[en] We present 2''-10'' imaging of 11 transitions from 9 molecular species across the nuclear bar in Maffei 2. The data were obtained with the BIMA and OVRO interferometers. The 10 detected transitions are compared with existing CO isotopologues, HCN, CS, and millimeter continuum data. Dramatic spatial variations among the mapped species are observed across the nuclear bar. A principal component analysis is performed to characterize correlations between the transitions, star formation, and molecular column density. The analysis reveals that HCN, HNC, HCO⁺, and 3 mm continuum are tightly correlated, indicating a direct connection to massive star formation. We find two main morphologically distinct chemical groups, CH₃OH, SiO, and HNCO comprising the grain chemistry molecules, versus HCN, HNC, HCO⁺, and C₂H, molecules strong in the presence of star formation. The grain chemistry molecules, HNCO, CH₃OH, and SiO, trace hydrodynamical bar shocks. The near constancy of the HNCO/CH₃OH, SiO/CH₃OH, and SiO/HNCO ratios argues that shock properties are uniform across the nucleus. HCN/HCO⁺, HCN/HNC, HCN/CS, and HCN/CO ratios are explained primarily by variations in density. High HCO⁺/N₂H⁺ ratios are correlated with the C₂H line, suggesting that this ratio may be a powerful new dense photon-dominated region probe in external galaxies. C₂H reveals a molecular outflow along the minor axis. The morphology and kinematics of the outflow are consistent with an outflow age of 6-7 Myr.

[en] The composite galaxy NGC 4102 hosts a LINER nucleus and a starburst. We mapped NGC 4102 in the 12.8 μm line of [Ne II], using the echelon spectrometer TEXES on the NASA IRTF, to obtain a data cube with 1.''5 spatial, and 25 km s^-1 spectral, resolution. Combining near-infrared, radio, and the [Ne II] data shows that the extinction to the starburst is substantial, more than 2 mag at the K band, and that the neon abundance is less than half solar. We find that the star formation in the nuclear region is confined to a rotating ring or disk of 4.''3 (∼300 pc) diameter, inside the inner Lindblad resonance. This region is an intense concentration of mass, with a dynamical mass ∼3 x 10⁹ M_sun, and of star formation. The young stars in the ring produce the [Ne II] flux reported by Spitzer for the entire galaxy. The mysterious blue component of line emission detected in the near-infrared is also seen in [Ne II]; it is not a normal active galactic nucleus outflow.

[en] Astrochemistry is rapidly becoming one of the most powerful tools to study the structure and evolution of the central kiloparsec in spiral nuclei. Imagining the distribution of quiescent ion-molecule, photon-dominated region (PDR) and shock chemistry permits the triggers of nuclear starbursts to be identified and the bursts subsequent feedback to be constrained. Two new chemical methods for identifying the evolutionary phase of the starbursts in IC 342, NGC 6946 and Maffei 2 are discussed. The first method is to use the HCO⁺/N₂H⁺ ratio to constrain the degree of penetration of dense clumps by UV radiation. The second determines the evolutionary phase by mapping the amount and physical conditions of the densest molecular component via multi-transition HC₃N observations. With the full capabilities of radio facilities such as the VLA and ALMA, probing the changing gas chemistry on sub arcsecond scales in external galaxies will very soon be routine.

