A resolved analysis of cold dust and gas in the nearby edge-on spiral NGC 891

et al. 2014

A&A

114

Context. Dust reprocesses about half of the stellar radiation in galaxies. The thermal re-emission by dust of absorbed energy is considered to be driven merely by young stars so is often applied to tracing the star formation rate in galaxies. Recent studies have argued that the old stellar population might be responsible for a non-negligible fraction of the radiative dust heating. Aims. In this work, we aim to analyze the contribution of young ( < ∼ 100 Myr) and old (∼10 Gyr) stellar populations to radiative dust heating processes in the nearby grand-design spiral galaxy M 51 using radiative transfer modeling. High-resolution 3D radiative transfer (RT) models are required to describe the complex morphologies of asymmetric spiral arms and clumpy star-forming regions and to model the propagation of light through a dusty medium. Methods. In this paper, we present a new technique developed to model the radiative transfer effects in nearby face-on galaxies. We construct a high-resolution 3D radiative transfer model with the Monte-Carlo code SKIRT to account for the absorption, scattering, and non-local thermal equilibrium (NLTE) emission of dust in M 51. The 3D distribution of stars is derived from the 2D morphology observed in the IRAC 3.6 µm, GALEX FUV, Hα, and MIPS 24 µm wavebands, assuming an exponential vertical distribution with an appropriate scale height. The dust geometry is constrained through the far-ultraviolet (FUV) attenuation, which is derived from the observed total-infrared-to-far-ultraviolet luminosity ratio. The stellar luminosity, star formation rate, and dust mass have been scaled to reproduce the observed stellar spectral energy distribution (SED), FUV attenuation, and infrared SED. Results. The dust emission derived from RT calculations is consistent with far-infrared and submillimeter observations of M 51, implying that the absorbed stellar energy is balanced by the thermal re-emission of dust. The young stars provide 63% of the energy for heating the dust responsible for the total infrared emission (8−1000 µm), while 37% of the dust emission is governed through heating by the evolved stellar population. In individual wavebands, the contribution from young stars to the dust heating dominates at all infrared wavebands but gradually decreases towards longer infrared and submillimeter wavebands for which the old stellar population becomes a non-negligible source of heating. Upon extrapolation of the results for M 51, we present prescriptions for estimating the contribution of young stars to the global dust heating based on a tight correlation between the dust heating fraction and specific star formation rate.

Section: Skirt Model Distribution: 3d Stellar and Dust Geometriesmentioning

confidence: 99%

Section: Dust Heating Prescriptionsmentioning

confidence: 99%

High-resolution, 3D radiative transfer modeling

Looze

Fritz

et al. 2014

A&A

114

“…As there is a known degeneracy between β and T dust (see e.g. Hughes et al 2014;Tabatabaei et al 2014;Galametz et al 2012;Smith et al 2012b, and references therein), this could explain the scatter in the relation. Furthermore, most of the pixels in Paper II had β < 2.…”

Section: Consistency With Previous Parameter Fitsmentioning

confidence: 99%

“…In particular, a modified black body can be fitted to the observed FIR (λ > 100 µm) data extracted from sub-kpc regions in galaxies (see e.g. Smith et al 2010;Hughes et al 2014). While still a step forward with respect to previous works, this approach inevitably suffers from some simplifications.…”

Section: Introductionmentioning

confidence: 99%

TheHerschelExploitation of Local Galaxy Andromeda (HELGA)

Viaene

Fritz

et al. 2014

A&A

Self Cite

108

Context. Dust and stars play a complex game of interactions in the interstellar medium and around young stars. The imprints of these processes are visible in scaling relations between stellar characteristics, star formation parameters, and dust properties. Aims. In the present work, we aim to examine dust scaling relations on a sub-kpc resolution in the Andromeda galaxy (M 31). The goal is to investigate the properties of M 31 on both a global and local scale and compare them to other galaxies of the local universe. Methods. New Herschel observations are combined with available data from GALEX, SDSS, WISE, and Spitzer to construct a dataset covering UV to submm wavelengths. All images were brought to the beam size and pixel grid of the SPIRE 500 µm frame. This divides M 31 in 22 437 pixels of 36 arcseconds in size on the sky, corresponding to physical regions of 137 × 608 pc in the galaxy's disk. A panchromatic spectral energy distribution was modelled for each pixel and maps of the physical quantities were constructed. Several scaling relations were investigated, focussing on the interactions of dust with starlight. Results. We find, on a sub-kpc scale, strong correlations between M dust /M and NUV-r, and between M dust /M and µ (the stellar mass surface density). Striking similarities with corresponding relations based on integrated galaxies are found. We decompose M 31 in four macro-regions based on their far-infrared morphology; the bulge, inner disk, star forming ring, and the outer disk region. In the scaling relations, all regions closely follow the galaxy-scale average trends and behave like galaxies of different morphological types. The specific star formation characteristics we derive for these macro-regions give strong hints of an inside-out formation of the bulge-disk geometry, as well as an internal downsizing process. Within each macro-region, however, a great diversity in individual micro-regions is found, regardless of the properties of the macro-regions. Furthermore, we confirm that dust in the bulge of M 31 is heated only by the old stellar populations. Conclusions. In general, the local dust scaling relations indicate that the dust content in M 31 is maintained by a subtle interplay of past and present star formation. The similarity with galaxy-based relations strongly suggests that they are in situ correlations, with underlying processes that must be local in nature.

“…: C. D. Wilson; see e.g. Parkin et al 2013Parkin et al , 2014Schirm et al 2014;Hughes et al 2014) is a Herschel Guaranteed Time Key Project that aims to study the gas and dust in the ISM of a diverse sample of 13 nearby galaxies with Herschel's PACS and SPIRE instruments (see Pilbratt et al 2010;Poglitsch et al 2010;Griffin et al 2010). This survey provides a unique dataset for investigating the spatially resolved [N] 205 µm line emission.…”

Section: Data and Image Processingmentioning

confidence: 99%

The spatially resolved correlation between [NII] 205μm line emission and the 24μm continuum in nearby galaxies

Hughes

Schirm

et al. 2016

A&A

Self Cite

A correlation between the 24 µm continuum and the [N] 205 µm line emission may arise if both quantities trace the star formation activity on spatially-resolved scales within a galaxy, yet has so far only been observed in the nearby edge-on spiral galaxy NGC 891. We therefore assess whether the [N] 205−24 µm emission correlation has some physical origin or is merely an artefact of line-ofsight projection effects in an edge-on disc. We search for the presence of a correlation in Herschel and Spitzer observations of two nearby face-on galaxies, M 51 and M 83, and the interacting Antennae galaxies NGC 4038 and 4039. We show that not only is this empirical relationship also observed in face-on galaxies, but also that the correlation appears to be governed by the star formation rate (SFR). Both the nuclear starburst in M 83 and the merger-induced star formation in NGC 4038/9 exhibit less [N] emission per unit SFR surface density than the normal star-forming discs. These regions of intense star formation exhibit stronger ionization parameters, as traced by the 70/160 µm far-infrared (FIR) colour. These observations suggest the presence of higher ionization lines that may become more important for gas cooling, thereby reducing the observed [N] 205 µm line emission in regions with higher star formation rates. Finally, we present a general relation between the [N] 205 µm line flux density and SFR density for normal star-forming galaxies, yet note that future studies should extend this analysis by including observations with wider spatial coverage for a larger sample of galaxies.