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$^{13}$CO (1-0) and HCN (1–0) spectra toward the center of the starburst galaxy samples. Two spectra are shown for each galaxy; the spectrum on the left-hand side denotes the $^{13}$CO (1–0) line, and that on the right-hand side denotes the HCN (1–0) line. The horizontal and vertical axes are $V_{\rm LSR}$ and $T_{\rm MB}$, respectively. Note that the scale for the vertical axis is the same for the $^{13}$CO and HCN spectra.
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We performed $^{12}$CO (1–0), $^{13}$CO (1–0), and HCN (1–0) single-dish observations (beam size $\sim $14–18) toward nearby starburst and non-starburst galaxies using the Nobeyama 45-m telescope. The $^{13}$CO (1–0) and HCN (1–0) emissions were detected from all seven starburst galaxies, with the intensities of both lines being
similar (i.e., the...
Context in source publication
Context 1
... observed region, (−45 , −9 ). The line intensity ratios are, on the other hand, different from the brightness temperature distributions. At the center of M82, the in- tensity ratios (HCN/ 13 CO, 12 CO/ 13 CO, and HCN/ 12 CO) are all high, but these ratios gradually decrease with in- creasing offsets from the center (Table 4). Fig. 3 shows 13 CO(1 -0) and HCN(1 -0) line spectra of the other starburst galaxy samples. Both lines have similar line shapes and intensities for all galaxies, which is similar to those for the central ±15 region of M82. We note again that since the HCN and 13 CO lines were ob- served simultaneously, there is no HCN/ 13 CO integrated ...
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... The HCN/ 13 CO intensity ratios of ETGs range from 0.18 to 0.58 (Krips et al. 2010;Crocker et al. 2012). They are consistent with the HCN/ 13 CO intensity ratios of spiral galaxies and dwarf galaxies with normal SFRs (e.g., Krips et al. 2010;Matsushita et al. 2010;Watanabe et al. 2014). No significant difference between the different morphological types of galaxies (i.e., ETG and LTG) has been found. ...
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 ¹³ CO (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 ¹³ CO and high dense gas fractions suggested by high HCN (1–0)/ ¹² CO (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) × 10 ³ L ☉ /(K km s ⁻¹ pc ² )). 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- ¹² CO (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.
... The HCN/ 13 CO intensity ratios of ETGs range from 0.18 to 0.58 (Krips et al. 2010;Crocker et al. 2012). They are consistent with the HCN/ 13 CO intensity ratios of spiral galaxies and dwarf galaxies with normal SFRs (e.g., Krips et al. 2010;Matsushita et al. 2010;Watanabe et al. 2014). No significant difference between the different morphological types of galaxies (i.e., ETG and LTG) has been found. ...
We present the 3 mm wavelength spectra of 28 local galaxy merger remnants obtained with the Large Millimeter Telescope. Fifteen molecular lines from 13 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 $^{13}$CO (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 $^{13}$CO and high dense gas fractions suggested by high HCN (1-0)/$^{12}$CO (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) $\times 10^{3}$ $L_{\odot}$/(K km s$^{-1}$ pc$^{2}$). 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 non-merging LIRGs, and it is comparable to that of the late-stage mergers. The IR-to-$^{12}$CO (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 post-merger phase.
... Observations of denser molecular gas traced by HCN and HCO+ have found low dense-gas fractions in post-starburst galaxies (French et al. 2018b), consistent with the low current SFRs, and which would explain their low CO-traced star formation efficiencies if something is preventing the CO-traced gas from collapse. However, different behavior is observed for the more nearby systems NGC 5195 (Kohno et al. 2002;Matsushita et al. 2010;Alatalo et al. 2016a) and NGC 1266 (Alatalo et al. 2015). Observations of NGC 5195 are complicated by its on-going merger with M51, but is observed to have a star formation efficiency similar to normal galaxies. ...
... This galaxy was observed in CO (1-0) and HCN (1-0) by Kohno et al. (2002) with the Nobeyama 45 m. Subsequent observations by Matsushita et al. (2010) did not detect HCN. Observations of NGC 5195 are complicated by the nearby spiral arm of M51 in this interacting system. ...
Post-starburst or "E + A" galaxies are rapidly transitioning from star-forming to quiescence. While the current star formation rate (SFR) of post-starbursts is already at the level of early-type galaxies, we recently discovered that many have large CO-traced molecular gas reservoirs consistent with normal star-forming galaxies. These observations raise the question of why these galaxies have such low SFRs. Here we present an ALMA search for the denser gas traced by HCN (1-0) and HCO⁺ (1-0) in two CO-luminous, quiescent post-starburst galaxies. Intriguingly, we fail to detect either molecule. The upper limits are consistent with the low SFRs and with early-type galaxies. The HCN/CO luminosity ratio upper limits are low compared to star-forming and even many early-type galaxies. This implied low dense gas mass fraction explains the low SFRs relative to the CO-traced molecular gas and suggests that the state of the gas in post-starburst galaxies is unusual, with some mechanism inhibiting its collapse to denser states. We conclude that post-starbursts galaxies are now quiescent because little dense gas is available, in contrast to the significant CO-traced lower density gas reservoirs that still remain. © 2018. The American Astronomical Society. All rights reserved..
