Fraction Of Dense Molecular Gas Research Articles

Dense molecular gas properties on 100 pc scales across the disc of NGC 3627

ABSTRACT It is still poorly constrained how the densest phase of the interstellar medium varies across galactic environment. A large observing time is required to recover significant emission from dense molecular gas at high spatial resolution, and to cover a large dynamic range of extragalactic disc environments. We present new NOrthern Extended Millimeter Array (NOEMA) observations of a range of high critical density molecular tracers (HCN, HNC, HCO+) and CO isotopologues (13CO, C18O) towards the nearby (11.3 Mpc) strongly barred galaxy NGC 3627. These observations represent the current highest angular resolution (1.85 arcsec; 100 pc) map of dense gas tracers across a disc of a nearby spiral galaxy, which we use here to assess the properties of the dense molecular gas, and their variation as a function of galactocentric radius, molecular gas, and star formation. We find that the HCN(1–0)/CO(2–1) integrated intensity ratio does not correlate with the amount of recent star formation. Instead, the HCN(1–0)/CO(2–1) ratio depends on the galactic environment, with differences between the galaxy centre, bar, and bar-end regions. The dense gas in the central 600 pc appears to produce stars less efficiently despite containing a higher fraction of dense molecular gas than the bar ends where the star formation is enhanced. In assessing the dynamics of the dense gas, we find the HCN(1–0) and HCO+(1–0) emission lines showing multiple components towards regions in the bar ends that correspond to previously identified features in CO emission. These features are cospatial with peaks of Hα emission, which highlights that the complex dynamics of this bar-end region could be linked to local enhancements in the star formation.

CO emission in distant galaxies on and above the main sequence

We present the detection of multiple carbon monoxide CO line transitions with ALMA in a few tens of infrared-selected galaxies on and above the main sequence atz = 1.1−1.7. We reliably detected the emission of CO (5 − 4), CO (2 − 1), and CO (7 − 6)+[C I](3P2 − 3P1) in 50, 33, and 13 galaxies, respectively, and we complemented this information with available CO (4 − 3) and [C I](3P1 − 3P0) fluxes for part of the sample, and by modeling of the optical-to-millimeter spectral energy distribution. We retrieve a quasi-linear relation betweenLIRand CO (5 − 4) or CO (7 − 6) for main-sequence galaxies and starbursts, corroborating the hypothesis that these transitions can be used as star formation rate (SFR) tracers. We find the CO excitation to steadily increase as a function of the star formation efficiency, the mean intensity of the radiation field warming the dust (⟨U⟩), the surface density of SFR (ΣSFR), and, less distinctly, with the distance from the main sequence (ΔMS). This adds to the tentative evidence for higher excitation of the CO+[C I] spectral line energy distribution (SLED) of starburst galaxies relative to that for main-sequence objects, where the dust opacities play a minor role in shaping the high-JCO transitions in our sample. However, the distinction between the average SLED of upper main-sequence and starburst galaxies is blurred, driven by a wide variety of intrinsic shapes. Large velocity gradient radiative transfer modeling demonstrates the existence of a highly excited component that elevates the CO SLED of high-redshift main-sequence and starbursting galaxies above the typical values observed in the disk of the Milky Way. This excited component is dense and it encloses ∼50% of the total molecular gas mass in main-sequence objects. We interpret the observed trends involving the CO excitation as to be mainly determined by a combination of large SFRs and compact sizes, as a large ΣSFRis naturally connected with enhanced dense molecular gas fractions and higher dust and gas temperatures, due to increasing ultraviolet radiation fields, cosmic ray rates, as well as dust and gas coupling. We release the full data compilation and the ancillary information to the community.

