arXiv subjects

F. S. Tabatabaei

Publications and source records attributed to F. S. Tabatabaei.

51 records · Page 3Linked to original sources

arXiv2011

The Herschel M33 extended survey (HerM33es): PACS spectroscopy of the star forming region BCLMP 302

Context: The emission line of [CII] at 158 micron is one of the strongest cooling lines of the interstellar medium (ISM) in galaxies. Aims: Disentangling the relative contributions of the different ISM phases to [CII] emission, is a major topic of the HerM33es program, a Herschel key project to study the ISM in the nearby spiral galaxy M33. Methods: Using PACS, we have mapped the emission of [CII] 158 micron, [OI] 63 micron, and other FIR lines in a 2'x2' region of the northern spiral arm of M33, centered on the HII region BCLMP302. At the peak of H-alpha emission, we have observed in addition a velocity resolved [CII] spectrum using HIFI. We use scatterplots to compare these data with PACS 160 micron continuum maps, and with maps of CO and HI data, at a common resolution of 12 arcsec or 50 pc. Maps of H-alpha and 24 micron emission observed with Spitzer are used to estimate the SFR. We have created maps of the [CII] and [OI] 63 micron emission and detected [NII] 122 micron and NIII 57 micron at individual positions. Results: The [CII] line observed with HIFI is significantly broader than that of CO, and slightly blue-shifted. In addition, there is little spatial correlation between [CII] observed with PACS and CO over the mapped region. There is even less spatial correlation between [CII] and the atomic gas traced by HI. Detailed comparison of the observed intensities towards the HII region with models of photo ionization and photon dominated regions, confirms that a significant fraction, 20--30%, of the observed [CII] emission stems from the ionized gas and not from the molecular cloud. The gas heating efficiency, using the ratio between [CII] and the TIR as a proxy, varies between 0.07 and 1.5%, with the largest variations found outside the HII region.

astro-ph.GA↗

arXiv2010

The local radio-IR correlation in M51

We observed M51 at three frequencies, 1.4GHz (20cm), 4.9GHz (6cm) and 8.4GHz (3.6cm), with the VLA and the Effelsberg 100m telescope to obtain the highest quality radio continuum images of a nearby spiral galaxy. These radio data were combined with deconvolved Spitzer IRAC 8mum and MIPS 24mum images to search for and investigate local changes in the radio-IR correlation. Utilizing wavelet decomposition, we compare the distribution of the radio and IR emission on spatial scales between 200pc and 30kpc. We show that the radio-IR correlation is not uniform across the galactic disk. It presents a complex behavior with local extrema corresponding to various galactic structures, such as complexes of HII regions, spiral arms and interarm filaments, indicating that the contribution of the thermal and non-thermal radio emission is a strong function of environment. In particular, the relation of the 24mum and 20cm emission presents a linear relation within the spiral arms and globally over the galaxy, while it deviates from linearity in the interarm and outer regions as well in the inner region, with two different behaviours: it is sublinear in the interarm and outer region and over-linear in the central 3.5kpc. Our analysis suggests that the changes in the radio/IR correlation reflect variations of ISM properties between spiral arms and interarm region. The good correlation in the spiral arms implies that 24mum and 20cm are tracing recent star formation, while a change in the dust opacity, 'Cirrus' contribution to the IR emission and/or the relation between the magnetic field strength and the gas density can explain the different relations found in the interarm, outer and inner regions.

astro-ph.CO↗

arXiv2010

Molecular and Atomic Gas in the Local Group Galaxy M33

We present high resolution large scale observations of the molecular and atomic gas in the Local Group Galaxy M33. The observations were carried out using the HERA at the 30m IRAM telescope in the CO(2-1) line achieving a resolution of 12"x2.6 km/s, enabling individual GMCs to be resolved. The observed region mainly along the major axis out to a radius of 8.5 kpc, and covers the strip observed with HIFI/PACS Spectrometers as part of the HERM33ES Herschel key program. The achieved sensitivity in main beam temperature is 20-50 mK at 2.6 km/s velocity resolution. The CO(2-1) luminosity of the observed region is 1.7\pm0.1x10^7 Kkm/s pc^2, corresponding to H2 masses of 1.9x10^8 Msun (including He), calculated with a NH2/ICO twice the Galactic value due to the half-solar metallicity of M33. HI 21 cm VLA archive observations were reduced and the mosaic was imaged and cleaned using the multi-scale task in CASA, yielding a series of datacubes with resolutions ranging from 5" to 25". The HI mass within a radius of 8.5 kpc is estimated to be 1.4x10^9 Msun. The azimuthally averaged CO surface brightness decreases exponentially with a scale length of 1.9\pm0.1 kpc whereas the atomic gas surface density is constant at Sigma_HI=6\pm2 Msun/pc^2 deprojected to face-on. The central kiloparsec H_2 surface density is Sigma_H2=8.5\pm0.2 Msun/pc^2. The star formation rate per unit molecular gas (SF Efficiency, the rate of transformation of molecular gas into stars), as traced by the ratio of CO to Halpha and FIR brightness, is constant with radius. The SFE appears 2-4 times greater than of large spiral galaxies. A morphological comparison of molecular and atomic gas with tracers of star formation shows good agreement between these maps both in terms of peaks and holes. A few exceptions are noted. Several spectra, including those of a molecular cloud situated more than 8 kpc from the galaxy center, are presented.

