The nearby dwarf starburst galaxy NGC 5253 hosts a number of young, massive star clusters, the two youngest of which are centrally concentrated and surrounded by thermal radio emission (the “radio ...nebula”). To investigate the role of these clusters in the starburst energetics, we combine new and archival Hubble Space Telescope images of NGC 5253 with wavelength coverage from 1500 Å to 1.9 μm in 13 filters. These include Hα, Pβ, and Pα, and the imaging from the Hubble Treasury Program LEGUS (Legacy Extragalactic UV Survey). The extraordinarily well-sampled spectral energy distributions enable modeling with unprecedented accuracy the ages, masses, and extinctions of the nine optically brightest clusters (MV < -8.8) and the two young radio nebula clusters. The clusters have ages ∼1-15 Myr and masses ∼1 × 104-2.5 × 105 M⊙. The clusters’ spatial location and ages indicate that star formation has become more concentrated toward the radio nebula over the last ∼15 Myr. The most massive cluster is in the radio nebula; with a mass ∼2.5 × 105 M⊙ and an age ∼1 Myr, it is 2-4 times less massive and younger than previously estimated. It is within a dust cloud with AV ∼ 50 mag, and shows a clear near-IR excess, likely from hot dust. The second radio nebula cluster is also ∼1 Myr old, confirming the extreme youth of the starburst region. These two clusters account for about half of the ionizing photon rate in the radio nebula, and will eventually supply about 2/3 of the mechanical energy in present-day shocks. Additional sources are required to supply the remaining ionizing radiation, and may include very massive stars. Based on observations obtained with the NASA/ESA Hubble Space Telescope, at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
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.
Aims. Observations of the molecular gas over scales of ~0.5 to several kpc provide crucial information on how molecular gas moves through galaxies, especially in mergers and interacting systems, ...where it ultimately reaches the galaxy center, accumulates, and feeds nuclear activity. Studying the processes involved in the gas transport is one of the important steps forward to understand galaxy evolution. Methods. super(12) CO, super(13) CO, and C super(18) O1-0 high-sensitivity ALMA observations (~4"x2") were used to assess the properties of the large-scale molecular gas reservoir and its connection to the circumnuclear molecular ring in the merger NGC 1614. Specifically, the role of excitation and abundances were studied in this context. We also observed the molecular gas high-density tracers CN and CS. Results. The spatial distributions of the detected super(12) CO1-0 and super(13) CO1-0 emission show significant differences. super(12) CO traces the large-scale molecular gas reservoir, which is associated with a dust lane that harbors infalling gas, and extends into the southern tidal tails. super(13) CO emission is for the first time detected in the large-scale dust lane. In contrast to super(12) CO, its line emission peaks between the dust lane and the circumnuclear molecular ring. A super(12) CO-to- super(13) CO1-0 intensity ratio map shows high values in the ring region (~30) that are typical for the centers of luminous galaxy mergers and even more extreme values in the dust lane (>45). Surprisingly, we do not detect C super(18) O emission in NGC 1614, but we do observe gas emitting the high-density tracers CN and CS. Conclusions. We find that the super(12) CO-to- super(13) CO1-0 line ratio in NGC 1614 changes from >45 in the 2 kpc dust lane to ~30 in the starburst nucleus. This drop in ratio with decreasing radius is consistent with the molecular gas in the dust lane being kept in a diffuse, unbound state while it is being funneled toward the nucleus. This also explains why there are no (or very faint) signs of star formation in the dust lane, despite its high super(12) CO luminosity. In the inner 1.5 kpc, the gas is compressed into denser and most likely self-gravitating clouds (traced by CN and CS emission), allowing it to power the intense central starburst. We find a high super(16) O-to- super(18) O abundance ratio in the starburst region (> or =900), typical of quiescent disk gas. This is surprising because by now, the starburst is expected to have enriched the nuclear interstellar medium in super(18) O relative to super(16) O. We suggest that the massive inflow of gas may be partially responsible for the low super(18) O/ super(16) O abundance since it will dilute the starburst enrichment with unprocessed gas from greater radial distances. The super(12) CO-to- super(13) CO abundance of >90 we infer from the line ratio is consistent with this scenario. It suggests that the nucleus of NGC 1614 is in a transient phase of its evolution where the starburst and the nuclear growth is still being fuelled by returning gas from the minor merger event.