We present $1''.4$ (98 pc) resolution ALMA observations of $^{13}$CO($J$=1-0), C$^{18}$O($J$=1-0), CS($J$=2-1) and CH$_3$OH($J_K$=$2_K$-$1_K$) molecular lines in the central $1'$ (4.2 kpc) region of NGC 1068 to study the physical and chemical properties of giant molecular clouds (GMCs) and to test whether these GMC-scale properties are linked to the larger-scale galactic environment. Using the derived $^{13}$CO cube, we have identified 187 high-significance ($> 8\sigma$) GMCs by employing the CLUMPFIND algorithm. The molecular gas masses of GMCs $M_{\rm ^{13}CO}$, derived from the $^{13}$CO data, range from $1.8 \times 10^4 ~ M_\odot$ to $4.2 \times 10^7 ~M_\odot$. A mass function of GMCs in NGC 1068 has been obtained for the first time at $\sim$100 pc resolution. We find the slope of the mass function $\gamma = -1.25 \pm 0.07$ for a mass range of $M_{\rm ^{13}CO} \geq 10^5M_\odot$. This is shallower than the GMCs in the disk regions of the Milky Way, M51 and NGC 300. Further, we find that the high mass cut-off of the GMC mass function occurs at $M_{\rm ^{13}CO} \sim 6 \times 10^7 M_\odot$, which is an order of magnitude larger than that in the nuclear bar region of M51, indicating that the more massive clouds dominate the mass budget in NGC 1068. The observed C$^{18}$O)/$^{13}$CO intensity ratios are found to be fairly uniform (0.27 $\pm$ 0.05) among the identified GMCs. In contrast, the CH$_3$OH/$^{13}$CO ratios exhibit striking spatial variation across the disk, with the smallest values around the bar-end ($<$ 0.03), and larger ratios along the spiral arms ($\sim$ 0.1-0.2). We find that GMCs with detectable methanol emission tend to have systematically larger velocity widths than those without methanol emission, suggesting that (relatively weak) shocks are responsible for the enhancement of the CH$_3$OH/$^{13}$CO ratios of GMCs in the disk of NGC 1068.
We present new molecular gas maps of NGC5195 (alternatively known as M51b) from the Combined Array for Research in Millimeter Astronomy (CARMA), including 12CO(1-0), 13CO(1-0), CN(1-0), and CS(2-1). NGC5195 has also been detected in 3mm continuum. NGC5195 has a 12CO/13CO ratio consistent with normal star-forming galaxies. The CN(1-0) intensity is higher than is seen in an average star-forming galaxy, possibly enhanced in the diffuse photo-dissociation regions. Stellar template fitting of the nuclear spectrum of NGC5195 shows two stellar populations: an 80% mass fraction of old (>10Gyr) and a 20% mass fraction of intermediate-aged (~1Gyr) stellar populations, providing a constraint on the timescale over which NGC5195 experienced enhanced star formation during its interaction with M51a. The average molecular gas depletion timescale in NGC5195 is: tdep=3.08Gyr, a factor of ~2 larger than the depletion timescales in nearby star-forming galaxies, but consistent with the depletion seen in CO-detected early-type galaxies. While the radio continuum emission at centimeter and millimeter wavelengths is present in the vicinity of the nucleus of NGC5195, we find it is most likely associated with nuclear star formation rather than a radio-loud AGN. Thus, despite having a substantial interaction with M51a ~1/2Gyr ago, the molecular gas in NGC5195 has resettled and is forming stars at an efficiency consistent with settled early-type galaxies at the present time.
In this work, we investigate the molecular gas and star formation properties
in the barred spiral galaxy NGC 6946 using multiple molecular lines and star
formation tracers. High-resolution image (100 pc) of $^{13}$CO (1-0) is created
by single dish NRO45 and interferometer CARMA for the inner 2 kpc disk, which
includes the central region (nuclear ring and bar) and the offset ridges of the
primary bar. Single dish HCN (1-0) observations were also made to constrain the
amount of dense gas. Physical properties of molecular gas are inferred by (1)
the Large Velocity Gradient (LVG) calculations using our observations and
archival $^{12}$CO (1-0), $^{12}$CO(2-1) data, (2) dense gas fraction suggested
by HCN to $^{12}$CO (1-0) luminosity ratio, and (3) infrared color. The results
show that the molecular gas in the central region is warmer and denser than
that of the offset ridges. Dense gas fraction of the central region is similar
with that of LIRGs/ULIRGs, while the offset ridges are close to the global
average of normal galaxies. The coolest and least dense region is found in a
spiral-like structure, which was misunderstood to be part of the southern
primary bar in previous low-resolution observations. Star formation efficiency
(SFE) changes by ~ 5 times in the inner disk. The variation of SFE agrees with
the prediction in terms of star formation regulated by galactic bar. We find a
consistency between star-forming region and the temperature inferred by the
infrared color, suggesting that the distribution of sub-kpc scale temperature
is driven by star formation.
