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Register a new userProbing the molecular interstellar medium of M82 with Herschel-SPIRE spectroscopy
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We present the observations of the starburst galaxy M82 taken with the Herschel SPIRE Fourier Transform Spectrometer. The spectrum (194-671 {mu}m) shows a prominent CO rotational ladder from J = 4-3 to 13-12 emitted by the central region of M82. The fundamental properties of the gas are well constrained by the high J lines observed for the first time. Radiative transfer modeling of these high-S/N 12CO and 13CO lines strongly indicates a very warm molecular gas component at ~500 K and pressure of ~3x10^6 K cm^-3, in good agreement with the H_2 rotational lines measurements from Spitzer and ISO. We suggest that this warm gas is heated by dissipation of turbulence in the interstellar medium (ISM) rather than X-rays or UV flux from the straburst. This paper illustrates the promise of the SPIRE FTS for the study of the ISM of nearby galaxies.
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The standard method of mapping the interstellar medium in a galaxy, by observing the molecular gas in the CO 1-0 line and the atomic gas in the 21-cm line, is largely limited with current telescopes to galaxies in the nearby universe. In this letter, we use SPIRE observations of the galaxies M99 and M100 to explore the alternative approach of mapping the interstellar medium using the continuum emission from the dust. We have compared the methods by measuring the relationship between the star-formation rate and the surface density of gas in the galaxies. We find the two methods give relationships with a similar dispersion, confirming that observing the continuum emission from the dust is a promising method of mapping the interstellar medium in galaxies.
NGC1569 has some of the most vigorous star formation among nearby galaxies. It hosts two super star clusters (SSCs) and has a higher star formation rate (SFR) per unit area than other starburst dwarf galaxies. Extended emission beyond the galaxys optical body is observed in warm and hot ionised and atomic hydrogen gas; a cavity surrounds the SSCs. We aim to understand the impact of the massive star formation on the surrounding interstellar medium in NGC1569 through a study of its stellar and dust properties. We use Herschel and ancillary multiwavelength observations, from the ultraviolet to the submillimeter regime, to construct its spectral energy distribution, which we model with magphys on ~300pc scales at the SPIRE250 {mu}m resolution. The multiwavelength morphology shows low levels of dust emission in the cavity, and a concentration of several dust knots in its periphery. The extended emission seen in the ionised and neutral hydrogen observations is also present in the far-infrared emission. The dust mass is higher in the periphery of the cavity, driven by ongoing star formation and dust emission knots. The SFR is highest in the central region, while the specific SFR is more sensitive to the ongoing star formation. The region encompassing the cavity and SSCs contains only 12 per cent of the dust mass of the central starburst, in accord with other tracers of the interstellar medium. The gas-to-dust mass ratio is lower in the cavity and fluctuates to higher values in its periphery.
We present far-infrared spectra and maps of the DR21 molecular cloud core between 196 and 671 microns, using the Herschel-SPIRE spectrometer. Nineteen molecular lines originating from CO, 13CO, HCO+ and H2O, plus lines of [N II] and [CI] were recorded, including several transitions not previously detected. The CO lines are excited in warm gas with Tkin ~ 125 K and nH2 ~ 7 x 10^4 cm-3, CO column density N(CO) ~ 3.5 x 10^18 cm^-2 and a filling factor of ~ 12%, and appear to trace gas associated with an outflow. The rotational temperature analysis incorporating observations from ground-based telescopes reveals an additional lower excitation CO compoment which has a temperature ~ 78 K and N(CO) ~ 4.5 x 10^21 cm^-2. Astronomy & Astrophysics HERSCHEL special Issue, in press.
We present a comprehensive analysis of interstellar absorption lines seen in moderately-high resolution, high signal-to-noise ratio optical spectra of SN 2014J in M82. Our observations were acquired over the course of six nights, covering the period from ~6 days before to ~30 days after the supernova reached its maximum B-band brightness. We examine complex absorption from Na I, Ca II, K I, Ca I, CH+, CH, and CN, arising primarily from diffuse gas in the interstellar medium (ISM) of M82. We detect Li I absorption over a range in velocity consistent with that exhibited by the strongest Na I and K I components associated with M82; this is the first detection of interstellar Li in a galaxy outside of the Local Group. There are no significant temporal variations in the absorption-line profiles over the 37 days sampled by our observations. The relative abundances of the various interstellar species detected reveal that the ISM of M82 probed by SN 2014J consists of a mixture of diffuse atomic and molecular clouds characterized by a wide range of physical/environmental conditions. Decreasing N(Na I)/N(Ca II) ratios and increasing N(Ca I)/N(K I) ratios with increasing velocity are indicative of reduced depletion in the higher-velocity material. Significant component-to-component scatter in the N(Na I)/N(Ca II) and N(Ca I)/N(Ca II) ratios may be due to variations in the local ionization conditions. An apparent anti-correlation between the N(CH+)/N(CH) and N(Ca I)/N(Ca II) ratios can be understood in terms of an opposite dependence on gas density and radiation field strength, while the overall high CH+ abundance may be indicative of enhanced turbulence in the ISM of M82. The Li abundance also seems to be enhanced in M82, which supports the conclusions of recent gamma-ray emission studies that the cosmic-ray acceleration processes are greatly enhanced in this starburst galaxy.
We present a power spectrum analysis of the Herschel-SPIRE observations of the Polaris flare, a high Galactic latitude cirrus cloud midway between the diffuse and molecular phases. The SPIRE images of the Polaris flare reveal for the first time the structure of the diffuse interstellar medium down to 0.01 parsec over a 10 square degrees region. These exceptional observations highlight the highly filamentary and clumpy structure of the interstellar medium even in diffuse regions of the map. The power spectrum analysis shows that the structure of the interstellar medium is well described by a single power law with an exponent of -2.7 +- 0.1 at all scales from 30 to 8 degrees. That the power spectrum slope of the dust emission is constant down to the SPIRE angular resolution is an indication that the inertial range of turbulence extends down to the 0.01 pc scale. The power spectrum analysis also allows the identification of a Poissonian component at sub-arcminute scales in agreement with predictions of the cosmic infrared background level at SPIRE wavelengths. Finally, the comparison of the SPIRE and IRAS 100 micron data of the Polaris flare clearly assesses the capability of SPIRE in maping diffuse emission over large areas.
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