ABSTRACT We present a study of the spatial distribution of the stellar cluster populations in the star-forming galaxy NGC 628. Using Hubble Space Telescope broadband WFC3/UVIS UV and optical images ...from the Treasury Program LEGUS (Legacy ExtraGalactic UV Survey), we have identified 1392 potential young ( Myr) stellar clusters within the galaxy using a combination of visual inspection and automatic selection. We investigate the clustering of these young stellar clusters and quantify the strength and change of clustering strength with scale using the two-point correlation function. We also investigate how image boundary conditions and dust lanes affect the observed clustering. The distribution of the clusters is well fit by a broken power law with negative exponent . We recover a weighted mean index of for all spatial scales below the break at 3 3 (158 pc at a distance of 9.9 Mpc) and an index of above 158 pc for the accumulation of all cluster types. The strength of the clustering increases with decreasing age and clusters older than 40 Myr lose their clustered structure very rapidly and tend to be randomly distributed in this galaxy, whereas the mass of the star cluster has little effect on the clustering strength. This is consistent with results from other studies that the morphological hierarchy in stellar clustering resembles the same hierarchy as the turbulent interstellar medium.
Spiral galaxies range from bulge-dominated early-type galaxies to late types with little or no bulge. Cosmological models do not predict the formation of disk-dominated, essentially bulgeless ...galaxies, yet these objects exist. A particularly striking and poorly understood example of bulgeless galaxies are flat or superthin galaxies with large axis ratios. We therefore embarked on a study aimed at a better understanding of these enigmatic objects, starting by compiling a statistically meaningful sample with well-defined properties. The disk axis ratios can be most easily measured when galaxies are seen edge-on. We used data from the Sloan Digital Sky Survey (SDSS) in order to identify edge-on galaxies with disks in a uniform, reproducible, automated fashion. In the five-color photometric database of the SDSS Data Release 1 (2099 deg2) we identified 3169 edge-on disk galaxies, which we subdivided into disk galaxies with bulge, intermediate types, and simple disk galaxies without any obvious bulge component. We subdivided these types further into subclasses: Sa(f), Sb(f), Sc(f), Scd(f), Sd(f), Irr(f), where the (f) indicates that these galaxies are seen edge-on. Here we present our selection algorithm and the resulting catalogs of the 3169 edge-on disk galaxies including the photometric, morphological, and structural parameters of our targets. A number of incompleteness effects affect our catalog, but it contains almost a factor of four more bulgeless galaxies with prominent simple disks (flat galaxies) within the area covered here than previous optical catalogs, which were based on the visual selection from photographic plates (cf. Karachentsev et al. 1999, Bull. Special Astrophys, Obs., 47, 5). We find that approximately 15% of the edge-on disk galaxies in our catalog are flat galaxies, demonstrating that these galaxies are fairly common, especially among intermediate-mass star-forming galaxies. Bulgeless disks account for roughly one third of our galaxies when also puffy disks and edge-on irregulars are included. Our catalog provides a uniform database for a multitude of follow-up studies of bulgeless galaxies in order to constrain their intrinsic and environmental properties and their evolutionary status.
