@article {2014ApJ...786..156H, title = {{A Multi-wavelength View of the Central Kiloparsec Region in the Luminous Infrared Galaxy NGC 1614}}, journal = {The Astrophysical Journal}, volume = {786}, year = {2014}, month = {may}, pages = {156}, abstract = {The Luminous Infrared Galaxy NGC 1614 hosts a prominent circumnuclear ring of star formation. However, the nature of the dominant emitting mechanism in its central \~{}100 pc is still under debate. We present sub-arcsecond angular resolution radio, mid-infrared, Pa{$\alpha$}, optical, and X-ray observations of NGC 1614, aimed at studying in detail both the circumnuclear ring and the nuclear region. The 8.4 GHz continuum emission traced by the Very Large Array and the Gemini/T-ReCS 8.7 {$μ$}m emission, as well as the Pa{$\alpha$} line emission, show remarkable morphological similarities within the star-forming ring, suggesting that the underlying emission mechanisms are tightly related. We used a Hubble Space Telescope/NICMOS Pa{$\alpha$} map of similar resolution to our radio maps to disentangle the thermal free-free and non-thermal synchrotron radio emission, from which we obtained the intrinsic synchrotron power law for each individual region within the central kiloparsec of NGC 1614. The radio ring surrounds a relatively faint, steep-spectrum source at the very center of the galaxy, suggesting that the central source is not powered by an active galactic nucleus (AGN), but rather by a compact (r {\l}t}\~{} 90 pc) starburst (SB). Chandra X-ray data also show that the central kiloparsec region is dominated by SB activity, without requiring the existence of an AGN. We also used publicly available infrared data to model-fit the spectral energy distribution of both the SB ring and a putative AGN in NGC 1614. In summary, we conclude that there is no need to invoke an AGN to explain the observed bolometric properties of the galaxy.}, keywords = {galaxies: individual: NGC 1614, galaxies: nuclei, galaxies: starburst, infrared: galaxies, radio continuum: galaxies, supernovae: general}, doi = {10.1088/0004-637X/786/2/156}, url = {http://adsabs.harvard.edu/abs/2014ApJ...786..156H}, author = {R. Herrero-Illana and M. A. P{\'e}rez-Torres and Alonso-Herrero, A. and Alberdi, A. and Colina, L. and Efstathiou, A. and Hern{\'a}ndez-Garc{\'\i}a, L. and Miralles-Caballero, D. and V{\"a}is{\"a}nen, P. and Packham, C.~C. and Rajpaul, V. and Zijlstra, A.~A.} } @article {2011A\&A...532A..83S, title = {{The mean infrared emission of Sagittarius A*}}, journal = {Astronomy and Astrophysics}, volume = {532}, year = {2011}, month = {aug}, pages = {A83}, abstract = {Context. The massive black hole at the center of the Milky Way, Sagittarius A* (Sgr A*) is, in relative terms, the weakest accreting black hole accessible to observations. It has inspired the theoretical models of radiatively inefficient accretion. Unfortunately, our knowledge of the mean SED and source structure of Sgr A* is very limited owing to numerous observational difficulties. At the moment, the mean SED of Sgr A* is only known reliably in the radio to mm regimes.
Aims: The goal of this paper is to provide constraints on the mean emission from Sgr A* in the near-to-mid infrared.
Methods: Sensitive images of the surroundings of Sgr A* at 8.6 {$μ$}m, 4.8 {$μ$}m , and 3.8 {$μ$}m were produced by combining large quantities of imaging data. Images were produced for several observing epochs. Excellent imaging quality was reached in the MIR by using speckle imaging combined with holographic image reconstruction, a novel technique for this kind of data.
Results: No counterpart of Sgr A* is detected at 8.6 {$μ$}m. At this wavelength, Sgr A* is located atop a dust ridge, which considerably complicates the search for a potential point source. An observed 3{$\sigma$} upper limit of \~{}10 mJy is estimated for the emission of Sgr A* at 8.6 {$μ$}m, a tighter limit at this wavelength than in previous work. The de-reddened 3{$\sigma$} upper limit, including the uncertainty of the extinction correction, is \~{}84 mJy . Based on the available data, it is argued that, with currently available instruments, Sgr A* cannot be detected in the MIR, not even during flares. At 4.8 {$μ$}m and 3.8 {$μ$}m, on the other hand, Sgr A* is detected at all times, at least when considering timescales of a few up to 13 min. We derive well-defined time-averaged, de-reddened flux densities of 3.8 {\plusmn} 1.3 mJy at 4.8 {$μ$}m and 5.0 {\plusmn} 0.6 mJy at 3.8 {$μ$}m. Observations with NIRC2/Keck and NaCo/VLT from the literature provide good evidence that Sgr A* also has a fairly well-defined de-reddened mean flux of 0.5-2.5 mJy at wavelengths of 2.1-2.2 {$μ$}m.
Conclusions: We present well-constrained anchor points for the SED of Sgr A* on the high-frequency side of the Terahertz peak. The new data are in general agreement with published theoretical SEDs of the mean emission from Sgr A*, but we expect them to have an appreciable impact on the model parameters in future theoretical work.}, keywords = {black hole physics, Galaxy: center, instrumentation: adaptive optics, instrumentation: high angular resolution}, doi = {10.1051/0004-6361/201116994}, url = {http://adsabs.harvard.edu/abs/2011A\%26A...532A..83S}, author = {Sch{\"o}del, R. and Morris, M.~R. and Muzi{\'c}, K. and Alberdi, A. and Meyer, L. and Eckart, A. and Gezari, D.~Y.} } @article {2011A\&A...531A..52M, title = {{A model for the thermal radio-continuum emission from radiative shocks in colliding stellar winds}}, journal = {Astronomy and Astrophysics}, volume = {531}, year = {2011}, month = {jul}, pages = {A52}, abstract = {Context. In massive-star binary systems, the interaction of the strong stellar winds results in a wind collision region (WCR) between the stars, which is limited by two shock fronts. Besides the nonthermal emission resulting from the shock acceleration, these shocks emit thermal (free-free) radiation detectable at radio frequencies that increase the expected emission from the stellar winds. Observations and theoretical studies of these sources show that the shocked gas is an important, but not dominant, contributor to the total emission in wide binary systems, while it plays a very substantial role in close binaries.
