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NASA NTRS1993

Kiloparsec-scale molecular gas excitation in spiral galaxies

We combine beam-matched (C-13)O, (C-12)O J = 3 - 2 and J = 2 - 1 line data to infer the molecular gas excitation conditions in the central 500 to 1600 pc diameters of a small sample of IR-bright external galaxies: NGC 253, IC 342, M83, Maffei 2, and NGC 6946. We find that the central 170 to 530 pc diameter regions have typical molecular gas densities ranging from approximately less than 10,000/cu cm (in M83) to approximately greater than 100,000/cu cm (in NGC 253) and that, outside of these regions, the densities are likely to be approximately less than 10,000/cu cm. The molecular clouds outside the inner 170-530 pc are at least as warm as the molecular clouds in our Galaxy. Column densities derived from integrated (C-13)O line strengths and H-alpha surface brightnesses suggest that the star formation rate is enhanced in the central 170-530 pc diameters by an order of magnitude over that inferred for the outer star-forming disks in spiral galaxies.

Wall, W. F.↗
NASA NTRS1986

Molecular gas in high-luminosity IRAS galaxies

The paper reports observations of CO(J = 1-0) emission from an unbiased sample of the highest-luminosity IRAS galaxies with the aim of measuring their molecular gas content and determining whether star formation is a viable energy source for these high luminosities. All of the observed galaxies are rich in molecular gas with H2 masses in the range (4 x 10 to the 9th)-(4 x 10 to the 10th) solar masses. Their primary luminosity source appears to be star formation in molecular clouds. The majority, if not all, of the most luminous IRAS galaxies (L-FIR greater than 10 to the 11th solar luminosities) appear to be strongly interacting systems; those with the highest L-FIR/M(H2) ratios are mergers or close contact pairs.

Sanders, D. B.↗
NASA NTRS1989

Molecular gas, the interstellar medium, and star formation in S0 and Sa galaxies

The results are presented of a survey for CO J = 1 - 0 emission from S0 and S0/a galaxies. The results show that molecular gas is abundant within some early-type disk galaxies, and that the range in the ratio of molecular gas to atomic gas mass is similar to those in other disk galaxies. In the S0 and S0/a galaxies studied, estimated rates of star formation are substantially smaller than, but efficiencies of star formation are roughly the same as, those in Sb or Sc galaxies. Although the rate of cooling of the hot, X-ray emitting gas may be close to the estimated stellar mass return rate in the sample, the star formation rate probably exceeds both by a significant factor.

Thronson, Harley A., Jr.↗
NASA NTRS1975

Molecular gas species in the lunar atmosphere

Evidence is presented from the data obtained by the Apollo 17 lunar mass spectrometer which indicates the presence of methane and perhaps very small amounts of ammonia and carbon dioxide in the lunar atmosphere. This evidence is based on predawn enhancement of the concentrations of the mass peaks at the parent position for these molecular gas compounds. Methane is shown to be the most abundant molecular gas, although its exceedingly low concentration (1000 mol/cu cm) is slightly less than that of Ar-36. Several reasons are considered for the very low concentration of methane in the lunar atmosphere.

Hoffman, J. H.↗
NASA NTRS1993

Molecular gas in powerful radio galaxies detected by IRAS

The present CO(J = 1 to 0) line survey of powerful radio galaxies has led to the detection of five sources in eight IRAS-determined radio galaxies. The range of the computed molecular gas masses is 1-7 times the H2 mass of the Milky Way and strikingly contrasts with the low molecular gas masses found in radio-quiet FIR-selected elliptical galaxies. These new CO observations lend support to the hypothesis that powerful radio galaxies result from disk galaxy collisions that evolve into gas-rich, peculiar E/S0 galaxies in the course of their merging.

Mazzarella, J. M.↗
NASA NTRS1989

Molecular gas in the powerful radio galaxies Perseus A and 4C 12.50

Perseus A, 4C 12.50, and Cygnus A have been observed in the CO(J = 1 to 0) line. In Perseus, an estimate of 3.2 x 10 to the 9th solar masses is found for the molecular gas in the giant galaxy in the cluster center. An upper limit of M(H2) = 9 x 10 to the 9th solar masses is obtained for the galaxy in the center of Cygnus A. The results suggest that large amounts of molecular gas, and thus high rates of star formation, are involved in the genesis of the central engines that power extragalactic radio sources.

