Dust, so little, so powerful!

Sub-micron solid particles, collectively known as dust, constitutes only a minor fraction of the mass of a galaxy, yet they have profound effects on its appearance and spectrum. Dust grains absorb starlight in the ultraviolet/optical and reemit this energy at longer wavelengths, mainly in the far-infrared and submm. On average, 20% of the total energy output of a galaxy is due to dust emission, with values running from a few percent for ellipticals up to 90% for bright spiral galaxies.

These are the results of a recent work led by Simone Bianchi and with the participation of Viviana Casasola, of the Arcetri Observatory. They measured the fraction of energy absorbed/emitted by dust for 814 galaxies of different morphological type. The objects are part of DustPedia, a sample including almost all the galaxies of the Local Universe observed in the far-infrared and submm by the Herschel Space Observatory.


Fig.1. A fit to the SED of one DustPedia galaxy. The blue curve is the CIGALE fit, used as a reference in the work. Other method have been used to confirm the results (red and purple lines).

Thanks to Herschel and ancillary data, Dustpedia galaxies have an extensive coverage of the Spectral Energy Distribution (SED), with an average of 21 photometric datapoints per object. Each SED (Fig. 1) was fitted with modelling tools to derive fabs as the ratio of the luminosity due to dust emission at wavelength larger than 4 micron, and the total luminosity from the UV to the submm.



Fig. 2. The fraction of absorbed energy (fabs) as a function of the total, bolometric, luminosity.

The large number of objects in DustPedia has allowed to detect unexpected trends: the fraction of absorbed energy has been found to correlate broadly with the total luminosity of the galaxy, at least for galaxies of later types (spirals and irregulars), with a structure dominated by a disk, rich in gas and with high specific star-formation rates (Fig. 2).  Instead, no correlation was found for Elliptical galaxies, while lenticular and earlier type spirals in part follow the correlation, in part do not. While it is expected that more luminous, evolved, spirals have a larger dust mass, it is not clear why that dust should be more effective in absorbing starlight, unless an evolution in the relative stellar and dust geometry is also at play. Various scenarios are currently being explored using samples from cosmological simulations coupled to the radiative transfer model SKIRT.


Fig.3: The fraction of absorbed energy (fabs) as a function of the gas fraction.

Indeed, evolution is suggested by analyzing fabs as a function of the gas fraction, fgas, the ratio between the mass of atomic gas and that of the baryons (gas and stars, Fig. 3). As more and more gas is converted into stars (a decreasing fgas), the fraction of absorbed energy increases. An abrupt drop in fabs is seen for the evolved lenticulars and ellipticals. If galaxies with fgas < 0.1 are supposed to have had, at an earlier time, a fabs along the trend for fgas>0.1, the luminosity-weighted average of the fraction of absorbed energy would rise to 45%. This value is close to what has been estimated from measurements of the extragalactic background light, showing that almost half of the starlight produced during the evolution of the Universe has been reprocessed by dust.


[1] Bianchi et al., "Fraction of bolometric luminosity absorbed by dust in DustPedia galaxies", A&A, in press

[2] DustPedia   is   a   collaborative   focused   research   project supported  by  the  European  Union  under  the  Seventh  Framework  Programme  (2007-  2013)  call  (proposal  no.  606824,  P.I.  J.  I.  Davies, http://www.dustpedia.com). The photometric data used in this work is publicly available at http://dustpedia.astro.noa.gr . Soon, ancillary data on gas and metallicity, as well as the result of global and resolved SED modelling will become available.

[3] A version of CIGALE, including the DustPedia reference grain model THEMIS  is available here