Major impact from a minor merger König, S.; Aalto, S.; Muller, S. ...
Astronomy and astrophysics (Berlin),
2018, Letnik:
615
Journal Article
Recenzirano
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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
(H
2
) > 10
4
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.
COSMIC RAY SAMPLING OF A CLUMPY INTERSTELLAR MEDIUM Boettcher, Erin; Zweibel, Ellen G; YOAST-HULL, TOVA M ...
Astrophysical journal/The Astrophysical journal,
12/2013, Letnik:
779, Številka:
1
Journal Article
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How cosmic rays sample the multi-phase interstellar medium (ISM) in starburst galaxies has important implications for many science goals, including evaluating the cosmic ray calorimeter model for ...these systems, predicting their neutrino fluxes, and modeling their winds. Here, we useMonte Carlo simulations to study cosmic ray sampling of a simple, two-phase ISM under conditions similar to those of the prototypical starburst galaxy M82. The assumption that cosmic rays sample the mean density of the ISM in the starburst region is assessed over a multi-dimensional parameter space where we vary the number of molecular clouds, the galactic wind speed, the extent to which the magnetic field is tangled, and the cosmic ray injection mechanism. We evaluate the ratio of the emissivity from pion production in molecular clouds to the emissivity that would be observed if the cosmic rays sampled the mean density, and seek areas of parameter space where this ratio differs significantly from unity. The assumption that cosmic rays sample the mean density holds over much of parameter space; however, this assumption begins to break down for high cloud density, injection close to the clouds, and a very tangled magnetic field.We conclude by evaluating the extent to which our simulated starburst region behaves as a proton calorimeter and constructing the time-dependent spectrum of a burst of cosmic rays.
We have obtained deep and wide field imaging of the Coma cluster of galaxies with the CFH12K camera at CFHT in the B, V, R and I filters. In this paper, we present the observations, data reduction, ...catalogs and first scientific results. We investigated the quality of our data by internal and external literature comparisons. We also checked the realisation of the observational requirements we set. Our observations cover two partially overlapping areas of 42 x 28 arcmin super(2), leading to a total area of 0.72 x 0.82 deg super(2). We have produced catalogs of objects that cover a range of more than 10 mag. and are complete at the 90% level at B similar to 25, V similar to 24, R similar to 24 and I similar to 23.5 for stellar-like objects, and at B similar to 22, V similar to 21, R similar to 20.75 and I similar to 20.5 for faint low-surface-brightness galaxy-like objects. Magnitudes are in good agreement with published values from R similar to 16 to R similar to 25. The photometric uncertainties are of the order of 0.1 mag at R similar to 20 and of 0.3 mag at R similar to 25. Astrometry is accurate to 0.5 arcsec and also in good agreement with published data. Our catalog provides a rich dataset that can be mined for years to come to gain new insights into the formation and evolution of the Coma cluster and its galaxy population. As an illustration of the data quality, we examine the bright part of the Colour Magnitude Relation (B-R versus R) derived from the catalog and find that it is in excellent agreement with that derived for galaxies with redshifts in the Coma cluster, and with previous CMRs estimated in the literature.
We present new spectroscopic observations of the stellar cluster population of region B in the prototype starburst galaxy M82 obtained with the Gemini North 8.1 m telescope. By coupling the ...spectroscopy with UBVI photometry acquired with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST), we derive ages, extinctions, and radial velocities for seven young massive clusters (YMCs) in region B. We find the clusters to have ages between 80 and 200 Myr and velocities in the range 230-350 km s super(-1), while the extinctions Av vary between similar to 1 and 2.5 mag. We also find evidence of differential extinction across the faces of some clusters, which hinders the photometric determination of ages and extinctions in these cases. The cluster radial velocities indicate that the clusters are located at different depths within the disk and are on regular disk orbits. Our results overall contradict the findings of previous studies, in which region B was thought to be a bound region populated by intermediate-age clusters that formed in an independent, offset starburst episode that commenced 600 Myr-1 Gyr ago. Our findings instead suggest that region B is optically bright because of low-extinction patches, and that this allows us to view the cluster population of the inner M82 disk, which probably formed as a result of the last encounter with M81. This study forms part of a series of papers whose aim is to study the cluster population of M82 using deep optical spectroscopy and multiband photometry.
