In external galaxies, molecular composition may be influenced by extreme environments such as starbursts and galaxy mergers. To study such molecular chemistry, we observed the luminous infrared ...galaxy and merger NGC 3256 using the Atacama Large Millimeter/submillimeter Array. We covered most of the 3 and 1.3 mm bands for a multispecies, multitransition analysis. We first analyzed intensity ratio maps of selected lines such as HCN/HCO+, which shows no enhancement at an active galactic nucleus. We then compared the chemical compositions within NGC 3256 at the two nuclei, tidal arms, and positions with influence from galactic outflows. We found the largest variation in SiO and CH3OH, species that are likely to be enhanced by shocks. Next, we compared the chemical compositions in the nuclei of NGC 3256, NGC 253, and Arp 220; these galactic nuclei have varying star formation efficiencies. Arp 220 shows higher abundances of SiO and HC3N than NGC 3256 and NGC 253. Abundances of most species do not show a strong correlation with star formation efficiencies, although the CH3CCH abundance seems to have a weak positive correlation with the star formation efficiency. Lastly, the chemistry of spiral arm positions in NGC 3256 is compared with that of W51, a Galactic molecular cloud complex in a spiral arm. We found higher fractional abundances of shock tracers, and possibly also a higher dense gas fraction in NGC 3256 compared with W51.
ABSTRACT
We present the highest resolution CO (2–1) observations obtained to date (0.25 arcsec) of NGC 3256 and use them to determine the detailed properties of the molecular interstellar medium in ...the central 6 kpc of this merger. Distributions of physical quantities are reported from pixel-by-pixel measurements at 55 and 120 pc scales and compared to disc galaxies observed by PHANGS-ALMA (Physics at High Angular resolution in Nearby GalaxieS with Atacama Large Millimeter/Submillimeter Array). Mass surface densities range from 8 to 5500 M⊙ pc−2 and velocity dispersions from 10 to 200 km s−1. Peak brightness temperatures as large as 37 K are measured, indicating the gas in NGC 3256 may be hotter than all regions in nearby disc galaxies measured by PHANGS-ALMA. Brightness temperatures even surpass those in the overlap region of NGC 4038/9 at the same scales. The majority of the gas appears unbound with median virial parameters of 7–19, although external pressure may bind some of the gas. High internal turbulent pressures of 105–1010 K cm−3 are found. Given the lack of significant trends in surface density, brightness temperature, and velocity dispersion with physical scale we argue the molecular gas is made up of a smooth medium down to 55 pc scales, unlike the more structured medium found in the PHANGS-ALMA disc galaxies.
We report ∼2″ resolution Atacama Large Millimeter/submillimeter Array observations of the HCN (1-0), HCO+ (1-0), CO (1-0), CO (2-1), and CO (3-2) lines toward the nearby merging double-nucleus galaxy ...NGC 3256. We find that the high-density gas outflow traced in HCN (1-0) and HCO+ (1-0) emission is colocated with the diffuse molecular outflow emanating from the southern nucleus, where a low-luminosity active galactic nucleus (AGN) is believed to be the dominant source of the far-infrared luminosity. On the other hand, the same lines were undetected in the outflow region associated with the northern nucleus, whose primary heating source is likely related to starburst activity without obvious signs of an AGN. Both the HCO+ (1-0)/CO (1-0) line ratio (i.e., dense gas fraction) and the CO (3-2)/CO (1-0) line ratio are larger in the southern outflow (0.20 0.04 and 1.3 0.2, respectively) than in the southern nucleus (0.08 0.01 and 0.7 0.1, respectively). By investigating these line ratios for each velocity component in the southern outflow, we find that the dense gas fraction increases and the CO (3-2)/CO (1-0) line ratio decreases toward the largest velocity offset. This suggests the existence of a two-phase (diffuse and clumpy) outflow. One possible scenario to produce such a two-phase outflow is an interaction between the jet and the interstellar medium, which possibly triggers shocks and/or star formation associated with the outflow.
