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Register a new userCool and warm dust emission from M33 (HerM33es)
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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 micron) of the Herschel Space Observatory and complementing our dataset with Spitzer-IRAC 5.8 and 8 micron and Spitzer-MIPS 24 and 70 micron 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 micron) is estimated to be 1.9x10^9 Lsun. 59% of the total luminosity of the galaxy is produced by the cool dust grains (~15 K) while the rest 41% is produced by warm dust grains (~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 micron) is 21%, while it gradually rises up to 57% in the submillimeter (500 micron). 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).
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We present an analysis of the first space-based far-IR-submm observations of M 33, which measure the emission from the cool dust and resolve the giant molecular cloud complexes. With roughly half-solar abundances, M33 is a first step towards young low-metallicity galaxies where the submm may be able to provide an alternative to CO mapping to measure their H$_2$ content. In this Letter, we measure the dust emission cross-section $sigma$ using SPIRE and recent CO and HI observations; a variation in $sigma$ is present from a near-solar neighborhood cross-section to about half-solar with the maximum being south of the nucleus. Calculating the total H column density from the measured dust temperature and cross-section, and then subtracting the HI column, yields a morphology similar to that observed in CO. The H$_2$/HI mass ratio decreases from about unity to well below 10% and is about 15% averaged over the optical disk. The single most important observation to reduce the potentially large systematic errors is to complete the CO mapping of M 33.
Power spectra of de-projected images of late-type galaxies in gas and/or dust emission are very useful diagnostics of the dynamics and stability of their interstellar medium. Previous studies have shown that the power spectra can be approximated as two power-laws, a shallow one at large scales (larger than 500 pc) and a steeper one at small scales, with the break between the two corresponding to the line-of-sight thickness of the galaxy disk. We present a thorough analysis of the power spectra of the dust and gas emission at several wavelengths in the nearby galaxy M33. In particular, we use the recently obtained images at five wavelengths by PACS and SPIRE onboard Herschel. The large dynamical range (2-3 dex in scale) of most images allow us to determine clearly the change in slopes from -1.5 to -4, with some variations with wavelength. The break scale is increasing with wavelength, from 100 pc at 24 and 100micron to 350 pc at 500micron, suggesting that the cool dust lies in a thicker disk than the warm dust, may be due to star formation more confined to the plane. The slope at small scale tends to be steeper at longer wavelength, meaning that the warmer dust is more concentrated in clumps. Numerical simulations of an isolated late-type galaxy, rich in gas and with no bulge, like M33, are carried out, in order to better interpret these observed results. Varying the star formation and feedback parameters, it is possible to obtain a range of power-spectra, with two power-law slopes and breaks, which nicely bracket the data. The small-scale power-law is indeed reflecting the 3D behaviour of the gas layer, steepening strongly while the feedback smoothes the structures, by increasing the gas turbulence. M33 appears to correspond to a fiducial model with an SFR of $sim$ 0.7 Mo/yr, with 10% supernovae energy coupled to the gas kinematics.
Within the key project Herschel M33 extended survey (HerM33es), we are studying the physical and chemical processes driving star formation and galactic evolution in the nearby galaxy M33, combining the study of local conditions affecting individual star formation with properties only becoming apparent on global scales. Here, we present recent results obtained by the HerM33es team. Combining Spitzer and Herschel data ranging from 3.6um to 500um, along with HI, Halpha, and GALEX UV data, we have studied the dust at high spatial resolutions of 150pc, providing estimators of the total infrared (TIR) brightness and of the star formation rate. While the temperature of the warm dust at high brightness is driven by young massive stars, evolved stellar populations appear to drive the temperature of the cold dust. Plane-parallel models of photon dominated regions (PDRs) fail to reproduce fully the [CII], [OI], and CO maps obtained in a first spectroscopic study of one 2x2 subregion of M33, located on the inner, northern spiral arm and encompassing the HII region BCLMP302.
Within the framework of the HERM33ES Key Project, using the high resolution and sensitivity of the Herschel photometric data, we study the compact emission in the Local Group spiral galaxy M33 to investigate the nature of the compact SPIRE emission sources. We extracted a catalogue of sources at 250um in order to investigate the nature of this compact emission. Taking advantage of the unprecedented Herschel resolution at these wavelengths, we also focus on a more precise study of some striking Halpha shells in the northern part of the galaxy. We present a catalogue of 159 compact emission sources in M33 identified by SExtractor in the 250um SPIRE band that is the one that provides the best spatial resolution. We also measured fluxes at 24um and Halpha for those 159 extracted sources. The morphological study of the shells also benefits from a multiwavelength approach including Halpha, far-UV from GALEX, and infrared from both Spitzer IRAC 8um and MIPS 24um in order to make comparisons. For the 159 compact sources selected at 250um, we find a very strong Pearson correlation coefficient with the MIPS 24um emission (r24 = 0.94) and a rather strong correlation with the Halpha emission, although with more scatter (rHa = 0.83). The morphological study of the Halpha shells shows a displacement between far-ultraviolet, Halpha, and the SPIRE bands. The cool dust emission from SPIRE clearly delineates the Halpha shell structures. The very strong link between the 250um compact emission and the 24um and Halpha emissions, by recovering the star formation rate from standard recipes for HII regions, allows us to provide star formation rate calibrations based on the 250um compact emission alone. The different locations of the Halpha and far-ultraviolet emissions with respect to the SPIRE cool dust emission leads to a dynamical age of a few Myr for the Halpha shells and the associated cool dust.
Over the past few years several studies have provided estimates of the SFR (star-formation rate) or the total infrared luminosity from just one infrared band. However these relations are generally derived for entire galaxies, which are known to contain a large scale diffuse emission that is not necessarily related to the latest star-formation episode. We provide new relations to estimate the SFR from resolved star-forming regions at 100 mum and 160 mum. We select individual star-forming regions in the nearby (840 kpc) galaxy M33. We estimate the SFR combining the emission in Halpha and at 24 mum to calibrate the emission at 100 mum and 160 mum as SFR estimators, as mapped with PACS/Herschel. The data are obtained in the framework of the HERM33ES open time key project. There is less emission in the HII regions at 160 mum than at 100 mum. Over a dynamic range of almost 2 dex in Sigma(SFR) we find that the 100 mum emission is a nearly linear estimator of the SFR, whereas that at 160 mum is slightly superlinear. The behaviour of individual star-forming regions is surprisingly similar to that of entire galaxies. At high Sigma(SFR), star formation drives the dust temperature, whereas uncertainties and variations in radiation-transfer and dust-heated processes dominate at low Sigma(SFR). Detailed modelling of both galaxies and individual star forming regions will be needed to interpret similarities and differences between the two and assess the fraction of diffuse emission in galaxies.
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