Spitzer’s perspective of polycyclic aromatic hydrocarbons in galaxies

ABSTRACT Polycyclic aromatic hydrocarbon (PAH) molecules are abundant and widespread throughout the Universe, as revealed by their distinctive set of emission bands at 3.3, 6.2, 7.7, 8.6,

11.3 and 12.7 μm, which are characteristic of their vibrational modes. They are ubiquitously seen in a wide variety of astrophysical regions, ranging from planet-forming disks around young

stars to the interstellar medium of the Milky Way and other galaxies out to high redshifts at _z_ ≳ 4. PAHs profoundly influence the thermal budget and chemistry of the interstellar medium

by dominating the photoelectric heating of the gas and controlling the ionization balance. Here I review the current state of knowledge of the astrophysics of PAHs, focusing on their

observational characteristics obtained from the Spitzer Space Telescope and their diagnostic power for probing the local physical and chemical conditions and processes. Special attention is

paid to the spectral properties of PAHs and their variations revealed by the Infrared Spectrograph onboard Spitzer across a much broader range of extragalactic environments (for example,

distant galaxies, early-type galaxies, galactic halos, active galactic nuclei and low-metallicity galaxies) than was previously possible with the Infrared Space Observatory or any other

telescope facilities. Also highlighted is the relation between the PAH abundance and the galaxy metallicity established for the first time by Spitzer. Access through your institution Buy or

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IN AROMATIC HYDROCARBON EMISSION IN A DUST-RICH GALAXY Article 05 June 2023 DETECTIONS OF INTERSTELLAR AROMATIC NITRILES 2-CYANOPYRENE AND 4-CYANOPYRENE IN TMC-1 Article Open access 05

November 2024 AN INVESTIGATION OF SPECTRAL LINE STACKING TECHNIQUES AND APPLICATION TO THE DETECTION OF HC11N Article 11 January 2021 REFERENCES * Gillett, F. C., Forrest, W. J. &

Merrill, K. M. 8–13 μm spectra of NGC 7027, BD+30 3639, and NGC 6572. _Astrophys. J._ 183, 87–93 (1973). ADS  Google Scholar  * Merrill, K. M., Soifer, B. T. & Russell, R. W. The 2–4 μm

spectrum of NGC 7027. _Astrophys. J._ 200, L37–L39 (1975). ADS  Google Scholar  * Russell, R. W., Soifer, B. T. & Willner, S. P. The 4–8 μm spectrum of NGC 7027. _Astrophys. J._ 217,

L149–L153 (1977). ADS  Google Scholar  * Willner, S. P., Soifer, B. T., Russell, R. W., Joyce, R. R. & Gillett, F. C. 2–8 μm spectrophotometry of M82. _Astrophys. J._ 217, L121–L124

(1977). ADS  Google Scholar  * Léger, A. & Puget, J. L. Identification of the “unidentified” IR emission features of interstellar dust? _Astron. Astrophys._ 137, L5–L8 (1984). ADS 

Google Scholar  * Allamandola, L. J., Tielens, A. G. G. M. & Baker, J. R. Polycyclic aromatic hydrocarbons and the unidentified infrared emission bands: Auto exhaust along the Milky Way.

_Astrophys. J._ 290, L25–L28 (1985). ADS  Google Scholar  * Allamandola, L. J., Tielens, A. G. G. M. & Baker, J. R. Interstellar polycyclic aromatic hydrocarbons: the infrared emission

bands, the excitation/emission mechanism, and the astrophysical implications. _Astrophys. J. Suppl._ 71, 733–775 (1989). ADS  Google Scholar  * Draine, B. T. & Li, A. Infrared emission

from interstellar dust. I. Stochastic heating of small grains. _Astrophys. J._ 551, 807–824 (2001). ADS  Google Scholar  * Li, A. & Draine, B. T. Infrared emission from interstellar

dust. II. The diffuse interstellar medium. _Astrophys. J._ 554, 778–802 (2001). ADS  Google Scholar  * Draine, B. T. & Li, A. Infrared emission from interstellar dust. IV. The

silicate–graphite–PAH model in the post-Spitzer era. _Astrophys. J._ 657, 810–837 (2007). ADS  Google Scholar  * Zubko, V., Dwek, E. & Arendt, R. G. Interstellar dust models consistent

with extinction, emission, and abundance constraints. _Astrophys. J. Suppl._ 152, 211–249 (2004). ADS  Google Scholar  * Siebenmorgen, R., Voshchinnikov, N. V. & Bagnulo, S. Dust in the

diffuse interstellar medium. Extinction, emission, linear and circular polarisation. _Astron. Astrophys._ 561, A82 (2014). ADS  Google Scholar  * Jones, A. P., Köhler, M., Ysard, N.,

Bocchio, M. & Verstraete, L. The global dust modelling framework THEMIS. _Astron. Astrophys._ 602, A46 (2017). ADS  Google Scholar  * Smith, J. D. T. et al. The mid-infrared spectrum of

star-forming galaxies: global properties of polycyclic aromatic hydrocarbon emission. _Astrophys. J._ 656, 770–791 (2007). ADS  Google Scholar  * Tielens, A. G. G. M. Interstellar polycyclic

aromatic hydrocarbon molecules. _Annu. Rev. Astron. Astrophys._ 46, 289–337 (2008). ADS  Google Scholar  * Joblin, C., Léger, A. & Martin, P. Contribution of polycyclic aromatic

hydrocarbon molecules to the interstellar extinction curve. _Astrophys. J._ 393, L79–L82 (1992). ADS  Google Scholar  * Cecchi-Pestellini, C., Malloci, G., Joblin, C. & Williams, D. A.

