The 3.5 meter diameter Herschel Space Observatory conducted a ∼720 square-degree survey of the Galactic plane, the Herschel Galactic plane survey (Hi-GAL). These data provide the most sensitive and highest resolution observations of the far-IR to sub-mm continuum from the central molecular zone (CMZ) at λ = 70, 160, 250, 350, and 500 μm obtained to date. Hi-GAL can be used to map the distributions of temperature and column density of dust in CMZ clouds, warm dust in Hii regions, and identify highly embedded massive protostars and clusters and the dusty shells ejected by supergiant stars. These data enable classification of sources and re-evaluation of the current and recent star-formation rate in the CMZ. The outer CMZ beyond |l| = 0.9 degrees (Rgal > 130 pc) contains most of the dense (n > 104 cm−3 gas in the Galaxy but supports very little star formation. The Hi-GAL and Spitzer data show that almost all star formation occurs in clouds moving on x2 orbits at Rgal < 100 pc. While the 106 M⊙ Sgr B2 complex, the 50 km s−1 cloud near Sgr A, and the Sgr C region are forming clusters of massive stars, other clouds are relatively inactive star formers, despite their high densities, large masses, and compact sizes. The asymmetric distribution of dense gas about Sgr A* on degree scales (most dense CMZ gas and dust is at positive Galactic longitudes and positive VLSR) and compact 24 μm sources (most are at negative longitudes) may indicate that eposidic mini-starbursts occasionally ‘blow-out’ a portion of the gas on these x2 orbits. The resulting massive-star feedback may fuel the compact 30 pc scale Galactic center bubble associated with the Arches and Quintuplet clusters, the several hundred pc scale Sofue-Handa lobe, and the kpc-scale Fermi/LAT bubble, making it the largest ‘superbubble’ in the Galaxy. A consequence of this model is that in our Galaxy, instead of the supermassive black hole (SMBH) limiting star formation, star formation may limit the growth of the SMBH.

Studying the molecular phase of the interstellar medium in galaxy nuclei is fundamental for the understanding of the onset and evolution of star formation and the growth of supermassive black holes. We can use molecules as observational tools exploiting them as tracers of chemical, physical and dynamical conditions. The molecular physical conditions in galaxy centers show large variety among galaxies, but in general the average gas densities (traced by e.g. HCN) and temperatures (probed by e.g. H2CO, NH3) are greater than in their disks. Molecular gas and dust is being funneled to the centers of galaxies by spiral arms, bars, and interactions - and one example of this is the minor merger NGC1614. Gas surface densities are also greater in galaxy nuclei and in extreme cases they become orders of magnitudes larger than what we find in the center of our own Milky Way. We can use IR excited molecular emission to probe the very inner regions of galaxies with deeply obscured nuclei where N(H2)>1024 cm−2 - for example the luminous infrared galaxy (LIRG) NGC4418. Abundances of key molecules such as HCN, HCO+, HNC, HC3N, CN, H3O+ are important tools in identifying the nature of buried activity and its evolution. Standard astrochemical scenarios (including X-ray Dominated regions (XDRs) and Photon Dominated Regions (PDRs)) are briefly discussed in this review and how we can use molecules to distinguish between them. High resolution studies are often necessary to separate effects of excitation and radiative transfer from those of chemistry - one example is absorption and effects of stimulated emission in the ULIRG Arp220. The nuclear activity in luminous galaxies often drives outflows and winds and in some cases molecular gas is being entrained in the outflows. Sometimes the molecular gas is carrying the bulk of the momentum. We can study the structure and physical conditions of the molecular gas to constrain the mass outflow rates and the evolution and nature of the driving source and two examples are discussed here: NGC1377 and Mrk231.

The vicinity of the Galactic center harbors many potential accelerators of cosmic rays (CR) that could shine in very-high-energy (VHE) γ-rays, such as pulsar wind nebulae, supernova remnants, binary systems and the central black hole Sgr A*, and is characterized by high gas density, large magnetic fields and a high rate of starburst activity similar to that observed in the core of starburst galaxies. In addition to these astrophysical sources, annihilation of putative WIMPs concentrated in the gravitational well could lead to significant high-energy emission at the Galactic center. The Galactic center region has been observed by atmospheric Cherenkov telescopes, and in particular by the H. E. S. S. array in Namibia for the last ten years above 150 GeV. This large data set, comprising more than 200 hours of observations, led to the discovery of a point-like source spatially compatible with the supermassive black hole Sgr A*, and to an extended diffuse emission, correlated with molecular clouds and attributed to the interaction of cosmic rays with the interstellar medium. Over the same time period, two starburst galaxies, namely M 82 and NGC 253, were detected at TeV energies after very deep exposures. Results from these ten years of observations of the Galactic center region and starburst galaxies at TeV energies are presented, and implications for the various very-high-energy emission mechanisms are discussed.

