Nuclear Starburst In NGC Research Articles

ALMA REVEALS THE MOLECULAR MEDIUM FUELING THE NEAREST NUCLEAR STARBURST

We use ALMA to derive the mass, length, and time scales associated with the nuclear starburst in NGC 253. This region forms ~2 M_sun/yr of stars and resembles other starbursts in scaling relations, with star formation consuming the gas reservoir 10 times faster than in galaxy disks. We present observations of CO, the high effective density transitions HCN(1-0), HCO+(1-0), CS(2-1), and their isotopologues. We identify ten clouds that appear as peaks in line emission and enhancements in the HCN-to-CO ratio. These clouds are massive (~10^7 M_sun) structures with sizes (~30 pc) similar to GMCs in other systems. Compared to disk galaxy GMCs, they show high line widths (~20-40 km/s) given their size, with implied Mach numbers ~90. The clouds also show high surface (~6,000 M_sun/pc^2) and volume densities (n_H2~2,000 cm^-3). Given these, self-gravity can explain the line widths. This short free fall time (~0.7 Myr) helps explain the more efficient star formation in NGC 253. We also consider the starburst region as a whole. The geometry is confused by the high inclination, but simple models support a non-axisymmetric, bar-like geometry with a compact, clumpy region of high gas density embedded in an extended CO distribution. Even for the whole region, the surface density still exceeds that of a disk galaxy GMC. The orbital time (~10 Myr), disk free fall time (<~ 3 Myr), and disk crossing time (<~ 3 Myr) are each much shorter than in a normal spiral galaxy disk. Some but not all aspects of the structure correspond to predictions from assuming vertical dynamical equilibrium or a marginally stable rotating disk. Finally, the CO-to-H2 conversion factor implied by our cloud calculations is approximately Galactic, contrasting with results showing a low value for the whole starburst region. The contrast provides resolved support for the idea of mixed molecular ISM phases in starburst galaxies.

The core flux of the brightest 10 $\mathsf{\mu}$m galaxies in the southern sky

Aims. Near diffraction-limited images have been taken at 8.9, 11.9, and 12.9 µm for the brightest extragalactic sources in the southern sky, in order to optimally plan N-band observations with MIDI (MID-infrared Interferometric instrument) at the VLTI. Methods. We have assembled a sample of 21 objects consisting of all the AGNs observable from Paranal observatory, Chile, plus three non-AGN objects, with an estimated N-band flux greater than 400 mJy. We used the TIMMI2 Mid Infrared instrument mounted on the ESO’s 3.6 m telescope to obtain near diffraction-limited images in order to establish the unresolved core flux within <0.5 arscsec. Results. Positions and core total fluxes were obtained for all sources in our sample and compared with similar investigations in the literature. We find that 15 AGN and the nuclear starburst in NGC 253 exhibit an unresolved core flux <300 mJy at 11.9 µm, making them promising targets for MIDI at the VLTI. For extended sources, near diffraction-limited images are presented and discussed.

Molecular Superbubbles in the Starburst Galaxy NGC 253

The central 2x1 kpc of the starburst galaxy NGC 253 has been imaged using the Submillimeter Array at a 60 pc resolution in the J=2-1 transitions of 12CO, 13CO, and C18O as well as in the 1.3 mm continuum. Molecular gas and dust are mainly in the circumnuclear disk of ~500 pc radius, with warm (~40 K) and high area-filling factor gas in its central part. Two gas shells or cavities have been discovered in the circumnuclear disk. They have ~100 pc diameters and have large velocity widths of 80-100 km/s, suggestive of expansion at ~50 km/s. Modeled as an expanding bubble, each shell has an age of ~0.5 Myr and needed kinetic energy of ~1E46 J as well as mean mechanical luminosity of ~1E33 W for its formation. The large energy allows each to be called a superbubble. A ~10^6 Msun super star cluster can provide the luminosity, and could be a building block of the nuclear starburst in NGC 253. Alternatively, a hypernova can also be the main source of energy for each superbubble, not only because it can provide the mechanical energy and luminosity but also because the estimated rate of superbubble formation and that of hypernova explosions are comparable. Our observations indicate that the circumnuclear molecular disk harboring the starburst is highly disturbed on 100 pc or smaller scales, presumably by individual young clusters and stellar explosions, in addition to globally disturbed in the form of the well-known superwind.