The Properties of the Interstellar Medium of Galaxies across Time as Traced by the Neutral Atomic Carbon [C i

Abstract We report Atacama Large Millimeter Array observations of the neutral atomic carbon transitions [C i] and multiple CO lines in a sample of ∼30 main-sequence galaxies at , including novel information on [C i] and CO for 7 of such normal objects. We complement our observations with a collection of >200 galaxies with coverage of similar transitions, spanning the z = 0–4 redshift interval and a variety of ambient conditions from local to high-redshift starbursts. We find systematic variations in the [C i]/IR and [C i]/high-J upper (J upper = 7) CO luminosity ratios among the various samples. We interpret these differences as increased dense molecular gas fractions and star formation efficiencies in the strongest high-redshift starbursts with respect to normal main-sequence galaxies. We further report constant / ratios across the galaxy populations and redshifts, suggesting that gas temperatures T exc traced by [C i] do not strongly vary. We find only a mild correlation with T dust and that, generally, T exc ≲ T dust. We fit the line ratios with classical photodissociation region models, retrieving consistently larger densities and intensities of the UV radiation fields in submillimeter galaxies than in main-sequence and local objects. However, these simple models fall short in representing the complexity of a multiphase interstellar medium and should be treated with caution. Finally, we compare our observations with the Santa Cruz semi-analytical model of galaxy evolution, recently extended to simulate submillimeter emission. While we confirm the success in reproducing the CO lines, we find systematically larger [C i] luminosities at fixed IR luminosity than predicted theoretically. This highlights the necessity of improving our understanding of the mechanisms regulating the [C i] emission on galactic scales. We release our data compilation to the community.

High Dense Gas Fraction in Intensely Star-forming Dusty Galaxies

Abstract We present ALMA and VLA observations of the dense molecular gas tracers HCN, HCO+, and HNC in two lensed, high-redshift starbursts selected from the Herschel-ATLAS survey: H-ATLAS J090740.0−004200 (SDP.9, ) and H-ATLAS J091043.1−000321 (SDP.11, ). In SDP.9 we have detected all transitions and also HCN(1–0) and HCO+(1–0). In SDP.11 we have detected HCN(3–2) and HCO+(3–2). The amplification factors for both galaxies have been determined from subarcsecond-resolution CO and dust emission observations carried out with NOEMA and the SMA. The HNC(1–0)/HCN(1–0) line ratio in SDP.9 suggests the presence of photon-dominated regions, as happens in most local (U)LIRGs. The CO, HCN, and HCO+ spectral line energy distribution (SLEDs) of SDP.9 are compatible to those found for many local, IR-bright galaxies, indicating that the molecular gas in local and high-redshift dusty starbursts can have similar excitation conditions. We obtain that the correlation between total IR ( ) and dense line ( ) luminosity in SDP.9 and SDP.11 and local star-forming galaxies can be represented by a single relation. We argue that the scatter of the correlation, together with the lack of sensitive dense molecular gas tracer observations for a homogeneous sample of high-redshift galaxies, prevents us from distinguishing differential trends with redshift. Our results suggest that the intense star formation found in some high-redshift, dusty, luminous starbursts is associated with more massive dense molecular gas reservoirs and higher dense molecular gas fractions.

Chemistry in isolation: High CCH/HCO+line ratio in the AMIGA galaxy CIG 638

Multi-molecule observations towards an increasing variety of galaxies have been showing that the relative molecular abundances are affected by the type of activity. However, these studies are biased towards bright active galaxies, which are typically in interaction. We study the molecular composition of one of the most isolated galaxies in the local Universe where the physical and chemical properties of their molecular clouds have been determined by intrinsic mechanisms. We present 3 mm broad band observations of the galaxy CIG 638, extracted from the AMIGA sample of isolated galaxies. The emission of the J=1-0 transitions of CCH, HCN, HCO+, and HNC are detected. Integrated intensity ratios between these line are compared with similar observations from the literature towards active galaxies including starburst galaxies (SB), active galactic nuclei (AGN), luminous infrared galaxies (LIRG), and GMCs in M33. A significantly high ratio of CCH with respect to HCN, HCO+, and HNC is found towards CIG 638 when compared with all other galaxies where these species have been detected. This points to either an overabundance of CCH or to a relative lack of dense molecular gas as supported by the low HCN/CO ratio, or both. The data suggest that the CIG 638 is naturally a less perturbed galaxy where a lower fraction of dense molecular gas, as well as a more even distribution could explain the measured ratios. In this scenario the dense gas tracers would be naturally dimmer, while the UV enhanced CCH, would be overproduced in a less shielded medium.

