Orateur
Description
Interstellar dust (ISD) grains are solid particles made of heavy elements (e.g., O, C, Si, Mg, Fe) available in the Interstellar Medium (ISM), with sizes ranging typically from 0.3 nm to 0.3 $\mu$m, and rather uniformly mixed with the gas. Although accounting just for 1% of the ISM mass, they have a radical impact on galaxy emission, since they scatter and absorb starlight, and re-radiate at longer wavelengths a large amount of the total stellar power (about the 30% in normal disk galaxies; up to the 99% in ultraluminous infrared galaxies) [1]. Dust grain seeds are formed and injected in the ISM during specific phases of stellar evolution, by asymptotic giant branch (AGB) stars and core-collapse supernovae (CC-SNe). At the same time, ISD constitutes one of the main fuels of star formation in galaxies. Moreover, dust grains are catalysts of numerous chemical reactions, including the formation of molecular hydrogen [2], and are responsible for the heating of the gas in Photodissociation Regions (PDR), by photoelectric effect [3]. As such, a detailed knowledge of grain properties is crucial to study both the evolution of galaxies and the ISM lifecycle. For example, it is needed to: unredden UV-visible observations; study deeply embedded regions; build reliable diagnostics of ISM physical conditions and of the evolutionary stage of galaxies; provide accurate prescriptions in photoionization and photodissociation models, and simulations of the star formation process.
Most of our knowledge of dust grain properties comes from studies of the Milky Way (MW). However, the latter is limited by a narrow range of environmental conditions (e.g. no extremely luminous star-forming region; narrow radial metallicity gradient; passive central black hole) and by confusion along the sightline (we have access just to the projected material of the entire disk). As a consequence, nearby galaxies (i.e., within 100 Mpc from the MW) are becoming more and more important to constrain dust properties in different ambients [4]. High latitude observations of face-on nearby galaxies can provide cleaner sightlines. Harbouring a wider diversity of metallicities, star-forming regions, etc., nearby galaxies allow us to study dust grains in extreme conditions and constitute a necessary intermediate step towards understanding distant galaxies, as they are spatially resolved and have a better wavelength coverage. This provides the scientific rationale for our study. The main objective is to put constraints on dust grain evolution and properties under the many environmental conditions that can be observed in local galaxies.
In this talk, I will present the major and latest results of our project, applied to a selection of nearby galaxies in the multi-wavelength DustPedia Archive [5], that is being observed at 1 and 2 mm by NIKA2 (IRAM 30-m telescope), at unprecedented resolution (i.e., 12'' and 18'' respectively), as a part of the European consortium of NIKA2 Guaranteed Time program, IMEGIN (PI Madden). Interstellar dust grain properties, such as composition, size, geometry, temperature, mass, etc., are derived by the pixel-by-pixel modelling of galaxy optical-to-cm Spectral Energy Distribution (SED) using the THEMIS dust evolution model [6], implemented within the hierarchical Bayesian SED fitting code HerBIE [7]. In this respect, the millimetre maps by NIKA2 play a crucial role, since they allow us to distinguish the dust emission from free-free and synchrotron radiation, to put constraints on the properties of cold dust in galaxies (e.g., dust millimetre opacity), and to investigate the origin of the observed sub-millimetre excess.
References:
[1] Clements DL et al. 1996. MNRAS 279:477-497 [2] Gould RJ, Salpeter EE. 1963. ApJ 138:393 [3] Draine BT. 1978. ApJS 36:595–619 [4] Galliano et al. 2018. ARA\&A 56:673-713 [5] http://dustpedia.astro.noa.gr/ [6] Jones et al. 2017. A\&A 602, A46 [7] Galliano 2018. MNRAS 476, 1445–1469