Aims: The interaction of two isotropic stellar winds is studied in order to calculate the free-free emission from the WCR. The effects of the binary separation and the wind momentum ratio on the emission from the wind-wind interaction region are investigated.
Methods: We developed a semi-analytical model for calculating the thermal emission from colliding stellar winds. Assuming radiative shocks for the compressed layer, which are expected in close binaries, we obtained the emission measure of the thin shell. Then, we computed the total optical depth along each line of sight to obtain the emission from the whole configuration.
Results: Here, we present predictions of the free-free emission at radio frequencies from analytic, radiative shock models in colliding wind binaries. It is shown that the emission from the WCR mainly arises from the optically thick region of the compressed layer and scales as \~{}D$^{4/5}$, where D is the binary separation. The predicted flux density S$_{nu}$ from the WCR becomes more important as the frequency {$ν$} increases, showing higher spectral indices than the expected 0.6 value (S$_{nu}$ {\prop} {$ν$}$^{alpha}$, where {$\alpha$} = 0.6) from the unshocked winds. We also investigate the emission from short-period WR+O systems calculated with our analytic formulation. In particular, we apply the model to the binary systems WR 98 and WR 113 and compare our results with the observations. Our theoretical results are in good agreement with the observed thermal spectra from these sources.}, keywords = {binaries: close, radiation mechanisms: thermal, radio continuum: stars, stars: Wolf-Rayet}, doi = {10.1051/0004-6361/201015988}, url = {http://adsabs.harvard.edu/abs/2011A\%26A...531A..52M}, author = {Montes, G. and Gonz{\'a}lez, R.~F. and Cant{\'o}, J. and M. A. P{\'e}rez-Torres and Alberdi, A.} } @article {2011A\&A...528A..30C, title = {{Molecules as tracers of galaxy evolution: an EMIR survey. I. Presentation of the data and first results}}, journal = {Astronomy and Astrophysics}, volume = {528}, year = {2011}, month = {apr}, pages = {A30}, abstract = {
Aims: We investigate the molecular gas properties of a sample of 23 galaxies in order to find and test chemical signatures of galaxy evolution and to compare them to IR evolutionary tracers.
Methods: Observation at 3 mm wavelengths were obtained with the EMIR broadband receiver, mounted on the IRAM 30 m telescope on Pico Veleta, Spain. We compare the emission of the main molecular species with existing models of chemical evolution by means of line intensity ratios diagrams and principal component analysis.
Results: We detect molecular emission in 19 galaxies in two 8 GHz-wide bands centred at 88 and 112 GHz. The main detected molecules are CO, $^{13}$CO, HCN, HNC, HCO$^{+}$, CN, and C$_{2}$H. We also detect HC$_{3}$N J = 10-9 in the galaxies IRAS 17208, IC 860, NGC 4418, NGC 7771, and NGC 1068. The only HC$_{3}$N detections are in objects with HCO$^{+}$/HCN {\l}t} 1. Galaxies with the highest HC$_{3}$N/HCN ratios have warm IRAS colours (60/100 {$μ$}m {\gt} 0.8). The brightest HC$_{3}$N emission is found in IC 860, where we also detect the molecule in its vibrationally excited state. We find low HNC/HCN line ratios ({\l}t}0.5), that cannot be explained by existing PDR or XDR chemical models. The intensities of HCO+ and HNC appear anti-correlated. No correlation is found between the HNC/HCN line ratio and dust temperature. All HNC-bright objects are either luminous IR galaxies (LIRG) or Seyferts. Galaxies with bright polycyclic aromatic hydrocarbons (PAH) emission show low HNC/HCO$^{+}$ ratios. The CO/$^{13}$CO ratio is positively correlated with the dust temperature and is generally higher than in our galaxy. The emission of CN and C$^{18}$O is correlated.
Conclusions: Bright HC$_{3}$N emission in HCO$^{+}$-faint objects may imply that these are not dominated by X-ray chemistry. Thus the HCN/HCO$^{+}$ line ratio is not, by itself, a reliable tracer of XDRs. Bright HC$_{3}$N and faint HCO$^{+}$ could be signatures of embedded star-formation, instead of AGN activity. Mechanical heating caused by supernova explosions may be responsible for the low HNC/HCN and high HCO$^{+}$/HCN ratios in some starbursts. We cannot exclude, however, that the discussed trends are largely caused by optical depth effects or excitation. Chemical models alone cannot explain all properties of the observed molecular emission. Better constraints to the gas spacial distribution and excitation are needed to distinguish abundance and excitation effects. Appendices are only available in electronic form http://www.aanda.org}, keywords = {evolution, galaxies: active, galaxies: evolution, galaxies: ISM, ISM: molecules}, doi = {10.1051/0004-6361/201015628}, url = {http://adsabs.harvard.edu/abs/2011A\%26A...528A..30C}, author = {F. Costagliola and Aalto, S. and M. Rodr{\'\i}guez and Muller, S. and Spoon, H.~W.~W. and Mart{\'\i}n, S. and M. A. P{\'e}rez-Torres and Alberdi, A. and Lindberg, J.~E. and Batejat, F. and J{\"u}tte, E. and van der Werf, P. and Lahuis, F.} }