We present the CO isotopologue Line Atlas within the Whirpool galaxy Survey (CLAWS), which is based on an IRAM 30 m large programme that provides a benchmark study of numerous faint CO isotopologues ...in the millimetre-wavelength regime across the full disc of the nearby grand-design spiral galaxy M 51 (NGC 5194). The survey’s core goal is to use the low-
J
CO isotopologue lines to constrain CO excitation and chemistry, and therefrom the local physical conditions of the gas. In this survey paper, we describe the CLAWS observing and data reduction strategies. We map the
J
= 1 → 0 and 2 → 1 transitions of the CO isotopologues
12
CO,
13
CO, C
18
O, and C
17
O, as well as several supplementary lines within the 1 mm and 3 mm window (CN(1-0), CS (2-1), CH
3
OH(2-1), N
2
H
+
(1–0), and HC
3
N (10–9)) at ~1 kpc resolution. A total observation time of 149 h offers unprecedented sensitivity. We use these data to explore several CO isotopologue line ratios in detail, study their radial (and azimuthal) trends, and investigate whether changes in line ratios stem from changes in interstellar medium properties such as gas temperatures, densities, or chemical abundances. For example, we find negative radial trends for the
13
CO/
12
CO, C
18
O/
12
CO, and C
18
O/
13
CO line ratios in their
J
= 1 → 0 transitions. We also find variations with the local environment, such as higher
12
CO (2–1)/(1–0) or
13
CO/
12
CO (1-0) line ratios in interarm regions compared to spiral arm regions. We propose that these aforementioned variations in CO line ratios are most likely due to a variation in the optical depth, though abundance variations due to selective nucleosynthesis on a galaxy-wide scale could also play a role. We also study the CO spectral line energy distribution (SLED) using archival JCMT
12
CO (3–2) data and find a variation in the SLED shape with local environmental parameters, suggesting changes in the optical depth, gas temperatures, or densities.
Aims. We present new IRAM Plateau de Bure Interferometer observations of Arp 220 in HCN, HCO+, HN13C J = 1 − 0, C2H N = 1 − 0, SiO J = 2 − 1, HNCO Jk,k′ = 50,4 − 40,4, CH3CN(6−5), CS J = 2 − 1 and ...5–4 and 13CO J = 1 − 0 and 2–1 and of NGC 6240 in HCN, HCO+J = 1 − 0 and C2H N = 1 − 0. In addition, we present Atacama Large Millimeter/submill-meter Array science verification observations of Arp 220 in CS J = 4 − 3 and CH3CN(10−9). Various lines are used to analyse the physical conditions of the molecular gas including the 12CO/13CO and 12CO/C18O abundance ratios. These observations will be made available to the public. Methods. We create brightness temperature line ratio maps to present the different physical conditions across Arp 220 and NGC 6240. In addition, we use the radiative transfer code RADEX and a Monte Carlo Markov chain likelihood code to model the 12CO, 13CO and C18O lines of Arp 220 at ~2′′ (~700 pc) scales, where the 12CO and C18O measurements were obtained from literature. Results. Line ratios of optically thick lines such as 12CO show smoothly varying ratios while the line ratios of optically thin lines such as 13CO show a east-west gradient across Arp 220. The HCN/HCO+ line ratio differs between Arp 220 and NGC 6240, where Arp 220 has line ratios above 2 and NGC 6240 below 1. The radiative transfer analysis solution is consistent with a warm (~40 K), moderately dense (~103.4 cm-3) molecular gas component averaged over the two nuclei. We find 12CO/13CO and 12CO/C18O abundance ratios of ~90 for both. The abundance enhancement of C18O can be explained by stellar nucleosynthesis enrichment of the interstellar medium.
We use the IRAM Large Program EMPIRE and new high-resolution ALMA data to measure 13CO(1-0)/C18O(1-0) intensity ratios across nine nearby spiral galaxies. These isotopologues of 12CO are typically ...optically thin across most of the area in galaxy disks, and this ratio allows us to gauge their relative abundance due to chemistry or stellar nucleosynthesis effects. Resolved 13CO/C18O gradients across normal galaxies have been rare due to the faintness of these lines. We find a mean 13CO/C18O ratio of 6.0 0.9 for the central regions of our galaxies. This agrees well with results in the Milky Way, but differs from results for starburst galaxies (3.4 0.9) and ultraluminous infrared galaxies (1.1 0.4). In our sample, the 13CO/C18O ratio consistently increases with increasing galactocentric radius and decreases with increasing star formation rate surface density. These trends could be explained if the isotopic abundances are altered by fractionation; the sense of the trends also agrees with those expected for carbon and oxygen isotopic abundance variations due to selective enrichment by massive stars.
ABSTRACT The cloud-scale density, velocity dispersion, and gravitational boundedness of the interstellar medium (ISM) vary within and among galaxies. In turbulent models, these properties play key ...roles in the ability of gas to form stars. New high-fidelity, high-resolution surveys offer the prospect to measure these quantities across galaxies. We present a simple approach to make such measurements and to test hypotheses that link small-scale gas structure to star formation and galactic environment. Our calculations capture the key physics of the Larson scaling relations, and we show good correspondence between our approach and a traditional "cloud properties" treatment. However, we argue that our method is preferable in many cases because of its simple, reproducible characterization of all emission. Using, low-J 12CO data from recent surveys, we characterize the molecular ISM at 60 pc resolution in the Antennae, the Large Magellanic Cloud (LMC), M31, M33, M51, and M74. We report the distributions of surface density, velocity dispersion, and gravitational boundedness at 60 pc scales and show galaxy-to-galaxy and intragalaxy variations in each. The distribution of flux as a function of surface density appears roughly lognormal with a 1 width of ∼0.3 dex, though the center of this distribution varies from galaxy to galaxy. The 60 pc resolution line width and molecular gas surface density correlate well, which is a fundamental behavior expected for virialized or free-falling gas. Varying the measurement scale for the LMC and M31, we show that the molecular ISM has higher surface densities, lower line widths, and more self-gravity at smaller scales.
