Understanding how molecules and dust might have formed within a rapidly expanding young supernova remnant is important because of the obvious application to vigorous supernova activity at very high redshift. In previous papers we have mapped the Crab Nebula in a roto-vibrational H 2 emission line, and then measured the molecular excitation temperature for a few of the brighter H 2 -emitting knots that we have found to be scattered throughout the Crab Nebula's filaments. We found that H 2 emission is often quite strong, correlates with optical low-ionization emission lines, and has a surprisingly high excitation temperature. Here we study Knot 51, a representative, bright example. It is a spatially isolated structure for which we have available long slit optical and NIR spectra covering emission lines from ionized, neutral, and molecular gas, as well as HST visible and SOAR Telescope NIR narrow-band images. We present a series of CLOUDY simulations to probe the excitation mechanisms, formation processes and dust content in environments that can produce the observed H 2 emission. There is still considerable ambiguity about the geometry of Knot 51, so we do not try for an exact match between model and observations. Rather, we aim to explain how the bright H 2 emission lines can be formed from within a cloud the size of Knot 51 that also produces the observed optical emission from ionized and neutral gas. Our models that are powered only by

We investigate the physical cause of the great range in the ionization level seen in the spectra of narrow lined active galactic nuclei (AGN). We used a recently developed technique called mean field independent component analysis to identify examples of individual SDSS galaxies whose spectra are not dominated by emission due to star formation (SF), which we therefore designate as AGN. We assembled high S/N ratio composite spectra of a sequence of these AGN defined by the ionization level of their narrow-line regions (NLR), and extending down to very low-ionization cases. We then used a local optimally emitting cloud (LOC) model to fit emission-line ratios in this AGN sequence, including the weak lines that can be measured only in the co-added spectra. These weak line ratios provide consistency checks on the density, temperature, abundances and ionizing continuum of Seyfert galaxies determined from strong-line ratios. After integrating over a wide range of clouds at different radii and densities, our models indicate that the radial extent of the NLR is the major parameter in determining the position of higher to moderate ionization AGN along our sequence. This provides a physical interpretation for their systematic variation. Higher ionization AGN contain optimally emitting clouds that are more concentrated towards the central continuum source than in lower ionization AGN. Our LOC models indicate that for the special set of objects that lie on our AGN sequence, the ionizing luminosity is anti-correlated with the NLR ionization level, and hence anticorrelated with the radial concentration and actual physical extent of the NLR. A possible interpretation that deserves further exploration is that the ionization sequence might be an age sequence where low ionization objects are older and have systematically cleared out their central regions by radiation pressure. We consider the alternative that our AGN sequence instead represents a mixing curve combining SF and AGN spectra in different proportions, but argue that while many galaxies in fact do have this type of composite spectra, our AGN sequence appears to be a special set of objects with negligible SF excitation.

We used K-band spectra to measure the H 2 excitation temperatures in six molecular knots associated with the filaments in the Crab Nebula. The temperatures are quite high -in the range T ∼ 2000-3000 K, just below the H 2 dissociation temperature. This is the temperature range over which the H 2 1-0 S(1) line at λ2.121 μm has its maximum emissivity per unit mass, so there may be many additional H 2 cores with lower temperatures that are too faint to detect. We also measured the electron density in adjacent ionized gas, which on the assumption of gas pressure balance indicates densities in the molecular region n mol ∼ 20 000 H baryons cm −3 , although this really is just a lower limit since the H 2 gas may be confined by other means. The excited region may be just a thin skin on a much more extensive blob of molecular gas that does not have the correct temperature and density to be as easily detectable. At the opposite extreme, the observed knots could consist of a fine mist of molecular gas in which we are detecting essentially all of the H 2 . Future CO observations could distinguish between these two cases. The Crab filaments serve as the nearby laboratories for understanding the very much larger filamentary structures that have formed in the intracluster medium of cool-core galaxy clusters.

We present an analysis of the optical spectra of narrow emission-line galaxies, based on mean field independent component analysis (MFICA), a blind source separation technique. Samples of galaxies were drawn from the Sloan Digital Sky Survey (SDSS) and used to generate compact sets of 'continuum' and 'emission-line' component spectra. These components can be linearly combined to reconstruct the observed spectra of a wider sample of galaxies. Only 10 components -five continuum and five emission line -are required to produce accurate reconstructions of essentially all narrow emission-line galaxies to a very high degree of accuracy; the median absolute deviations of the reconstructed emission-line fluxes, given the signal-to-noise ratio (S/N) of the observed spectra, are 1.2-1.8σ for the strong lines. After applying the MFICA components to a large sample of SDSS galaxies we identify the regions of parameter space that correspond to pure star formation and pure active galactic nucleus (AGN) emission-line spectra, and produce high S/N reconstructions of these spectra.The physical properties of the pure star formation and pure AGN spectra are investigated by means of a series of photoionization models, exploiting the faint emission lines that can be measured in the reconstructions. We are able to recreate the emission line strengths of the most extreme AGN case by assuming the central engine illuminates a large number of individual clouds with radial distance and density distributions, f (r) ∝ r γ and g(n) ∝ n β , respectively. The best fit is obtained with γ = −0.75 and β = −1.4. From the reconstructed star formation spectra we are able to estimate the starburst ages. These preliminary investigations serve to demonstrate the success of the MFICA-based technique in identifying distinct emission sources, and its potential as a tool for the detailed analysis of the physical properties of galaxies in large-scale surveys.

Current observational facilities have yet to conclusively detect 103–104 M ⊙ intermediate-mass black holes (IMBHs) that fill in the evolutionary gap between seed black holes in the early universe and z ∼ 0 supermassive black holes. Dwarf galaxies present an opportunity to reveal active IMBHs amidst persistent star formation. We introduce photoionization simulations tailored to address key physical uncertainties: coincident versus noncoincident mixing of IMBH and starlight excitation, open versus closed geometries of surrounding gas clouds, and different shapes of the spectral energy distribution of active galactic nuclei (AGN). We examine possible AGN emission line diagnostics in the optical and mid-IR, and find that the diagnostics are often degenerate with respect to the investigated physical uncertainties. In spite of these setbacks, and in contrast to recent work, we are able to show that [O iii]/Hβ typically remains bright for dwarf AGN powered by IMBHs down to 103 M ⊙. Dwarf AGN are predicted to have inconsistent star-forming and Seyfert/LINER classifications using the most common optical diagnostics. In the mid-IR, [O iv] 25.9 μm and [Ar ii] 6.98 μm are less sensitive to physical uncertainties than are optical diagnostics. Based on these emission lines, we provide several diagrams of mid-IR emission line diagnostic diagrams with demarcations for separating starbursts and AGN with varying levels of activity. The diagrams are valid over a wide range of ionization parameters and metallicities out to z ∼ 0.1, so will prove useful for future JWST observations of local dwarf AGN in the search for IMBHs. We make our photoionization simulation suite freely available.