astro-ph.CO↗

arXiv2010

PACS and SPIRE photometer maps of M33: First results of the Herschel M33 extended survey (HERM33ES)

Within the framework of the HERM33ES key project, we are studying the star forming interstellar medium in the nearby, metal-poor spiral galaxy M33, exploiting the high resolution and sensitivity of Herschel. We use PACS and SPIRE maps at 100, 160, 250, 350, and 500 micron wavelength, to study the variation of the spectral energy distributions (SEDs) with galacto-centric distance. Detailed SED modeling is performed using azimuthally averaged fluxes in elliptical rings of 2 kpc width, out to 8 kpc galacto-centric distance. Simple isothermal and two-component grey body models, with fixed dust emissivity index, are fitted to the SEDs between 24 and 500 micron using also MIPS/Spitzer data, to derive first estimates of the dust physical conditions. The far-infrared and submillimeter maps reveal the branched, knotted spiral structure of M33. An underlying diffuse disk is seen in all SPIRE maps (250-500 micron). Two component fits to the SEDs agree better than isothermal models with the observed, total and radially averaged flux densities. The two component model, with beta fixed at 1.5, best fits the global and the radial SEDs. The cold dust component clearly dominates; the relative mass of the warm component is less than 0.3% for all the fits. The temperature of the warm component is not well constrained and is found to be about 60K plus/minus 10K. The temperature of the cold component drops significantly from about 24K in the inner 2 kpc radius to 13K beyond 6 kpc radial distance, for the best fitting model. The gas-to-dust ratio for beta=1.5, averaged over the galaxy, is higher than the solar value by a factor of 1.5 and is roughly in agreement with the subsolar metallicity of M33.

astro-ph.CO↗

arXiv2010

Properties of compact 250 μm emission and HII regions in M33 (HERM33ES)

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.

astro-ph.CO↗

arXiv2010

100 mum and 160 mum emission as resolved star-formation rate estimators in M33 (HERM33ES)

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.

astro-ph.CO↗

arXiv2010

Relating dust, gas and the rate of star formation in M31

We derive distributions of dust temperature and dust opacity across M31 at 45" resolution using the Spitzer data. With the opacity map and a standard dust model we de-redden the Ha emission yielding the first de-reddened Ha map of M31. We compare the emissions from dust, Ha, HI and H2 by means of radial distributions, pixel-to-pixel correlations and wavelet cross-correlations. The dust temperature steeply decreases from 30K near the center to 15K at large radii. The mean dust optical depth at the Ha wavelength along the line of sight is about 0.7. The radial decrease of the dust-to-gas ratio is similar to that of the oxygen abundance. On scales<2kpc, cold dust emission is best correlated with that of neutral gas and warm dust emission with that of ionized gas. Ha emission is slightly better correlated with emission at 70um than at 24um. In the area 6kpc<R< 17kpc, the total SFR is ~0.3Msun/yr. The Kennicutt-Schmidt law between SFR and total gas has a power-law index of 1.30+-0.05 in the radial range of R=7-11kpc increasing by about 0.3 for R=11-13kpc. The lack of H2 in the central region could be related to the lack of HI and the low opacity/high temperature of the dust. Since neither SFR nor SFE is well correlated with the surface density of H2 or total gas, other factors than gas density must play an important role in the formation of massive stars in M31. The molecular depletion time scale of 1.1 Gyr indicates that M31 is about three times less efficient in forming young massive stars than M33.

