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Register a new userCold dust clumps in dynamically hot gas
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We present clumps of dust emission from Herschel observations of the Large Magellanic Cloud (LMC) and their physical and statistical properties. We catalog cloud features seen in the dust emission from Herschel observations of the LMC, the Magellanic type irregular galaxy closest to the Milky Way, and compare these features with HI catalogs from the ATCA+Parkes HI survey. Using an automated cloud-finding algorithm, we identify clouds and clumps of dust emission and examine the cumulative mass distribution of the detected dust clouds. The mass of cold dust is determined from physical parameters that we derive by performing spectral energy distribution fits to 250, 350, and 500 micronm emission from SPIRE observations using DUSTY and GRASIL radiative transfer calculation with dust grain size distributions for graphite/silicate in low-metallicity extragalactic environments. The dust cloud mass spectrum follows a power law distribution with an exponent of gamma=-1.8 for clumps larger than 400 solar mass and is similar to the HI mass distribution. This is expected from the theory of ISM structure in the vicinity of star formation.
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Data from the Herschel Space Observatory have revealed an unusual elliptical galaxy, NGC 4125, which has strong and extended submillimeter emission from cold dust but only very strict upper limits to its CO and HI emission. Depending on the dust emissivity, the total dust mass is 2-5x10^6 Msun. While the neutral gas-to-dust mass ratio is extremely low (< 12-30), including the ionized gas traced by [CII] emission raises this limit to < 39-100. The dust emission follows a similar r^{1/4} profile to the stellar light and the dust to stellar mass ratio is towards the high end of what is found in nearby elliptical galaxies. We suggest that NGC 4125 is currently in an unusual phase where evolved stars produced in a merger-triggered burst of star formation are pumping large amounts of gas and dust into the interstellar medium. In this scenario, the low neutral gas-to-dust mass ratio is explained by the gas being heated to temperatures >= 10^4 K faster than the dust is evaporated. If galaxies like NGC 4125, where the far-infrared emission does not trace neutral gas in the usual manner, are common at higher redshift, this could have significant implications for our understanding of high redshift galaxies and galaxy evolution.
The Planck Catalogue of Galactic Cold Clumps (PGCC) contains over 13000 sources detected based on their cold dust signature. They are believed to consist of a mixture of quiescent, pre-stellar, and already star-forming objects. We extracted PGCC-type objects from cloud simulations and examined their physical and polarisation properties. The comparison with the PGCC catalogue helps to characterise the PGCC sample and provides valuable tests for numerical simulations of interstellar medium. We used several MHD snapshots to define the density field of our models. Sub-millimetre images of the surface brightness and polarisation were obtained with radiative transfer calculations. We examined the statistics of synthetic cold clump catalogues and examined the variations of the clump polarisation fraction p. The clump sizes, aspect ratios, and temperatures in the synthetic catalogue are similar to the PGCC. The fluxes and column densities are smaller by a factor of a few. Rather than with an increased dust opacity, this could be explained by increasing the average column density of the models by a factor of two to three, close to N(H2)= 10^22 cm-2. When the line of sight is parallel to the mean magnetic field, the polarisation fraction tends to increase towards the clump centres, contrary to observations. When the field is perpendicular, the polarisation fraction tends to decrease towards the clumps, but the drop in $p$ is small (e.g. from p~8% to p~7%). Magnetic field geometry reduces the polarisation fraction in the simulated clumps by only Delta p~1% on average. The larger drop seen towards the actual PGCC clumps suggests some loss of grain alignment in the dense medium, such as predicted by the radiative torque mechanism. The statistical study is not able to quantify dust opacity changes at the scale of the PGCC clumps.