Barred galaxies contain substructures such as a pair of dust lanes and nuclear rings, with the latter being sites of intense star formation. We study the substructure formation as well as star formation in nuclear rings using numerical simulations. We find that nuclear rings form not by the Lindblad resonances, as previously thought, but by the centrifugal barrier that inflowing gas along dust lanes cannot overcome. This predicts a smaller ring in a more strongly barred galaxy, consistent with observations. Star formation rate (SFR) in a nuclear ring is determined primarily by the mass inflow rate to the ring. In our models, the SFR typically shows a short strong burst associated with the rapid gas infall and stays very small for the rest of the evolution. When the SFR is low, ages of young star clusters exhibit an azimuthal gradient along the ring since star formation takes place mostly near the contact points between the dust lanes and the nuclear ring. When the SFR is large, on the other hand, star formation is widely distributed throughout the whole length of the ring, with no apparent age gradient of star clusters. Since observed ring star formation appears long-lived with episodic bursts, our results suggest that the bar region should be replenished continually with fresh gas from outside.

We present 10 μm – 35μm Spitzer spectra of the interstellar medium in the central molecular zone (CMZ), the central 210 pc × 60 pc of the Galactic center (GC). We present maps of the CMZ in ionic and H2 emission, covering a more extensive area than earlier spectroscopic surveys in this region. We compare diagnostic line ratios measured in the Spitzer Infrared Nearby Galaxies Survey to our data. Previous work shows that forbidden line ratios can distinguish star-forming galaxies from low-ionization nuclear emission-line regions (LINERs) and active galactic nuclei (AGNs). Our GC line ratios agree with star-forming galaxies and not with LINERs or AGNs.

Recently, Clarkson et al. (2012) measured the intrinsic velocity dispersion of the Arches cluster, a young and massive star cluster in the Galactic center. Using the observed velocity dispersion profile and the surface brightness profile of Espinoza et al. (2009), they estimate the cluster's present-day mass to be ∼ 1.5×104 M⊙ by fitting an isothermal King model. In this study, we trace the best-fit initial mass for the Arches cluster using the same observed data set and also the anisotropic Fokker-Planck calculations for the dynamical evolution.

The Milky Way appears as a typical barred spiral, and comparisons can be made between its nuclear region and those of structurally similar nearby spirals. Maffei 2, M83, IC 342 and NGC 253 are nearby systems whose nuclear region properties contrast with those of the Milky Way. Stellar masses derived from NIR photometery, molecular gas masses and star formation rates allow us to assess the evolutionary states of this set of nuclear regions. These data suggest similarities between nuclear regions in terms of their stellar content while highlighting significant differences in current star formation rates. In particular current star formation rates appear to cover a larger range than expected based on the molecular gas masses. This behavior is consistent with nuclear region star formation experiencing episodic variations. Under this hypothesis the Milky Way's nuclear region currently may be in a low star formation rate phase.

The Galactic center (GC), being the closest nucleus, holds a position of privilege in the study of galaxy centers, but because it is edge-on and hidden behind 30 magnitudes of visual extinction it is often difficult to understand the overall structure of the region. Nearby galactic nuclei potentially provide a guide to understanding the large-scale structure of the GC. High resolution maps of molecular line emission along with radio and optical continuum towards the nucleus of the nearby, face-on spiral IC 342 are discussed. Attention is focused on a comparison of the large-scale morphology, gas chemistry, and star formation between the two nuclei. The case is made that IC 342 is one of the best extragalactic templates for the GC. Both have a star formation rate within a factor of two of each other and an ISM morphology characterized by a R 300 pc central molecular zone formed from a pair of arms laced with a collection of dense star forming molecular clouds. IC 342 also exhibits a nuclear cluster and associated circumnuclear disk. Whether the nuclear morphology and chemistry in IC 342 is an extension of the disk bar, a separate nuclear bar, or results from radiative/mechanical feedback remains unsettled.

We present a data cube of the [NeII] (12.8 μm) emission from the inner 2 pc of Sgr A West with 1″ and 4 km s−1 resolution, and with substantially better SNR and velocity resolution than previous observations of the ionized gas. We compare the observations to two proposed models of the gas motions and distribution: flows along tidally stretched streamers, and more nearly circular motions with density wave compression. The density wave model provides a considerably better fit to the kinematics of the northern arm and western arc. Neither model fits the eastern arm and bar kinematics well.

