scholarly journals Cold Molecular Gas in Merger Remnants. II. The Properties of Dense Molecular Gas

2021 ◽  
Vol 257 (2) ◽  
pp. 57
Author(s):  
Junko Ueda ◽  
Daisuke Iono ◽  
Min S. Yun ◽  
Tomonari Michiyama ◽  
Yoshimasa Watanabe ◽  
...  
Keyword(s):  
Star Formation ◽  
Control Sample ◽  
Molecular Gas ◽  
Dense Gas ◽  
Luminous Infrared Galaxies ◽  
Formation Mode ◽  
Merging Process ◽  
Molecular Lines ◽  
Formation Efficiency ◽  
Gas Fractions

Abstract We present the 3 mm wavelength spectra of 28 local galaxy merger remnants obtained with the Large Millimeter Telescope. Sixteen molecular lines from 14 different molecular species and isotopologues were identified, and 21 out of 28 sources were detected in one or more molecular lines. On average, the line ratios of the dense gas tracers, such as HCN (1–0) and HCO+(1–0), to 13CO (1–0) are 3–4 times higher in ultra/luminous infrared galaxies (U/LIRGs) than in non-LIRGs in our sample. These high line ratios could be explained by the deficiency of 13CO and high dense gas fractions suggested by high HCN (1–0)/12CO (1–0) ratios. We calculate the IR-to-HCN (1–0) luminosity ratio as a proxy of the dense gas star formation efficiency. There is no correlation between the IR/HCN ratio and the IR luminosity, while the IR/HCN ratio varies from source to source ((1.1–6.5) × 103 L ☉/(K km s−1 pc2)). Compared with the control sample, we find that the average IR/HCN ratio of the merger remnants is higher by a factor of 2–3 than those of the early/mid-stage mergers and nonmerging LIRGs, and it is comparable to that of the late-stage mergers. The IR-to-12CO (1–0) ratios show a similar trend to the IR/HCN ratios. These results suggest that star formation efficiency is enhanced by the merging process and maintained at high levels even after the final coalescence. The dynamical interactions and mergers could change the star formation mode and continue to impact the star formation properties of the gas in the postmerger phase.

scholarly journals Molecular Gas in Hickson Compact Groups

1999 ◽  
Vol 186 ◽  
pp. 414-414
Author(s):  
S. Leon ◽  
F. Combes ◽  
T.K. Menon
Keyword(s):  
Star Formation ◽  
Compact Group ◽  
Gas Flow ◽  
Control Sample ◽  
Gas Content ◽  
Cloud Formation ◽  
Molecular Gas ◽  
Compact Groups ◽  
Hickson Compact Groups ◽  
Formation Efficiency

Compact groups are ideal sites to study the influence of strong dynamical evolution due to environment on molecular cloud formation and star formation efficiency. We have observed 70 galaxies belonging to 45 Hickson compact groups (HCGs) in the 12CO(1→0) and 12CO(2→1) lines, in order to determine their molecular content. We compare the gas content relative to blue and LFIR luminosities of galaxies in compact groups with respect to other samples in the literature, including various environments and morphological types. We find that there is some hint, of enhanced MH2/LB and Mdust/LB ratios in the galaxies from compact group with respect to our control sample, especially for the most compact groups, suggesting that tidal interactions can drive the gas component inwards, by removing its angular momentum, and concentrating it in the dense central regions, where it is easily detected. The threshold at 20–30 kpc in mean galaxy separation for the enhancement of H2 suggests that it must correspond to an acceleration of the merging process and a significant inward gas flow. The molecular gas content in compact group galaxies is similar to that in pairs and starburst samples. However, the total LFIR luminosity of HCGs is quite similar to that of the control sample, and therefore the star formation efficiency appears lower than in the control galaxies. However this assumes that the FIR spatial distributions are similar in both samples which is not the case at radio frequencies. Higher spatial resolution FIR data are needed to make a valid comparison. Given their short dynamical friction time-scale, it is possible that some of these systems are in the final stage before merging, leading to ultraluminous starburst phases. We also find for all galaxy samples that the H2 content (normalized to blue luminosity) is strongly correlated with LFIR, while the total gas content (H2+HI) is not.

scholarly journals Dense gas is not enough: environmental variations in the star formation efficiency of dense molecular gas at 100 pc scales in M 51

