CO Spectral Line Energy Distributions Research Articles

Submillimeter line emission from LMC 30 Doradus: The impact of a starburst on a low-metallicity environment

(Abridged) The 30 Dor region in the Large Magellanic Cloud (LMC) is the most vigorous star-forming region in the Local Group. Star formation in this region is taking place in low-metallicity molecular gas that is exposed to an extreme far--ultraviolet (FUV) radiation field powered by the massive compact star cluster R136. We used the NANTEN2 telescope to obtain high-angular resolution observations of the 12CO 4-3, 7-6, and 13CO 4-3 rotational lines and [CI] 3P1-3P0 and 3P2-3P1 fine-structure submillimeter transitions in 30Dor-10, the brightest CO and FIR-emitting cloud at the center of the 30Dor region. We derived the properties of the low-metallicity molecular gas using an excitation/radiative transfer code and found a self-consistent solution of the chemistry and thermal balance of the gas in the framework of a clumpy cloud PDR model. We compared the derived properties with those in the N159W region, which is exposed to a more moderate far-ultraviolet radiation field compared with 30Dor-10, but has similar metallicity. We also combined our CO detections with previously observed low-J CO transitions to derive the CO spectral-line energy distribution in 30Dor-10 and N159W. The separate excitation analysis of the submm CO lines and the neutral carbon fine structure lines shows that the mid-J CO and [CI]-emitting gas in the 30Dor-10 region has a temperature of about 160 K and a H2 density of about 10^4 cm^-3. We find that the molecular gas in 30Dor-10 is warmer and has a lower beam filling factor compared to that of N159W, which might be a result of the effect of a strong FUV radiation field heating and disrupting the low--metallicity molecular gas. We use a clumpy PDR model (including the [CII] line intensity reported in the literature) to constrain the FUV intensity to about chi_0 ~ 3100 and an average total H density of the clump ensemble of about 10^5 cm^-3 in 30Dor-10.

THE MOLECULAR GAS IN LUMINOUS INFRARED GALAXIES. II. EXTREME PHYSICAL CONDITIONS AND THEIR EFFECTS ON THEXcoFACTOR

In this work we conclude the analysis of our CO line survey of Luminous Infrared Galaxies (LIRGs: L_{IR}>=10^{11}L_{sol}) in the local Universe (Paper\,I), by focusing on the influence of their average ISM properties on the total molecular gas mass estimates via the so-called X_{co}=M(H_2)/L_{co,1-0} factor. One-phase radiative transfer models of the global CO Spectral Line Energy Distributions (SLEDs) yield an X_{co} distribution with: <X_{co}>\sim(0.6+/-0.2) M_{sol}(K km s^{-1} pc^2)^{-1} over a significant range of average gas densities, temperatures and dynamical states. The latter emerges as the most important parameter in determining X_{co}, with unbound states yielding low values and self-gravitating states the highest ones. Nevertheless in many (U)LIRGs where available higher-J CO lines (J=3--2, 4--3, and/or J=6--5) or HCN line data from the literature allow a separate assessment of the gas mass at high densities (>=10^{4} cm^{-3}) rather than a simple one-phase analysis we find that {\it near-Galactic X_{co} (3-6)\, M_sol\,(K\,km^{-1}\,pc^2)^{-1} values become possible.} We further show that in the highly turbulent molecular gas in ULIRGs a high-density component will be common and can be massive enough for its high X_{co} to dominate the average value for the entire galaxy. ......... ...this may have thus resulted to systematic underestimates of molecular gas mass in ULIRGs.