On the Nature of the NGC 1275 System Conselice, Christopher J; Gallagher III, John S; Wyse, Rosemary F. G
The Astronomical journal,
11/2001, Letnik:
122, Številka:
5
Journal Article
We present a multiwavelength study of IC 860, a nearby post-starburst galaxy at the early stage of transitioning from blue and star forming to red and quiescent. Optical images reveal a galaxy-wide, ...dusty outflow originating from a compact core. We find evidence for a multiphase outflow in the molecular and neutral gas phase from the CO position-velocity diagram and NaD absorption features. We constrain the neutral mass outflow rate to be ∼0.5 M ⊙ yr−1, and the total hydrogen mass outflow rate to be ∼12 M ⊙ yr−1. Neither outflow component seems able to escape the galaxy. We also find evidence for a recent merger in the optical images, CO spatial distribution, and kinematics, and evidence for a buried active galactic nucleus in the optical emission line ratios, mid-IR properties, and radio spectral shape. The depletion time of the molecular gas reservoir under the current star formation rate is ∼7 Gyr, indicating that the galaxy could stay at the intermediate stage between the blue and red sequence for a long time. Thus the timescales for a significant decline in star formation rate (quenching) and gas depletion are not necessarily the same. Our analysis supports the quenching picture where outflows help suppress star formation by disturbing rather than expelling the gas and shed light on possible ongoing activities in similar quenching galaxies.
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.
Major impact from a minor merger König, S.; Aalto, S.; Muller, S. ...
Astronomy and astrophysics (Berlin),
07/2018, Letnik:
615
Journal Article
Recenzirano
Odprti dostop
Context. Minor mergers are important processes contributing significantly to how galaxies evolve across the age of the Universe. Their impact on the growth of supermassive black holes and star ...formation is profound – about half of the star formation activity in the local Universe is the result of minor mergers. Aims. The detailed study of dense molecular gas in galaxies provides an important test of the validity of the relation between star formation rate and HCN luminosity on different galactic scales – from whole galaxies to giant molecular clouds in their molecular gas-rich centers. Methods. We use observations of HCN and HCO+ 1−0 with NOEMA and of CO3−2 with the SMA to study the properties of the dense molecular gas in the Medusa merger (NGC 4194) at 1′′ resolution. In particular, we compare the distribution of these dense gas tracers with CO2−1 high-resolution maps in the Medusa merger. To characterize gas properties, we calculate the brightness temperature ratios between the three tracers and use them in conjunction with a non-local thermodynamic equilibrium (non-LTE) radiative line transfer model. Results. The gas represented by HCN and HCO+ 1−0, and CO3−2 does not occupy the same structures as the less dense gas associated with the lower-J CO emission. Interestingly, the only emission from dense gas is detected in a 200 pc region within the “Eye of the Medusa”, an asymmetric 500 pc off-nuclear concentration of molecular gas. Surprisingly, no HCN or HCO+ is detected for the extended starburst of the Medusa merger. Additionally, there are only small amounts of HCN or HCO+ associated with the active galactic nucleus. The CO3−2/2−1 brightness temperature ratio inside “the Eye” is ~2.5 – the highest ratio found so far – implying optically thin CO emission. The CO2−1/HCN 1−0 (~9.8) and CO2−1/HCO+ 1−0 (~7.9) ratios show that the dense gas filling factor must be relatively high in the central region, consistent with the elevated CO3−1/2−1 ratio. Conclusions. The line ratios reveal an extreme, fragmented molecular cloud population inside the Eye with large bulk temperatures (T > 300 K) and high gas densities (n(H2) > 104 cm-3). This is very different from the cool, self-gravitating structures of giant molecular clouds normally found in the disks of galaxies. The Eye of the Medusa is found at an interface between a large-scale minor axis inflow and the central region of the Medusa. Hence, the extreme conditions inside the Eye may be the result of the radiative and mechanical feedback from a deeply embedded, young and massive super star cluster formed due to the gas pile-up at the intersection. Alternatively, shocks from the inflowing gas entering the central region of the Medusa may be strong enough to shock and fragment the gas. For both scenarios, however, it appears that the HCN and HCO+ dense gas tracers are not probing star formation, but instead a post-starburst and/or shocked ISM that is too hot and fragmented to form newstars. Thus, caution is advised in taking the detection of emission from dense gas tracers as evidence of ongoing or imminent star formation.
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