue to be addressed in future works. Hence the AKARI 9 μm map reveals that PAHs are widely distributed throughout the Galactic plane, similar to large grains. Yet the maps also exhibit significant variations in the relation between the PAH and the far-IR dust emission, depending on local interstellar conditions. For example, in shocked regions associated with supernova remnants (SNRs), PAH emission is extremely suppressed as compared to far-IR dust emission (e.g., Ishihara et al., 2010b), which is attributed to a large difference in the lifetime against strong shocks between PAHs and large grains (Micelotta et al., 2010). In post-shock hot plasmas, lifetimes of PAHs are two to three orders of magnitude shorter than those for equivalent dust grains of roughly the same size, because the sputtering yields of 3-dimensional grains (i.e. the number of sputtered atoms per incident high-energy particle) are much smaller than unity, while the dissociation yields of 2-dimensional PAHs are close to unity. Other than SNRs, Kaneda et al. (2012a) found that the ratios of the PAH to far-IR dust emission show a significant depression (by a factor of 5) near the foot points of the molecular loops revealed by the NANTEN 12CO ðJ ¼ 1 0Þ observations in the Galactic center region (Fukui et al., 2006). Because the CO observations indicated that a violent motion and shock heating of gas took place in the loops (Torii et al., 2010), the relative decrease in the PAH emission suggests the destruction of PAHs by shocks at the foot points of the molecular loops. External galaxies provide much wider ranges of physical conditions for the ISM than our Galaxy. Among them, nearby edge-on starburst galaxies with prominent galactic superwinds are important targets to understand the processing of dust in high energetic phenomena. For example, in M 82 (Kaneda et al., 2010) and NGC 253 (Kaneda et al., 2009), we find that copious amounts of large grains and PAHs are flowing out of the galaxies by galactic superwinds, both of which are likely being shattered and destroyed in the galactic haloes. Fig. 2a and b shows correlation plots between the AKARI 9 μm and 140 μm band intensities for M 82 and NGC 253, respectively, where the different symbols and colors are used to discriminate the center, disk, northern and southern halo regions. In each panel, the solid line represents the relationship obtained for our Galaxy (i.e., Fig. 1b). As can be seen in the figure, they exhibit global relations quite similar to our Galaxy, despite the fact that the environments are much harsher in these galaxies. In the halo regions, it appears that the 9 μm to 140 μm ratios are systematically shifted toward lower values. Moreover the data points for NGC 253 exhibit an apparently larger scatter than those for M 82, which may reflect that NGC 253 is a starburst galaxy in a later evolutionary stage than M 82, causing difference in the degree of the dust processing. In the center of M 82, the 9 μm to 140 μm ratios are well above the other regions, which is likely to be caused by a large increase in interstellar radiation field intensity (see Fig. 13 in Draine and Li, 2007). In interpreting the above results, we have to take into account the photo-dissociation of PAHs exposed to a strong ultraviolet (UV) radiation field (Boulanger et al., 1988; Bendo et al., 2008), although it is much less effective than destruction by shocks; the lifetimes of PAHs against the photo-dissociation are 105 years in massive starforming regions, while they are 108 years for the diffuse ISM (Allain et al., 1996). Even for a constant abundance ratio of PAHs to large grains, the 9 μm to 140 μm intensity ratio will change with other parameters such as the UV radiation field and dust extinction (Kaneda et al., 2012a). The ratios increase with the UV radiation field as long as it is stronger than that in the solar neighborhood (Draine and Li, 2007). Larger interstellar extinction in the mid-IR than in the far-IR can systematically lower the ratios of 9 μm to 140 μm intensities in dense gas regions. The dominance
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Fig. 2. (a) A correlation plot between the AKARI 9 μm and 140 μm band intensities for M 82, where the images in both bands are regridded to a spatial scale of 90 arc s 90 arc s. (b) Same as panel (a), but for NGC 253. The plus (black), diamond (green), triangle (blue), and square (magenta) symbols correspond to data points for the galactic center, disk, northern halo, and southern halo regions, respectively. In each panel, the solid line represents the relationship for our Galaxy. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)
of neutral PAHs over ionized ones in very dense gas regions can also lower the ratios, because neutral PAHs emit much less in the 6–9 μm region than ionized PAHs (Szczepanski and Vala, 1993; Joblin et al., 1994; Hudgins and Allamandola, 1995). Condensation of PAHs in grain ice mantles would also be an important factor in lowering the relative intensity of the PAH emission features in dense gas regions. In short, even for a constant relative abundance of PAHs, the 9 μm to 140 μm intensity ratio can decline in UV-poor, dense gas regions, the situation of which does not seem to be applicable to the cases presented above.
