Aims. Our goal was to investigate the molecular gas distribution and kinematics in the extreme far-infrared (FIR) excess galaxy NGC 1377 and to address the nature and evolutionary status of the ...buried source. Methods. We used high-(0.'' 65 x 0.'' 52, (65 x 52 pc)) and low-(4.'' 88 x 2.'' 93) resolution SubMillimeter Array (SMA) observations to image the (CO)-C-12 and (CO)-C-13 2-1 line emission. Results. We find bright, complex (CO)-C-12 2-1 line emission in the inner 400 pc of NGC 1377. The (CO)-C-12 2-1 line has wings that are tracing a kinematical component that appears to be perpendicular to the component traced by the line core. Together with an intriguing X-shape of the integrated intensity and dispersion maps, this suggests that the molecular emission of NGC 1377 consists of a disk-outflow system. Lower limits to the molecular mass and outflow rate are M-out(H-2) > 1 x 10(7) M-circle dot and (M) over dot > 8 M-circle dot yr(-1). The age of the proposed outflow is estimated to be 1.4 Myr, the extent to be 200 pc and the outflow speed to be V-out = 140 km s(-1). The total molecular mass in the SMA map is estimated to M-tot(H-2) = 1.5 x 10(8) M-circle dot (on a scale of 400 pc) while in the inner r = 29 pc the molecular mass is M-core(H-2) = 1.7 x 10(7) M-circle dot with a corresponding H-2 column density of N(H-2) = 3.4 x 10(23) cm(-2) and an average (CO)-C-12 2-1 brightness temperature of 19 K. (CO)-C-13 2-1 emission is found at a factor 10 fainter than (CO)-C-12 in the low-resolution map while (CO)-O-18 2-1 remains undetected. We find weak 1 mm continuum emission of 2.4 mJy with spatial extent less than 400 pc. Conclusions. Observing the molecular properties of the FIR-excess galaxy NGC 1377 allows us to probe the early stages of nuclear activity and the onset of feedback in active galaxies. The age of the outflow supports the notion that the current nuclear activity is young - a few Myr. The outflow may be powered by radiation pressure from a compact, dust enshrouded nucleus, but other driving mechanisms are possible. The buried source may be an active galactic nucleus (AGN) or an extremely young (1 Myr) compact star-burst. Limitations on size and mass lead us to favor the AGN scenario, but additional studies are required to settle this question. In either case, the wind with its implied mass outflow rate will quench the nuclear power source within the very short time of 5-25 Myr. It is possible, however, that the gas is unable to escape the galaxy and may eventually fall back onto NGC 1377 again.
We study the star cluster population of NGC 2997, a giant spiral galaxy located at 9.5 Mpc and targeted by the Snapshot Hubble U-band Cluster Survey (SHUCS). Combining our U-band imaging from SHUCS ...with archival BVI imaging from the Hubble Space Telescope, we select a high confidence sample of clusters in the circumnuclear ring and disk through a combination of automatic detection procedures and visual inspection. The cluster luminosity functions in all four filters can be approximated by power laws with indices of -1.7 to -2.3. Some deviations from pure power-law shape are observed, hinting at the presence of a high-mass truncation in the cluster mass function. However, upon inspection of the cluster mass function, we find it is consistent with a pure power law of index -2.2 + or - 0.2 despite a slight bend at ~2.5 x 10 super(4) M sub(middot in circle). No statistically significant truncation is observed. From the cluster age distributions, we find a low rate of disruption (zeta ~ -0.1) in both the disk and circumnuclear ring. Finally, we estimate the cluster formation efficiency (Gamma) over the last 100 Myr in each region, finding 7% + or - 2% for the disk, 12% + or - 4% for the circumnuclear ring, and 10% + or - 3% for the entire UBVI footprint. This study highlights the need for wide-field UBVI coverage of galaxies to study cluster populations in detail, though a small sample of clusters can provide significant insight into the characteristics of the population.
