Quiescent Molecular Gas Research Articles

Cloud-scale radio surveys of star formation and feedback in Triangulum Galaxy M 33: VLA observations

ABSTRACT Studying the interplay between massive star formation and the interstellar medium (ISM) is paramount to understand the evolution of galaxies. Radio continuum (RC) emission serves as an extinction-free tracer of both massive star formation and the energetic components of the ISM. We present a multiband RC survey of the Local Group galaxy M 33 down to ≃30 pc linear resolution observed with the Karl G. Jansky Very Large Array (VLA). We calibrate the star formation rate surface density and investigate the impact of diffuse emission on this calibration using a structural decomposition. Separating the thermal and non-thermal emission components, the correlation between different phases of the ISM, and the impact of massive star formation are being investigated. Radio sources with sizes ≲200 pc constitute about 36 per cent (46 per cent) of the total RC emission at 1.5 GHz (6.3 GHz) in the inner 18 × 18 arcmin2 (or 4 kpc × 4 kpc) disc of M 33. The non-thermal spectral index becomes flatter with increasing star formation rate surface density, indicating the escape of cosmic ray electrons from their birth places. The magnetic field strength also increases with star formation rate following a bi-modal relation, indicating that the small-scale turbulent dynamo acts more efficiently at higher luminosities and star formation rates. Although the correlations are tighter in star-forming regions, the non-thermal emission is also correlated with the more quiescent molecular gas in the ISM. An almost linear molecular star formation law exists in M 33 when excluding diffuse structures. Massive star formation amplifies the magnetic field and increases the number of high-energy cosmic ray electrons, which can help the onset of winds and outflows.

The Gas–Star Formation Cycle in Nearby Star-forming Galaxies. I. Assessment of Multi-scale Variations

Abstract The processes regulating star formation in galaxies are thought to act across a hierarchy of spatial scales. To connect extragalactic star formation relations from global and kiloparsec-scale measurements to recent cloud-scale resolution studies, we have developed a simple, robust method that quantifies the scale dependence of the relative spatial distributions of molecular gas and recent star formation. In this paper, we apply this method to eight galaxies with ∼1″ resolution molecular gas imaging from the Physics at High Angular resolution in Nearby GalaxieS–ALMA (PHANGS–ALMA) survey and PdBI Arcsecond Whirlpool Survey (PAWS) that have matched resolution, high-quality narrowband Hα imaging. At a common scale of 140 pc, our massive (log(M ⋆[M ⊙]) = 9.3–10.7), normally star-forming (SFR[M ⊙ yr−1] = 0.3–5.9) galaxies exhibit a significant reservoir of quiescent molecular gas not associated with star formation as traced by Hα emission. Galactic structures act as backbones for both molecular gas and H ii region distributions. As we degrade the spatial resolution, the quiescent molecular gas disappears, with the most rapid changes occurring for resolutions up to ∼0.5 kpc. As the resolution becomes poorer, the morphological features become indistinct for spatial scales larger than ∼1 kpc. The method is a promising tool to search for relationships between the quiescent or star-forming molecular reservoir and galaxy properties, but requires a larger sample size to identify robust correlations between the star-forming molecular gas fraction and global galaxy parameters.

A 3 mm Spectral Line Survey toward the Barred Spiral Galaxy NGC 3627

Abstract We conduct spectral line survey observations in the 3 mm band toward a spiral arm, a bar-end, and a nuclear region of the nearby barred spiral galaxy NGC 3627 with the IRAM 30 m telescope and the Nobeyama 45 m telescope. Additional observations are performed toward the spiral arm and the bar-end in the 2 mm band. We detect 8, 11, and 9 molecular species in the spiral arm, the bar-end, and the nuclear region, respectively. Star formation activities are different among the three regions, and in particular, the nucleus of NGC 3627 is known as a low-ionization nuclear emission region/Seyfert 2 type nucleus. In spite of these physical differences, the chemical composition shows impressive similarities among the three regions. This result means that the characteristic chemical composition associated with these regions is insensitive to the local physical conditions such as star formation rate, because such local effects are smeared out by extended quiescent molecular gas on scales of 1 kpc. Moreover, the observed chemical compositions are also found to be similar to those of molecular clouds in our Galaxy and the spiral arm of M51, whose elemental abundances are close to those in NGC 3627. Therefore, this study provides us with a standard template of the chemical composition of extended molecular clouds with the solar metallicity in nearby galaxies.