Mirabel, I. F.↗
NASA NTRS1989

Optical depth of molecular gas in starburst galaxies - Is M82 the prototype?

An attempt is made to survey the CO(2-1) emission toward the centers of 17 IR-luminous galaxies which have previously been detected in CO(1-0). These galaxies span a wide range of size and L(FIR)/L(B) ratio, many have multiple-wavelength studies establishing them as starbursts, and some bear a morphological resemblance to M 82. Nine galaxies are detected and useful upper limits are placed on the remaining eight. Using the CO(2-1)/CO(1-0) ratio of antenna temperature as a diagnostic of optical depth, it is found that all of the galaxies contain predominantly optically thick molecular gas. This implies that the phase of starburst during which the molecular gas is optically thin, currently witnessed in M 82, is either uncommon or short-lived.

Verter, F.↗
NASA NTRS1993

Circumstellar molecular gas of the HH 34 and HH 111 exciting stars

The HH 34 and HH 111 exciting stars, two pre-main-sequence objects which are the sources of highly collimated optical jets, have been observed in the J = 1-0 transition of (C-13)O and in the 2.7 mm continuum using the Owens Valley Radio Observatory Millimeter Interferometer. The high-resolution aperture synthesis maps reveal dense molecular gas concentrations at the positions of both stars. The morphology and kinematics of the bright emission-line cores suggest that the molecular gas is distributed in circumstellar disks, each about 2000 AU in diameter, elongated perpendicular to each system's outflow axis. A fainter component of (C-13)O emission extends along each source's outflow axis. Independent mass determinations based on the dust continuum and molecular line fluxes indicate for HH 34, a disk mass of 0.2 solar mass; and for HH 111, a disk mass of 0.3 solar mass. The inferred circumstellar disks of these two jet sources are significantly more massive than those found associated with T Tauri stars and are therefore among the most massive disks known in association with low-mass pre-main-sequence stars.

Stapelfeldt, Karl R.↗
NASA NTRS1999

A New Probe of the Molecular Gas Content in Galaxies: Application to M101

Studies of nearby spiral galaxies suggest that photodissociation regions (PDRs) are capable of producing the observed large--scale distribution of HI (Allen et al.- 1997, and references therein). The column density of HI in a PDR is fundamentally linked to the amount of far--ultraviolet (FUV) emission produced by nearby young stars and the local molecular gas volume density. Measurements of the HI column density and the FUV emission associated with PDRs thus provide a new probe of the molecular gas distribution in nearby galaxies. Advantages of this method include its insensitivity to assumptions about the CO/{\rm H2} conversion factor or the gas temperature. We discuss the application of this method to M101. The HI column density and FUV emission have been measured for 35 PDRs from VLA data (Braun 1997) and Ultraviolet Imaging Telescope data (Waller et al.-1997). We derive volume densities ranging from n-100 {\rm cm(exp -3)} in the central HI--poor regions of M101 to n -3000 {\rm cm(exp -3)} in the HI--rich periphery of the galaxy.

Smith, D. A.↗
NASA NTRS1989

No molecular gas disk in S106

The radio and optical bipolar H II region, S106, is bisected by a dark lane. The premain-sequence object, S106 IR, which is the source of a powerful ionized stellar wind, is the exciting source of this region and is found at the center of the equatorial emission gap. The existence of a massive, extended, molecular gas disk has previously been suggested as an explanation for the peculiar morphology of this source, and S106 has widely been quoted as the best example of theoretically posited accretion disks. The new, high-resolution, CS and (C-13)O observations presented show that the molecular emission, previously attributed to a disk structure, actually originates from distinct masses of molecular gas, swept up from the ambient cloud core by the ionized lobes.

