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Liu H.B.,Harvard - Smithsonian Center for Astrophysics | Liu H.B.,Academia Sinica, Taiwan | Liu H.B.,National Taiwan University | Quintana-Lacaci G.,Institute Radioastronomia Milimetrica | And 7 more authors.
Astrophysical Journal | Year: 2012

We observe the 1.2mm continuum emission around the OB cluster-forming region G10.6-0.4, using the MAMBO-2 bolometer array of the IRAM 30 m telescope and the Submillimeter Array (SMA). Comparison of the Spitzer 24 μm and 8 μm images with our 1.2mm continuum maps reveal an ionization front of an H II region, the photon-dominated layer, and several 5pc scale filaments that follow the outer edge of the photon-dominated layer. The filaments, which are resolved in the MAMBO-2 observations, show regularly spaced parsec-scale molecular clumps, embedded with a cluster of dense molecular cores as shown in the SMA 0.87mm observations. Toward the center of the G10.6-0.4 region, the combined SMA+IRAM 30 m continuum image reveals several parsec-scale protrusions. They may continue down to within 0.1pc of the geometric center of a dense 3pc scale structure, where a 200 M O OB cluster resides. The observed filaments may facilitate mass accretion onto the central cluster-forming region in the presence of strong radiative and mechanical stellar feedback. Their filamentary geometry may also facilitate fragmentation. We did not detect any significant polarized emission at 0.87mm in the inner 1pc region with SMA. © 2012. The American Astronomical Society. All rights reserved. Source


Okada Y.,University of Cologne | Pilleri P.,CSIC - National Institute of Aerospace Technology | Pilleri P.,Los Alamos National Laboratory | Berne O.,Toulouse 1 University Capitole | And 11 more authors.
Astronomy and Astrophysics | Year: 2013

Aims. We observationally investigate the relation between the photoelectric heating efficiency in photodissociation regions (PDRs) and the charge of polycyclic aromatic hydrocarbons (PAHs), which are considered to play a key role in photoelectric heating. Methods. Using PACS onboard Herschel, we observed six PDRs spanning a wide range of far-ultraviolet radiation fields (G0 = 100-105). To measure the photoelectric heating efficiency, we obtained the intensities of the main cooling lines in these PDRs, i.e., the [O i] 63 μm, 145 μm, and [C ii] 158 μm, as well as the far-infrared (FIR) continuum intensity. We used Spitzer/IRS spectroscopic mapping observations to investigate the mid-infrared (MIR; 5.5-14 μm) PAH features in the same regions. We decomposed the MIR PAH emission into that of neutral (PAH 0) and positively ionized (PAH+) species to derive the fraction of the positively charged PAHs in each region, and compare it to the photoelectric heating efficiency. Results. The heating efficiency traced by ([O i] 63 μm + [O i] 145 μm + [C ii] 158 μm)/TIR, where TIR is the total infrared flux, ranges between 0.1% and 0.9% in different sources, and the fraction of PAH+ relative to (PAH0+ PAH+) spans from 0 (+11)% to 87 (±10)%. All positions with a high PAH+ fraction show a low heating efficiency, and all positions with a high heating efficiency have a low PAH+ fraction, supporting the scenario in which a positive grain charge results in a decreased heating efficiency. Theoretical estimates of the photoelectric heating efficiency show a stronger dependence on the charging parameter γ = G0T1/2/ne than the observed efficiency reported in this study, and the discrepancy is significant at low γ. The photoelectric heating efficiency on PAHs, traced by ([O i] 63 μm + [O i] 145 μm + [C ii] 158 μm)/(PAH-band emission + [O i] 63 μm + [O i] 145 μm + [C ii] 158 μm), shows a much better match between the observations and the theoretical estimates. Conclusions. The good agreement of the photoelectric heating efficiency on PAHs with a theoretical model indicates the dominant contribution of PAHs to the photoelectric heating. This study demonstrates the fundamental role that PAHs have in photoelectric heating. More studies of their charging behavior are crucial to understand the thermal balance of the interstellar medium. © 2013 ESO. Source


Lebron M.,University of Puerto Rico at San Juan | Lebron M.,Max Planck Institute for Radio Astronomy | Mangum J.G.,U.S. National Radio Astronomy Observatory | Mauersberger R.,Joint Alma Observatory | And 7 more authors.
Astronomy and Astrophysics | Year: 2011

Context. The central few 100 pc of galaxies often contain large amounts of molecular gas. The chemical and physical properties of these extragalactic star formation regions differ from those in galactic disks, but are poorly constrained. Aims. This study aims to develop a better knowledge of the spatial distribution and kinetic temperature of the dense neutral gas associated with the nuclear regions of three prototypical spiral galaxies, NGC 253, IC 342, and Maffei 2. Methods. VLA CnD and D configuration measurements have been made of three ammonia (NH3) inversion transitions. Results. The (J,K) = (1, 1) and (2, 2) transitions of NH3 were imaged toward IC 342 and Maffei 2. The (3, 3) transition was imaged toward NGC 253. The entire flux obtained from single-antenna measurements is recovered for all three galaxies observed. Derived lower limits to the kinetic temperatures determined for the giant molecular clouds in the centers of these galaxies are between 25 and 50 K. There is good agreement between the distributions of NH3 and other H 2 tracers, such as rare CO isotopologues or HCN, suggesting that NH3 is representative of the distribution of dense gas. The "Western Peak" in IC 342 is seen in the (6, 6) line but not in lower transitions, suggesting maser emission in the (6, 6) transition. © 2011 ESO. Source