Molecular-cloud-scale Chemical Composition. I. A Mapping Spectral Line Survey toward W51 in the 3 mm Band

Abstract We have conducted a mapping spectral line survey toward the Galactic giant molecular cloud W51 in the 3 mm band with the Mopra 22 m telescope in order to study an averaged chemical composition of the gas extended over a molecular-cloud scale in our Galaxy. We have observed the area of 25′ × 30′, which corresponds to 39 pc × 47 pc. The frequency ranges of the observation are 85.1–101.1 GHz and 107.0–114.9 GHz. In the spectrum spatially averaged over the observed area, spectral lines of 12 molecular species and 4 additional isotopologues are identified. An intensity pattern of the spatially averaged spectrum is found to be similar to that of the spiral arm in the external galaxy M51, indicating that these two sources have similar chemical compositions. The observed area has been classified into five subregions according to the integrated intensity of 13CO(J = 1 − 0) ( ), and contributions of the fluxes of 11 molecular lines from each subregion to the averaged spectrum have been evaluated. For most of the molecular species, 50% or more of the flux comes from the subregions with from 25 to 100 K km s−1, which does not involve active star-forming regions. Therefore, the molecular-cloud-scale spectrum observed in the 3 mm band hardly represents the chemical composition of star-forming cores, but mainly represents the chemical composition of an extended quiescent molecular gas. The present result constitutes a sound base for interpreting the spectra of external galaxies at a resolution of a molecular-cloud scale (∼10 pc) or larger.

Tracing the molecular gas in distant submillimetre galaxies via CO(1-0) imaging with the Expanded Very Large Array

We report the results of a pilot study with the EVLA of 12CO J=1-0 emission from four SMGs at z=2.2-2.5, each with an existing detection of CO J=3-2. Using the EVLA's most compact configuration we detect strong, broad J=1-0 line emission from all of our targets. The median line width ratio, sigma(1-0)/sigma(3-2) = 1.15 +/- 0.06, suggests that the J=1-0 is more spatially extended than the J=3-2 emission, a situation confirmed by our maps which reveal velocity structure in several cases and typical sizes of ~16 kpc FWHM. The median Tb ratio is r(3-2/1-0) = 0.55 +/- 0.05, noting that our value may be biased high because of the J=3-2-based sample selection. Naively, this suggests gas masses ~2x higher than estimates made using higher-J transitions of CO, with the discrepency due to the difference in assumed Tb ratio. We also estimate masses using the 12CO J=1-0 line and the observed global Tb ratios, assuming standard underlying Tb ratios as well as a limiting SFE, i.e. without calling upon X(CO). Using this new method, we find a median molecular gas mass of (2.5 +/- 0.8) x 10^10 Msun, with a plausible range stretching 3x higher. Even larger masses cannot be ruled out, but are not favoured by dynamical constraints: the median dynamical mass for our sample is (2.3 +/- 1.4) x 10^11 Msun. We examine the Schmidt-Kennicutt relation for all the distant galaxy populations for which CO J=1-0 or J=2-1 data are available, finding small systematic differences. These have previously been interpreted as evidence for different modes of star formation, but we argue that these differences are to be expected, given the still considerable uncertainties. Finally, we discuss the morass of degeneracies surrounding molecular gas mass estimates, the possibilities for breaking them, and the future prospects for imaging and studying cold, quiescent molecular gas at high redshifts [abridged].