Barsony, M.↗
NASA NTRS2013

ALMA Observations of the Molecular Gas in the Debris Disk of the 30 Myr Old Star HD 21997

The 30 Myr old A3-type star HD 21997 is one of the two known debris dust disks having a measurable amount of cold molecular gas. With the goal of understanding the physical state, origin, and evolution of the gas in young debris disks, we obtained CO line observations with the Atacama Large Millimeter/submillimeter Array (ALMA). Here, we report on the detection of (12)CO and (13)CO in the J = 2-1 and J = 3-2 transitions and C(18)O in the J = 2-1 line. The gas exhibits a Keplerian velocity curve, one of the few direct measurements of Keplerian rotation in young debris disks. The measured CO brightness distribution could be reproduced by a simple star+disk system, whose parameters are r(sub in) < 26 AU, r(sub out) = 138 +/- 20 AU, Stellar M = 1.8 +0.5/−0.2 Solar M, and i = 32. Deg. 6 +/- 3 deg..1. The total CO mass, as calculated from the optically thin C(18)O line, is about (4-8) ×10(exp −2 ) Solar M, while the CO line ratios suggest a radiation temperature on the order of 6-9 K. Comparing our results with those obtained for the dust component of the HD 21997 disk from ALMA continuum observations by Moor et al., we conclude that comparable amounts of CO gas and dust are present in the disk. Interestingly, the gas and dust in the HD 21997 system are not colocated, indicating a dust-free inner gas disk within 55 AU of the star. We explore two possible scenarios for the origin of the gas. A secondary origin, which involves gas production from colliding or active planetesimals, would require unreasonably high gas production rates and would not explain why the gas and dust are not colocated. We propose that HD 21997 is a hybrid system where secondary debris dust and primordial gas coexist. HD 21997, whose age exceeds both the model predictions for disk clearing and the ages of the oldest T Tauri-like or transitional gas disks in the literature, may be a key object linking the primordial and the debris phases of disk evolution.

molecular gas↗
NASA NTRS1990

Dissociation and ionization of molecular gas in the spiral arms of M51

Researchers derive the star formation rate and efficiency in the arm and interarm regions of M51 from observations of the molecular (Lo et al. 1987) and ionized (van der Hulst et al. 1988) phases of the interstellar medium, and show that the HI observations of Tilanus and Allen (1989) are consistent with dissociation of molecular gas by these young, massive stars if n sub H greater than or equal to 200 cm (-2). However, these stars are not able to dissociate or ionize all the gas, and at least 60 percent must remain molecular in the interarm regions. The efficiency of star formation in M51 seems to be similar to that in the Galaxy, and does not appear to be enhanced in the spiral arms. Therefore, the effect of the strong density wave may be only to concentrate the gas, and hence the young stars, to the arm regions.

Lees, J. F.↗
NASA NTRS1994

(abstract) Circumstellar Molecular Gas of the HH 1-2 and HH 24 Exciting Stars

13 CO and C 18 O observations of HH 1-2 VLA and SSV 63, the exciting stars of the HH 1-2 and HH 24 outflows, have been obtained with the Owens Valley Millimeter Array. The HH 1-2 source has been detected in both lines and in the 2.7 mm continuum. In the continuum, the central source appears unresolved in the 5" beam with a flux density of 20 mJy. A secondary peak of 8 mJy appears 12" SE of VLA 1. A strong peak of molecular line emission appears at the position of VLA 1. Molecular gas associated with SSV 63 has also been detected. Maps of circumstellar material of both sources will be presented.

circumstellar↗
NASA NTRS1987

Molecular gas and star formation in HI-deficient Virgo cluster galaxies

Mapping of the CO emission line in 42 Virgo cluster galaxies reveals that the molecular gas contents and distributions are roughly normal in severaly HI-deficient Virgo spirals. The survival of the molecular component mitigates the impact of the HI-stripping on star formation and subsequent galactic evolution. For spirals which are deficient in HI by a factor of 10, far-infrared, H alpha line, and nonthermal radio continuum luminosities are lower by no more than a factor of 2. The fact that the inner galactic disks are stripped of HI, while CO is normal, suggests that the lifetime of the molecular phase is approximately one billion years in the inner regions of luminous spirals.