[en] We present the first images of the J = 5-4 and J = 16-15 lines of the dense gas tracer, cyanoacetylene, HC₃N, in an external galaxy. The central 200 pc of the nearby star-forming spiral galaxy, IC 342, was mapped using the Very Large Array and the Plateau de Bure Interferometer. HC₃N(5-4) line emission is found across the nuclear mini-spiral, but is very weak toward the starburst site, the location of the strongest mid-IR and radio emission. The J = 16-15 and 10-9 lines are also faint near the large H II region complex, but are brighter relative to the 5-4 line, consistent with higher excitation. The brightest HC₃N emission is located in the northern arm of the nuclear mini-spiral, 100 pc away from the radio/IR source to the southwest of the nucleus. This location appears less affected by ultraviolet radiation and may represent a more embedded, earlier stage of star formation. HC₃N excitation temperatures are consistent with those determined from C¹⁸O; the gas is dense 10⁴ - 10⁵ cm^-3 and cool, T_k < 40 K. So as to not violate limits on the total H₂ mass determined from C¹⁸O, at least two dense components are required to model IC 342's giant molecular clouds. These observations suggest that HC₃N(5-4) is an excellent probe of the dense, quiescent gas in galaxies. The high excitation combined with faint emission toward the dense molecular gas at the starburst indicates that it currently lacks large masses of very dense gas. We propose a scenario where the starburst is being caught in the act of dispersing or destroying its dense gas in the presence of the large H II region. This explains the high star formation efficiency seen in the dense component. The little remaining dense gas appears to be in pressure equilibrium with the starburst H II region.

[en] We present subarcsecond images of submillimeter CO and continuum emission from a local galaxy forming massive star clusters: the blue compact dwarf galaxy II Zw 40. At ∼0.″4 resolution (20 pc), the CO(3-2), CO(1-0), 3 mm, and 870 μ m continuum maps illustrate star formation on the scales of individual molecular clouds. Dust contributes about one-third of the 870 μ m continuum emission, with free–free accounting for the rest. On these scales, there is not a good correspondence between gas, dust, and free–free emission. Dust continuum is enhanced toward the star-forming region as compared to the CO emission. We suggest that an unexpectedly low and spatially variable gas-to-dust ratio is the result of rapid and localized dust enrichment of clouds by the massive clusters of the starburst.

[en] We present a survey of 3 mm molecular lines in IRAS 04296+2923, one of the brightest known molecular-line emitting galaxies, and one of the closest luminous infrared galaxies (LIRGs). Data are from the Owens Valley and CARMA millimeter interferometers. Species detected at ≲ 4'' resolution include C¹⁸O, HCN, HCO⁺, HNC, CN, CH₃OH, and, tentatively, HNCO. Along with existing CO, ¹³CO, and radio continuum data, these lines constrain the chemical properties of the inner disk. Dense molecular gas in the nucleus fuels a star formation rate ≳10 M _☉ yr^–1 and is traced by lines of HCN, HCO⁺, HNC, and CN. A correlation between HCN and star formation rate is observed on sub-kiloparsec scales, consistent with global relations. Toward the nucleus, CN abundances are similar to those of HCN, indicating emission comes from a collection (∼40-50) of moderate visual extinction, photon-dominated-region clouds. The CO isotopic line ratios are unusual: CO(1-0)/¹³CO(1-0) and CO(1-0)/C¹⁸O(1-0) line ratios are large toward the starburst, as is commonly observed in LIRGs, but farther out in the disk these ratios are remarkably low (≲ 3). ¹³CO/C¹⁸O abundance ratios are lower than in Galactic clouds, possibly because the C¹⁸O is enriched by massive star ejecta from the starburst. ¹³CO is underabundant relative to CO. Extended emission from CH₃OH indicates that dynamical shocks pervade both the nucleus and the inner disk. The unusual CO isotopologue ratios, the CO/HCN intensity ratio versus L _IR, the HCN/CN abundance ratio, and the gas consumption time versus inflow rate all indicate that the starburst in IRAS 04296+2923 is in an early stage of development.

[en] The nearby dwarf starburst galaxy NGC 5253 hosts a deeply embedded radio-infrared supernebula excited by thousands of O stars. We have observed this source in the 10.5 μm line of S⁺³ at 3.8 km s^–1 spectral and 1.''4 spatial resolution, using the high-resolution spectrometer TEXES on the IRTF. The line profile cannot be fit well by a single Gaussian. The best simple fit describes the gas with two Gaussians, one near the galactic velocity with FWHM 33.6 km s^–1 and another of similar strength and FWHM 94 km s^–1 centered ∼20 km s^–1 to the blue. This suggests a model for the supernebula in which gas flows toward us out of the molecular cloud, as in a 'blister' or 'champagne flow' or in the H II regions modelled by Zhu.

