Tuesday, 06/11/13

4 papers of interest today, goodness gracious!

The first paper (Costagliola et al.) investigates whether the compact nucleus of NGC 4418 (nearby LIRG) is fueled by a starburst or an AGN using mm and cm lines. Possibly relevant for investigating NGC 1266 in a similar way? The second paper (Pope et al.) discusses PAHs, IR and CO emission from z~1 ULIRGs. They find that for a given IR luminosity, L_PAH/L_IR is higher in the higher redshift galaxies. Bruce Draine and company apply his-type dust modeling to Andromeda in the third paper – will be the highest resolution dust maps of a giant galaxy, for, well, a good long while (excluding the MW). A fourth paper I’ve included mostly to see what Eric thinks – an exposition on measuring nebular temperatures by Nicholls, Dopita and crew. Enjoy!

Title: A high-resolution mm and cm study of the obscured LIRG NGC 4418 – A compact obscured nucleus fed by in-falling gas?
Authors: Costagliola, F.; Aalto, S.; Sakamoto, K.; Martín, S.; Beswick, R.; Muller, S.; Klöckner, H.-R.
Publication: eprint arXiv:1306.2211
Publication Date: 06/2013
Origin: ARXIV
Keywords: Astrophysics – Galaxy Astrophysics, Astrophysics – Cosmology and Extragalactic Astrophysics
Comment: 18 pages, 11 figures, Accepted for publication by A&A on 10/6/2013
Bibliographic Code: 2013arXiv1306.2211C

Abstract

The aim of this study is to constrain the dynamics, structure and feeding of the compact nucleous of NGC4418, and to reveal the nature of the main hidden power source: starburst or AGN. We obtained high spatial resolution observations of NGC4418 at 1.4 and 5 GHz with MERLIN, and at 230 and 270 GHz with the SMA very extended configuration. We use the continuum morphology and flux density to estimate the size of the emitting region, the star formation rate and the dust temperature. Emission lines are used to study the kinematics through position-velocity diagrams. Molecular emission is studied with population diagrams and by fitting an LTE synthetic spectrum. We detect bright 1mm line emission from CO, HC3N, HNC and C34S, and 1.4 GHz absorption from HI. The CO 2-1 emission and HI absorption can be fit by two velocity components at 2090 and 2180 km s-1. We detect vibrationally excited HC3N and HNC, with Tvib 300K. Molecular excitation is consistent with a layered temperature structure, with three main components at 80, 160 and 300 K. For the hot component we estimate a source size of less than 5 pc. The nuclear molecular gas surface density of 1e4 Msun pc-2 is extremely high, and similar to that found in the ultra-luminous infrared galaxy (ULIRG) Arp220. Our observations confirm the the presence of a molecular and atomic in-flow, previously suggested by Herschel observations, which is feeding the activity in the center of NGC4418. Molecular excitation confirms the presence of a very compact, hot dusty core. If a starburst is responsible for the observed IR flux, this has to be at least as extreme as the one in Arp220, with an age of 3-10 Myr and a star formation rate >10 Msun yr-1. If an AGN is present, it must be extremely Compton-thick.

Title: Probing the Interstellar Medium of z~1 Ultra-luminous Infrared Galaxies through Interferometric Observations of CO and Spitzer Mid-infrared Spectroscopy
Authors: Pope, Alexandra; Wagg, Jeff; Frayer, David; Armus, Lee; Chary, Ranga-Ram; Daddi, Emanuele; Desai, Vandana; Dickinson, Mark E.; Elbaz, David; Gabor, Jared;Kirkpatrick, Allison
Publication: eprint arXiv:1306.1831
Publication Date: 06/2013
Origin: ARXIV
Keywords: Astrophysics – Cosmology and Extragalactic Astrophysics, Astrophysics – Galaxy Astrophysics
Comment: 16 pages, 6 figures, 5 tables, accepted for publication in ApJ
Bibliographic Code: 2013arXiv1306.1831P