The role of the charge state of PAHs in ultraviolet extinction. _Astron. Astrophys._ 486, 25–29 (2008). ADS  Google Scholar  * Mulas, G., Zonca, A., Casu, S. & Cecchi-Pestellini, C.

Modeling galactic extinction with dust and “real” polycyclic aromatic hydrocarbons. _Astrophys. J. Suppl._ 207, 7 (2013). ADS  Google Scholar  * Steglich, M. et al. Electronic spectroscopy

of medium-sized polycyclic aromatic hydrocarbons: implications for the carriers of the 2175 Å UV bump. _Astrophys. J._ 712, L16–L20 (2011). ADS  Google Scholar  * Salama, F. et al.

Polycyclic aromatic hydrocarbons and the diffuse interstellar bands: a survey. _Astrophys. J._ 728, 154 (2011). ADS  Google Scholar  * Witt, A. N. Blue luminescence and extended red

emission: possible connections to the diffuse interstellar bands. _Proc. Int. Astronom. Union_ 9, 173–179 (2014). ADS  Google Scholar  * Draine, B. T. Interstellar dust grains. _Annu. Rev.

Astron. Astrophys._ 41, 241–289 (2003). ADS  Google Scholar  * Dickinson, C. et al. The state-of-play of anomalous microwave emission (AME) research. _New Astron. Rev._ 80, 1–28 (2018). ADS

  Google Scholar  * Bakes, E. & Tielens, A. G. G. M. The photoelectric heating mechanism for very small graphitic grains and polycyclic aromatic hydrocarbons. _Astrophys. J._ 427,

822–838 (1994). ADS  Google Scholar  * Weingartner, J. C. & Draine, B. T. Photoelectric emission from interstellar dust: grain charging and gas heating. _Astrophys. J. Suppl._ 134,

263–282 (2001). ADS  Google Scholar  * Kamp, I. & Dullemond, C. P. The gas temperature in the surface layers of protoplanetary disks. _Astrophys. J._ 615, 991–999 (2004). ADS  Google

Scholar  * Verstraete, L. The role of PAHs in the physics of the interstellar medium. _EAS Publ. Ser._ 46, 415–426 (2011). Google Scholar  * Hudgins, D. M. & Allamandola, L. J. Steps

toward identifying PAHs: A summary of some recent results. _IAUS_ 231, 443–454 (2005). ADS  Google Scholar  * Sellgren, K., Werner, M. W. & Dinerstein, H. L. Extended near-infrared

emission from visual reflection nebulae. _Astrophys. J._ 271, L13–L17 (1983). ADS  Google Scholar  * Greenberg, J. M. in _Stars and Stellar Systems_ Vol. 7 (eds Middlehurst, B. M. &

Aller, L. H.) 221–364 (Univ. Chicago Press, 1968). * Sellgren, K. The near-infrared continuum emission of visual reflection nebulae. _Astrophys. J._ 277, 623–633 (1984). ADS  Google Scholar

  * Geballe, T. R., Lacy, J. H., Persson, S. E., McGregor, P. J. & Soifer, B. T. Spectroscopy of the 3 μm emission features. _Astrophys. J._ 292, 500–505 (1985). ADS  Google Scholar  *

Jourdain de Muizon, M., Geballe, T. R., d’Hendecourt, L. B. & Baas, F. New emission features in the infrared spectra of two IRAS sources. _Astrophys. J._ 306, L105–L108 (1986). ADS 

Google Scholar  * Joblin, C., Tielens, A. G. G. M., Allamandola, L. J. & Geballe, T. R. Spatial variation of the 3.29 and 3.40 μm emission bands within reflection nebulae and the

photochemical evolution of methylated polycyclic aromatic hydrocarbons. _Astrophys. J._ 458, 610–620 (1996). ADS  Google Scholar  * Cohen, M., Tielens, A. G. G. M. & Allamandola, L. J. A

new emission feature in IRAS spectra and the polycyclic aromatic hydrocarbon spectrum. _Astrophys. J._ 299, L93–L97 (1985). ADS  Google Scholar  * Tokunaga, A. T. A summary of the “UIR”

bands. _ASP Conf. Ser._ 124, 149–160 (1997). ADS  Google Scholar  * Cohen, M. et al. The infrared emission bands. III. Southern IRAS sources. _Astrophys. J._ 341, 246–269 (1989). ADS  Google

Scholar  * Peeters, E., Allamandola, L. J., Hudgins, D. M., Hony, S. & Tielens, A. G. G. M. The unidentified infrared features after ISO. _ASP Conf. Ser._ 309, 141–162 (2004). ADS 

Google Scholar  * Moutou, C., Verstraete, L., Léger, A., Sellgren, K., Schmidt, W. & New, P. A. H. mode at 16.4 μm. _Astron. Astrophys._ 354, L17–L20 (2000). ADS  Google Scholar  *

Schutte, W. A. et al. ISO-SWS observations of infrared absorption bands of the diffuse interstellar medium: the 6.2 μm feature of aromatic compounds. _Astron. Astrophys._ 337, 261–274

(1998). ADS  Google Scholar  * Chiar, J. E. et al. The composition and distribution of dust along the line of sight toward the Galactic Center. _Astrophys. J._ 537, 749–762 (2000). ADS 

Google Scholar  * Peeters, E. et al. The rich 6 to 9 μm spectrum of interstellar PAHs. _Astron. Astrophys._ 390, 1089–1113 (2002). ADS  Google Scholar  * Mattila, K. et al. Spectrophotometry

of UIR bands in the diffuse emission of the Galactic Disk. _Astron. Astrophys._ 315, L353–L356 (2000). ADS  Google Scholar  * Onaka, T., Yamamura, I., Tanabe, T., Roellig, T. L. & Yuen,

L. Detection of the mid-infrared unidentified bands in the diffuse galactic emission by IRTS. _Publ. Astron. Soc. Jpn_ 48, L59–L63 (1996). ADS  Google Scholar  * Tanaka, M. et al. IRTS

observation of the unidentified 3.3 μm band in the diffuse galactic emission. _Publ. Astron. Soc. Jpn_ 48, L53–L57 (1996). ADS  Google Scholar  * Siebenmorgen, R., Prusti, T., Natta, A.