Molecular gas chemistry in AGN

<i>Context. <i/>This paper is part of a multi-species survey of line emission from the molecular gas in the circum-nuclear disk (CND) of the Seyfert 2 galaxy NGC 1068. Unlike in other active galaxies, the intensely star-forming regions in NGC 1068 and the CND can be resolved with current instrumentation. This makes this galaxy an optimal test-bed to probe the effects of AGN on the molecular medium at ~100 pc scales. <i>Aims. <i/> Single-dish observations have provided evidence that the abundance of silicon monoxide (SiO) in the CND of NGC 1068 is enhanced by 3–4 orders of magnitude with respect to the values typically measured in quiescent molecular gas in the Galaxy. We aim at unveiling the mechanism(s) underlying the SiO enhancement. <i>Methods. <i/> We have imaged the emission of the SiO(2-1) (86.8 GHz) and CN(2-1) (226.8 GHz) lines in NGC 1068 at ~<i>150<i/> pc and <i>60<i/> pc spatial resolution with the IRAM Plateau de Bure interferometer (PdBI). We have also obtained complementary IRAM 30 m observations of HNCO and methanol (CH<sub>3<sub/>OH) lines. These species are known as tracers of shocks in the Galaxy. <i>Results. <i/> SiO is detected in a disk of ~400 pc size around the AGN. SiO abundances in the CND of ~(1–5) × 10<sup>-9<sup/> are about 1-2 orders of magnitude above those measured in the starburst ring. The overall abundance of CN in the CND is high: ~(0.2–1) × 10<sup>-7<sup/>. The abundances of SiO and CN are enhanced at the extreme velocities of gas associated with non-circular motions close to the AGN (<i>r<i/> <i><<i/> 70 pc). On average, HNCO/SiO and CH<sub>3<sub/>OH/SiO line ratios in the CND are similar to those measured in prototypical shocked regions in our Galaxy. Yet the strength and abundance of CN in NGC 1068 can be explained neither by shocks nor by photon-dominated region (PDR) chemistry. Abundances measured for CN and SiO and the correlation of CN/CO and SiO/CO ratios with hard X-ray irradiation suggest that the CND of NGC 1068 has become a giant X-ray-dominated region (XDR). <i>Conclusions. <i/> The extreme properties of molecular gas in the circum-nuclear molecular disk of NGC 1068 result from the interplay between different processes directly linked to nuclear activity. The results presented here highlight in particular the footprint of shocks and X-ray irradiation on the properties of molecular gas in this Seyfert. Whereas XDR chemistry offers a simple explanation for CN and SiO in NGC 1068, the relevance of shocks deserves further scrutiny. The inclusion of dust grain chemistry would help solve the controversy regarding the abundances of other molecular species, like HCN, which are under-predicted by XDR models.

CO emission and variable CH and CH+absorption towards HD 34078: evidence for a nascent bow shock?

The runaway star HD34078, initially selected to investigate small scale structure in a foreground diffuse cloud has been shown to be surrounded by highly excited H2. We first search for an association between the foreground cloud and HD34078. Second, we extend previous investigations of temporal absorption line variations (CH, CH+, H2) in order to better characterize them. We have mapped the CO(2-1) emission at 12 arcsec resolution around HD34078's position, using the 30 m IRAM antenna. The follow-up of CH and CH+ absorption lines has been extended over 5 more years. In parallel, CH absorption towards the reddened star Zeta Per have been monitored to check the homogeneity of our measurements. Three more FUSE spectra have been obtained to search for N(H2) variations. CO observations show a pronounced maximum near HD34078's position, clearly indicating that the star and diffuse cloud are associated. The optical spectra confirm the reality of strong, rapid and correlated CH and CH+ fluctuations. On the other hand, N(H2, J=0) has varied by less than 5 % over 4 years. We also discard N(CH) variations towards Zeta Per at scales less than 20 AU. Observational constraints from this work and from 24 micron dust emission appear to be consistent with H2 excitation but inconsistent with steady-state bow shock models and rather suggest that the shell of compressed gas surrounding HD34078, is seen at an early stage of the interaction. The CH and CH+ time variations as well as their large abundances are likely due to chemical structure in the shocked gas layer located at the stellar wind/ambient cloud interface. Finally, the lack of variations for both N(H2, J=0) towards HD34078 and N(CH) towards Zeta Per suggests that quiescent molecular gas is not subject to pronounced small-scale structure.