astro-ph.GA↗

arXiv2009

The radio-infrared correlation in galaxies

The radio-infrared correlation was explained as a direct and linear relationship between star formation and IR emission. However, one fact making the IR-star formation linkage less obvious is that the IR emission consists of at least two emission components, cold dust and warm dust. The cold dust emission may not be directly linked to the young stellar population. Furthermore, understanding the origin of the radio-IR correlation requires to discriminate between the two main components of the radio continuum emission, free-free and synchrotron emission. Here, we present a multi-scale study of the correlation of IR with both the thermal and non-thermal (synchrotron) components of the radio continuum emission from the nearby galaxies M33 and M31.

astro-ph.GA↗

arXiv2009

Magnetism in the nearby galaxy M33

Using high-resolution data of the linearly polarized intensity and polarization angle at 3.6, 6.2, and 20 cm together with a 3-D model of the regular magnetic field, we study variations of the structure, strength, and energy density of the magnetic field in the Scd galaxy M33. The regular magnetic field consists of a horizontal component (represented by an axisymmetric mode from 1 to 3 kpc radius and a superposition of axisymmetric and bisymmetric modes from 3 to 5 kpc radius) and a vertical component. However, the inferred `vertical field' may be partly due to a galactic warp. We estimate the average total and regular magnetic field strengths as ~ 6.4 and 2.5 $μ$G, respectively. Generation of interstellar magnetic fields by turbulent gas motion in M33 is indicated as the turbulent and magnetic energy densities are about equal.

astro-ph.GA↗

arXiv2008

High-resolution radio continuum survey of M33: III. Magnetic fields

Using the linearly polarized intensity and polarization angle data at 3.6, 6.2 and 20 cm, we determine variations of Faraday rotation and depolarization across the nearby galaxy M33. A 3-D model of the regular magnetic field is fitted to the observed azimuthal distribution of polarization angles. Faraday rotation, measured between 3.6 and 6.2 cm at a linear resolution of 0.7 kpc, shows more variation in the south than in the north of the galaxy. About 10% of the nonthermal emission from M33 at 3.6 cm is polarized. We estimate the average total and regular magnetic field strengths in M33 as ~ 6.4 and 2.5 $μ$G, respectively. Under the assumption that the disk of M33 is flat, the regular magnetic field consists of horizontal and vertical components: however the inferred vertical field may be partly due to a galactic warp. The horizontal field is represented by an axisymmetric (m=0) mode from 1 to 3 kpc radius and a superposition of axisymmetric and bisymmetric (m=0+1) modes from 3 to 5 kpc radius. An excess of differential Faraday rotation in the southern half together with strong Faraday dispersion in the southern spiral arms seem to be responsible for the north-south asymmetry in the observed wavelength dependent depolarization. The presence of an axisymmetric m=0 mode of the regular magnetic field in each ring suggests that a galactic dynamo is operating in M33. The pitch angles of the spiral regular magnetic field are generally smaller than the pitch angles of the optical spiral arms but are twice as big as simple estimates based on the mean-field dynamo theory and M33's rotation curve. Generation of interstellar magnetic fields from turbulent gas motions in M33 is indicated by the equipartition of turbulent and magnetic energy densities.

astro-ph↗

arXiv2008

A Multi-Scale Study of IR and Radio Emission from M33

The origin of the tight radio--IR correlation in galaxies has not been fully understood. One reason is the uncertainty about which heating sources (stars or diffuse interstellar radiation field)provide the energy that is absorbed by dust and re-radiated in IR. Another problem is caused by comparing the IR emission with the thermal and nonthermal components of the radio continuum emission separated by simplistically assuming a constant nonthermal spectral index. We use the data at the $Spitzer$ MIPS wavelengths of 24, 70, and 160 $μ$m, as well as recent radio continuum map at 3.6 cm observed with the 100--m Effelsberg telescope. Using the wavelet transformation, we separate diffuse emission components from compact sources and study the radio-IR correlation at various scales. We also investigate the IR correlations with the thermal and nonthermal radio emissions separated by our developed method. A H$α$ map serves as a tracer of star forming regions.

astro-ph↗

arXiv2008

Variations of the Radio Synchrotron Spectral Index in M33

We determine the variation in the nonthermal radio spectral index in the nearby spiral galaxy M33. We separate the thermal and nonthermal components of the radio continuum emission without the assumption of a constant nonthermal spectral index. Using the Spitzer FIR data at 70 and 160 $μ$m and a standard dust model, we de-redden the H$α$ emission. The extinction-corrected H$α$ emission serves as a template for the thermal free-free radio emission. Subtracting from the observed 3.6 and 20cm emission (Effelsberg and the VLA) this free-free emission, we obtain maps of the nonthermal intensity and spectral index.

astro-ph↗

arXiv2007

High-resolution radio continuum survey of M33 II. Thermal and nonthermal emission