& Müller, T. G. Mid infrared emission of nearby Herbig Ae/Be stars. _Astron. Astrophys._ 361, 258–264 (2006). ADS  Google Scholar  * Furlan, E. et al. A survey and analysis of Spitzer

Infrared Spectrograph spectra of T Tauri stars in Taurus. _Astrophys. J. Suppl._ 165, 568–605 (2006). ADS  Google Scholar  * Geers, V. C. et al. C2D Spitzer-IRS spectra of disks around T

Tauri stars. II. PAH emission features. _Astron. Astrophys._ 459, 545–556 (2006). ADS  Google Scholar  * Seok, J. Y. & Li, A. Polycyclic aromatic hydrocarbons in protoplanetary disks

around Herbig Ae/Be and T Tauri stars. _Astrophys. J._ 835, 291 (2017). ADS  Google Scholar  * Sandstrom, K. M. et al. The Spitzer spectroscopic survey of the Small Magellanic Cloud (S4MC):

probing the physical state of polycyclic aromatic hydrocarbons in a low-metallicity environment. _Astrophys. J._ 744, 20 (2012). ADS  Google Scholar  * Hemachandra, D. et al. Mid-infrared

spectroscopy of the Andromeda galaxy. _Astron. Astrophys._ 454, 818–830 (2015). Google Scholar  * Boersma, C., Rubin, R. H. & Allamandola, L. J. Spatial analysis of the polycyclic

aromatic hydrocarbon features southeast of the Orion Bar. _Astrophys. J._ 753, 168 (2012). ADS  Google Scholar  * Boersma, C., Bregman, J. & Allamandola, L. J. Properties of polycyclic

aromatic hydrocarbons in the northwest photon dominated region of NGC 7023. III. Quantifying the traditional proxy for PAH charge and assessing its role. _Astrophys. J._ 806, 121 (2015). ADS

  Google Scholar  * Shannon, M. J., Stock, D. J. & Peeters, E. Interpreting the subtle spectral variations of the 11.2 and 12.7 μm polycyclic aromatic hydrocarbon bands. _Astrophys. J._

824, 111 (2016). ADS  Google Scholar  * Smith, J. D. T. et al. Mid-infrared IRS spectroscopy of NGC 7331: a first look at the Spitzer Infrared Nearby Galaxies Survey (SINGS) legacy.

_Astrophys. J. Suppl._ 154, 199–203 (2004). ADS  Google Scholar  * Werner, M. W. et al. New infrared emission features and spectral variations in NGC 7023. _Astrophys. J. Suppl._ 154,

309–314 (2004). ADS  Google Scholar  * Beintema, D. A. et al. The rich spectrum of circumstellar PAHs. _Astron. Astrophys._ 315, L369–L372 (1996). ADS  Google Scholar  * van Kerckhoven, C.

et al. The C–C–C bending modes of PAHs: a new emission plateau from 15 to 20 μm. _Astrophys. J. Suppl._ 357, 1013–1019 (2000). Google Scholar  * Cami, J., Bernard-Salas, J., Peeters, E.

& Malek, S. E. Detection of C60 and C70 in a young planetary nebula. _Science_ 329, 1180–1182 (2010). ADS  Google Scholar  * Sellgren, K. et al. C60 in reflection nebulae. _Astrophys.

J._ 722, L54–L57 (2010). ADS  Google Scholar  * Witteborn, F. C. et al. New emission features in the 11–13 μm region and their relationship to polycyclic aromatic hydrocarbons. _Astrophys.

J._ 341, 270–277 (1989). ADS  Google Scholar  * Hony, S. et al. The CH out-of-plane bending modes of PAH molecules in astrophysical environments. _Astron. Astrophys._ 370, 1030–1043 (2001).

ADS  Google Scholar  * Hudgins, D. M. & Allamandola, L. J. Interstellar PAH emission in the 11–14 μm region: new insights from laboratory data and a tracer of ionized PAHs. _Astrophys.

J._ 516, L41–L44 (1999). ADS  Google Scholar  * Matsuura, M. et al. Spitzer Space Telescope spectra of post-AGB stars in the Large Magellanic Cloud — polycyclic aromatic hydrocarbons at low

metallicities. _Mon. Not. R. Astron. Soc._ 439, 1472–1493 (2014). ADS  Google Scholar  * Sloan, G. C. et al. Carbon-rich dust past the asymptotic giant branch: aliphatics, aromatics, and

fullerenes in the Magellanic Clouds. _Astrophys. J._ 791, 28 (2014). ADS  Google Scholar  * Hudgins, D. M., Bauschlicher, J. C. W. & Allamandola, L. J. Variations in the peak position of

the 6.2 μm interstellar emission feature: a tracer of N in the interstellar polycyclic aromatic hydrocarbon population. _Astrophys. J._ 632, 316–332 (2005). ADS  Google Scholar  * Canelo,