We present optical spectroscopy obtained with the Space Telescope Imaging Spectrograph of five young massive star clusters in the starburst galaxy M82. A detailed analysis is performed for one ...cluster 'M82-A1' and its immediate environment in the starburst core. From Hubble Space Telescope archive images, we find that it is elliptical with an effective radius of 3.0 ± 0.5-pc and is surrounded by a compact (r= 4.5 ± 0.5-pc) H-ii region. We determine the age and reddening of M82-A1 using synthetic spectra from population synthesis models by fitting both the continuum energy distribution and the depth of the Balmer jump. We find an age of 6.4 ± 0.5-Myr and a photometric mass estimate of M= 7-13 × 105-M⊙. We associate its formation with the most recent starburst event 4-6-Myr ago. We find that the oxygen abundance of the H-ii region surrounding M82-A1 is solar or slightly higher. The H-ii region has a high pressure P/k= 1-2 × 107-cm−3-K. The diffuse gas in region A has a slightly lower pressure, which together with the broad Hα emission-line width, suggests that both the thermal and turbulent pressures in the M82 starburst core are unusually high. We discuss how this environment has affected the evolution of the cluster wind for M82-A1. We find that the high pressure may have caused the pressure-driven bubble to stall. We also obtain spectroscopic ages for clusters B1-2 and B2-1 in the 'fossil' starburst region and for the intermediate age clusters F and L. These are consistent with earlier studies and demonstrate that star formation activity, sufficiently intense to produce super star clusters, has been going on in M82 during the past Gyr, perhaps in discrete and localized episodes.
The conditions in the Galactic Center are often compared with those in starburst systems, which contain higher supernova rates, stronger magnetic fields, more intense radiation fields, and larger ...amounts of dense molecular gas than in our own Galactic disk. Interactions between such an augmented interstellar medium and cosmic rays result in brighter radio and gamma -ray emission. Here, we test how well the comparisons between the Galactic Center and starburst galaxies hold by applying a model for cosmic-ray interactions to the Galactic Center to predict the resulting gamma -ray emission. The model only partially explains the observed gamma -ray and radio emission. The model for the gamma -ray spectrum agrees with the data at TeV energies but not at GeV energies. Additionally, as the fits of the model to the radio and gamma -ray spectra require significant differences in the optimal wind speed and magnetic field strength, we find that the single-zone model alone cannot account for the observed emission from the Galactic Center. Our model is improved by including a soft, additional cosmic-ray population. We assess such a cosmic-ray population and its potential sources and find that a cosmic-ray electron spectrum is energetically favored over a cosmic-ray proton spectrum.
High-resolution submillimeter line and continuum observations are important in probing the morphology, column density, and dynamics of the molecular gas and dust around obscured active galactic ...nuclei (AGNs). With high-resolution (0.̋06 × 0.̋05 (6 × 5 pc)) ALMA 690 GHz observations we have found bright (TB > 80 K) and compact (full width half maximum size (FWHM) size of 10 × 7 pc) CO 6−5 line emission in the nuclear region of the extremely radio-quiet galaxy NGC 1377. The CO 6−5 intensity is partially aligned with the previously discovered jet/outflow of NGC 1377 and is tracing dense (n > 104cm-3) hot molecular gas at the base of the outflow. The velocity structure is complex and shifts across the jet/outflow are discussed in terms of separate overlapping kinematical components or rotation. High-velocity gas (Δv ± 145km s-1) is detected inside r < 2−3 pc and we suggest that it is emerging from an inclined rotating disk or torus of position angle PA = 140° ± 20° with a dynamical mass of 3 × 106M⊙. This mass is consistent with that of a supermassive black hole (SMBH), as inferred from the M−σ relation. The gas mass of the proposed disk/torus constitutes <3% of the dynamical mass inside a radius of 3 pc. In contrast to the intense CO 6−5 line emission, we do not detect 690 GHz dust continuum in the nuclear region of NGC 1377. The upper limit of S(690 GHz) ≲ 2 mJy implies an H2 column density N(H2) < 3 × 1023cm-2 (averaged in the central 6 × 5 pc beam). This is inconsistent with a Compton thick (CT) source and we discuss the possibility that CT obscuration may instead be occurring on smaller subparsec scales or in a larger foreground structure. From SED fitting we suggest that half of the IR emission of NGC 1377 is nuclear and the rest, mostly the far-infrared (FIR), is emerging from larger scales. The extreme radio quietness, and the lack of emission from other star formation tracers, raise questions on the origin of the FIR emission. We discuss the possibility that it arises from AGN-heated dust along the minor axis.