We report on observations of redshifted CO(1-0) line emission and observed-frame $rmsim$ 30GHz radio continuum emission from five ultra-luminous, mid-IR selected hot, Dust-Obscured Galaxies (Hot DOGs) at $zrmgtrsim$ 3 using the Karl G. Jansky Very Large Array. We detect CO(1-0) line emission in all five Hot DOGs, with one of them at high signal to noise. We analyse FIR-radio spectral energy distributions, including dust, free-free and synchrotron emission for the galaxies. We find that most of the 115 GHz rest-frame continuum is mostly due to synchrotron or free-free emission, with only a potentially small contribution from thermal emission. We see a deficit in the rest-frame 115 GHz continuum emission compared to dusty star-forming galaxies (DSFGs) and sub-millimetre galaxies (SMGs) at high redshift, suggesting that Hot DOGs do not have similar cold gas reserves compared with star-forming galaxies. One target, W2305-0039, is detected in the FIRST 1.4 GHz survey, and is likely to possess compact radio jets. We compare to the FIR-radio correlation, and find that at least half of the Hot DOGs in our sample are radio-quiet with respect to normal galaxies. These findings suggest that Hot DOGs have comparably less cold molecular gas than star-forming galaxies at lower, $zrmsim$ 2 redshifts, and are dominated by powerful, yet radio-quiet AGN.
We present IRAM 30m sensitive upper limits on CO emission in the ram pressure stripped dwarf Virgo galaxy IC3418 and in a few positions covering HII regions in its prominent 17 kpc UV/Ha gas-stripped tail. In the central few arcseconds of the galaxy, we report a possible marginal detection of about 1x10^6 M_sun of molecular gas (assuming a Galactic CO-to-H_2 conversion factor) that could correspond to a surviving nuclear gas reservoir. We estimate that there is less molecular gas in the main body of IC3418, by at least a factor of 20, than would be expected from the pre-quenching UV-based star formation rate assuming the typical gas depletion timescale of 2 Gyr. Given the lack of star formation in the main body, we think the H_2-deficiency is real, although some of it may also arise from a higher CO-to-H_2 factor typical in low-metallicity, low-mass galaxies. The presence of HII regions in the tail of IC3418 suggests that there must be some dense gas; however, only upper limits of < 1x10^6 M_sun were found in the three observed points in the outer tail. This yields an upper limit on the molecular gas content of the whole tail < 1x10^7 M_sun, which is an amount similar to the estimates from the observed star formation rate over the tail. We also present strong upper limits on the X-ray emission of the stripped gas in IC3418 from a new Chandra observation. The measured X-ray luminosity of the IC3418 tail is about 280 times lower than that of ESO 137-001, a spiral galaxy in a more distant cluster with a prominent ram pressure stripped tail. Non-detection of any diffuse X-ray emission in the IC3418 tail may be due to a low gas content in the tail associated with its advanced evolutionary state and/or due to a rather low thermal pressure of the surrounding intra-cluster medium.
Analysis of all-sky Planck submillimetre observations and the IRAS 100um data has led to the detection of a population of Galactic cold clumps. The clumps can be used to study star formation and dust properties in a wide range of Galactic environments. Our aim is to measure dust spectral energy distribution (SED) variations as a function of the spatial scale and the wavelength. We examine the SEDs at large scales using IRAS, Planck, and Herschel data. At smaller scales, we compare with JCMT/SCUBA-2 850um maps with Herschel data that are filtered using the SCUBA-2 pipeline. Clumps are extracted using the Fellwalker method and their spectra are modelled as modified blackbody functions. According to IRAS and Planck data, most fields have dust colour temperatures T_C ~ 14-18K and opacity spectral index values of beta=1.5-1.9. The clumps/cores identified in SCUBA-2 maps have T~ 13K and similar beta values. There are some indications of the dust emission spectrum becoming flatter at wavelengths longer than 500um. In fits involving Planck data, the significance is limited by the uncertainty of the corrections for CO line contamination. The fits to the SPIRE data give a median beta value slightly above 1.8. In the joint SPIRE and SCUBA-2 850um fits the value decreases to beta ~1.6. Most of the observed T-beta anticorrelation can be explained by noise. The typical submillimetre opacity spectral index beta of cold clumps is found to be ~1.7. This is above the values of diffuse clouds but lower than in some previous studies of dense clumps. There is only tentative evidence of T-beta anticorrelation and beta decreasing at millimetre wavelengths.
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