3. Spectroscopy of the near-IR PAH emission In Fig. 1a, we plot the positions of the AKARI near-IR spectroscopic observations. The properties of PAHs are probed by the spectroscopy of the 3.3 μm feature and the 3.4–3.5 μm features. Both of them are attributed to the C–H stretching vibration. The former is due to aromatic (sp2) hydrocarbons, while the latter is attributed to aliphatic (sp3) hydrocarbons (e.g., Duley and Williams, 1981; Wagner et al., 2000). To be exact, the 3.4–3.5 μm feature intensities give upper limits on the aliphatic fractions in hydrocarbon grains (Li and Draine, 2012; Yang et al., 2013). The
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aromatic feature at 3.3 μm is sensitive to smallest PAHs (Schutte et al., 1993), as compared to the other PAH features at longer wavelengths. For about 200 star-forming galaxies in a redshift range of 0.01–0.1, we systematically investigated a global relation between the PAH 3.3 μm luminosity, L3:3 , and the total IR (8–1000 μm) luminosity, LIR (Yamada et al., 2013). We classified the samples into IR galaxies (IRGs; LIR o 1011 L ), luminous IR galaxies (LIRGs: LIR 1011 1012 L ) and ultra-luminous IR galaxies (ULIRGs: LIR 41012 L ). M82 is categorized into IRGs. We confirm that many of the IRGs and LIRGs follow the relationship, L3:3 =LIR C10 3 , which is a ratio typical of starburst galaxies (Mouri et al., 1990). However we also find that the L3:3 =LIR ratio considerably decreases toward the luminous end in the ULIRG population. We conclude that local ULIRGs intrinsically possess smaller amounts of PAHs relative to large grains, as a result of PAH processing through recent galaxy mergers. Some fraction of PAHs may have been destroyed once by a shock induced during a merging process, whereas large grains survive. Hence our result is consistent with the observational fact that local ULIRGs are merging galaxies (e.g., Clements et al., 1996). Considering a starburst age typical of local ULIRGs ( 10–100 Myr; Genzel et al., 1998) as well as a lifetime of intermediate-mass stars ( 100–1000 Myr) responsible for the production of PAHs at their late stages, it is unlikely that PAHs have been reproduced and replenished by the stars that were newly born after the merger. Therefore the observed PAHs are likely to be merger remnants in local ULIRGs. The intensity ratios of the aliphatic to the aromatic feature are known to show regional variations in the ISM. A huge dataset of AKARI near-IR spectroscopy reveals that they indeed change very much, depending on the interstellar conditions, which implies structural changes of hydrocarbon grains. Fig. 3 displays examples of the spectra with relatively normal aliphatic/aromatic ratios, while Fig. 4 shows those with unusually high aliphatic/aromatic ratios. As for Galactic sources, comparing the spectra in Figs. 3a and 4a, we find that the spectrum for the foot point of the molecular loop in the Galactic center region has spectral properties notably different from a typical Galactic diffuse spectrum at 3.2–3.6 μm; the properties in Fig. 4a are characterized by the faint PAH 3.3 μm emission and the broad excess above the linear baseline (Kaneda et al., 2012a). The broad excess may be explained by a combination of a series of the features at 3.47, 3.51, and 3.56 μm accompanying the 3.4 μm aliphatic feature (e.g., Sloan et al., 1997). Assuming that all the features are due to aliphatic C–H, the spectrum suggests that hydrogenated amorphous carbon grains may have been produced near the foot point of the molecular loop by shattering of larger carbonaceous grains (Jones et al., 1996), while pre-existing small PAHs may have been destroyed there. These hydrocarbon grains are likely to be H-rich and aliphatic-rich, and have not yet been evolved into H-poor, aromatic-rich materials (Jones et al., 2013); the aromatization requires subsequent UV radiation and/or thermal annealing. For M 82, we clearly detect the aromatic 3.3 μm emission and the aliphatic 3.4–3.6 μm features even in the galactic halo regions, which are located at a distance of 2 kpc away from the galactic center (Yamagishi et al., 2012). We thus confirm the presence of PAHs even in the harsh environment of the M82 halo. We find that the aliphatic 3.4–3.5 μm features are unusually abundant in the halo spectra, comparing Fig. 4b and c with Fig. 3b and c. The spectra of 34 regions in M 82 reveal that the aliphatic/aromatic ratio significantly increases with the distance from the galactic center (Yamagishi et al., 2012), which again indicates the dominance of aliphatic structures over aromatic ones by shattering of hydrogenated amorphous carbon grains in the galactic superwind. As for the mid-IR PAH emission, we found that there is an excellent correlation between the PAH and Hα distributions
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Fig. 3. AKARI near-IR spectra with relatively normal aliphatic/aromatic ratios: (a) Galactic diffuse emission observed at the galactic longitude of 311:51, (b) center and (c) disk in M 82, (d) starburst ring in NGC 1097, (e) J1224360þ 392258 (IRG), and (f) J1456077 þ833122 (LIRG). All the spectra are shown in the observed, but not rest-frame wavelengths. The emission at 4.05 μm is the hydrogen Brα line.