The Hubble Tarantula Treasury Project (HTTP) is an ongoing panchromatic imaging survey of stellar populations in the Tarantula Nebula in the Large Magellanic Cloud that reaches into the sub-solar ...mass regime (<0.5 Modot). HTTP utilizes the capability of the Hubble Space Telescope to operate the Advanced Camera for Surveys and the Wide Field Camera 3 in parallel to study this remarkable region in the near-ultraviolet, optical, and near-infrared spectral regions, including narrow-band H alpha images. The combination of all these bands provides a unique multi-band view. The resulting maps of the stellar content of the Tarantula Nebula within its main body provide the basis for investigations of star formation in an environment resembling the extreme conditions found in starburst galaxies and in the early universe. Access to detailed properties of individual stars allows us to begin to reconstruct the temporal and spatial evolution of the stellar skeleton of the Tarantula Nebula over space and time on a sub-parsec scale. In this first paper we describe the observing strategy, the photometric techniques, and the upcoming data products from this survey and present preliminary results obtained from the analysis of the initial set of near-infrared observations.
Context. Understanding the nuclear growth and feedback processes in galaxies requires investigating their often obscured central regions. One way to do this is to use (sub)millimeter line emission ...from vibrationally excited HCN (HCN-vib), which is thought to trace warm and highly enshrouded galaxy nuclei. It has been suggested that the most intense HCN-vib emission from a galaxy is connected to a phase of nuclear growth that occurs before the nuclear feedback processes have been fully developed. Aims. We aim to investigate if there is a connection between the presence of strong HCN-vib emission and the development of feedback in (ultra)luminous infrared galaxies ((U)LIRGs). Methods. We collected literature and archival data to compare the luminosities of rotational lines of HCN-vib, normalized to the total infrared luminosity, to the median velocities of 119 μm OH absorption lines, potentially indicating outflows, in a total of 17 (U)LIRGs. Results. The most HCN-vib luminous systems all lack signatures of significant molecular outflows in the far-infrared OH absorption lines. However, at least some of the systems with bright HCN-vib emission have fast and collimated outflows that can be seen in spectral lines at longer wavelengths, including in millimeter emission lines of CO and HCN (in its vibrational ground state) and in radio absorption lines of OH. Conclusions. We conclude that the galaxy nuclei with the highest LHCN − vib/LIR do not drive wide-angle outflows that are detectable using the median velocities of far-infrared OH absorption lines. This is possibly because of an orientation effect in which sources oriented in such a way that their outflows are not along our line of sight also radiate a smaller proportion of their infrared luminosity in our direction. It could also be that massive wide-angle outflows destroy the deeply embedded regions responsible for bright HCN-vib emission, so that the two phenomena cannot coexist. This would strengthen the idea that vibrationally excited HCN traces a heavily obscured stage of evolution before nuclear feedback mechanisms are fully developed.