REDSHIFT DETERMINATION AND CO LINE EXCITATION MODELING FOR THE MULTIPLY LENSED GALAXY HLSW-01

We report on the redshift measurement and CO line excitation of HERMES J105751.1+573027 (HLSW-01), a strongly lensed submillimeter galaxy discovered in Herschel/SPIRE observations as part of the Herschel Multi-tiered Extragalactic Survey (HerMES). HLSW-01 is an ultra-luminous galaxy with an intrinsic far-infrared luminosity of LFIR = 1.4 × 1013 L☉, and is lensed by a massive group of galaxies into at least four images with a total magnification of μ = 10.9 ± 0.7. With the 100 GHz instantaneous bandwidth of the Z-Spec instrument on the Caltech Submillimeter Observatory, we robustly identify a redshift of z = 2.958 ± 0.007 for this source, using the simultaneous detection of four CO emission lines (J = 7 → 6, J = 8 → 7, J = 9 → 8, and J = 10 → 9). Combining the measured line fluxes for these high-J transitions with the J = 1 → 0, J = 3 → 2, and J = 5 → 4 line fluxes measured with the Green Bank Telescope, the Combined Array for Research in Millimeter Astronomy, and the Plateau de Bure Interferometer, respectively, we model the physical properties of the molecular gas in this galaxy. We find that the full CO spectral line energy distribution is described well by warm, moderate-density gas with Tkin = 86–235 K and cm−3. However, it is possible that the highest-J transitions are tracing a small fraction of very dense gas in molecular cloud cores, and two-component models that include a warm/dense molecular gas phase with Tkin ∼ 200 K, cm−3 are also consistent with these data. Higher signal-to-noise measurements of the Jup ⩾ 7 transitions with high spectral resolution, combined with high spatial resolution CO maps, are needed to improve our understanding of the gas excitation, morphology, and dynamics of this interesting high-redshift galaxy.

CO SPECTRAL LINE ENERGY DISTRIBUTIONS OF INFRARED-LUMINOUS GALAXIES AND ACTIVE GALACTIC NUCLEI

We report on new sensitive CO J=6-5 line observations of several luminous infrared Galaxies (LIRGs: L$_{\rm IR}$(8-1000$\mu $m$)\ga 10^{11}$ L$_{\odot}$), 36% (8/22) of them ULIRGs (L$_{\rm IR}$$>10^{12}$ L$_{\odot}$), and two powerful local AGN: the optically luminous QSO PG 1119+120, and the powerful radio galaxy 3C 293 using the James Clerk Maxwell Telescope (JCMT) on Mauna Kea in Hawaii. We combine these observations with existing low-J CO data and dust emission Spectral Energy Distributions (SEDs) in the far-infrared - submillimetre from the literature to constrain the properties of the star-forming ISM in these systems. We then build the first {\it local} CO Spectral Line Energy Distributions (SLEDs) for the {\it global} molecular gas reservoirs that reach up to high J-levels. These CO SLEDs are neither biased by strong lensing (which affects many of those constructed for high-redshift galaxies), nor suffer from undersampling of CO-bright regions (as most current high-J CO observations of nearby extended systems do). We find: ...

COJ= 6-5 IN Arp 220: STRONG EFFECTS OF DUST ON HIGH-JCO LINES

We report new single dish CO J=6--5 line observations for the archetypal Ultra Luminous Infrared Galaxy (ULIRG) Arp 220 with the James Clerk Maxwell Telescope atop Mauna Kea in Hawaii. The J=6--5 line is found to be faint, with brightness temperature ratios (6--5)/(1--0), (6--5)/(3--2) of R_{65/10} = 0.080+/-0.017 and R_{65/32} = 0.082+/-0.019, suggesting very low excitation conditions that cannot be reconciled with the warm and very dense molecular gas present in one of the most extreme starbursts in the local Universe. We find that an optically thick dust continuum, with tau(f>350 GHz)>=1 for the bulk of the warm dust and gas in Arp 220, submerges this line to an almost black body curve, reducing its flux, and {\it affecting its CO Spectral Line Energy Distribution (SLED) at high frequencies}. This also resolves the CII line deficiency in this object, first observed by {\it ISO}: the near absence of that line is a dust optical depth effect, not a dense Photodissociation Region (PDR) phenomenon. Finally we briefly comment on the possibility of such extreme ISM states in other ULIRGs in the distant Universe, and its consequences for the diagnostic utility of high frequency molecular and atomic ISM lines in such systems. In the case of Arp 220 we anticipate that the now spaceborne Herschel Space Observatory will find faint high-J CO lines at f>=690 GHz that would appear as sub-thermally excited with respect to the low-J ones as a result of the effects of dust absorption.