Molecular gas in SAURON early-type galaxies: detection of 13CO and HCN emission★

In a pilot project to study the relationship between star formation and molecular gas properties in nearby normal early-type galaxies, we used the IRAM 30m telescope to observe the 13CO(J=1-0), 13CO(J=2-1), HCN(J=1-0) and HCO+(J=1-0) line emission in the four galaxies of the SAURON sample with the strongest 12CO emission. We report the detection of 13CO emission in all four SAURON sources and HCN emission in three sources, while no HCO+ emission was found to our detection limits in any of the four galaxies. We find that the 13CO/12CO ratios of three SAURON galaxies are somewhat higher than those in galaxies of different Hubble types. The HCN/12CO and HCN/13CO ratios of all four SAURON galaxies resemble those of nearby Seyfert and dwarf galaxies with normal star formation rates, rather than those of starburst galaxies. The HCN/HCO+ ratio is found to be relatively high (i.e., >1) in the three SAURON galaxies with detected HCN emission, mimicking the behaviour in other star-forming galaxies but being higher than in starburst galaxies. When compared to most galaxies, it thus appears that 13CO is enhanced (relative to 12CO) in three out of four SAURON galaxies and HCO+ is weak (relative to HCN) in three out of three galaxies. All three galaxies detected in HCN follow the standard HCN-infrared luminosity and dense gas fraction-star formation efficiency correlations. As already suggested by 12CO observations, when traced by infrared radiation, star formation in the three SAURON galaxies thus appears to follow the same physical laws as in galaxies of different Hubble types. The star formation rate and fraction of dense molecular gas however do not reach the high values found in nearby starburst galaxies, but rather resemble those of nearby normal star-forming galaxies.

HCN(1–0) enhancement in the bar of NGC 2903

We have mapped the HCN(1-0) emission from two spiral galaxies, NGC 2903 and NGC 3504 to study the gas properties in the bars. The HCN(1-0) emission is detected in the center and along the bar of NGC 2903. The line ratio HCN(1-0)/ 12 CO(1-0) ranges from 0.07 to 0.12 with the lowest value in the center. The enhancement of HCN(1-0) emission along the bar indicates a higher fraction of dense molecular gas in the bar than at the center. The mass of dense molecular gas in the center (2.2 × 10 7 M� ) is about 6 times lower than that in the bar (1.2 × 10 8 M� ). The total star formation rate (SFR) is estimated to be 1.4 Myr −1 , where the SFR at the center is 1.9 times higher than that in the bar. The time scale of consumption of the dense molecular gas in the center is about ∼3 × 10 7 yr which is much shorter than that in the bar of about 2 to 10 × 10 8 yr. The dynamical time scale of inflow of the gas from the bar to the center is shorter than the consumption time scale in the bar, which suggests that the star formation (SF) activity at the center is not deprived of fuel. In the bar, the fraction of dense molecular gas mass relative to the total molecular gas mass is twice as high along the leading edge than along the central axis of the bar. The HCN(1-0) emission has a large velocity dispersion in the bar, which can be attributed partially to the streaming motions indicative of shocks along the bar. In NGC 3504, the HCN(1-0) emission is detected only at the center. The fraction of dense molecular gas mass in the center is about 15%. Comparison of the SFR with the predictions from numerical simulations suggest that NGC 2903 harbors a young type B bar with a strong inflow of gas toward the center whereas NGC 3504 has an older bar and has already passed the phase of inflow of gas toward the center.