[en] We present a three-dimensional data cube of the K-band continuum and the Br γ, H₂ S(0), and S(1) lines within the central 18.''5 × 13.''8 (520 pc × 390 pc) region of NGC 6946. Data were obtained using OSIRIS, a near-infrared Integral Field Spectrograph at Keck Observatory, with Laser Guide Star Adaptive Optics. The 0.''3 resolution allows us to investigate the stellar bulge and the forming star clusters in the nuclear region on 10 pc scales. We detect giant H II regions associated with massive young star clusters in the nuclear spiral/ring (R ∼ 30 pc) and in the principal shocks along the nuclear bar. Comparisons of the Br γ fluxes with Pa α line emission and radio continuum indicate A_K ∼ 3, A_V ∼ 25 for the nuclear star-forming regions. The most luminous H II regions are restricted to within 70 pc of the center, despite the presence of high gas columns at larger radii (R ∼ 200 pc). H₂ emission is restricted to clouds within R ∼ 60 pc of the center, resembling the distribution of HCN line emission. We propose that gas-assisted migration of the young star clusters is contributing to the buildup of the nuclear bar and nuclear star cluster (R < 30 pc) in this galaxy

[en] We present 7 mm Very Large Array continuum images of the starburst galaxy M82. On arcsecond scales, two-thirds of the 7 mm continuum consists of free-free emission from H II regions. In the subarcsecond resolution map, we identify 14 compact sources, including 9 bright H II regions with N _lyc > 10⁵¹ s^-1. Four of the H II regions have rising spectra, implying emission measures >10⁸ cm^-6 pc. Except for one compact source with peculiar features, all other compact radio sources are found in dust lanes and do not have optical or near-infrared continuum counterparts. Four regions of extended, high brightness (EM >10⁷ cm^-6 pc) radio emission are found in our high-resolution map, including some as large as ∼2'', or 30 pc, representing either associations of small H II regions, or sheet-like structures of denser gas. The good correlation between 7 mm emission and Spitzer IRAC 8 μm continuum-removed polycyclic aromatic hydrocarbon (PAH) feature suggests that PAH emission may track the recently formed OB stars. We find an excellent correlation between molecular gas and star formation, particularly dense gas traced by HCN, down to the ∼45 pc scale in M82. We also find star formation efficiencies (SFEs) of 1%-10% on the same scale, based on CO maps. The highest SFE are found in regions with the highest dense gas fractions.

[en] We present observations of CO(3–2) and ¹³CO(3–2) emission near the supernebula in the dwarf galaxy NGC 5253, which contains one of the best examples of a potential globular cluster in formation. The 0.″3 resolution images reveal an unusual molecular cloud, “Cloud D1,” that is coincident with the radio-infrared supernebula. The ∼6 pc diameter cloud has a linewidth, Δ v = 21.7 $\mathrm{k}\mathrm{m}{\mathrm{s}}^{-<!--\; ???\; -->1}$, that reflects only the gravitational potential of the star cluster residing within it. The corresponding virial mass is 2.5 × 10⁵ $M_{\odot <!--\; ???\; -->}$. The cluster appears to have a top-heavy initial mass function, with M _* ≳ 1–2 $M_{\odot <!--\; ???\; -->}$. Cloud D1 is optically thin in CO(3–2), probably because the gas is hot. Molecular gas mass is very uncertain but constitutes <35% of the dynamical mass within the cloud boundaries. In spite of the presence of an estimated ∼1500–2000 O stars within the small cloud, the CO appears relatively undisturbed. We propose that Cloud D1 consists of molecular clumps or cores, possibly star-forming, orbiting with more evolved stars in the core of the giant cluster.