C. M., Friaça, A. C. S., Sales, D. A., Pastoriza, M. G. & Ruschel-Dutra, D. Variations in the 6.2 μm emission profile in starburst-dominated galaxies: a signature of polycyclic aromatic

nitrogen heterocycles (PANHs)? _Mon. Not. R. Astron. Soc._ 475, 3746–3763 (2018). ADS  Google Scholar  * Berné, O. et al. Analysis of the emission of very small dust particles from Spitzer

spectro-imagery data using blind signal separation methods. _Astron. Astrophys._ 469, 575–586 (2007). ADS  Google Scholar  * Povich, M. S. et al. A Multiwavelength study of M17: the spectral

energy distribution and PAH emission morphology of a massive star formation region. _Astrophys. J._ 660, 346–362 (2007). ADS  Google Scholar  * Elbaz, D., Le Floc’h, E., Dole, H. &

Marcillac, D. Observational evidence for the presence of PAHs in distant luminous infrared galaxies using ISO and Spitzer. _Astron. Astrophys._ 434, L1–L4 (2004). ADS  Google Scholar  * Yan,

L. et al. Spitzer detection of polycyclic aromatic hydrocarbon and silicate dust features in the mid-infrared spectra of _z_ ~ 2 ultraluminous infrared galaxies. _Astrophys. J._ 628,

604–610 (2005). ADS  Google Scholar  * Lutz, D. et al. Mid-infrared spectroscopy of two luminous submillimeter galaxies at _z_ ~ 2.8. _Astrophys. J._ 625, L83–L86 (2005). ADS  Google Scholar

  * Siana, B. et al. Detection of far-infrared and polycyclic aromatic hydrocarbon emission from the Cosmic Eye: probing the dust and star formation of Lyman break galaxies. _Astrophys. J._

698, 1273–1281 (2009). ADS  Google Scholar  * Riechers, D. A. et al. Polycyclic aromatic hydrocarbon and mid-infrared continuum emission in a _z_ > 4 submillimeter galaxy. _Astrophys. J._

786, 31 (2014). ADS  Google Scholar  * Bernstein, M. P. et al. UV irradiation of polycyclic aromatic hydrocarbons in ices: production of alcohols, quinones, and ethers. _Science_ 283,

1158–1138 (1999). Google Scholar  * Kwok, S. Complex organics in space from Solar System to distant galaxies. _Astron. Astrophys. Rev._ 24, 8 (2016). ADS  Google Scholar  * Shipley, H. V. et

al. A new star formation rate calibration from polycyclic aromatic hydrocarbon emission features and application to high-redshift galaxies. _Astrophys. J._ 818, 60 (2016). ADS  Google

Scholar  * Pope, A. et al. Mid-infrared spectral diagnosis of submillimeter galaxies. _Astrophys. J._ 675, 1171–1193 (2008). ADS  Google Scholar  * Calzetti, D. et al. The calibration of

mid-infrared star formation rate indicators. _Astrophys. J._ 666, 870–895 (2007). ADS  Google Scholar  * Xie, Y. & Ho, L. C. A new calibration of star formation rate in galaxies based on

polycyclic aromatic hydrocarbon emission. _Astrophys. J._ 884, 136 (2019). ADS  Google Scholar  * Kaneda, H. et al. Unbiased large spectroscopic surveys of galaxies selected by SPICA using

dust bands. _Publ. Astron. Soc. Aust._ 34, e059 (2017). ADS  Google Scholar  * Kaneda, H., Onaka, T. & Sakon, I. Detection of PAH emission features from nearby elliptical galaxies with

the Spitzer Infrared Spectrograph. _Astrophys. J._ 632, L83–L86 (2005). ADS  Google Scholar  * Kaneda, H. et al. Properties of polycyclic aromatic hydrocarbons in local elliptical galaxies

revealed by the Infrared Spectrograph on Spitzer. _Astrophys. J._ 684, 270–281 (2008). ADS  Google Scholar  * Vega, O. et al. Unusual PAH emission in nearby early-type galaxies: a signature

of an intermediate-age stellar population? _Astrophys. J._ 721, 1090–1104 (2010). ADS  Google Scholar  * Irwin, J. A. & Madden, S. C. Discovery of PAHs in the halo of NGC 5907. _Astron.

Astrophys._ 445, 123–141 (2005). ADS  Google Scholar  * Engelbracht, C. W. et al. Extended mid-infrared aromatic feature emission in M82. _Astrophys. J._ 642, 127–132 (2006). ADS  Google

Scholar  * Beirão, P. et al. Spatially resolved Spitzer-IRS spectral maps of the superwind in M82. _Mon. Not. R. Astron. Soc._ 451, 2640–2655 (2015). ADS  Google Scholar  * Yamagishi, M. et

al. AKARI near-infrared spectroscopy of the aromatic and aliphatic hydrocarbon emission features in the galactic superwind of M82. _Astron. Astrophys._ 541, A10 (2012). Google Scholar  *

Schutte, W. A., Tielens, A. G. G. M. & Allamandola, L. J. Theoretical modeling of the infrared fluorescence from interstellar polycyclic aromatic hydrocarbons. _Astrophys. J._ 415,

397–414 (1993). ADS  Google Scholar  * Bernstein, M. P., Sandford, S. A. & Allamandola, L. J. Hydrogenated polycyclic aromatic hydrocarbons and the 2940 and 2850 wavenumber (3.40 and

3.51 μm) infrared emission features. _Astrophys. J._ 472, L127–L130 (1996). ADS  Google Scholar  * Sandford, S. A. The infrared spectra of polycyclic aromatic hydrocarbons with excess

peripheral H atoms (H_n_-PAHs) and their relation to the 3.4 and 6.9 μm PAH emission features. _Astrophys. J. Suppl._ 205, 8 (2013). ADS  Google Scholar  * Steglich, M. et al. The abundances

of hydrocarbon functional groups in the interstellar medium inferred from laboratory spectra of hydrogenated and methylated polycyclic aromatic hydrocarbons. _Astrophys. J. Suppl._ 208, 26

(2013). ADS  Google Scholar  * Yang, X. J., Li, A. & Glaser, R. Superhydrogenated polycyclic aromatic hydrocarbon molecules: vibrational spectra in the infrared. _Astrophys. J. Suppl_.