2019 ◽  
Vol 625 ◽  
pp. A19 ◽  
Author(s):  
M. Querejeta ◽  
E. Schinnerer ◽  
A. Schruba ◽  
E. Murphy ◽  
S. Meidt ◽  
...  
Keyword(s):  
Star Formation ◽  
Surface Density ◽  
Formation Rate ◽  
Radio Continuum ◽  
Molecular Gas ◽  
Dense Gas ◽  
Environmental Variations ◽  
Formation Efficiency ◽  
Density Threshold ◽  
Spiral Arm

It remains unclear what sets the efficiency with which molecular gas transforms into stars. Here we present a new VLA map of the spiral galaxy M 51 in 33 GHz radio continuum, an extinction-free tracer of star formation, at 3″ scales (∼100 pc). We combined this map with interferometric PdBI/NOEMA observations of CO(1–0) and HCN(1–0) at matched resolution for three regions in M 51 (central molecular ring, northern and southern spiral arm segments). While our measurements roughly fall on the well-known correlation between total infrared and HCN luminosity, bridging the gap between Galactic and extragalactic observations, we find systematic offsets from that relation for different dynamical environments probed in M 51; for example, the southern arm segment is more quiescent due to low star formation efficiency (SFE) of the dense gas, despite its high dense gas fraction. Combining our results with measurements from the literature at 100 pc scales, we find that the SFE of the dense gas and the dense gas fraction anti-correlate and correlate, respectively, with the local stellar mass surface density. This is consistent with previous kpc-scale studies. In addition, we find a significant anti-correlation between the SFE and velocity dispersion of the dense gas. Finally, we confirm that a correlation also holds between star formation rate surface density and the dense gas fraction, but it is not stronger than the correlation with dense gas surface density. Our results are hard to reconcile with models relying on a universal gas density threshold for star formation and suggest that turbulence and galactic dynamics play a major role in setting how efficiently dense gas converts into stars.

scholarly journals Testing the universality of the star-formation efficiency in dense molecular gas

2017 ◽  
Vol 604 ◽  
pp. A74 ◽  
Author(s):  
Y. Shimajiri ◽  
Ph. André ◽  
J. Braine ◽  
V. Könyves ◽  
N. Schneider ◽  
...  
Keyword(s):  
Star Formation ◽  
Independent Study ◽  
Molecular Gas ◽  
Wide Field ◽  
Dense Gas ◽  
Filamentary Structure ◽  
Moderate Density ◽  
Wide Range ◽  
External Galaxies ◽  
Formation Efficiency

Context. Recent studies with, for example, Spitzer and Herschel have suggested that star formation in dense molecular gas may be governed by essentially the same “law” in Galactic clouds and external galaxies. This conclusion remains controversial, however, in large part because different tracers have been used to probe the mass of dense molecular gas in Galactic and extragalactic studies. Aims. We aimed to calibrate the HCN and HCO+ lines commonly used as dense gas tracers in extragalactic studies and to test the possible universality of the star-formation efficiency in dense gas (≳104 cm-3), SFEdense. Methods. We conducted wide-field mapping of the Aquila, Ophiuchus, and Orion B clouds at ~0.04 pc resolution in the J = 1 − 0 transition of HCN, HCO+, and their isotopomers. For each cloud, we derived a reference estimate of the dense gas mass MHerschelAV > 8, as well as the strength of the local far-ultraviolet (FUV) radiation field, using Herschel Gould Belt survey data products, and estimated the star-formation rate from direct counting of the number of Spitzer young stellar objects. Results. The H13CO+(1–0) and H13CN(1–0) lines were observed to be good tracers of the dense star-forming filaments detected with Herschel. Comparing the luminosities LHCN and LHCO+ measured in the HCN and HCO+ lines with the reference masses MHerschelAV > 8, the empirical conversion factors αHerschel − HCN (=MHerschelAV > 8/LHCN) and αHerschel − HCO+ (=MHerschelAV > 8/LHCO+) were found to be significantly anti-correlated with the local FUV strength. In agreement with a recent independent study of Orion B by Pety et al., the HCN and HCO+ lines were found to trace gas down to AV ≳ 2. As a result, published extragalactic HCN studies must be tracing all of the moderate density gas down to nH2 ≲ 103 cm-3. Estimating the contribution of this moderate density gas from the typical column density probability distribution functions in nearby clouds, we obtained the following G0-dependent HCN conversion factor for external galaxies: αHerschel − HCNfit′ = 64 × G0-0.34. Re-estimating the dense gas masses in external galaxies with αHerschel − HCNfit′(G0), we found that SFEdense is remarkably constant, with a scatter of less than 1.5 orders of magnitude around 4.5 × 10-8 yr-1, over eight orders of magnitude in dense gas mass. Conclusions. Our results confirm that SFEdense of galaxies is quasi-universal on a wide range of scales from ~ 1–10 pc to > 10 kpc. Based on the tight link between star formation and filamentary structure found in Herschel studies of nearby clouds, we argue that SFEdense is primarily set by the “microphysics” of core and star formation along filaments.