We present HST/STIS optical and Gemini/NIFS near-IR IFU spectroscopy and archival Hubble Space Telescope (HST) imaging of the triplet of super star clusters (A1, A2, and A3) in the core of the M82 ...starburst. Using model fits to the Space Telescope Imaging Spectrograph (STIS) spectra and the weakness of red supergiant CO absorption features (appearing at ~6 Myr) in the NIFS H-band spectra, the ages of A2 and A3 are 4.5 + or - 1.0 Myr. A1 has strong CO bands, consistent with our previously determined age of 6.4 + or - 0.5 Myr. The photometric masses of the three clusters are 4-7 x 10 super(5) M sub(middot in circle), and their sizes are R sub(eff) = 159, 104, 59 mas (~2.8, 1.8, 1.0 pc) for A1, A2, and A3. The STIS spectra yielded radial velocities of 320 + or - 2, 330 + or - 6, and 336 + or - 5 km s super(-1) for A1, A2, and A3, placing them at the eastern end of the x sub(2) orbits of M82's bar. Clusters A2 and A3 are in high-density (800-1000 cm super(-3)) environments, and like A1, are surrounded by compact H II regions. We suggest the winds from A2 and A3 have stalled, as in A1, due to the high ISM ambient pressure. We propose that the three clusters were formed in situ on the outer x sub(2) orbits in regions of dense molecular gas subsequently ionized by the rapidly evolving starburst. The similar radial velocities of the three clusters and their small projected separation of ~25 pc suggest that they may merge in the near future unless this is prevented by velocity shearing.
Evolution of dwarf-elliptical galaxies Hensler, G.; Theis, Ch; Gallagher, J. S.
Astronomy & astrophysics,
10/2004, Letnik:
426, Številka:
1
Journal Article
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Here we present as an extension to already published models of massive spherical galaxies one-dimensional chemo-dynamical models of dwarf elliptical galaxies in the mass range between 109 and ...1010 $M_{\odot}$ and initial density fluctuation of 1σ and 3σ. Because of their vanishing angular momentum the models are restricted to spherical symmetry. Due to this limitation the dynamics of the different components consider only radial motions and also their interaction processes have only one degree of freedom. Therefore, the galaxy evolution is preferably determined by radial oscillatory phenomena caused by heating and cooling of the interstellar gas, which reenforce effects like starbursts. Nevertheless, the low gravitational binding energy of dwarf ellipticals can easily be exceeded by thermal and turbulent energy production in the interstellar medium, leading to gas expansion and even to galactic winds. Furthermore, gas phases as well as gas and stars can decouple dynamically during the galactic evolution. For comparison with observational signatures like stellar kinematics, radial densities and metallicity distributions of the different components, the chemo-dynamical treatment of galaxy evolution must take the multi-phase character of the interstellar medium and self-regulation of star formation at low gravitation into account. Since non-rotating low-mass galaxies with stochastic star-formation episodes are seen as dwarf ellipticals, the aim of this paper is to compare the 1d chemo-dynamical models with observed characteristics of this type of dwarf galaxies. Several features like stellar populations from separate formation episodes, low metallicities, significant mass loss, etc. can be reproduced by the models in a global manner. Quantitative disagreements between model predictions and observations provide insight into necessary improvements of subsequent chemo-dynamical models implying rotation, dark matter halos, and external effects by an intergalactic gas, like its pressure, gas infall and stripping of galactic gas. The results can be summarized as follows: 109 $M_{\odot}$ models are characterised by a single star-formation event during the initial collapse with an active self-regulation leading to reexpansion and massive gas loss as well as remaining at very low metallicities; 1010 $M_{\odot}$ dwarf elliptical models become almost gas-free due to large initial star formation and a strong galactic wind. These as well as a $5 \times 10^9$ $M_{\odot}$ and 3σ model experience oscillatory but non-negligible later star-formation histories that can account for the observed intermediate-age stellar populations.
M82-F: a doomed super star cluster? Smith, Linda J.; Gallagher, John S.
Monthly Notices of the Royal Astronomical Society,
09/2001, Letnik:
326, Številka:
3
Journal Article
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We present high-dispersion echelle spectroscopy of the very luminous, young super star cluster (SSC) ‘F’ in M82, obtained with the 4.2-m William Herschel Telescope, for the purpose of deriving its ...dynamical mass and assessing whether it will survive to become an old globular cluster. In addition to the stellar lines, the spectrum contains complex Na i absorption and broad emission lines from the ionized gas. We measure a stellar velocity dispersion of 13.4±0.7 km s−1 and a projected half-light radius of 2.8±0.3 pc from archival HST/WFPC2 images, and derive a dynamical mass of 1.2±0.1×106M⊙, demonstrating that M82-F is a very massive, compact cluster. We determine that the current luminosity-to-mass ratio (LV/M)⊙ for M82-F is 45±13. Comparison with spectral synthesis models shows that (LV/M)⊙ is a factor of ∼ 5 higher than that predicted for a standard Kroupa initial mass function (IMF) at the well-determined age for M82-F of 60±20 Myr. This high value of (LV/M)⊙ indicates a deficit of low-mass stars in M82-F; the current mass function (MF) evidently is ‘top-heavy’. We find that a lower mass cut-off of 2–3 M⊙ is required to match the observations for a MF with a slope α=2.3. Since the cluster apparently lacks long-lived low-mass stars, it will not become an old globular cluster, but probably will dissolve at an age of ≤ 2 Gyr. We also derive updated luminosity-to-mass ratios for the younger SSCs NGC 1569A and 1705-1. For the first object, the observations are consistent with a slightly steeper MF (α=2.5), whereas for NGC 1705-1 the observed ratio requires the MF to be truncated near 2 M⊙ for a slope of α=2.3. We discuss the implications of our findings in the context of large-scale IMF variations; with the present data the top-heavy MF could reflect a local mass segregation effect during the birth of the cluster. M82-F probably formed in a dense molecular cloud; however, its high radial velocity with respect to the centre of M82 (∼−175 km s−1) suggests that it is on an eccentric orbit and now far from its birthplace, so the environment of its formation is unknown.