247, 1 (2020). * Baker, J. R., Allamandola, L. J. & Tielens, A. G. G. M. Anharmonicity and the interstellar polycyclic aromatic hydrocarbon infrared emission spectrum. _Astrophys. J._

315, L61–L65 (1987). ADS  Google Scholar  * Maltseva, E. et al. High-resolution IR absorption spectroscopy of polycyclic aromatic hydrocarbons in the 3 μm region: role of periphery.

_Astrophys. J. Suppl._ 831, 58 (2016). ADS  Google Scholar  * Li, A. & Draine, B. T. The carriers of the interstellar unidentified infrared emission features: aromatic or aliphatic?

_Astrophys. J._ 760, L35 (2012). ADS  Google Scholar  * Yang, X. J., Glaser, R., Li, A. & Zhong, J. X. The carriers of the interstellar unidentified infrared emission features:

constraints from the interstellar C–H stretching features at 3.2–35 μm. _Astrophys. J._ 776, 110 (2013). ADS  Google Scholar  * Yang, X. J., Glaser, R., Li, A. & Zhong, J. X. The

carriers of the unidentified infrared emission features: clues from polycyclic aromatic hydrocarbons with aliphatic sidegroups. _New Astron. Rev._ 77, 1–22 (2016). ADS  Google Scholar  *

Yang, X. J., Glaser, R., Li, A. & Zhong, J. X. On the aliphatic versus aromatic content of the carriers of the ‘unidentified’ infrared emission features. _Mon. Not. R. Astron. Soc._ 462,

1551–1562 (2016). ADS  Google Scholar  * Micelotta, E. R., Jones, A. P. & Tielens, A. G. G. M. Polycyclic aromatic hydrocarbon processing in interstellar shocks. _Astron. Astrophys._

510, A36 (2010). ADS  Google Scholar  * Shannon, M. J., Peeters, E., Cami, J. & Blommaert, J. A. D. L. Polycyclic aromatic hydrocarbon emission toward the galactic bulge. _Astrophys. J._

855, 32 (2018). ADS  Google Scholar  * Rand, R. J., Wood, K. & Benjamin, R. A. Infrared spectroscopy of the diffuse ionized halo of NGC 891. _Astrophys. J._ 680, 263–275 (2008). ADS 

Google Scholar  * Sanders, D. B. & Mirabel, I. F. Luminous infrared galaxies. _Annu. Rev. Astron. Astrophys._ 34, 749–792 (1996). ADS  Google Scholar  * Higdon, S. J., Higdon, J. L.

& Marshall, J. First detection of PAHs and warm molecular hydrogen in tidal dwarf galaxies. _Astrophys. J._ 640, 768–783 (2006). ADS  Google Scholar  * Haan, S. et al. Spitzer IRS

spectral mapping of the Toomre sequence: spatial variations of PAH, gas, and dust properties in nearby major mergers. _Astrophys. J. Suppl._ 197, 27 (2011). ADS  Google Scholar  * Murata, K.

L. et al. A relationship of polycyclic aromatic hydrocarbon features with galaxy merger in star-forming galaxies at _z_ < 0.2. _Mon. Not. R. Astron. Soc._ 472, 39–50 (2017). ADS  Google

Scholar  * Onaka, T. et al. Near-infrared to mid-infrared observations of galaxy mergers: NGC 2782 and NGC 7727. _Astrophys. J._ 853, 31 (2018). ADS  Google Scholar  * Voit, G. M.

Destruction and survival of polycyclic aromatic hydrocarbons in active galaxies. _Mon. Not. R. Astron. Soc._ 258, 841–848 (1992). ADS  Google Scholar  * Siebenmorgen, R., Krügel, E. &

Spoon, H. W. W. Mid-infrared emission of galactic nuclei. TIMMI2 versus ISO observations and models. _Astron. Astrophys._ 414, 123–139 (2004). ADS  Google Scholar  * Roche, P. F., Aitken, D.

K., Smith, C. H. & Ward, M. J. An atlas of mid-infrared spectra of galaxy nuclei. _Mon. Not. R. Astron. Soc._ 248, 606–629 (1991). ADS  Google Scholar  * Genezel, R. et al. What powers

ultraluminous IRAS galaxies? _Astrophys. J._ 498, 579–605 (1998). ADS  Google Scholar  * Esquej, C. W. et al. Nuclear star formation activity and black hole accretion in nearby Seyfert

galaxies. _Astrophys. J._ 780, 86–100 (2014). ADS  Google Scholar  * Jensen, J. J. et al. PAH features within few hundred parsecs of active galactic nuclei. _Mon. Not. R. Astron. Soc._ 470,

3071–3094 (2017). ADS  Google Scholar  * Tommasin, S. et al. Spitzer-IRS high-resolution spectroscopy of the 12 μm Seyfert galaxies. II. Results for the complete data set. _Astrophys. J._

709, 1257–1283 (2010). ADS  Google Scholar  * Murata, K. L. et al. Evolution of the fraction of clumpy galaxies at 0.2 < z < 1.0 in the COSMOS field. _Astrophys. J._ 786, 15 (2014).