A Dense Gas Trigger for OH Megamasers

HCN and CO line diagnostics provide new insight into the OH megamaser (OHM) phenomenon, suggesting a dense gas trigger for OHMs. We identify three physical properties that differentiate OHM hosts from other starburst galaxies: (1) OHMs have the highest mean molecular gas densities among starburst galaxies; nearly all OHM hosts have (H2) = 103-104 cm-3 (OH line-emitting clouds likely have n(H2) > 104 cm-3). (2) OHM hosts are a distinct population in the nonlinear part of the IR-CO relation. (3) OHM hosts have exceptionally high dense molecular gas fractions, LHCN/LCO > 0.07, and comprise roughly half of this unusual population. OH absorbers and kilomasers generally follow the linear IR-CO relation and are uniformly distributed in dense gas fraction and LHCN, demonstrating that OHMs are independent of OH abundance. The fraction of non-OHMs with high mean densities and high dense gas fractions constrains beaming to be a minor effect: OHM emission solid angle must exceed 2π steradians. Contrary to conventional wisdom, IR luminosity does not dictate OHM formation; both star formation and OHM activity are consequences of tidal density enhancements accompanying galaxy interactions. The OHM fraction in starbursts is likely due to the fraction of mergers experiencing a temporal spike in tidally driven density enhancement. OHMs are thus signposts marking the most intense, compact, and unusual modes of star formation in the local universe. Future high-redshift OHM surveys can now be interpreted in a star formation and galaxy evolution context, indicating both the merging rate of galaxies and the burst contribution to star formation.

HCN Observations of Dense Star-forming Gas in High-Redshift Galaxies

We present here the sensitive HCN(1-0) observations made with the VLA of two submillimeter galaxies and two QSOs at high-redshift. HCN emission is the signature of dense molecular gas found in GMC cores, the actual sites of massive star formation. We have made the first detection of HCN in a submillimeter galaxy, SMM J16359+6612. The HCN emission is seen with a signal to noise ratio of 4$\sigma$ and appears to be resolved as a double-source of $\approxlt 2''$ separation. Our new HCN observations, combined with previous HCN detections and upper limits, show that the FIR/HCN ratios in these high redshift sources lie systematically above the FIR/HCN correlation established for nearby galaxies by about a factor of 2. Even considering the scatter in the data and the presence of upper limits, this is an indication that the FIR/HCN ratios for the early Universe molecular emission line galaxies (EMGs) deviate from the correlation that fits Galactic giant molecular cloud cores, normal spirals, LIRGs, and ULIRGs. This indicates that the star formation rate per solar mass of dense molecular gas is higher in the high-$z$ objects than in local galaxies including normal spirals LIRGs and ULIRGs. The limited HCN detections at high-redshift show that the HCN/CO ratios for the high-$z$ objects are high and are comparable to those of the local ULIRGs rather than those of normal spirals. This indicates that EMGs have a high fraction of dense molecular gas compared to total molecular gas traced by CO emission.

Is HCN a True Tracer of Dense Molecular Gas in Luminous and Ultraluminous Infrared Galaxies?

We present the results of the first HCO+ survey probing the dense molecular gas content of a sample of 16 luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). Previous work, based on HCN(1-0) observations, had shown that LIRGs and ULIRGs posses a significantly higher fraction of dense molecular gas compared to normal galaxies. While the picture issued from HCO+ partly confirms this result, we have discovered an intriguing correlation between the HCN(1-0)/HCO+(1-0) luminosity ratio and the IR luminosity of the galaxy (L(IR)). This trend casts doubts on the use of HCN as an unbiased quantitative tracer of the dense molecular gas content in LIRGs and ULIRGs. A plausible scenario explaining the observed trend implies that X-rays coming from an embedded AGN may play a dominant role in the chemistry of molecular gas at L(IR) > 1e12 Lsun. We discuss the implications of this result for the understanding of LIRGs, ULIRGs and high redshift gas-rich galaxies.