extended in the halo (Kaneda et al., 2010). The spectro-polarimetry showed that Hα is significantly (5–15%) polarized (Yoshida et al., 2011), implying that Hα photons from the galactic disk are scattered by dust grains in the halo. Thus the excellent PAH–Hα correlation can be explained if the grains are being fragmented to produce PAHs by the galactic superwinds (Jones et al., 1996, 2013). Moreover the estimated dust flow velocity was found to decrease with the height from the disk (Yoshida et al., 2011), suggesting that the grains thus processed may be falling back toward the disk. We also detect strong aliphatic emission from the central region of the barred spiral galaxy NGC 1097 (Kondo et al., 2012). The galaxy is categorized as Seyfert 1 with the starburst ring of 2 kpc in diameter and the inner bar structure of 1 kpc in length connecting the ring and the nucleus (Hsieh et al., 2008). Hence NGC 1097 is an ideal laboratory to study the ISM in galaxies showing both active galactic nucleus (AGN) and circumnuclear starburst activities. Figs. 3d and 4d present the spectra for the starburst ring and inner bar regions in NGC 1097, respectively, from which we find that the spectrum taken from the inner bar exhibits a relatively high aliphatic/aromatic ratio. By spectral mapping in the 3.3 μm feature and the 3.4–3.5 μm features, we find that the distribution of the aliphatic relative to the aromatic feature spatially corresponds to the inner bar connecting the ring and the nucleus (Kondo et al., 2012). Thus the local enhancements of the aliphatic features are, again, consistent with the picture that small hydrocarbon grains, not much aromatized, may be newly formed through shattering of carbonaceous grains in the inner bar (Jones et al., 1996, 2013), which might provide observational
evidence that the gas and dust in the bar is in a turbulent motion, likely fueling the central AGN from the starburst ring. Among the afore-mentioned AKARI near-IR spectral sample of star-forming galaxies, Fig. 3e and f presents examples of the spectra of IRGs and LIRGs, respectively, with relatively normal aliphatic/aromatic ratios, while Fig. 4e and f shows those with unusually high aliphatic/aromatic ratios. A significant fraction of the LIRG and ULIRG samples is found to exhibit similarly high aliphatic/aromatic ratios. However we do not find any clear dependence of the aliphatic/aromatic ratio on LIR or the galaxy population. The variation of the ratio can be explained by difference in the scale of the past merger, i.e., minor or major, or difference in the stage of the current merging process. Since most of the sample galaxies are not spatially resolved, it is notable that not only particular regions in galaxies but also galaxies as a whole exhibit such unusually high aliphatic/aromatic ratios. We estimate the ratios of the integrated intensities of the aliphatic to the aromatic feature to be 0.1–0.4 for the spectra in Fig. 3 and 0.6–3 for the spectra in Fig. 4. Using A3:4 =A3:3 C 1:76 (Yang et al., 2013), where A3:3 and A3:4 are the band strengths of the 3.4 μm aliphatic and 3.3 μm aromatic C–H bonds, along with the assumption that one aliphatic and one aromatic C atom correspond to 2.5 aliphatic and 0.75 aromatic C–H bonds, respectively (Yang et al., 2013), we derive the fraction of C atoms in aliphatic form to be 2–7% for the spectra in Fig. 3 and 10–50% for the spectra in Fig. 4. Thus a significant fraction of C atoms are in aliphatic form for the latter (Fig. 4), while they are predominantly aromatic for the former targets (Fig. 3). It should be noted,
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Fig. 4. AKARI near-IR spectra with unusually high aliphatic/aromatic ratios: (a) the foot point of one of the molecular loops in the Galactic center region, (b) southern and (c) northern haloes in M 82, (d) inner bar in NGC 1097, (e) J0055449-502749 (IRG), and (f) NGC 3110 (LIRG). All the spectra are shown in the observed, but not in rest-frame wavelengths. In panel (a), instrumental artifacts are masked out.
however, that a significant fraction of aromatic or aliphatic C atoms can be locked inside of grains, which cannot be probed by the 3.3 and 3.4–3.5 μm emission features. As pointed out by Yang et al. (2013), in benign, UV-poor environments, fragile species such as aliphatic hydrocarbon chains tend to attach to an aromatic skeleton as side groups. Therefore we can expect that PAHs are rich with aliphatic chains in quiescent ISM conditions. Our AKARI results on interstellar PAHs in galaxies, however, appear to favor rather opposite situations for the presence of aliphatic-rich PAHs, i.e. they are detected in harsh interstellar environments, the presence of which can be explained by shock fragmentation of grains with carbonaceous mantles (Jones et al., 2013).
4. Summary With the AKARI's unique capabilities of mid-/far-IR all-sky surveys and near-IR spectroscopy, we have revealed various phenomena about interstellar dust grains in galaxies including our Galaxy. Based on the all-sky maps, we find that the intensity ratios of PAHs to far-IR large grains systematically decrease in harsh environments, which can be used as probes to study the conditions of interstellar shocks on both global and local scales. With the near-IR spectroscopy we find that the ratios of the aliphatic to aromatic feature strength considerably change, depending on the interstellar conditions; all the cases presented above indicate structural changes of hydrocarbon grains by shocks in harsh interstellar environments.
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Please cite this article as: Kaneda, H., et al., AKARI observations of interstellar dust grains in our Galaxy and nearby galaxies. Planetary and Space Science (2014), http://dx.doi.org/10.1016/j.pss.2014.01.017i