ADS  Google Scholar  * Maragkoudakis, A. et al. PAHs and star formation in the H ii regions of nearby galaxies M83 and M33. _Mon. Not. R. Astron. Soc._ 481, 5370–5393 (2018). ADS  Google

Scholar  * O’Dowd, M. J. et al. Polycyclic aromatic hydrocarbons in galaxies at _z_ ~ 0.1: the effect of star formation and active galactic nuclei. _Astrophys. J._ 705, 885–898 (2009). ADS 

Google Scholar  * Diamond-Stanic, A. M. & Rieke, G. H. The effect of active galactic nuclei on the mid-infrared aromatic features. _Astrophys. J._ 724, 140–153 (2010). ADS  Google

Scholar  * Wu, Y. et al. Infrared luminosities and aromatic features in the 24 μm flux-limited sample of 5MUSES. _Astrophys. J._ 723, 895–914 (2010). ADS  Google Scholar  * Allamandola, L.

J., Hudgins, D. M. & Sandford, S. A. Modeling the unidentified infrared emission with combinations of polycyclic aromatic hydrocarbons. _Astrophys. J._ 511, L115–L119 (1999). ADS  Google

Scholar  * Bauschlicher, C. W., Ricca, A., Boersma, C. & Allamandola, L. J. The NASA Ames PAH IR spectroscopic database: computational version 3.00 with updated content and the

introduction of multiple scaling factors. _Astrophys. J. Suppl._ 234, 32 (2018). ADS  Google Scholar  * Galliano, F., Madden, S. C., Tielens, A. G. G. M., Peeters, E. & Jones, A. P.

Variations of the mid-IR aromatic features inside and among galaxies. _Astrophys. J._ 679, 310–345 (2008). ADS  Google Scholar  * Mattioda, A. L., Hudgins, D. M., Bauschlicher, C. W., Rosi,

M. & Allamandola, L. J. Infrared spectroscopy of matrix-isolated polycyclic aromatic compounds and their ions. 6. Polycyclic aromatic nitrogen heterocycles. _J. Phys. Chem. A_ 107,

1486–1498 (2003). Google Scholar  * Mattioda, A. L. et al. Infrared spectroscopy of matrix-isolated neutral polycyclic aromatic nitrogen heterocycles: the acridine series. _Spectrochim. Acta

A_ 181, 286–308 (2017). ADS  Google Scholar  * Thuan, T. X., Sauvage, M. & Madden, S. Dust in an extremely metal-poor galaxy: mid-infrared observations of SBS 0335−052. _Astrophys. J._

516, 783–787 (1999). ADS  Google Scholar  * Houck, J. R. et al. The extraordinary mid-infrared spectrum of the blue compact dwarf galaxy SBS 0335−052. _Astrophys. J. Suppl._ 154, 211–214

(2004). ADS  Google Scholar  * Madden, S. C., Galliano, F., Jones, A. P. & Sauvage, M. ISM properties in low-metallicity environments. _Astron. Astrophys._ 446, 877–896 (2006). ADS 

Google Scholar  * Wu, Y. et al. Mid-infrared properties of low-metallicity blue compact dwarf galaxies from the Spitzer Infrared Spectrograph. _Astrophys. J._ 639, 157–172 (2006). ADS 

Google Scholar  * Hunt, L. K., Thuan, T. X., Izotov, Y. I. & Sauvage, M. The Spitzer view of low-metallicity star formation. III. Fine-structure lines, aromatic features, and molecules.

_Astrophys. J._ 712, 164–187 (2010). ADS  Google Scholar  * Engelbracht, C. W. et al. Metallicity effects on mid-infrared colors and the 8 μm PAH emission in galaxies. _Astrophys. J._ 628,

29–32 (2005). ADS  Google Scholar  * Draine, B. T. et al. Dust masses, PAH abundances, and starlight intensities in the SINGS galaxy sample. _Astrophys. J._ 663, 866–894 (2007). ADS  Google

Scholar  * Asplund, M., Grevesse, N., Sauval, A. J. & Scott, P. The chemical composition of the Sun. _Annu. Rev. Astron. Astrophys._ 47, 481–522 (2009). ADS  Google Scholar  * Gordon, K.

D. et al. The behavior of the aromatic features in M101 H ii regions: evidence for dust processing. _Astrophys. J._ 682, 336–354 (2008). ADS  Google Scholar  * Wu, R., Hogg, D. W. &

Moustakas, J. The aromatic features in very faint dwarf galaxies. _Astrophys. J._ 730, 111 (2011). ADS  Google Scholar  * Seok, J. Y., Hirashita, H. & Asano, R. S. Formation history of

polycyclic aromatic hydrocarbons in galaxies. _Mon. Not. R. Astron. Soc._ 439, 2186–2196 (2014). ADS  Google Scholar  * Galliano, F., Dwek, E. & Chanial, P. Stellar evolutionary effects

on the abundances of polycyclic aromatic hydrocarbons and supernova-condensed dust in galaxies. _Astrophys. J._ 672, 214–243 (2008). ADS  Google Scholar  * Shivaei, I. et al. The MOSDEF

survey: Metallicity dependence of PAH emission at high redshift and implications for 24 μm inferred IR luminosities and star formation rates at _z_ ~ 2. _Astrophys. J._ 837, 157 (2017). ADS

  Google Scholar  * Jackson, D. C. et al. Hot dust and polycyclic aromatic hydrocarbon emission at low metallicity: a Spitzer survey of Local Group and other nearby dwarf galaxies.

