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Context. Compact Obscured Nuclei (CONs) are an extreme phase of galaxy evolution where rapid supermassive black hole growth and$/$or compact star-forming activity is completely obscured by gas and dust. Aims. We investigate the properties of CONs in the mid-infrared and explore techniques aimed at identifying these objects such as through the equivalent width (EW) ratios of their Polycyclic Aromatic Hydrocarbon (PAH) features. Methods. We model Spitzer spectra by decomposing the continua into nuclear and star-forming components from which we then measure the nuclear optical depth, $\tau_N$, of the $9.8 \mu$m silicate absorption feature. We also use Spitzer spectral maps to investigate how PAH EW ratios vary with aperture size for objects hosting CONs. Results. We find that the nuclear optical depth, $\tau_N$, strongly correlates with the HCN-vib emission line in the millimetre for CONs with a Pearson correlation coefficient of 0.91. We find the PAH EW ratios technique to be effective at selecting CONs and robust against highly inclined galaxies where strong dust lanes may mimic a CON like spectrum by producing a high $\tau_N$. Our analysis of the Spitzer spectral maps showed that the efficacy of the PAH EW ratios to isolate CONs is reduced when there is a strong star-forming component from the host galaxy. In addition, we find that the use of the inferred nuclear optical depth is a reliable method to identify CONs in $36^{+8}_{-7}\%$ of ULIRGs and $17^{+3}_{-3}\%$ of LIRGs, consistent with previous work. Conclusions. We confirm mid-IR spectra to be a powerful diagnostic of CONs where the increased sensitivity of JWST will allow identification of CONs at cosmic noon revealing this extreme but hidden phase of galaxy evolution., Comment: Accepted for publication in A&A. 23 pages, 15 figures, 3 tables

We present Gran Telescopio CANARIAS CanariCam 8.7$��$m imaging and 7.5-13$��$m spectroscopy of six local systems known to host an active galactic nucleus (AGN) and have nuclear star formation. Our main goal is to investigate whether the molecules responsible for the 11.3$��$m polyclyclic aromatic hydrocarbon (PAH) feature are destroyed in the close vicinity of an AGN. We detect 11.3$��$m PAH feature emission in the nuclear regions of the galaxies as well as extended PAH emission over a few hundred parsecs. The equivalent width (EW) of the feature shows a minimum at the nucleus but increases with increasing radial distances, reaching typical star-forming values a few hundred parsecs away from the nucleus. The reduced nuclear EW are interpreted as due to increased dilution from the AGN continuum rather than destruction of the PAH molecules. We conclude that at least those molecules responsible for the 11.3$��$m PAH feature survive in the nuclear environments as close as 10pc from the AGN and for Seyfert-like AGN luminosities. We propose that material in the dusty tori, nuclear gas disks, and/or host galaxies of AGN is likely to provide the column densities necessary to protect the PAH molecules from the AGN radiation field., Accepted for publication in MNRAS, 19 pages, 13 figures

We present mid-infrared (MIR) 8-13micron spectroscopy of the nuclear regions of the interacting galaxy Arp299 (IC694+NGC3690) obtained with CanariCam (CC) on the 10.4m Gran Telescopio Canarias (GTC). The high angular resolution (~0.3-0.6arcsec) of the data allows us to probe nuclear physical scales between 60 and 120pc, which is a factor of 10 improvement over previous MIR spectroscopic observations of this system. The GTC/CC spectroscopy displays evidence of deeply embedded Active Galactic Nucleus (AGN) activity in both nuclei. The GTC/CC nuclear spectrum of NGC3690/Arp299-B1 can be explained as emission from AGN-heated dust in a clumpy torus with both a high covering factor and high extinction along the line of sight. The estimated bolometric luminosity of the AGN in NGC3690 is 3.2(+/-0.6)x10^44 erg/s. The nuclear GTC/CC spectrum of IC694/Arp299-A shows 11.3micron polycyclic aromatic hydrocarbon (PAH) emission stemming from a deeply embedded (A_V~24mag) region of less than 120pc in size. There is also a continuum-emitting dust component. If associated with th putative AGN in IC694, we estimate that it would be approximately 5 times less luminous than the AGN in NGC3690. The presence of dual AGN activity makes Arp299 a good example to study such phenomenon in the early coalescence phase of interacting galaxies., Comment: Accepted for publication in ApJ Letters