Ion Chemistry in Photon‐dominated Regions: Examining the [HCO+]/[HOC+]/[CO+] Chemical Network

HOC+ and CO+ have been detected toward two well-known photon-dominated regions (PDRs), S140 and NGC 2023, using the Arizona Radio Observatory 12 m telescope. The J = 1 → 0 transition of HOC+ at 89 GHz and the two spin components of the N = 2 → 1 line of CO+ near 236 GHz were observed, as well as the J = 1 → 0 transitions of H13CO+ and HC18O+. The J = 3 → 2 line of HOC+ at 268 GHz was also mapped across the Orion Bar. The [HCO+]/[HOC+] ratios determined in S140 and NGC 2023 were ~12,408 and 1913, respectively, values indicative of quiescent molecular gas rather than PDR sources, where the abundance of HOC+ is thought to be enhanced. However, the beam in both these measurements may contain material from the adjoining molecular cloud, favoring HCO+. Alternatively, the [HCO+]/[HOC+] ratio may vary with Av in PDR regions. The [CO+]/[HOC+] ratio in S140 and NGC 2023, at several positions in the Orion Bar, and in other PDRs, on the other hand, falls uniformly in the range ~1-10. In addition, the line profiles of CO+ and HOC+ in the Orion Bar look remarkably similar. The syntheses of HOC+ and CO+ appear to be correlated in PDRs, most likely through the common precursor, C+. The reaction of C+ and H2O is thought to preferentially create HOC+, as opposed to HCO+, and C+ + OH or O2 leads favorably to CO+.

Cold Dust and Very Cold Excess Emission in the Galaxy

We provide insight into the origin of the far-IR to mm emission from the Galaxy by presenting a decomposition of the emission where we separate the contributions from dust in the atomic gas and dust associated with quiescent molecular gas, as a function of Galacto-centric distance. We find that the sub-mm dust emissivity per hydrogen change by a factor 3 between the diffuse ISM and molecular clouds, first observed on localized clouds in the Solar Neighborhood, applies on large scale to the Molecular Ring. The decomposition leaves an emission excess in the sub-mm with a very cold effective temperature, concentrated in the plane but with a brightness independent of Galactic longitude. This emission might be associated with interstellar matter in the outer Galaxy not traced by H I nor CO emission.

Cold Dust and Very Cold Excess Emission in the Galaxy

We provide insight in the origin of the far-IR to mm emission from galaxies by presenting a decomposition of the Galaxy emission where we separate the contributions from dust in the atomic gas and dust associated with quiescent molecular gas as a function of Galacto-centric distance. This decomposition leaves a puzzling very cold emission excess with a brightness independent of Galactic longitude that might be pointing at interstellar matter in the outer Galaxy not traced by H I nor CO emission.

Implications of [ITAL]Submillimeter Wave Astronomy Satellite[/ITAL] Observations for Interstellar Chemistry and Star Formation

A long-standing prediction of steady state gas-phase chemical theory is that H2O and O2 are important reservoirs of elemental oxygen and major coolants of the interstellar medium. Analysis of Submillimeter Wave Astronomy Satellite (SWAS) observations has set sensitive upper limits on the abundance of O2 and has provided H2O abundances toward a variety of star-forming regions. Based on these results, we show that gaseous H2O and O2 are not dominant carriers of elemental oxygen in molecular clouds. Instead, the available oxygen is presumably frozen on dust grains in the form of molecular ices, with a significant portion potentially remaining in atomic form, along with CO, in the gas phase. H2O and O2 are also not significant coolants for quiescent molecular gas. In the case of H2O, a number of known chemical processes can locally elevate its abundance in regions with enhanced temperatures, such as warm regions surrounding young stars or in hot shocked gas. Thus, water can be a locally important coolant. The new information provided by SWAS, when combined with recent results from the Infrared Space Observatory, also provides several hard observational constraints for theoretical models of the chemistry in molecular clouds, and we discuss various models that satisfy these conditions.

CO lines toward NGC 2024 and other star-forming regions - A closer look at the warm gas component