_Astrophys. J._ 646, 192–204 (2006). ADS  Google Scholar  * Tappe, A., Rho, J. & Reach, W. T. Shock processing of interstellar dust and polycyclic aromatic hydrocarbons in the supernova

remnant N132D. _Astrophys. J._ 653, 267–279 (2006). ADS  Google Scholar  * Seok, J. Y., Koo, B.-C. & Onaka, T. Detection of the 3.3 μm aromatic feature in the supernova remnant N49 with

AKARI. _Astrophys. J._ 744, 160 (2012). ADS  Google Scholar  * Andersen, M. et al. Dust processing in supernova remnants: Spitzer MIPS spectral energy distribution and Infrared Spectrograph

observations. _Astrophys. J._ 742, 7 (2011). ADS  Google Scholar  * Andrews, H. et al. PAH emission at the bright locations of PDRs: the grand PAH hypothesis. _Mon. Not. R. Astron. Soc._

807, 99 (2015). Google Scholar  * Kwok, S. & Zhang, Y. Mixed aromatic-aliphatic organic nanoparticles as carriers of unidentified infrared emission features. _Nature_ 479, 80–83 (2011).

ADS  Google Scholar  * Salama, F., Joblin, C. & Allamandola, L. J. Neutral and ionized PAHs: contribution to the interstellar extinction. _Planet. Space Sci._ 43, 1165–1173 (1995). ADS 

Google Scholar  * Clayton, G. C. et al. The role of polycyclic aromatic hydrocarbons in ultraviolet extinction. I. Probing small molecular polycyclic aromatic hydrocarbons. _Astrophys. J._

592, 947–952 (2003). ADS  Google Scholar  * Buss, R. H., Tielens, A. G. G. M. & Snow, T. P. The mid-infrared spectrum of the carbon star HD 38218 and its possible relation to polycyclic

aromatic hydrocarbons. _Astrophys. J._ 372, 281–290 (1991). ADS  Google Scholar  * Speck, A. K. & Barlow, M. J. UIR bands in carbon star spectra. _Astrophys. Space. Sci._ 251, 115–121

(1997). ADS  Google Scholar  * Boersma, C., Hony, S. & Tielens, A. G. G. M. UIR bands in the ISO SWS spectrum of the carbon star TU Tauri. _Astron. Astrophys._ 447, 213–220 (2006). ADS 

Google Scholar  * Sloan, G. C. et al. The unusual hydrocarbon emission from the early carbon star HD 100764: the connection between aromatics and aliphatics. _Astrophys. J._ 664, 1144–1153

(2007). ADS  Google Scholar  * Li, A. & Draine, B. T. Do the infrared emission features need ultraviolet excitation? The polycyclic aromatic hydrocarbon model in UV-poor reflection

nebulae. _Astrophys. J._ 572, 232–237 (2002). ADS  Google Scholar  * Mattioda, A. L., Hudgins, D. M. & Allamandola, L. J. Experimental near-infrared spectroscopy of polycyclic aromatic

hydrocarbons between 0.7 and 2.5 μm. _Astrophys. J._ 629, 1188–1210 (2005). ADS  Google Scholar  * Uchida, K. I., Sellgren, K. & Werner, M. W. Do the infrared emission features need

ultraviolet excitation? _Astrophys. J._ 493, L109–L112 (1998). ADS  Google Scholar  * Jura, M. et al. Polycyclic aromatic hydrocarbons orbiting HD 233517, an evolved oxygen-rich red giant.

_Astrophys. J._ 637, L45–L47 (2006). ADS  Google Scholar  * Sandstrom, K. M. et al. The Spitzer survey of the Small Magellanic Cloud (S3MC): insights into the life cycle of polycyclic

aromatic hydrocarbons. _Astrophys. J._ 715, 701–723 (2010). ADS  Google Scholar  * Paradis, D. et al. Spatial variations of dust abundances across the Large Magellanic Cloud. _Astron. J._

138, 196–209 (2009). ADS  Google Scholar  * Lau, R. M., Werner, M. W., Sahai, R. & Ressler, M. E. Evidence from SOFIA imaging of polycyclic aromatic hydrocarbon formation along a recent

outflow in NGC 7027. _Astrophys. J._ 833, 115 (2016). ADS  Google Scholar  * Xie, Y., Ho, L. C., Li, A. & Shangguan, J. Y. The widespread presence of nanometer-size dust grains in the

interstellar medium of galaxies. _Astrophys. J._ 867, 91 (2018). ADS  Google Scholar  * Kwok, S., Volk, K. & Bernath, P. On the origin of infrared plateau features in proto-planetary

nebulae. _Astrophys. J._ 554, L87–L90 (2001). ADS  Google Scholar  * Uchida, K. I., Sellgren, K., Werner, M. W. & Houdashelt, M. L. Infrared Space Observatory mid-infrared spectra of

reflection nebulae. _Astrophys. J._ 530, 817–833 (2000). ADS  Google Scholar  * Rapacioli, M., Joblin, C. & Boissel, P. Spectroscopy of polycyclic aromatic hydrocarbons and very small

grains in photodissociation regions. _Astron. Astrophys._ 429, 193–204 (2005). ADS  Google Scholar  * Peeters, E. et al. The PAH emission characteristics of the reflection nebula NGC 2023.

