Interferometric CO observations of the ultraluminous IRAS galaxies ARP 220, IC 694/NGC 3690, NGC 6420 and NGC 7469

High resolution CO observations of the IRAS galaxies Arp 220, IC 694/NGC 3690, NGC 6240 and NGC 7469 were made with the Millimeter Wave Interferometer of the Owen Valley Radio Observatory. These yield spatial information on scales of 1 to 5 kpc and allow the separation of compact condensations from the more extended emission in the galaxies. In the case of the obviously interacting system IC 694/NGC 3690 the contributions of each component can be discerned. For that galaxy, and also for Arp 220, the unusually high lumonisities may be produced by nonthermal processes rather than by intense bursts of star formation

H₂ and Infrared in Global Starburst Galaxies

Far infrared measurements from the IRAS survey combined with estimates of the molecular gas content provide a fundamental basis for the analysis of the starburst phenomena in galaxies. When the ratio of far infrared luminosity to molecular gas mass significantly exceeds that in normal galaxies like the Milky Way (4 L_⊙ M_⊙⁻¹), star formation is occurring on a shorter timescale, possibly with an initial mass function-biased towards high mass stars. In the highest luminosity IRAS galaxies (L_(IR) ≥ 10¹¹ L_⊙), the luminosity to H₂ mass ratio is typically 40 L_⊙ M_⊙⁻¹, indicating star formation rates of 10- 100 M_⊙ yr⁻¹ and cycling times for the ISM much less than 10⁹ yr. In the very luminous infrared galaxies, the optical morphology almost invariably shows evidence of a strong galactic interaction and a substantial fraction of the total molecular gas content is seen at radii ~ 1 kpc. Dense molecular gas probably plays a pivotal role in the evolution of such dynamically disturbed systems: being dissipative, the gas can readily sink to the center of the interacting system where it may fuel a nuclear starburst and/or build up and fuel a central active nucleus. We show that the shape of the high luminosity end of the infrared galaxy luminosity function can be reproduced by a model in which normal spiral galaxies, represented in the Schecter function, undergo collision-induced starbursts. Statistics from the IRAS survey are consistent with the percentage of all spiral galaxies currently undergoing a global starburst being approximately 0.2% and the lifetime of the starburst being a dynamical time, approximately 2 x 10⁸ years. The present epoch rate is therefore such that 2% of all galaxies participate in a merger every 109 years and with standard cosmological evolution, nearly all galaxies would be undergoing such merger-induced starbursts at z= 1. Galactic merging and starburst activity must therefore play a central role in galactic evolution

CO aperture synthesis of NGC 4038/9 (ARP 244)

Researchers present high-resolution (approx. 6 seconds) CO observations of the merging galaxies NGC 4038/9 made with the Owens Valley Radio Observatory (OVRO) Millimeter Wave Interferometer. The CO observations of Arp 244 were obtained between April and June 1988 using the OVRO Millimeter Wave Interferometer. Two fields with phase centers near the NGC 4039 nucleus and near the NGC 4038 nucleus were observed. The size of the synthesized beam is approximately 6.5 x 7 seconds at PA=72 degrees. The rms in a single cleaned channel map is 0.06 Jy beam(exp -1), corresponding to a brightness temperature of 0.12 K over the synthesized beam. Contour maps of the integrated CO intensity for both interferometer fields are shown. Three CO concentrations are evident. Two are centered near the nuclei of NGC 4038 and NGC 4039, closely correlated with H alpha and radio continuum maxima. A third CO emission region lies about 25 seconds northeast of the NGC 4039 nucleus. A number of radio continuum, H alpha, and 10 micron emission knots appear in this region. The total integrated intensity at the northern nuclear source, 302 K km/s, leads to a molecular mass of 8.3 by 10 to the 8th power solar mass assuming a Galactic CO to H2 conversion factor of 3.0 x 10 to the 20th power H2 cm(-2) (K km/s)(-1). The integrated CO intensity of the southern nuclear source leads to a molecular mass of 2.4 x 10 to the 8th solar mass. The extranuclear CO concentration contains 1.2 x 10 to the 9th power solar mass of molecular gas, extending over 170 km/s, and is resolved in a number of channels. Its large size, mass, and morphology strongly suggest that it is an agglomeration of several clumps