To help understand the origin of the spiral pattern we calculated orbits in the potential of a black hole in a star cluster and find that the orbits naturally evolve to set up a one-armed spiral wave very similar to that observed, both spatially and kinematically. Magnetic or other perturbing forces may influence the formation of the spiral wave, but self gravity is not required. Because a density wave evolves on the orbit precession timescale, rather than the orbital timescale, a wave pattern should persist for several 105 yr. No net inward motion of the gas is required by the model. If there is inflow, it is much smaller than is suggested by the infalling streamer model.

We present Herschel/HIFI sub-mm atomic carbon ([Ci] 3P1 – 3P0 and [Ci] 3P2 – 3P1), ionized carbon ([Cii] 2P3/2 – 2P1/2), and ionized nitrogen ([Nii] 3P1 – 3P0) line observations obtained in the frame of the Herschel Guaranteed Time HEXGAL (Herschel EXtraGALactic) key program (P. I. Rolf Güsten, MPIfR), and NANTEN2/SMART carbon monoxide (CO(J = 4 – 3)) observations of the warm gas around the Sgr A region. The spectrally resolved emission from all lines, and the corresponding line intensity ratios, show a very complex morphology. The determination of spatial and spectral (anti)correlation with known sources in the Sgr A region such as the Arched Filaments, NTF filaments, the Sickle, Quintuplet cluster, CND clouds, is ongoing work.

The diffuse interstellar bands (DIBs) are a set of absorption features, some of which are broad (“diffuse”), that are formed in the diffuse ISM. Since their discovery nearly a century ago their numbers have increased to over 500. The strongest of these are known to be ubiquitous in the universe. There is general consensus that they are produced by large carbon-bearing molecules; however, no specific identification of any single DIB has survived scrutiny. The overwhelming majority of DIBs are at optical and very near infrared wavelengths. In 1990 two DIBs were identified in J-band spectra, at 1.18 μm and 1.31 μm by Joblin et al. (1990); until recently these were the longest wavelength examples known.

Within a few parsecs around the central black hole A*, chemistry in the dense molecular cloud material of the circumnuclear disk (CND) can be affected by many energetic phenomena such as high UV-flux from the massive central star cluster, X-rays from A*, shock waves, and an enhanced cosmic-ray flux. Recently, spectroscopic surveys with the IRAM 30 meter and the APEX 12 meter telescopes of substantial parts of the 80–500 GHz frequency range were made toward selected positions in and near the CND. These data sets contain lines from the molecules HCN, HCO+, HNC, CS, SO, SiO, CN, H2CO, HC3N, N2H+, H3O+ and others. We conduct Large Velocity Gradient analyses to obtain column densities and total hydrogen densities, n, for each species in molecular clouds located in the southwest lobe of the CND. The data for the above mentioned molecules indicate 105 cm−3 ≲ n < 106 cm−3, which shows that the CND is tidally unstable. The derived chemical composition is compared with a chemical model calculated using the UCL_CHEM code that includes gas and grain reactions, and the effects of shock waves. Models are run for varying shock velocities, cosmic-ray ionization rates, and number densities. The resulting chemical composition is fitted best to an extremely high value of cosmic-ray ionization rate ζ ∼ 10−14 s−1, 3 orders of magnitude higher than the value in regular Galactic molecular clouds, if the pre-shock density is n=105 cm−3.

The cool gas in the central parsec of the Galaxy is organized in the surrounding circumnuclear disk, made of neutral gas, and the internal minispiral, composed of dust and ionized gas. In order to study the transition between them we have investigated the presence of H2 neutral gas in this area, through NIR spectro-imaging data observed with SPIFFI. To preserve the spatial resolution we implemented a new method consisting of a regularized 3D fit. We concentrated on the supposedly fully ionized central cavity and the very inner edge of the CND. H2 is detected everywhere: at the boundary of the CND and in the central cavity, where it seems to split in two components, one in the background of the minispiral and one inside the Northern arm.

We present the very first detection of N2H+J = (1 – 0) and CH3OH(2k−1k) line emission on 5″ scales in the circumnuclear disk (CND) around Sgr A*. The emission matches the position and shape of the dark clouds in the near-infrared. Our findings suggest that these molecular clouds in the eastern CND are significantly colder and denser than the rest of the CND, and partially shocked. The research on these dark clouds will contribute to understanding the processes of star formation close to a supermassive black hole.