scholarly journals CO Observations of Luminous IR Galaxy Mergers

1999 ◽  
Vol 186 ◽  
pp. 275-276
Author(s):  
Y. Gao ◽  
P.M. Solomon
Keyword(s):  
Gas Content ◽  
Molecular Gas ◽  
Galaxy Mergers ◽  
Galaxy Interactions ◽  
Luminous Infrared Galaxies ◽  
The Galaxy ◽  
Merging Process ◽  
Formation Efficiency ◽  
The Relationship ◽  
Co Observations

Luminous starbursts are observed to occur mostly as a result of a collision/merger in gas-rich galaxies, and most luminous infrared galaxies (LIGs) are indeed gas-rich mergers. In order to determine the relationship between the IR and molecular gas properties and the galaxy-galaxy interactions, we study LIG mergers in the intermediate merging process. We have observed nearly 20 LIG mergers and together with the CO data in the literature, we have found a correlation between the CO luminosity, LCO, and the projected separation of merger nuclei, RSep, in > 50 LIG mergers. The correlation suggests the molecular content is decreasing as merging advances and is better established with ~ 40 LIG mergers excluding ultraluminous ones, which resembles more a volume-limited, statistically complete sample of LIG mergers. In addition, an anti-correlation between LIR/LCO (the measure of star formation efficiency, SFE) and RSep is evident. One interpretation is that the molecular gas content of LIG mergers is being rapidly depleted due to the merger-induced starbursts and the increase of SFE as merging progresses.

From global to local scales in galaxies

2020 ◽  
Vol 15 (S359) ◽  
pp. 391-395
Author(s):  
Sebastian F. Sánchez ◽  
Carlos Lopez Cobá
Keyword(s):  
Star Formation ◽  
Spatial Scales ◽  
Bimodal Distribution ◽  
Spectroscopic Properties ◽  
Molecular Gas ◽  
Ionized Gas ◽  
Galaxy Surveys ◽  
Quenching Process ◽  
Formation Efficiency ◽  
Integral Field

AbstractWe summarize here some of the results reviewed recently by Sanchez (2020) comprising the advances in the comprehension of galaxies in the nearby universe based on integral field spectroscopic galaxy surveys. In particular we explore the bimodal distribution of galaxies in terms of the properties of their ionized gas, showing the connection between the star-formation (quenching) process with the presence (absence) of molecular gas and the star-formation efficiency. We show two galaxy examples that illustrates the well known fact that ionization in galaxies (and the processes that produce it), does not happen monolitically at galactic scales. This highlight the importance to explore the spectroscopic properties of galaxies and the evolutionary processes unveiled by them at different spatial scales, from sub-kpc to galaxy wide.

scholarly journals Molecules as tracers of galaxy evolution

2012 ◽  
Vol 8 (S292) ◽  
pp. 199-208 ◽  
Author(s):  
Susanne Aalto
Keyword(s):  
Black Holes ◽  
Star Formation ◽  
High Resolution ◽  
Galaxy Evolution ◽  
Large Scale ◽  
Short Review ◽  
Stimulated Emission ◽  
Molecular Gas ◽  
Infrared Galaxies ◽  
Luminous Infrared Galaxies

AbstractStudying the molecular phase of the interstellar medium in galaxies is fundamental for the understanding of the onset and evolution of star formation and the growth of supermassive black holes. We can use molecules as observational tools exploiting them as tracers of chemical, physical and dynamical conditions. In this short review, key molecules (e.g. HCN, HCO+, HNC, HC3N, CN, H3O+) in identifying the nature of buried activity and its evolution are discussed including some standard astrochemical scenarios. Furthermore, we can use IR excited molecular emission to probe the very inner regions of luminous infrared galaxies (LIRGs) allowing us to get past the optically thick dust barrier of the compact obscured nuclei, e.g. in the dusty LIRG NGC4418. High resolution studies are often necessary to separate effects of excitation and radiative transport from those of chemistry - one example is absorption and effects of stimulated emission in the ULIRG Arp220. Finally, molecular gas in large scale galactic outflows is briefly discussed.