Radiation Pressure Supported Starburst Disks and AGN Fueling

We consider the structure of marginally Toomre-stable starburst disks under the assumption that radiation pressure on dust grains provides the dominant vertical support against gravity. This is particularly appropriate when the disk is optically thick to its own IR radiation, as in the central regions of ULIRGs. Because the disk radiates at its Eddington limit, the Schmidt-law for star formation changes in the optically-thick limit, with the star formation rate per unit area scaling as Sigma_g/kappa, where Sigma_g is the gas surface density and kappa is the mean opacity. We show that optically thick starburst disks have a characteristic flux and dust effective temperature of F ~ 10^{13} L_sun/kpc^2 and T_eff ~ 90K, respectively. We compare our predictions with observations and find good agreement. We extend our model from many-hundred parsec scales to sub-parsec scales and address the problem of fueling AGN. We assume that angular momentum transport proceeds via global torques rather than a local viscosity. We account for the radial depletion of gas due to star formation and find a strong bifurcation between two classes of disk models: (1) solutions with a starburst on large scales that consumes all of the gas with little fueling of a central AGN and (2) models with an outer large-scale starburst accompanied by a more compact starburst on 1-10 pc scales and a bright central AGN. The luminosity of the latter models is in many cases dominated by the AGN. We show that the vertical thickness of the starburst disk on pc scales can approach h ~ r, perhaps accounting for the nuclear obscuration in some Type 2 AGN. We also argue that the disk of young stars in the Galactic Center may be the remnant of such a compact nuclear starburst.Comment: 26 pages, 9 figures, emulateapj, accepted to ApJ, minor changes, discussion tightened, references adde

VIMOS Ultra-Deep Survey (VUDS): Witnessing the assembly of a massive cluster at z ~ 3.3

Using new spectroscopic observations obtained as part of the VIMOS Ultra-Deep Survey (VUDS), we performed a systematic search for overdense environments in the early universe (z > 2) and report here on the discovery of C1 J0227-0421, a massive protocluster at z = 3.29. This protocluster is characterized by both the large overdensity of spectroscopically confirmed members, δ_(gal) = 10.5 ± 2.8, and a significant overdensity in photometric redshift members. The halo mass of this protocluster is estimated by a variety of methods to be ~3 x 10^(14) M_☉ at z ~ 3.3, which, evolved to z = 0 results in a halo mass rivaling or exceeding that of the Coma cluster. The properties of 19 spectroscopically confirmed member galaxies are compared with a large sample of VUDS/VVDS galaxies in lower density field environments at similar redshifts. We find tentative evidence for an excess of redder, brighter, and more massive galaxies within the confines of the protocluster relative to the field population, which suggests that we may be observing the beginning of environmentally induced quenching. The properties of these galaxies are investigated, including a discussion of the brightest protocluster galaxy, which appears to be undergoing vigorous coeval nuclear and starburst activity. The remaining member galaxies appear to have characteristics that are largely similar to the field population. Though we find weaker evidence of the suppression of the median star formation rates among and differences in the stacked spectra of member galaxies with respect to the field, we defer any conclusions about these trends to future work with the ensemble of protostructures that are found in the full VUDS sample

Arp 299 - Two Interacting Galaxies

Arp 299 (Mrk 171), is an interacting system at 42 Mpc, comprising the galaxies IC 694 and NGC 3690. Interferometric CO maps at 6″ resolution (Sargent et al. 1987) showed compact molecular condensations at the nucleus of IC694 and in the overlap region of the galaxy disks [positions A and C – C in the terminology of Gehrz, Sramek, and Weedman (1983)]

The galactic distribution (in radius and Z) of interstellar molecular hydrogen

Observations of the galactic longitude and latitude distributions of lambda = 2.6 mm CO emission are presented. Analysis of these spectral-line data yields the large-scale distribution of molecular clouds in the galactic disk and their z-distribution out of the disk. Strong maxima in the number of molecular clouds occur in the galactic nucleus and at galactic radii 4 to 8 kpc. The peak at 4 to 8 kpc correlates well with a region of enhanced 100-MeV γ-ray emissivity. This correlation strongly supports the conclusion that the γ-rays are produced as a result of cosmic ray interactions in molecular H_2 clouds rather than in H(I). The width of the cloud layer perpendicular to the galactic plane between half-density points is 105 ± 15 pc near the 5.5-kpc peak. The total mass of molecular gas in the interior of the galaxy exceeds that of atomic hydrogen and is 3·10^9 M⊙ based on these observations