We determine the variation in the nonthermal radio spectral index in the nearby spiral galaxy M33 at a linear resolution of 360 pc. We separate the thermal and nonthermal components of the radio continuum emission without the assumption of a constant nonthermal spectral index. Using the Spitzer FIR data at 70 and 160 $μ$m and a standard dust model, we deredden the H$α$ emission. The extinction corrected H$α$ emission serves as a template for the thermal free-free radio emission. Subtracting from the observed 3.6 cm and 20 cm emission (Effelsberg and the VLA) this free-free emission, we obtain the nonthermal maps. A constant electron temperature used to obtain the thermal radio intensity seems appropriate for M~33 which, unlike the Milky Way, has a shallow metallicity gradient. For the first time, we derive the distribution of the nonthermal spectral index across a galaxy, M33. We detect strong nonthermal emission from the spiral arms and star-forming regions. Wavelet analysis shows that at 3.6 cm the nonthermal emission is dominated by contributions from star-forming regions, while it is smoothly distributed at 20 cm. For the whole galaxy, we obtain thermal fractions of 51% and 18% at 3.6 cm and 20 cm, respectively. The thermal emission is slightly stronger in the southern than in the northern half of the galaxy. We find a clear radial gradient of mean extinction in the galactic plane. The nonthermal spectral index map indicates that the relativistic electrons suffer energy-loss when diffusing from their origin in star-forming regions towards interarm regions and the outer parts of the galaxy. We also conclude that the radio emission is mostly nonthermal at R $>$ 5 kpc in M33.

astro-ph↗

arXiv2007

High resolution radio continuum survey of M33: I. The radio maps

We study the exponential scale length of total radio emission, the spectral index distribution, and the linear radio polarization in the Scd galaxy M33. Observations were carried out using the 3.6 cm dual channel and the 6.2 cm four channel receivers of the 100--m Effelsberg telescope along with the L-band VLA D--array at 20 cm. High spatial resolution and sensitivity in both total and linearly polarized radio continuum emission from M33 were achieved. We found considerable extended emission, not only from the main arms I S and I N, but also from the weaker arms. The large--scale magnetic field exhibits well--ordered spiral structure with almost the same orientation as that of the optical spiral arms, however, it does not show a clear structural correlation or anti--correlation with the optical arms. There is a north-south asymmetry in polarization that is frequency-dependent. We found that the ring mean spectral index versus radius increases faster beyond $R$ = 4 kpc. At each wavelength, the exponential scale length is larger inside than outside $R$ = 4 kpc. From the larger scales lengths at $R$ $<$ 4 kpc, we conclude that star forming regions are mainly spread over the region $R$ $<$ 4 kpc without a dominant nuclear concentration. Furthermore, at $R$ $<$ 4 kpc, a spatial correlation between cosmic rays and star forming regions may exist. From the behaviour of the mean spectral indices obtained from different pairs of the radio continuum data at 3.6, 6.2, and 20 cm, we confirm that a decrease in the thermal fraction causes an increase in the spectral index. The frequency-dependent asymmetry in the polarization hints to an asymmetry in Faraday depolarization.

astro-ph↗

arXiv2007

A multi-scale study of infrared and radio emission from Scd galaxy M33

We investigate the energy sources of the infrared (IR) emission and their relation to the radio continuum emission at various spatial scales within the Scd galaxy M33. We use the wavelet transform to analyze IR data at the Spitzer wavelengths of 24, 70, and 160$μ$m, as well as recent radio continuum data at 3.6cm and 20cm. An H$α$ map serves as a tracer of the star forming regions and as an indicator of the thermal radio emission. We find that the dominant scale of the 70$μ$m emission is larger than that of the 24$μ$m emission, while the 160$μ$m emission shows a smooth wavelet spectrum. The radio and H$α$ maps are well correlated with all 3 MIPS maps, although their correlations with the 160$μ$m map are weaker. After subtracting the bright HII regions, the 24 and 70$μ$m maps show weaker correlations with the 20cm map than with the 3.6cm map at most scales. We also find a strong correlation between the 3.6cm and H$α$ emission at all scales. Comparing the results with and without the bright HII regions, we conclude that the IR emission is influenced by young, massive stars increasingly with decreasing wavelength from 160 to 24$μ$m. The radio-IR correlations indicate that the warm dust-thermal radio correlation is stronger than the cold dust-nonthermal radio correlation at scales smaller than 4kpc. A perfect 3.6cm-H$α$ correlation implies that extinction has no significant effect on H$α$ emitting structures.

astro-ph↗