scholarly journals Major impact from a minor merger

2018 ◽  
Vol 615 ◽  
pp. A122 ◽  
Author(s):  
S. König ◽  
S. Aalto ◽  
S. Muller ◽  
J. S. Gallagher III ◽  
R. J. Beswick ◽  
...  
Keyword(s):  
Star Formation ◽  
Brightness Temperature ◽  
Central Region ◽  
Molecular Clouds ◽  
Molecular Gas ◽  
Transfer Model ◽  
Dense Gas ◽  
Giant Molecular Clouds ◽  
Minor Mergers ◽  
Co Emission

Context. Minor mergers are important processes contributing significantly to how galaxies evolve across the age of the Universe. Their impact on the growth of supermassive black holes and star formation is profound – about half of the star formation activity in the local Universe is the result of minor mergers. Aims. The detailed study of dense molecular gas in galaxies provides an important test of the validity of the relation between star formation rate and HCN luminosity on different galactic scales – from whole galaxies to giant molecular clouds in their molecular gas-rich centers. Methods. We use observations of HCN and HCO+ 1−0 with NOEMA and of CO3−2 with the SMA to study the properties of the dense molecular gas in the Medusa merger (NGC 4194) at 1′′ resolution. In particular, we compare the distribution of these dense gas tracers with CO2−1 high-resolution maps in the Medusa merger. To characterize gas properties, we calculate the brightness temperature ratios between the three tracers and use them in conjunction with a non-local thermodynamic equilibrium (non-LTE) radiative line transfer model. Results. The gas represented by HCN and HCO+ 1−0, and CO3−2 does not occupy the same structures as the less dense gas associated with the lower-J CO emission. Interestingly, the only emission from dense gas is detected in a 200 pc region within the “Eye of the Medusa”, an asymmetric 500 pc off-nuclear concentration of molecular gas. Surprisingly, no HCN or HCO+ is detected for the extended starburst of the Medusa merger. Additionally, there are only small amounts of HCN or HCO+ associated with the active galactic nucleus. The CO3−2/2−1 brightness temperature ratio inside “the Eye” is ~2.5 – the highest ratio found so far – implying optically thin CO emission. The CO2−1/HCN 1−0 (~9.8) and CO2−1/HCO+ 1−0 (~7.9) ratios show that the dense gas filling factor must be relatively high in the central region, consistent with the elevated CO3−1/2−1 ratio. Conclusions. The line ratios reveal an extreme, fragmented molecular cloud population inside the Eye with large bulk temperatures (T > 300 K) and high gas densities (n(H2) > 104 cm-3). This is very different from the cool, self-gravitating structures of giant molecular clouds normally found in the disks of galaxies. The Eye of the Medusa is found at an interface between a large-scale minor axis inflow and the central region of the Medusa. Hence, the extreme conditions inside the Eye may be the result of the radiative and mechanical feedback from a deeply embedded, young and massive super star cluster formed due to the gas pile-up at the intersection. Alternatively, shocks from the inflowing gas entering the central region of the Medusa may be strong enough to shock and fragment the gas. For both scenarios, however, it appears that the HCN and HCO+ dense gas tracers are not probing star formation, but instead a post-starburst and/or shocked ISM that is too hot and fragmented to form newstars. Thus, caution is advised in taking the detection of emission from dense gas tracers as evidence of ongoing or imminent star formation.

scholarly journals A large amount of diffuse molecular gases in the bar of the strongly barred galaxy NGC 1300: cause of the low star formation efficiency

2020 ◽  
Vol 495 (4) ◽  
pp. 3840-3858 ◽  
Author(s):  
Fumiya Maeda ◽  
Kouji Ohta ◽  
Yusuke Fujimoto ◽  
Asao Habe ◽  
Kaito Ushio
Keyword(s):  
Star Formation ◽  
Molecular Gas ◽  
Full Width ◽  
Half Maximum ◽  
Barred Galaxies ◽  
Molecular Gases ◽  
Formation Efficiency