The Massive Stellar Content in the Starburst NGC 3049: A Test for Hot‐Star Models

The objective of this work is twofold. First, we seek evidence for or against the depletion of massive stars in metal-rich starbursts. A second, equally important goal is to perform a consistency test of the latest generation of starburst models in such a high-metallicity environment. We have obtained high spatial resolution ultraviolet and optical STIS spectroscopy and imaging of the metal-rich nuclear starburst in NGC 3049. The stellar continuum and the absorption-line spectrum in the ultraviolet are used to constrain the massive stellar population. The strong, blueshifted stellar lines of C IV and Si IV detected in the UV spectra indicate a metal-rich, compact, massive (~106 M☉) cluster of age 3-4 Myr emitting the UV-optical continuum. We find strong evidence against a depletion of massive stars in this metal-rich cluster. The derived age and the upper mass-limit cutoff of the initial mass function are also consistent with the detection of Wolf-Rayet (W-R) features at optical wavelengths. As a second, independent constraint on the massive stellar content, the nebular emission-line spectrum is modeled with photoionization codes using stellar spectra from evolutionary synthesis models. The morphology of the nuclear starburst of NGC 3049 from the STIS images indicates a simple geometry for the nebular emission-line region. However, the nebular lines are badly reproduced by 3-4 Myr instantaneous bursts, as required by the UV line spectrum, when unblanketed W-R and/or Kurucz stellar atmospheres are used. The corresponding number of photons above 24 and 54 eV in the synthetic models is too high in comparison with values suggested by the observed line ratios. Since the ionizing spectrum in this regime is dominated by emission from W-R stars, this discrepancy between observations and models is most likely the result of incorrect assumptions about the W-R stars. Thus, we conclude that the nebular spectrum of high-metallicity starbursts is poorly reproduced by models for W-R-dominated populations. However, the new model set of Smith et al. with blanketed W-R and O atmospheres and adjusted W-R temperatures predicts a softer far-UV radiation field, providing a better match to the data.

Observations and Modeling of the Nuclear Starburst in NGC 6946

Observations and Modeling of the Nuclear Starburst in NGC 6946

The interstellar medium of the hot-spot galaxy NGC 2903

We report 21 cm radio continuum, H-alpha, IRAS, and J = 1 --> 0 and 2 --> 1 CO observations of the hot-spot galaxy NGC 2903. The radio continuum, which traces synchrotron emission, H-alpha, which traces thermal H II regions, and CO, which traces neutral molecular gas, are distributed almost identically in a central bar. The CO data are consistent with emission from giant molecular clouds with properties typical of Milky Way clouds; that is, with large optical depths, excitation temperatures near 10 K, and sizes about 100 pc. If the standard CO luminosity to H-2 mass conversion factor holds in NGC 2903, then the central 770 pc contains about 3 x 10(8) M. in molecular gas, comparable to the central 500 pc of the Galaxy, and a mass surface density of approximately 240 M. pc-2, comparable to Orion and the Galactic center. The star formation rate per unit area is enhanced by an order of magnitude in the nuclear region. The global far-infrared-to-CO luminosity ratio, which is often interpreted as a measure of star formation efficiency, is typical of that for normal spirals. However, the local FIR/CO ratio varies considerably. In particular, 100-mu-m emission extends well outside the molecular bar into a region of H I emission. We associate this emission with cirrus, which contributes about 30% of the global 100-mu-m flux. Where cirrus emission dominates, the FIR/CO ratio does not reflect the local star formation efficiency. The nuclear starburst in NGC 2903 may result from a large amount of molecular gas with normal star formation efficiency collected into a small nuclear region, due to streaming motions induced by the bar.