We study the far-infrared emission from the nearby spiral galaxy M33 in order to investigate the dust physical properties such as the temperature and the luminosity density across the galaxy .
Taking advantage of the unique wavelength coverage ( 100 , 160 , 250 , 350 and 500 \mu m ) of the Herschel Space Observatory and complementing our dataset with Spitzer-IRAC 5.8 and 8 \mu m and Spitzer-MIPS 24 and 70 \mu m data , we construct temperature and luminosity density maps by fitting two modified blackbodies of a fixed emissivity index of 1.5 .
We find that the “ cool ” dust grains are heated at temperatures between 11 and 28 K with the lowest temperatures found in the outskirts of the galaxy and the highest ones in the center and in the bright HII regions .
The infrared/submillimeter total luminosity ( 5 - 1000 \mu m ) is estimated to be 1.9 \times 10 ^ { 9 } _ { -4.4 \times 10 ^ { 8 } } ^ { +4.0 \times 10 ^ { 8 } } L _ { \odot } .
59 % of the total luminosity of the galaxy is produced by the “ cool ” dust grains ( \sim 15 K ) while the rest 41 % is produced by “ warm ” dust grains ( \sim 55 K ) .
The ratio of the cool-to-warm dust luminosity is close to unity ( within the computed uncertainties ) , throughout the galaxy , with the luminosity of the cool dust being slightly enhanced in the center of the galaxy .
Decomposing the emission of the dust into two components ( one emitted by the diffuse disk of the galaxy and one emitted by the spiral arms ) we find that the fraction of the emission in the disk in the mid-infrared ( 24 \mu m ) is 21 % , while it gradually rises up to 57 % in the submillimeter ( 500 \mu m ) .
We find that the bulk of the luminosity comes from the spiral arm network that produces 70 % of the total luminosity of the galaxy with the rest coming from the diffuse dust disk .
The “ cool ” dust inside the disk is heated at a narrow range of temperatures between 18 and 15 K ( going from the center to the outer parts of the galaxy ) .