ABSTRACT In many barred galaxies, star formation efficiency (SFE) in the bar is lower than those in the arm and bar-end, and its cause has still not been clear. Focusing on the strongly barred galaxy NGC 1300, we investigate the possibility that the presence of a large amount of diffuse molecular gas, which would not contribute to the SF, makes the SFE low in appearance. We examine the relation between the SFE and the diffuse molecular gas fraction (fdif), which is derived using the 12CO(1–0) flux obtained from the interferometer of ALMA 12-m array, which has no sensitivity on diffuse (extended; full width at half-maximum ⪆700 pc) molecular gases due to the lack of ACA, and the total 12CO(1–0) flux obtained from Nobeyama 45-m single-dish telescope. We find that the SFE decreases with increasing fdif. The fdif and SFE are 0.74−0.91 and $0.06\!-\!0.16 ~\rm Gyr^{-1}$ in the bar regions, and 0.28−0.65 and $0.23\!-\!0.96 ~\rm Gyr^{-1}$ in the arm and bar-end regions, respectively. This result supports the idea that the presence of a large amount of diffuse molecular gas makes the SFE low. The suppression of the SFE in the bar has also been seen even when we exclude the diffuse molecular gas components. This suggests that the low SFE appears to be caused not only by a large amount of diffuse molecular gases but also by other mechanisms such as fast cloud–cloud collisions.

scholarly journals Galaxies hosting an active galactic nucleus: a view from the CALIFA survey

2020 ◽  
Vol 492 (3) ◽  
pp. 3073-3090 ◽  
Author(s):  
Eduardo A D Lacerda ◽  
Sebastián F Sánchez ◽  
R Cid Fernandes ◽  
Carlos López-Cobá ◽  
Carlos Espinosa-Ponce ◽  
...  
Keyword(s):  
Star Formation ◽  
Morphological Type ◽  
Molecular Gas ◽  
Main Role ◽  
Type I ◽  
Massive Galaxies ◽  
Star Forming ◽  
Integral Field Spectroscopy ◽  
Almost All ◽  
Formation Efficiency

ABSTRACT We study the presence of optically-selected active galactic nuclei (AGNs) within a sample of 867 galaxies extracted from the extended Calar-Alto Legacy Integral Field spectroscopy Area (eCALIFA) spanning all morphological classes. We identify 10 Type-I and 24 Type-II AGNs, amounting to ∼4 per cent of our sample, similar to the fraction reported by previous explorations in the same redshift range. We compare the integrated properties of the ionized and molecular gas, and stellar population of AGN hosts and their non-active counterparts, combining them with morphological information. The AGN hosts are found in transitory parts (i.e. green-valley) in almost all analysed properties which present bimodal distributions (i.e. a region where reside star-forming galaxies and another with quiescent/retired ones). Regarding morphology, we find AGN hosts among the most massive galaxies, with enhanced central stellar-mass surface density in comparison to the average population at each morphological type. Moreover, their distribution peaks at the Sab-Sb classes and none are found among very late-type galaxies (>Scd). Finally, we inspect how the AGN could act in their hosts regarding the quenching of star-formation. The main role of the AGN in the quenching process appears to be the removal (or heating) of molecular gas, rather than an additional suppression of the already observed decrease of the star-formation efficiency from late-to-early type galaxies.

scholarly journals Evolution of the star formation efficiency in galaxies

2012 ◽  
Vol 10 (H16) ◽  
pp. 341-341
Author(s):  
Jonathan Braine
Keyword(s):  
Star Formation ◽  
Chemical Evolution ◽  
Conversion Factor ◽  
Star Formation Rate ◽  
Formation Rate ◽  
Molecular Gas ◽  
Present Evidence ◽  
Ngc 6822 ◽  
Physical And Chemical ◽  
Formation Efficiency

AbstractThe physical and chemical evolution of galaxies is intimately linked to star formation, We present evidence that molecular gas (H2) is transformed into stars more quickly in smaller and/or subsolar metallicity galaxies than in large spirals – which we consider to be equivalent to a star formation efficiency (SFE). In particular, we show that this is not due to uncertainties in the N(H2)/Ico conversion factor. Several possible reasons for the high SFE in galaxies like the nearby M33 or NGC 6822 are proposed which, separately or together, are the likely cause of the high SFE in this environment. We then try to estimate how much this could contribute to the increase in cosmic star formation rate density from z = 0 to z = 1.

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