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.
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
Recenzirano
Odprti dostop
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.
Hierarchical structure in ultraviolet images of 12 late-type LEGUS galaxies is studied by determining the numbers and fluxes of nested regions as a function of size from ~1 to ~200 pc, and the number ...as a function of flux. Two starburst dwarfs, NGC 1705 and NGC 5253, have steeper number-size and flux-size distributions than the others, indicating high fractions of the projected areas filled with star formation. Nine subregions in seven galaxies have similarly steep number-size slopes, even when the whole galaxies have shallower slopes. The results suggest that hierarchically structured star-forming regions several hundred parsecs or larger represent common unit structures. Small galaxies dominated by only a few of these units tend to be starbursts. The self-similarity of young stellar structures down to parsec scales suggests that star clusters form in the densest parts of a turbulent medium that also forms loose stellar groupings on larger scales. The presence of super star clusters in two of our starburst dwarfs would follow from the observed structure if cloud and stellar subregions more readily coalesce when self-gravity in the unit cell contributes more to the total gravitational potential.
NGC 602 is a young stellar cluster located in a peripheral region of the Small Magellanic Cloud (SMC) known as the wing. Far from the main body of the galaxy and abutting the Magellanic Bridge, the ...SMC's wing is characterized by low gas and stellar content. With deep optical imaging from the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope (HST), we have discovered an extensive pre-main-sequence (PMS) population, with stellar masses in the range 0.6-3 M unk. These low-mass PMS stars formed coevally with the central cluster about 4 Myr ago. Spitzer IRAC images of the same region also reveal a population of young stellar objects, some of which are still embedded in nebular material and most of which likely formed even more recently than the young stars detected with HST ACS imaging. We infer that star formation started in this region similar to 4 Myr ago with the formation of the central cluster and gradually propagated toward the outskirts where star formation is presently ongoing.
We present optical/near-IR integral field unit observations of a gas pillar in the Galactic H ii region NGC 6357 containing the young open star cluster Pismis 24. These observations have allowed us ...to examine in detail the gas conditions of the strong wind–clump interactions taking place on its surface. By accurately decomposing the Hα line profile, we identify the presence of a narrow (∼20 km s−1) and broad (50–150 km s−1) component, both of which we can associate with the pillar and its surroundings. Furthermore, the broadest broad component widths are found in a region that follows the shape of the eastern pillar edge. These connections have allowed us to firmly associate the broad component with emission from ionized gas within turbulent mixing layers on the pillar's surface set up by the shear flows of the winds from the O stars in the cluster. We discuss the implications of our findings in terms of the broad emission-line component that is increasingly found in extragalactic starburst environments. Although the broad linewidths found here are narrower, we conclude that the mechanisms producing both must be the same. The difference in linewidths may result from the lower total mechanical wind energy produced by the O stars in Pismis 24 compared to that from a typical young massive star cluster found in a starburst galaxy. The pillar's edge is also clearly defined by dense (≲5000 cm−3), hot (≳20 000 K) and excited (via the N ii/Hα and S ii/Hα ratios) gas conditions, implying the presence of a D-type ionization front propagating into the pillar surface. Although there must be both photoevaporation outflows produced by the ionization front and mass loss through mechanical ablation, we see no evidence for any significant bulk gas motions on or around the pillar. We postulate that the evaporated/ablated gas must be rapidly heated before being entrained.
The NGC 1614 interacting galaxy Konig, S; Aalto, S; Muller, S ...
Astronomy and astrophysics (Berlin),
05/2013, Letnik:
553
Journal Article
Recenzirano
Odprti dostop
Minor mergers frequently occur between giant and gas-rich low-mass galaxies and can provide significant amounts of interstellar matter to refuel star formation and power active galactic nuclei (AGN) ...in the giant systems. Major starbursts and/or AGN result when fresh gas is transported and compressed in the central regions of the giant galaxy. This is the situation in the starburst minor merger NGC 1614, whose molecular medium we explore at half-arcsecond angular resolution through our observations of sup 12CO(2-1) emission using the Submillimeter Array (SMA). The astrophysical process for producing a ring structure for the final resting place of accreted gas in NGC 1614 is not fully understood, but the presence of numerous GMAs suggests an orbit-crowding or resonance phenomenon. There is some evidence that star formation is progressing radially outward within the ring, indicating that a self-triggering mechanism may also affect star formation processes. The net result of this merger therefore very likely increases the central concentration of stellar mass in the NGC 1614 remnant giant system.
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.
PDF