Kenney, Jeffrey D.↗
NASA NTRS1993

Molecular gas temperature and density in spiral galaxies

We combine beam-matched CO-13, CO-12 J = 3 yields 2 and J = 2 yields 1 line data to infer the molecular gas excitation conditions in the central 500 to 1600 pc diameters of a small sample of infrared-bright external galaxies: NGC253, IC342, M 83, Maffei 2, and NGC6946. Additional observations of the J = 1 yields 0 lines of C-18O and CO-13 set limits on the opacity of the CO-13 J = 1 yields 0 line averaged over the central kiloparsec of these spiral galaxies.

Wall, W. F.↗
NASA NTRS2020

High-resolution Infrared Spectroscopy of Hot Molecular Gas in AFGL 2591 and AFGL 2136: Accretion in the Inner Regions of Disks around Massive Young Stellar Objects

We have performed a high-resolution 4–13 μm spectral survey of the hot molecular gas associated with the massive protostars AFGL 2591 and AFGL 2136, utilizing the Echelon Cross Echelle Spectrograph (EXES) on board the Stratospheric Observatory for Infrared Astronomy, and the iSHELL instrument and Texas EchelonCross Echelle Spectrograph (TEXES) on the NASA Infrared Telescope Facility (IRTF). Here we present the results of this survey with analysis of CO, HCN, C2H2, NH3, and CS, deriving the physical conditions for each species. Also from the IRTF, iSHELL data at 3 μm for AFGL 2591are presented that show HCN and C2H2 in emission. In the EXES and TEXES data, all species are detected in absorption, and temperatures and abundances are found to be high (600 K and 10−6, respectively). Differences of up to an order of magnitude in the abundances of transitions that trace the same ground-state level are measured for HCN and C2H2. The mid-infrared continuum is known to originate in a disk, hence we attribute the infrared absorption to arise in the photosphere of the disk. As absorption lines require an outwardly decreasing temperature gradient, we conclude that the disk is heated in the midplane by viscous heating due to accretion. We attribute the near-IR emission lines to scattering by molecules in the upper layers of the disk photosphere. The absorption lines trace the disk properties at 50 au where high-temperature gasphase chemistry is taking place. Abundances are consistent with chemical models of the inner disk of Herbig disks.

Andrew Barr↗
NASA NTRS1991

Orbit crowding of molecular gas at a bar-spiral arm transition zone in M83

The southwestern bar-spiral arm transition zone in M83 is been studied in CO, H-alpha, H I, red light, and the radio continuum. A massive molecular gas complex in the heart of the transition zone is composed or two principal components which have the morphology and kinematics expected from orbit crowding, where gas on highly elliptical orbits form the bar region converges with gas on more circular orbits from the spiral arm region. Three mechanisms for the origin of the orbit crowding are investigated, and it is proposed that the crowding is due primarily to density wave streaming motions caused by the bar and spiral arms. The inner CO component is partially coincident with a region of highly polarized radio continuum emission which precedes the H-alpha spiral arm by 15-25 arcsec, indicating that it lies on or just downstream from a shock front. This suggests that the bar gas approaching the transition zone is shocked and explains the ridge of dense gas seen upstream from the spiral arm.

Kenney, Jeffrey D. P.↗
NASA NTRS1987

Compact molecular gas structure in the interacting galaxy pair Arp 299 (IC 694-NGC 3690)

High-resolution (about 5-arcsec) CO observations of the interacting galaxy pair Arp 299 (IC 694-NGC 3690) show that about 40 percent of the total molecular gas content is concentrated in two compact regions each of mass 1.4 x 10 to the 9th solar mass. One component lies at the nucleus of IC 694, while the other spans the region of overlap between the two galaxies. The properties of the latter component are consistent with its being a region of greatly enhanced star formation. However, the unusually high L(FIR)/M(H2) ration for IC 694, together with the presence of a flat-spectrum radio source, suggest that its remarkable luminosity may be largely produced by nonthermal processes.

Sargent, A. I.↗