The Young Cluster Population of M82 Region B Smith, L. J; Bastian, N; Konstantopoulos, I. S ...
Astrophysical journal/The Astrophysical journal,
10/2007, Letnik:
667, Številka:
2
Journal Article
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We present observations obtained with the Advanced Camera for Surveys on board the Hubble Space Telescope of the "fossil" starburst region B in the nearby starburst galaxy M82. By comparing UBVI ...photometry with models, we derive ages and extinctions for 35 U-band-selected star clusters. We find that the peak epoch of cluster formation occurred 6150 Myr ago, in contrast to earlier work that found a peak formation age of 1.1 Gyr. The difference is most likely due to our inclusion of U-band data, which are essential for accurate age determinations of young cluster populations. We further show that the previously reported turnover in the cluster luminosity function is probably due to the neglect of the effect of extended sources on the detection limit. The much younger cluster ages we derive clarifies the evolution of the M82 starburst. The M82-B age distribution now overlaps with the ages of the nuclear starburst, the clusters formed on the opposite side of the disk, and the last encounter with M81, some 220 Myr ago.
Sk 183 is the visually brightest star in the N90 nebula, a young star-forming region in the Wing of the Small Magellanic Cloud (SMC). We present new optical spectroscopy from the Very Large Telescope ...which reveals Sk 183 to be one of the most massive O-type stars in the SMC. Classified as an 03-type dwarf on the basis of its nitrogen spectrum, the star also displays broadened He I absorption, which suggests a later type. We propose that Sk 183 has a composite spectrum and that it is similar to another star in the SMC, MPG 324. This brings the number of rare O2- and O3-type stars known in the whole of the SMC to a mere four. We estimate physical parameters for Sk 183 from analysis of its spectrum. For a single-star model, we estimate an effective temperature of 46 + or - 2 kK, a low mass-loss rate of ~10 super(-7) M sub(middot in circle) yr super(-1), and a spectroscopic mass of (ProQuest: Formulae and/or non-USASCII text omitted) (for an adopted distance modulus of 18.7 mag to the young population in the SMC Wing). An illustrative binary model requires a slightly hotter temperature (~47.5 kK) for the primary component. In either scenario, Sk 183 is the earliest-type star known in N90 and will therefore be the dominant source of hydrogen-ionizing photons. This suggests Sk 183 is the primary influence on the star formation along the inner edge of the nebula.
We report the discovery of extended X-ray emission within the young star cluster NGC 602a in the Wing of the Small Magellanic Cloud (SMC) based on observations obtained with the Chandra X-Ray ...Observatory. X-ray emission is detected from the cluster core area with the highest stellar density and from a dusty ridge surrounding the H II region. We use a census of massive stars in the cluster to demonstrate that a cluster wind or wind-blown bubble is unlikely to provide a significant contribution to the X-ray emission detected from the central area of the cluster. We therefore suggest that X-ray emission at the cluster core originates from an ensemble of low- and solar-mass pre-main-sequence (PMS) stars, each of which would be too weak in X-rays to be detected individually. We attribute the X-ray emission from the dusty ridge to the embedded tight cluster of the newborn stars known in this area from infrared studies. Assuming that the levels of X-ray activity in young stars in the low-metallicity environment of NGC 602a are comparable to their Galactic counterparts, then the detected spatial distribution, spectral properties, and level of X-ray emission are largely consistent with those expected from low- and solar-mass PMS stars and young stellar objects (YSOs). This is the first discovery of X-ray emission attributable to PMS stars and YSOs in the SMC, which suggests that the accretion and dynamo processes in young, low-mass objects in the SMC resemble those in the Galaxy.
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