Tuesday 6/25/13

Sorry – late!

Title: Herschel-SPIRE-Fourier Transform Spectroscopy of the nearby spiral galaxy IC342
Authors: Rigopoulou, D.; Hurley, P. D.; Swinyard, B. M.; Virdee, J.; Croxall, K. V.; Hopwood, R. H. B.; Lim, T.; Magdis, G. E.; Pearson, C. P.; Pellegrini, E.;Polehampton, E.; Smith, J-D.
Publication: eprint arXiv:1306.5485
Publication Date: 06/2013
Origin: ARXIV
Keywords: Astrophysics – Galaxy Astrophysics, Astrophysics – Cosmology and Extragalactic Astrophysics
Comment: 9 pages, 8 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society (MNRAS)
Bibliographic Code: 2013arXiv1306.5485R


We present observations of the nearby spiral galaxy IC342 with the Herschel Spectral and Photometric Imaging Receiver (SPIRE) Fourier Transform Spectrometer. The spectral range afforded by SPIRE, 196-671 microns, allows us to access a number of 12CO lines from J=4–3 to J=13–12 with the highest J transitions observed for the first time. In addition we present measurements of 13CO, [CI] and [NII]. We use a radiative transfer code coupled with Bayesian likelihood analysis to model and constrain the temperature, density and column density of the gas. We find two 12CO components, one at 35 K and one at 400 K with CO column densities of 6.3×10^{17} cm^{-2} and 0.4×10^{17} cm^{-2} and CO gas masses of 1.26×10^{7} Msolar and 0.15×10^{7} Msolar, for the cold and warm components, respectively. The inclusion of the high-J 12CO line observations, indicate the existence of a much warmer gas component (~400 K) confirming earlier findings from H_{2} rotational line analysis from ISO and Spitzer. The mass of the warm gas is 10% of the cold gas, but it likely dominates the CO luminosity. In addition, we detect strong emission from [NII] 205microns and the {3}P_{1}->{3}P_{0} and {3}P_{2} ->{3}P_{1} [CI] lines at 370 and 608 microns, respectively. The measured 12CO line ratios can be explained by Photon-dominated region (PDR) models although additional heating by e.g. cosmic rays cannot be excluded. The measured [CI] line ratio together with the derived [C] column density of 2.1×10^{17} cm^{-2} and the fact that [CI] is weaker than CO emission in IC342 suggests that [CI] likely arises in a thin layer on the outside of the CO emitting molecular clouds consistent with PDRs playing an important role.

Title: On column density thresholds and the star formation rate
Authors: Clark, Paul C.; Glover, Simon C. O.
Publication: eprint arXiv:1306.5714
Publication Date: 06/2013
Origin: ARXIV
Keywords: Astrophysics – Galaxy Astrophysics
Comment: 18 pages and 14 figures. Submitted to MNRAS
Bibliographic Code: 2013arXiv1306.5714C


We present the results of a numerical study designed to address the question of whether there is a column density threshold for star formation within molecular clouds. We have simulated a large number of different clouds, with volume and column densities spanning a wide range of different values, using a state-of-the-art model for the coupled chemical, thermal and dynamical evolution of the gas. We show that for low-mass clouds, around 1000 solar masses and below, star formation is only possible if the mean cloud column density exceeds 10^21 cm^-2. In more massive clouds, the required mean column density is a factor of a few lower. We demonstrate that this behaviour is well-described by a simple Jeans mass argument: clouds must contain multiple Jeans masses in order to form stars, and hence star-forming clouds cannot have arbitrarily low column densities. We have also examined the question of whether there is a column density threshold for the regions within clouds where star formation occurs. We show that there is a good correlation between the mass of gas above a K-band extinction A_K = 0.8 and the star formation rate (SFR), in agreement with recent observational work. Previously, this relationship has been explained in terms of a correlation between the SFR and the mass in dense gas. However, we find that in our simulations, this correlation is weaker and more time-dependent than that between the SFR and the column density. We argue that this points to dust shielding as the key process: the true correlation is one between the SFR and the mass in cold, well-shielded gas, and the latter correlates better with the column density than the volume density.

Title: PAH and Mid-Infrared Continuum Emission in a z>4 Submillimeter Galaxy
Authors: Riechers, Dominik A.; Pope, Alexandra; Daddi, Emanuele; Armus, Lee; Carilli, Christopher L.; Walter, Fabian; Hodge, Jacqueline; Chary, Ranga-Ram; Morrison, Glenn E.; Dickinson, Mark; Dannerbauer, Helmut; Elbaz, David
Publication: eprint arXiv:1306.5235
Publication Date: 06/2013
Origin: ARXIV
Keywords: Astrophysics – Cosmology and Extragalactic Astrophysics
Comment: 7 pages, 3 figures, submitted to ApJ
Bibliographic Code: 2013arXiv1306.5235R


We report the detection of 6.2um polycyclic aromatic hydrocarbon (PAH) and rest-frame 4-7um continuum emission in the z=4.055 submillimeter galaxy GN20, using the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. This represents the first detection of PAH emission at z>4. The strength of the PAH emission feature is consistent with a very high star formation rate of ~1800Msun/yr. We find that this intense starburst powers at least ~1/3 of the faint underlying 6um continuum emission, with an additional, significant (and perhaps dominant) contribution due to a power-law-like hot dust source, which we interpret to likely be a faint, dust-obscured active galactic nucleus (AGN). Despite the strong power-law component enhancing the mid-infrared continuum emission, the intense starburst associated with the photon-dominated regions that give rise to the PAH emission appears to dominate the total energy output in the infrared. By comparing the 6um AGN continuum luminosity to an upper limit on the hard X-ray emission as measured by the Chandra X-Ray Observatory, we also find evidence that the previously undetected AGN in this source is Compton-thick, consistent with the finding at optical/infrared wavelengths that the galaxy and its nucleus are heavily dust-obscured. GN20 is one of the most luminous starburst galaxies known at any redshift, embedded in a rich protocluster of star-forming galaxies. This investigation provides an improved understanding of the energy sources that power such exceptional systems, which represent the extreme end of massive galaxy formation at early cosmic times.

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