Abstract

We explore the relationship between gas, dust and star formation in a sample of 12 ultra-luminous infrared galaxies (ULIRGs) at high redshift compared to a similar sample of local galaxies. We present new CO observations and/or Spitzer mid-IR spectroscopy for 6 70 micron selected galaxies at z~1 in order to quantify the properties of the molecular gas reservoir, the contribution of an active galactic nuclei (AGN) to the mid-IR luminosity and the star formation efficiency (SFE=LIR/L’CO). The mid-IR spectra show strong polycyclic aromatic hydrocarbon (PAH) emission and our spectral decomposition suggests that the AGN makes a minimal contribution (<25%) to the mid-IR luminosity. The 70 micron selected ULIRGs which we find to be spectroscopic close pairs, are observed to have high SFE, similar to local ULIRGs and high redshift submillimeter galaxies, consistent with enhanced IR luminosity due to an ongoing major merger. Combined with existing observations of local and high redshift ULIRGs, we further compare the PAH, IR and CO luminosities. We show that the ratio LPAH6.2/LIR decreases with increasing IR luminosity for both local and high redshift galaxies but the trend for high redshift galaxies is shifted to higher IR luminosities; the average LPAH6.2/LIR ratio at a given LIR is ~3 times higher at high redshift. When we normalize by the molecular gas, we find this trend to be uniform for galaxies at all redshifts and that the molecular gas is correlated with the PAH dust emission.The similar trends seen in the [CII] to molecular gas ratios in other studies suggests that PAH emission, like [CII], continues to be a good tracer of photodissociation regions even at high redshift. Together the CO, PAH and far-IR fine structure lines should be useful for constraining the interstellar medium conditions in high redshift galaxies.

Title: Andromeda’s Dust
Authors: Draine, B. T.; Aniano, G.; Krause, Oliver; Groves, Brent; Sandstrom, Karin; Braun, Robert; Leroy, Adam; Klaas, Ulrich; Linz, Hendrik; Rix, Hans-Walter; Schinnerer, Eva;Schmiedeke, Anika; Walter, Fabian
Publication: eprint arXiv:1306.2304
Publication Date: 06/2013
Origin: ARXIV
Keywords: Astrophysics – Cosmology and Extragalactic Astrophysics, Astrophysics – Galaxy Astrophysics
Comment: 36 pages, 14 figures. Submitted to ApJ
Bibliographic Code: 2013arXiv1306.2304D

Abstract

Spitzer Space Telescope and Herschel Space Observatory imaging of M31 is used, with a physical dust model, to construct maps of dust surface density, dust-to-gas ratio, starlight heating intensity, and PAH abundance, out to R=25kpc. The global dust mass is M_d=5.4×10^7Msol, the global dust/H mass ratio is M_d/M_H=0.0081, and the global PAH abundance is <q_PAH>=0.039. The dust surface density has an inner ring at R=5.6kpc, a maximum at R=11.2kpc, and an outer ring at R=15.1kpc. The dust/gas ratio varies from M_d/M_H=0.026 at the center to 0.0027 at R=25kpc. From the dust/gas ratio, we estimate the ISM metallicity to vary by a factor ~10, from Z/Zsol=3 at R=0 to ~0.3 at R=25kpc. The dust heating rate parameter <U> peaks at the center, with <U> approx 35, declining to <U> approx 0.25 at R=20kpc. Within the central kpc, the starlight heating intensity inferred from the dust modeling is close to what is estimated from the stars in the bulge. The PAH abundance reaches a peak q_PAH=0.045 at R=11.2kpc. When allowance is made for the different spectrum of the bulge stars, q_PAH for the dust in the central kpc is similar to the overall value of q_PAH in the disk. The silicate-graphite-PAH dust model used here is generally able to reproduce the observed dust spectral energy distribution across M31, but overpredicts 500um emission at R=2-6kpc, suggesting that at R=2-6kpc, the dust opacity varies more steeply with frequency (with beta approx 2.3 between 200 and 600um) than in the model.

Title: Measuring nebular temperatures: the effect of new collision strengths with equilibrium and kappa-distributed electron energies
Authors: Nicholls, David C.; Dopita, Michael A.; Sutherland, Ralph S.; Kewley, Lisa J.; Palay, Ethan
Publication: eprint arXiv:1306.2023
Publication Date: 06/2013
Origin: ARXIV
Keywords: Astrophysics – Galaxy Astrophysics
Comment: 15 figures. Accepted for publication in ApJS
Bibliographic Code: 2013arXiv1306.2023N

Abstract

In this paper we develop tools for observers to use when analysing nebular spectra for temperatures and metallicities, with two goals: to present a new, simple method to calculate equilibrium electron temperatures for collisionally excited line flux ratios, using the latest atomic data; and to adapt current methods to include the effects of possible non-equilibrium ‘{\kappa}’ electron energy distributions. Adopting recent collision strength data for [O
iii], [S iii], [O ii], [S ii], and [N ii], we find that existing methods based on older atomic data seriously overestimate the electron temperatures, even when considering purely Maxwellian statistics. If {\kappa} distributions exist in H ii regions and planetary nebulae as they do in solar system plasmas, it is important to investigate the observational consequences. This paper continues our previous work on the {\kappa} distribution (Nicholls et al. 2012). We present simple formulaic methods that allow observers to (a) measure equilibrium electron temperatures and atomic abundances using the latest atomic data, and (b) to apply simple corrections to existing equilibrium analysis techniques to allow for possible non-equilibrium effects. These tools should lead to better consistency in temperature and abundance measurements, and a clearer understanding of the physics of H ii regions and planetary nebulae.

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