ABSTRACT
We present new high-resolution 12CO J = 1–0, J = 2–1, and 13CO J = 1–0 maps of the early stage merger Arp 240 (NGC 5257/8) obtained with the Atacama Large Millimeter/submillimeter Array ...(ALMA). Simulations in the literature suggest that the merger has just completed its first passage; however, we find that this system has a lower global gas fraction but a higher star formation efficiency (SFE) compared to typical close galaxy pairs, which suggests that this system may already be in an advanced merger stage. We combine the ALMA data with 12CO J = 3–2 observations from the Submillimeter Array and carry out RADEX modelling on several different regions. Both, the RADEX modelling and a local thermal equilibrium (LTE) analysis show that the regions are most likely to have a CO-to-H2 conversion factor αCO close to or perhaps even smaller than the typical value for (ultra)luminous infrared galaxies. Using 33-GHz data from the Very Large Array to measure the star formation rate, we find that most star-forming regions have molecular gas depletion times of less than 100 Myr. We calculated the SFE per free-fall time for different regions and find some regions appear to have values greater than 100 per cent. We find these regions generally show evidence for young massive clusters (YMCs). After exploring various factors, we argue that this is mainly due to the fact that radio continuum emission in those regions is dominated by that from YMCs, which results in an overestimate of the SFE per free-fall time.
We present new high-resolution ALMA (13CO J = 1−0 and J = 2−1) and CARMA (12CO and 13CO J = 1−0) observations of two luminous infrared galaxies (LIRGs), Arp 55 and NGC 2623. The new data are ...complementary to published and archival submillimeter array observations of 12CO J = 2−1 and J = 3−2. We perform a Bayesian likelihood non-local thermodynamic equilibrium analysis to constrain the molecular gas physical conditions such as temperature, column, and volume densities and the 12CO/13CO abundance ratio. For Arp 55, an early/intermediate-staged merger, the line measurements are consistent with cold (∼10-20 K), dense (> cm−3) molecular gas. For NGC 2623, the molecular gas is warmer (∼110 K) and less dense ( cm−3). Because Arp 55 is an early/intermediate stage merger, while NGC 2623 is a merger remnant, the difference in physical conditions may be an indicator of merger stage. Comparing the temperature and volume density of several LIRGs shows that the molecular gas, averaged over ∼kiloparsec scales, of advanced mergers is in general warmer and less dense than early/intermediate stage mergers. We also find that the 12CO/13CO abundance ratio of NGC 2623 is unusually high (>250) when compared with the Milky Way; however, it follows a trend seen with other LIRGs in the literature. This high 12CO/13CO value is very likely due to stellar nucleosynthesis enrichment of the interstellar medium. On the other hand, Arp 55 has a more Galactic 12CO/13CO value with the most probable 12CO/13CO value being 20-30. We measure the CO-to-H2 conversion factor, , to be ∼0.1 and ∼0.7 (3 × 10−4/ ) M (K km s−1 pc2)−1 for Arp 55 and NGC 2623, respectively. Because Arp 55 is an early/intermediate-stage merger, this suggests that the transition from a Galactic conversion factor to a LIRG value happens at an even earlier merger stage.
We present new Atacama Large Millimeter/submillimeter Array Band 7 (∼340 GHz) observations of the dense gas tracers HCN, HCO+, and CS in the local, single-nucleus, ultraluminous infrared galaxy IRAS ...13120-5453. We find centrally enhanced HCN (4-3) emission, relative to HCO+ (4-3), but do not find evidence for radiative pumping of HCN. Considering the size of the starburst (0.5 kpc) and the estimated supernovae rate of ∼1.2 yr−1, the high HCN/HCO+ ratio can be explained by an enhanced HCN abundance as a result of mechanical heating by the supernovae, though the active galactic nucleus and winds may also contribute additional mechanical heating. The starburst size implies a high IR of 4.7 × 1012 L kpc−2, slightly below predictions of radiation-pressure limited starbursts. The HCN line profile has low-level wings, which we tentatively interpret as evidence for outflowing dense molecular gas. However, the dense molecular outflow seen in the HCN line wings is unlikely to escape the Galaxy and is destined to return to the nucleus and fuel future star formation. We also present modeling of Herschel observations of the H2O lines and find a nuclear dust temperature of ∼40 K. IRAS 13120-5453 has a lower dust temperature and IR than is inferred for the systems termed "compact obscured nuclei (CONs)" (such as Arp 220 and Mrk 231). If IRAS 13120-5453 has undergone a CON phase, we are likely witnessing it at a time when the feedback has already inflated the nuclear ISM and diluted star formation in the starburst/active galactic nucleus core.
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