We have used the Atacama Pathfinder Experiment (APEX) 12 m telescope at 218 GHz to observe molecular clouds simultaneously in the JKaKc = 303→202, 322→221, and 321→220 transitions of para-H2CO to determine kinetic temperatures of the dense gas in the central molecular zone of the Galaxy. Gas kinetic temperatures for individual molecular clouds range from 55 to 125 K or even higher. The molecular clouds at high temperatures may be heated by turbulent dissipation and/or cosmic-rays.

We report results of a study of the ionized gas towards the Galactic center with radio recombination lines at cm wavelengths. Both the Green Bank Telescope and the Very Large Array were utilized to probe the kinematics of the ionized gas on a global scale for both diffuse and discrete sources within the inner 2.0° × 0.5° (l × b). A diffuse ∼0 km s−1 gas, a thermal flux continuum fraction exceeding 40%, and an asymmetry where ∼70% of the ionized gas is found at positive Galactic longitudes are the preliminary results briefly discussed here.

The bulk of the FeKα emission detected in the central molecular zone (CMZ) is thought to be associated with reflection by the central molecular clouds of enhanced past emission from an external X-ray source, most likely Sgr A*. In order to follow the propagation of the reflected emission through the Galactic center (GC), we analyzed all XMM-Newton observations carried out from 2000 to 2012. Preliminary results indicate that while most of the regions that were bright at 6.4 keV in 2000–2001 have a significantly lower flux in 2012, a few other experienced a flux increase. We report for the first time a significant decrease of the FeKα emission in the Sgr C complex, supporting the reflection origin of the 6.4 keV emission detected in this region.

We observed Hydroxyl, water, ammonia, carbon monoxide and neutral carbon towards the +50 km s−1 cloud (M−0.02−0.07), the circumnuclear disk (CND) and the +20 km s−1 (M−0.13−0.08) cloud in the Sgr A complex with the VLA, Odin and SEST. Strong OH absorption, H2O emission and absorption lines were seen at all three positions. Strong C18O emissions were seen towards the +50 and +20 km s−1 clouds. The CND is rich in H2O and OH, and these abundances are considerably higher than in the surrounding clouds, indicating that shocks, star formation and clump collisions prevail in those objects. A comparison with the literature reveals that it is likely that PDR chemistry including grain surface reactions, and perhaps also the influences of shocks has led to the observed abundances of the observed molecular species studied here. In the redward high-velocity line wings of both the +50 and +20 km s−1 clouds and the CND, the very high H2O abundances are suggested to be caused by the combined action of shock desorption from icy grain mantles and high-temperature, gas-phase shock chemistry. Only three of the molecules are briefly discussed here. For OH and H2O three of the nine observed positions are shown, while a map of the C18O emission is provided. An extensive paper was recently published with Open Access (Karlsson et al. 2013, A&A 554, A141).

Despite many investigations, the physical characteristics of the molecular gas in the Galactic center circumnuclear disk (CND) remain a topic of debate. Its mass is highly uncertain, between 104 (from dust) and 105–6 M⊙ (derived from gas tracers), and depending on the probe, density estimates for the dense clumps are 105–8 cm−3 and gas temperatures run from 50 to a few hundred K. The range of physical parameters leaves open many questions about the nature and fate of the CND. Using several ground-based observatories, together with Herschel and SOFIA, we have studied the physical conditions of the dense clumps using CO, HCN and HCO+, finding that most of them are transient. Their densities are not large enough for them to be gravitationally bound in the tidal field in the center of our Galaxy.

We present maps of a large number of dense molecular gas tracers across the central molecular zone of our Galaxy. The data were taken with the CSIRO/CASS Mopra telescope in Large Projects in the 1.3 cm, 7 mm, and 3 mm wavelength regimes. Here, we focus on the brightness of the shock tracers SiO and HNCO, molecules that are liberated from dust grains under strong (SiO) and weak (HNCO) shocks. The shocks may have occurred when the gas enters the bar regions and the shock differences could be due to differences in the moving cloud masses. Based on tracers of ionizing photons, it is unlikely that the morphological differences are due to selective photo-dissociation of the molecules. We also observe direct heating of molecular gas in strongly shocked zones, with high SiO/HNCO ratios, where temperatures are determined from the transitions of ammonia. Strong shocks appear to be the most efficient heating source of molecular gas, apart from high energy emission emitted by the central supermassive black hole Sgr A* and the processes within the extreme star formation region Sgr B2.