We present optical HST STIS observations made with two slits crossing four of the optically brightest starburst clumps near the nucleus of M82. These provide Ha kinematics, extinction, electron ...density, and emission measures. From the radial velocity curves derived from both slits we confirm the presence of a stellar bar. We derive a new model for the orientation of the bar and disk with respect to the main starburst clumps and the cluster M82-A1. We propose that clump A has formed within the bar region as a result of gas interactions between the bar orbits, whereas region C lies at the edge of the bar and regions D and E are located farther out from the nucleus but heavily obscured. We derive extremely high interstellar densities of 500 900 cm unk, corresponding to ISM pressures of P/k approximately (0.5-1.0) x 10 super(7) cm super(-3) K, and discuss the implications of the measured gas properties on the production and evolution of the galactic wind. Despite varying pressures, the ionization parameter is uniform down to parsec scales, and we discuss why this might be so. Where the signal-to-noise ratios of our spectra are high enough, we identify multiple emission-line components. Through detailed Gaussian line fitting, we identify a ubiquitous broad (200-300 km s super(-1)) underlying component to the bright Ha line and discuss the physical mechanlsm(s) that could be responsible for such widths. We conclude that evaporation and/or ablation of material from interstellar gas clouds caused by the impact of high-energy photons and fast flowing cluster winds produce a highly turbulent layer on the surface of the clouds from which the emission arises.
How galaxies regulate nuclear growth through gas accretion by supermassive black holes (SMBHs) is one of the most fundamental questions in galaxy evolution. One potential way to regulate nuclear ...growth is through a galactic wind that removes gas from the nucleus. It is unclear whether galactic winds are powered by jets, mechanical winds, radiation, or via magnetohydrodynamic (MHD) processes. Compact obscured nuclei represent a significant phase of galactic nuclear growth. These galaxies hide growing SMBHs or unusual starbursts in their very opaque, extremely compact ( r < 100 pc) centres. They are found in approximately 30% of the luminous and ultra-luminous infrared galaxy population. Here, we present high-resolution ALMA observations (∼30 mas, ∼5 pc) of ground-state and vibrationally excited HCN towards ESO 320-G030 (IRAS 11506-3851). ESO 320-G030 is an isolated luminous infrared galaxy known to host a compact obscured nucleus and a kiloparsec-scale molecular wind. Our analysis of these high-resolution observations excludes the possibility of a starburst-driven wind, a mechanically or energy driven active galactic nucleus wind, and exposes a molecular MDH wind. These results imply that the nuclear evolution of galaxies and the growth of SMBHs are similar to the growth of hot cores or protostars where gravitational collapse of the nuclear torus drives a MHD wind. These results mean galaxies are capable, in part, of regulating the evolution of their nuclei without feedback.
Context. Arp 220 is the prototypical ultra luminous infrared galaxy (ULIRG). Despite extensive studies, the structure at MHz-frequencies has remained unknown because of limits in spatial ...resolution.Aims: This work aims to constrain the flux and shape of radio emission from Arp 220 at MHz frequencies.Methods: We analyse new observations with the International Low Frequency Array (LOFAR) telescope, and archival data from the Multi-Element Radio Linked Interferometer Network (MERLIN) and the Karl G. Jansky Very Large Array (VLA). We model the spatially resolved radio spectrum of Arp 220 from 150 MHz to 33 GHz.Results: We present an image of Arp 220 at 150 MHz with resolution 0.̋65 × 0.̋35, sensitivity 0.15 mJy beam-1, and integrated flux density 394 ± 59 mJy. More than 80% of the detected flux comes from extended (6''≈ 2.2 kpc) steep spectrum (α = -0.7) emission, likely from star formation in the molecular disk surrounding the two nuclei. We find elongated features extending 0.3'' (110 pc) and 0.9'' (330 pc) from the eastern and western nucleus respectively, which we interpret as evidence for outflows. The extent of radio emission requires acceleration of cosmic rays far outside the nuclei. We find that a simple three component model can explain most of the observed radio spectrum of the galaxy. When accounting for absorption at 1.4 GHz, Arp 220 follows the FIR/radio correlation with q = 2.36, and we estimate a star formation rate of 220 M⊙ yr-1. We derive thermal fractions at 1 GHz of less than 1% for the nuclei, which indicates that a major part of the UV-photons are absorbed by dust.Conclusions: International LOFAR observations shows great promise to detect steep spectrum outflows and probe regions of thermal absorption. However, in LIRGs the emission detected at 150 MHz does not necessarily come from the main regions of star formation. This implies that high spatial resolution is crucial for accurate estimates of star formation rates for such galaxies at 150 MHz.
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