We used multi-wavelength analysis of the newly observed molecular gas (12CO
and 13CO (1-0)) with interferometer CARMA and archival star formation tracers
to constrain the interaction, merging, and star formation history of an
off-center minor merger, a three-spiral barred galaxy NGC 5430 and its
satellite embedded in the bar. Morphology of the molecular gas in the bar of
NGC 5430 shows minimal signs of recent interactions in our resolution. The
apparent morphological remnant of the past galaxy interaction is an asymmetric
spiral arm, containing more molecular gas and exhibiting higher star formation
rate (SFR) surface density than the two primary arms. Rotation curve analysis
suggests that NGC 5430 and its satellite collided several Gyr ago. History of
star formation was constrained by using SFRs that trace different timescales
(infrared, radio continuum, and H-alpha). The collision occurred 5 - 10 Myr
ago, triggering a transient off-center starburst of Wolf-Rayet stars at the
eastern bar end. In the past, the global SFR during the Wolf-Rayet starburst
peaked at 35 Msun/yr. At present, the merger-driven starburst is rapidly
decaying and the current global SFR has decreased to the Galactic value. The
SFR will continue to decay as suggested by the present amount of dense gas
(traced by HCN (1-0)). Nonetheless, the global SFR is still dominated by the
Wolf-Rayet region rather than the circumnuclear region. Compared with other
barred galaxies, the circumnuclear region exhibits particularly low dense gas
fraction, low star formation activity and high concentration of gas. Physical
properties of the molecular gas are inferred by using the large velocity
gradient (LVG) calculations. The initial mass ratio of the NGC 5430 and its
satellite are suggested to be in the intermediate ratio range of 7:1-20:1.
The central kiloparsec region of the Andromeda galaxy is relatively gas poor,
while the interstellar medium appears to be concentrated in a ring-like
structure at about 10 kpc radius. The central gas depletion has been attributed
to a possible head-on collision 200 Myr ago, supported by the existence of an
offset inner ring of warm dust. We present new IRAM-30m observations of the
molecular gas in the central region, and the detection of CO and its isotopes
$^{13}$CO(2-1) and C$^{18}$O(2-1), together with the dense gas tracers,
HCN(1-0) and HCO+(1-0). A systematic study of the observed peak temperatures
with non-LTE equilibrium simulations shows that the detected lines trace dense
regions with n$_{H_2}$ in the range 2.5 $10^4 - 5.6 10^5$ cm$^{-3}$, while the
gas is very clumpy with a beam filling factor of 0.5-2 10$^{-2}$. We also show
that the gas is optically thin in all lines, except the $^{12}$CO(1-0) and
$^{12}$CO(2-1) lines, that the CO lines are close to the thermal equilibrium at
17.5-20 K and a molecular hydrogen density larger than critical and that the
HCN and HCO+ lines have a subthermal excitation temperature of 8-10 K with a
density smaller than critical. The molecular mass we derive is compatible with
the dust mass derived from the far-infrared emission, assuming a dust-to-gas
mass ratio of 0.01. In one of the regions, the $^{12}$CO/$^{13}$CO line ratio
is quite high (~20), and equals to the $^{12}$CO/C$^{18}$O ratio. The fact that
the optically thin $^{13}$CO and C$^{18}$O lines have comparable intensities
means that the secondary element $^{13}$C is depleted with respect to the
primary $^{12}$C, as is expected just after a recent star formation. This
suggests that there has been a recent starburst in the central region,
supporting the head-on collision scenario.
We study the global SF law - the relation between gas and SFRs in a sample of
181 local galaxies with L_IR spanning almost five orders of magnitude, which
includes 115 normal galaxies and 66 (U)LIRGs. We derive their atomic, molecular
gas and dense molecular gas masses using newly available HI, CO and HCN data
from the literature, and SFRs are determined both from total IR and 1.4 GHz
radio continuum (RC) luminosities. In order to derive the disk-averaged surface
densities of gas and SFRs, we have used high-resolution RC observations to
measure the radio sizes for all galaxies. We find that dense molecular gas (as
traced by HCN) has the tightest correlation with that of SFRs, and is linear in
(N=1.01 +/- 0.02) across the full galaxy sample. The correlation between
densities of molecular gas (traced by CO) and SFRs is sensitive to the adopted
value of the alpha_CO used to infer molecular gas masses from CO luminosities.
For a fixed value of alpha_CO, a slope of 1.14+/-0.02 is found. If instead we
adopt values of 4.6 and 0.8 for disk galaxies and (U)LIRGs, respectively, we
find the two distinct relations. If applying a continuously varying alpha_CO to
our sample, we recover a single relation with slope of 1.60+/-0.03. The SFRs is
a steeper function of total gas than that of molecular gas, and is tighter
among low-luminosity galaxies. We find no correlation between SFRs and atomic
gas.