The masses and temperatures of young low mass stars and brown dwarfs in star- forming regions are not yet well established because of uncertainties in the age of individual objects and the spectral type vs. temperature scale appropriate for objects with ages of only a few Myr. Using multi-object optical spectroscopy, 45 low-mass stars and brown dwarfs in the Trapezium Cluster in Orion have been classified and 44 of these confirmed as bona fide cluster members. The spectral types obtained have been converted to effective temperatures using a temperature scale intermediate between those of dwarfs and giants, which is suitable for young pre-main sequence objects. The objects have been placed on an H-R diagram overlaid with theoretical isochrones. The low mass stars and the higher mass substellar objects are found to be clustered around the 1 Myr isochrone, while many of the lower mass substellar objects are located well above this isochrone. An average age of 1 Myr is found for the majority of the objects. Assuming coevality of the sources and an average age of 1 Myr, the masses of the objects have been estimated and range from 0.018 to 0.44Msun. The spectra also allow an investigation of the surface gravity of the objects by measurement of the sodium doublet equivalent width. With one possible exception, all objects have low gravities, in line with young ages, and the Na indices for the Trapezium objects lie systematically below those of young stars and brown dwarfs in Chamaeleon, suggesting that the 820 nm Na index may provide a sensitive means of estimating ages in young clusters., Comment: 19 pages, accepted by MNRAS

The dusty molecular torus, cornerstone of the unified model of AGN, has been the subject of much observational and theoretical scrutiny in recent years, and much progress has been made with observations and models aiming to reveal, explain and predict its properties. ©2008: Instituto de Astronomía.

We present 10 and 20-μm spectroscopic observations of a sample of F- and G-type carbon-rich post-AGB objects, three of which were known to exhibit the unidentified emission feature at 21 μm. We also find the F3I post-AGB object SAO 163075 to exhibit a (weak) 21-μm emission feature. We additionally obtained a 10-μm spectrum of IRAS 05341+0852, which has been reported to have a possible 21-μm emission band in its IRAS LRS spectrum, and obtained new 10 and 20-μm spectra of the carbon-rich bipolar post-AGB sources GL 2688 and GL 618, the extreme carbon star GL 3068, and the planetary nebulae IRAS 21282+5050 and He 2-447, in order to study the evolution of C-rich dust from the early post-AGB through to the planetary nebula (PN) phases. The 7.5-13-μm spectra of the 21-μm band objects exhibit broad plateau emission, shortwards of 9μm and from 10-13μm, superposed upon which, in addition to the well-known UIR-band at 11.3 μm, are several new features, at 7.9, 8.2, 10.6, 11.5 and 12.2 μm, differing from those observed in standard UIR-band spectra. An excellent match is found between the wavelengths of these new features and those of bands in the spectrum of chrysene (C18H12), one of the simplest PAH molecules. The absence of the new features in the spectra of earlier spectral-type post-AGB objects and PN is consistent with the expected complete dehydrogenation of any PAH molecules having less than 20-25 carbon atoms when exposed to the ultraviolet radiation fields of stars with spectral types earlier than F. Chrysene is not responsible for the 21-μm emission bands observed in the spectra of the cool post-AGB objects. Possible identifications for the 21-μm band in terms of highly hydrogenated 2-dimensional PAH molecules or 3-dimensional fulleranes (hydrogenated fullerenes) are discussed. The mid-infrared spectrum of GL 2688 is largely featureless, apart from two broad weak emission features between 9.5-10.5 and 10.5-12.2 μm. The profiles of the broad 10-13-μm absorption features in the spectra of the extreme carbon star GL 3068 and the C-rich bipolar post-AGB object GL 618 are compared. For GL 3068 the profile shape and the wavelength of peak absorption, near 11 μm, are consistent with absorption by silicon carbide particles. However, the absorption observed towards GL 618 is considerably broader and peaks at 12 μm. Its profile is very similar to that of the 10-13-μm emission plateau observed in the spectra of the 21-μm band objects, suggesting that it arises from absorption by a large column of highly-hydrogenated PAH-type species.