view Abstract Citations (48) References (58) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS CO Lines toward NGC 2024 and Other Star-forming Regions: A Closer Look at the Warm Gas Component Graf, U. U. ; Eckart, A. ; Genzel, R. ; Harris, A. I. ; Poglitsch, A. ; Russell, A. P. G. ; Stutzki, J. Abstract Observations of (C-13)O J = 6 yields 5 emission from a variety of galactic star-forming regions are reported. Sources spanning a luminosity range of six orders of magnitude were observed in this transition. These observations confirm the existence of large amounts of warm, dense, quiescent molecular gas in most Galactic star-forming regions. Mid-J lines of (C-12)O are very optically thick. The lower limit to the kinetic gas temperature is around 100 K. Typical column densities in the warm gas components are as high as 10 exp 23/sq cm or higher. The molecular hydrogen densities are high enough to keep the mid-J CO level population close to thermalized. The new measurements provide further evidence that the warm, dense, quiescent gas found in star formation regions is most likely to be heated by the energetic UV radiation escaping the H II regions around young stars. Publication: The Astrophysical Journal Pub Date: March 1993 DOI: 10.1086/172357 Bibcode: 1993ApJ...405..249G Keywords: Carbon Monoxide; Interstellar Gas; Line Spectra; Molecular Gases; Radio Emission; Star Formation; H Ii Regions; Submillimeter Waves; Astrophysics; ISM: INDIVIDUAL NGC NUMBER: NGC 2024; ISM: INDIVIDUAL ALPHANUMERIC: W51; ISM: INDIVIDUAL ALPHANUMERIC: S106; ISM: INDIVIDUAL MESSIER NUMBER: M17; RADIO LINES: ISM; STARS: FORMATION full text sources ADS | data products SIMBAD (14)

Carbon Monoxide in the Magellanic Clouds

Molecular gas is a major constituent of the interstellar medium of all late-type galaxies. Virtually all of it is in the form of cold molecular hydrogen (H2) which today cannot be observed directly. However, the tracer molecule carbon monoxide (CO) (relative abundance 10−5) is easily detected. For the Magellanic Clouds (MCs), CO studies are of specific importance. The Clouds are rich in HI, and if we can establish the presence of significant amounts of H2 as well, this will influence our estimates of the global rate of star formation and its history. Complexes of presently quiescent molecular gas may betray regions primed for star formation, but not yet in action. Detailed studies of the HI, HII and H2 and young star content may provide estimates of star formation efficiences on scales of a kiloparsec.

Submillimeter and far-infrared spectroscopy of M17 and S106 - UV-heated, quiescent molecular gas?

The authors report measurements of 372 μm J = 7→6 and 186 μm J = 14→13 CO line emission toward the interface between the molecular cloud and the H II region in M17, and toward the center of the bipolar nebula S106. The ratio of 14→13/7→6 lines indicates gas temperatures between 200 and 500 K, and hydrogen densities ≥10<SUP>4</SUP>cm<SUP>-3</SUP>. The 7→6 lines are remarkably narrow, with widths of ≡5 - 10 km s<SUP>-1</SUP>, identical to those of the cool quiescent molecular cloud cores. The warm quiescent molecular gas is in the interface between the exciting OB stars and the surrounding molecular cloud. Possible heating mechanisms of the warm CO gas are slow shocks and heating by photoelectrons in UV-illuminated, photodissociation regions.

12C18O in OMC-1: Kinematics, Molecular Column Density, and Kinetic Temperature Distribution

A fully sampled map of size ∼1′×3′ (R.A. Dec), centered on BN-KL has been made in the J = 1-0 line of 12C18O with 21″ angular resolution. The 12C18O emission is concentrated in a ← 40″ wide continuous strip running S to NE. Several maxima are superposed on the ridge, but none exceeds the average emission level by more than 40%. There is no intense peak of 12C18O J = 1-0 line emission centered on BN-KL, in contrast to maps of the dust emission. The dust and 12C18O results can be reconciled with a constant (CO/H2) ratio if there are variations in the kinetic temperature and column density of ∼50%. Peaks in both temperature and column density are then located near BN-KL, and 90″ to the south. From the estimated CO column density, about 10% of the carbon is in the form of CO. Near the BN-KL region, the 12C18O line profiles tend to become wider. These wider lines appear to be superposed on a weak, 18 km s−1 (FWHP) wide pedestal. In regions 40″ NE and 30″ S of BN-KL, the 12C18O lines have widths of less than 2 km s−1. Presumably, these are the locations of high density, quiescent molecular gas. The radial velocity of the CO emission increases from 6.5 km s−1 (at 90″ S) to 10.5 km s−1 (at 60″ NE) of BN-KL. Close to BN-KL, however, there is evidence that this trend is reversed.