Multi-Line Observations of Molecular Gas in the Central Region of the Low Star-Formation Efficiency “Starburst” Galaxy NGC 4527

Abstract We performed high-resolution CO($J = 2 \hbox{--} 1$), CO($J = 1 \hbox{--} 0$), HCN($J = 1 \hbox{--} 0$), $\mathrm{HCO}^{+}$($J = 1 \hbox{--} 0$), and 3 mm continuum observations of the low star formation efficiency (SFE) “starburst” galaxy NGC 4527 with the Nobeyama Millimeter Array. From CO($J = 1 \hbox{--} 0$) observations, we found two offset ridges with non-circular motion and a central gas concentration. The distribution and kinematics of NGC 4527 are very similar to those of barred spiral galaxies. The distribution of CO($J = 2 \hbox{--} 1$) emission resembles the features of the concentrated component of CO($J = 1 \hbox{--} 0$). HCN($J = 1 \hbox{--} 0$) and $\mathrm{HCO}^{+}$($J = 1 \hbox{--} 0$) emissions concentrated toward the center have also been detected. The total mass of molecular gas detected in the central $r < {30{}^{\mathrm {\prime \prime }}}$ is $2.5 \times 10^9 \,{{{M}_{\odot}}}$. The $\mathrm{HCN}\,(J = 1 \hbox{--} 0) / \mathrm{CO}\,(J = 1 \hbox{--} 0)$ integrated intensity ratio of $0.075 \pm 0.008$ in the central $r \lesssim {3{}^{\mathrm {\prime \prime }}} \hbox{--} {4{}^{\mathrm {\prime \prime }}}$ region of NGC 4527 could suggest that the fraction of dense molecular gas to the total molecular gas is small compared with those of prototypical starburst galaxies, such as NGC 253. This suggests that the small dense gas fraction is responsible for the low SFE in the center of NGC 4527, though this interpretation could depend on aperture sizes of the observations. The ratio of the dynamical mass to molecular gas mass is 13% within the central region ($r < 500 \,\mathrm{pc}$). This suggests that molecular gas within the $r < 500 \,\mathrm{pc}$ region could be in a state of gravitational instability. The low SFE, the small dense gas fraction, and the large amount of gravitationally unstable molecular gas can be explained if NGC 4527 is in the very early phase of starburst phenomena or a pre-starburst phase, and the formation of dense molecular gas will begin and the SFE will increase within a short time scale.

Dense Molecular Gas in Lenticular Galaxies

We made CO and HCN simultaneous observations of lenticular galaxies (NGC 404, NGC 3593, and NGC 4293) and detected HCN emission in NGC 3593 and NGC 4293 as well as CO in all the galaxies. The $I_\mathrm{HCN} / I_\mathrm{CO}$ ratios were $0.025 \pm 0.006$ and $0.066 \pm 0.005$ in NGC 3593 and NGC 4293, respectively, which are comparable to that of late-type spiral galaxies. The average of the $I_\mathrm{HCN} / I_\mathrm{CO}$ ratios at the center of 12 nearby spiral galaxies including the late type was $0.055 \pm 0.028$. The line profiles of CO and HCN emission showed different shape in both galaxies. The HCN peaks were not at the systemic velocity of these galaxies, while the CO peaks were near the systemic velocity. These results suggest that the fraction of dense molecular gas is high around the center of these galaxies.

Radial Distributions of the HCN/CO Ratio in Nearby CO Bright Galaxies

Abstract Simultaneous observations of HCN and CO lines were made along the major axes of six nearby CO bright galaxies with the Nobeyama 45-m telescope. The beam-averaged HCN/CO ratios are 0.01 to 0.1. The distributions of the HCN/CO ratio along the major axes (and the minor axis for M 51) show different patterns from one galaxy to another. By adding data obtained from the outer disk, however, we found gradual decreases in the HCN and CO intensities and the HCN/CO ratio with increasing radius in M 51 from the nuclear region to 3 kpc, similar to our previous result for NGC 253. The HCN/CO ratio is very sensitive to the observing beam size on a linear scale. The ratio, which reflects the fraction of dense molecular gas, correlates with the star-formation activity derived from the far-infrared luminosity.