Aladro R.,European Southern Observatory | Martin S.,Institut Universitaire de France | Riquelme D.,Max Planck Institute for Radio Astronomy | Henkel C.,Max Planck Institute for Radio Astronomy | And 9 more authors.
Astronomy and Astrophysics | Year: 2015

Aims. We aim to better understand the imprints that the nuclear activity in galaxies leaves in the molecular gas. Methods. We used the IRAM 30? m telescope to observe the frequency range ∼[86-116] GHz towards the central regions of the starburst galaxies M? 83, M? 82, and NGC? 253, the galaxies hosting an active galactic nucleus (AGN) M? 51, NGC? 1068, and NGC? 7469, and the ultra-luminous infrared galaxies (ULIRGs) Arp 220 and Mrk 231. Assuming local thermodynamic equilibrium (LTE), we calculated the column densities of 27 molecules and 10 isotopologues (or their upper limits in case of non-detections). Results. Among others, we report the first tentative detections of CH3CHO, HNCO, and NS in M? 82 and, for the first time in the extragalactic medium, HC5N in NGC? 253. Hα recombination lines were only found in M? 82 and NGC? 253. Vibrationally excited lines of HC3N were only detected in Arp? 220. CH3CCH emission is only seen in the starburst-dominated galaxies. By comparison of the fractional abundances among the galaxies, we looked for the molecules that are best suited to characterise the chemistry of each group of galaxies (starbursts, AGNs and ULIRGs), as well as the differences among galaxies within the same group. Conclusions. Suitable species for characterising and comparing starburst galaxies are CH3OH and HNCO as tracers of large-scale shocks, which dominate early to intermediate starburst stages, and CH3CCH, c-C3H2, and HCO as tracers of UV fields, which control the intermediate-to-old or post starburst phases. M? 83 shows signs of a shock-dominated environment. NGC? 253 is characterised by both strong shocks and some UV fields. M? 82 stands out for its bright photo-dissociated region tracers, which indicate an UV field-dominated environment. Regarding AGNs, the abundances of HCN and CN (previously claimed as enhanced in AGNs) in M? 51 are similar to those in starburst galaxies, while the HCN/HCO+ ratio is high in M? 51 and NGC? 1068, but not in NGC? 7469. We did not find a correlation between the HCN/CS ratio (recently claimed as a possible starburst/AGN discriminator) and the AGN activity. However, a high enough spatial resolution to separate their circumnuclear disks from the surrounding star-forming regions is needed to find molecular abundance trends in AGNs. High abundances of H13CN and HC3N, as well as a similarity between the column densities of 13CO and C18O, are representative of the molecular interstellar medium in the ULIRGs. Furthermore, the chemistry of Arp? 220 points towards a more starburst-dominated environment, while that of Mrk? 231 more resembles the AGNs of our sample. © ESO 2015. Source


Aladro R.,European Southern Observatory | Aladro R.,University College London | Viti S.,University College London | Bayet E.,University of Oxford | And 7 more authors.
Astronomy and Astrophysics | Year: 2012

Aims. We study the molecular composition of the interstellar medium (ISM) surrounding an active galactic nucleus(AGN), by making an inventory of molecular species and their abundances, to establish a chemical differentiation between starburst galaxies and AGN. Methods. We used the IRAM-30m telescope to observe the central 1.5-2kpc region of NGC 1068, covering the frequencies between 86.2GHz and 115.6GHz. Using Boltzmann diagrams, we calculated the column densities of the detected molecules. We used a chemical model to reproduce the abundances found in the AGN, to determine the origin of each detected species, and to test the influence of UV fields, cosmic rays, and shocks on the ISM. Results. We identified 24different molecular species and isotopologues, among which HC3N, SO, N2H+, CH3CN, NS, 13CN, and HN13C are detected for the first time in NGC 1068. A comparison of the abundances in the nuclear regions of NGC 1068, M 82, and NGC 253 allowed us to establish a chemical differentiation between starburst galaxies and AGN. Two abundant species in starburst galaxies, H2CO and CH3CCH, are not detected in NGC 1068, probably because they are destroyed by UV fields or shocks. On the other hand, species such as CN, SiO, HCO+, and HCN, are enhanced by cosmic ray radiation fields. We obtained the upper limits to the isotopic ratios 12C/ 13C=49, 16O/18O=177, and 32S/ 34S=5. These ratios are much lower in this AGN than in starburst galaxies. Our chemical models suggest that the chemistry in the nucleus of NGC 1068 is strongly influenced by cosmic rays, although high values of both cosmic rays and far ultraviolet (FUV) radiation fields also explain the observations well. C-shocks can explain the abundances of C2H and H2CO, but do not strongly affect the abundances of the other detected species. Conclusions. The gas in the nucleus of NGC 1068 has a different chemical composition than starburst galaxies. The distinct physical processes dominating galaxy nuclei (e.g. C-shocks, UV fields, X-rays, cosmic rays) leave clear imprints in the chemistry of the gas, which allow the nucleus activity to be characterised by its molecular abundances. © ESO, 2012. Source

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