_Astrophys. J._ 836, 198 (2017). ADS  Google Scholar  * Xie, Y., Ho, L. C., Li, A. & Shangguan, J. Y. A New technique for measuring polycyclic aromatic hydrocarbon emission in different

environments. _Astrophys. J._ 860, 154 (2018). ADS  Google Scholar  * Cruz-Diaz, G. A. et al. PAH products and processing by different energy sources. _Astrophys. J._ 882, 44 (2019). ADS 

Google Scholar  * An, J. H. & Sellgren, K. Spatial separation of the 3.29 μm emission feature and associated 2 μm continuum in NGC 7023. _Astrophys. J._ 599, 312–323 (2003). ADS  Google

Scholar  * Geballe, T. R. et al. Detection of the overtone of the 3.3 μm emission feature in IRAS 21282+5050. _Astrophys. J_. 434, L15–L18 (1994). * Chen, T., Luo, Y. & Li, A. The

infrared bands of polycyclic aromatic hydrocarbons in the 1.6–17 μm wavelength region. _Astron. Astrophys_. 632, A71 (2019). * Draine, B. T. Can dust explain variations in the D/H ratio?

_ASP Conf. Ser._ 348, 58–69 (2006). ADS  Google Scholar  * Peeters, E. et al. Deuterated interstellar polycyclic aromatic hydrocarbons. _Astrophys. J._ 604, 252–257 (2004). ADS  Google

Scholar  * Onaka, T. et al. Search for the infrared emission features from deuterated interstellar polycyclic aromatic hydrocarbons. _Astrophys. J._ 780, 114 (2014). ADS  Google Scholar  *

Doney, K. D., Candian, A., Mori, T., Onaka, T. & Tielens, A. G. G. M. Deuterated polycyclic aromatic hydrocarbons: revisited. _Astron. Astrophys._ 586, 65–74 (2016). ADS  Google Scholar

  * Peeters, E., Tielens, A. G. G. M., Boogert, A. C. A., Hayward, T. L. & Allamandola, L. J. The prominent dust emission feature near 8.9 μm in four H ii regions. _Astrophys. J._ 620,

774–785 (2005). ADS  Google Scholar  * Verstraete, L. et al. The aromatic infrared bands as seen by ISO-SWS: probing the PAH model. _Astron. Astrophys._ 372, 981–997 (2001). ADS  Google

Scholar  * van Diedenhoven, B. et al. The profiles of the 3–12 μm polycyclic aromatic hydrocarbon features. _Astron. Astrophys._ 611, 928–939 (2004). Google Scholar  * Schütz, O., Meeus, G.,

Sterzik, M. F. & Peeters, E. Mid-IR observations of circumstellar disks. Part III. A mixed sample of PMS stars and Vega-type objects. _Astron. Astrophys._ 507, 261–276 (2009). ADS 

Google Scholar  * Sloan, G. C. et al. Mid-infrared spectra of polycyclic aromatic hydrocarbon emission in Herbig Ae/Be stars. _Astrophys. J._ 632, 956–963 (2005). ADS  Google Scholar  *

Ingalls, J. G. et al. Spitzer Infrared Spectrograph detection of molecular hydrogen rotational emission towards translucent clouds. _Astrophys. J._ 743, 174 (2011). ADS  Google Scholar  *

Mathis, J. S., Mezger, P. G. & Panagia, N. Interstellar radiation field and dust temperatures in the diffuse interstellar matter and in giant molecular clouds. _Astron. Astrophys._ 128,

212–229 (1983). ADS  Google Scholar  * Berné, O. & Tielens, A. G. G. M. Formation of buckminsterfullerene (C60) in interstellar space. _Proc. Natl Acad. Sci. USA_ 109, 401–406 (2012).

ADS  Google Scholar  * García-Hernández, D. A. et al. The formation of fullerenes: clues from new C60, C70, and (possible) planar C24 detections in Magellanic Cloud planetary nebulae.

_Astrophys. J._ 737, L30 (2011). ADS  Google Scholar  * Li, Q., Li, A. & Jiang, B. W. How much graphene in space? _Mon. Not. R. Astron. Soc._ 490, 3875–3881 (2019). ADS  Google Scholar 

* Chen, T. & Li, A. Synthesizing carbon nanotubes in space. _Astron. Astrophys._ 631, A54 (2019). ADS  Google Scholar  * Derenne, S. & Robert, F. Model of molecular structure of the

insoluble organic matter isolated from Murchison meteorite. _Meteorit. Planet. Sci._ 45, 1461–1475 (2010). ADS  Google Scholar  Download references ACKNOWLEDGEMENTS I dedicate this article

to the 60th anniversary of the Department of Astronomy of Beijing Normal University, the 2nd astronomy programme in the modern history of China. I thank B. T. Draine, L. C. Ho, M. Karouzos

and X. J. Yang for useful comments and suggestions. I thank L. Armus, P. Beirão, J. G. Ingalls, H. Kaneda, D. Lutz, K. Mattila, D. A. Riechers, B. Siana, O. Vega, M. Yamagishi and L. Yan for

providing the PAH spectra shown in Figs. 1–4. This work is supported in part by NASA grants 80NSSC19K0572 and 80NSSC19K0701. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Department of

Physics and Astronomy, University of Missouri, Columbia, MO, USA Aigen Li Authors * Aigen Li View author publications You can also search for this author inPubMed Google Scholar

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Li, A. Spitzer’s perspective of polycyclic aromatic hydrocarbons in galaxies. _Nat Astron_ 4, 339–351 (2020). https://doi.org/10.1038/s41550-020-1051-1 Download citation * Received: 27

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