DENSE MOLECULAR GAS IN GALAXIES AND THE EVOLUTION OF LUMINOUS INFRARED GALAXIES

ABSTRACT This thesis explores the molecular gas (the raw material for star formation), especially the dense molecular gas content of luminous infrared galaxies (LIGs) and ''normal'' galaxies using millimeter line observations. Most LIGs are closely interacting/merging galaxies and ''normal'' spiral galaxies are believed to be the building blocks of LIGs. We here study and compare the distributions and masses of the total molecular gas and the dense molecular gas, traced by CO and HCN emission respectively, in LIGs, starburst galaxies and ''normal'' spiral galaxies. The molecular gas properties are then compared with the stages of galaxy- galaxy interaction and the far-IR luminosity to understand the star formation process and evolution of LIGs. We first study the dense molecular gas extent and distribution in ~ 10 nearby ''normal'' galaxies by mapping HCN emission (complementary with CO) at least along the major axes. We present the first detailed observational evidence that HCN emission in galaxies, \\ie, the dense molecular gas, is not confined to the inner ~ 1 kpc nuclear region, although the highest concentrations of dense molecular gas are in the center. A significant fraction of dense molecular gas is distributed in the inner disks of galaxies outside the nuclear or inner ring starburst regions, and can be detected to radii as large as a few kpc, perhaps to diameters of ~ D25/4. Then we have further surveyed HCN emission in more than ~ 40 relatively distant IR/CO bright ''normal'' galaxies and LIGs. Therefore, a statistically significant sample of HCN data in galaxies is established. We find that LIGs, especially ultraluminous ones, contain tremendous amount of dense molecular gas fueling the starbursts. Although LIGs are rich in molecular gas, some ''normal'' galaxies could have as much molecular gas as LIGs. However, the dense molecular gas content of even gas-rich ''normal'' galaxies is much less than that of LIGs of comparable molecular gas content. We show that HCN emission is better correlated with IR emission than that of CO. The total dense molecular content, the ratio of HCN/CO luminosity and the distribution of the surface brightness ratio IHCN/ICO are important high mass star formation indicators. We also confirm that the star formation efficiency indicated by LIR/LCO depends on the fraction of dense molecular gas (LHCN/LCO) and that the LIR/LHCN ratio is similar in all galaxies, ultraluminous or not, hot or cold in dust temperature, illustrating the starburst nature of ultraluminous IR galaxies. A second goal of this thesis is to study LIGs in the intermediate merging process, to determine the relationship between the various IR/CO properties and galaxy-galaxy interactions. We find a correlation between the CO luminosity and the projected separation of merger nuclei in a sample of more than ~ 50 LIG mergers, which suggests that the molecular content is decreasing as merging advances. Although the correlation is weak for the truly ultraluminous (LIR>1012 ls ) IR mergers, which could simply be due to the incompleteness of the sample since ultraluminous mergers are at great distances, the correlation is better established with less luminous LIG mergers, more close to a volume-limited statistically complete sample. The correlation slope for this nearby almost complete sample is the same as that of the large, heterogeneous sample of 50 mergers. We conducted new CO observation in ~ 20 LIG mergers to provide the CO data for this statistically complete sample of nearby LIG mergers. This correlation seems to have important constraints on both the merger- induced star formation models and the evolution scenario of LIGs. We have also conducted high resolution interferometry CO imaging in two spectacular LIGs, Arp 118 and Arp 119. We detected strong CO emission from rings/tails more than 10 kpc away from the nuclei, and most molecular gas is extranuclear. Detailed study of the distribution and kinematics of the molecular gas can provide the dynamical clues to the origin and evolution of these LIGs. Finally, we study the environment and spatial distribution of LIGs. We found that LIGs have stronger clustering than that of \\IRAS galaxies, yet most of them are not inside the groups of galaxies. Using various previous studies along with our own present investigations, we suggest that some LIGs are remnants of the merged groups of gas-rich galaxies.