Abstract
The dependence of the star formation efficiency (SFE) on galactic structures—especially whether the SFE in the bar region is lower than those in other regions—has recently been debated. We ...report the SFEs of 18 nearby gas-rich massive star-forming barred galaxies with large apparent bar major axes (≧75″). We statistically measure the SFE by distinguishing the center, the bar end, and the bar regions for the first time. The molecular gas surface density is derived from archival CO(1–0) and/or CO(2–1) data by assuming a constant CO-to-H
2
conversion factor (
α
CO
), and the star formation rate surface density is derived from a linear combination of far-UV and mid-IR intensities. The angular resolution is 15″, which corresponds to 0.3–1.8 kpc. We find that the ratio of the SFE in the bar to that in the disk was systematically lower than unity (typically 0.6–0.8), which means that the star formation in the bar is systematically suppressed. Our results are inconsistent with similar recent statistical studies, which have reported that the SFE tends to be independent of galactic structures. This inconsistency can be attributed to the differences in the definitions of the bar region, the spatial resolutions, the
α
CO
, and the sample galaxies. Furthermore, we find a negative correlation between the SFE and the velocity width of the CO spectrum, which is consistent with the idea that the large dynamical effects—such as strong shocks, large shears, and fast cloud–cloud collisions caused by the noncircular motion of the bar—result in a low SFE.
Abstract
High-resolution observations of ionized and molecular gas in the nuclear regions of galaxies are indispensable for delineating the interplay of star formation, gaseous inflows, stellar ...radiation, and feedback processes. Combining our new Atacama Large Millimeter/submillimeter Array band 3 mapping and archival Very Large Telescope/MUSE data, we present a spatially resolved analysis of molecular and ionized gas in the central 5.4 kpc region of NGC 1365. We find the star formation rate/efficiency (SFR/SFE) in the inner circumnuclear ring is about 0.4/1.1 dex higher than in the outer regions. At a linear resolution of 180 pc, we obtain a superlinear Kennicutt–Schmidt law, demonstrating a steeper slope (1.96 ± 0.14) than previous results presumably based on lower-resolution observations. Compared to the northeastern counterpart, the southwestern dust lane shows lower SFE, but denser molecular gas and larger virial parameters. This is consistent with an interpretation of negative feedback from an active galactic nucleus (AGN) and/or starburst, in the sense that the radiation/winds can heat and interact with the molecular gas even in relatively dense regions. After subtracting the circular motion component of the molecular gas and the stellar rotation, we detect two prominent noncircular motion components of molecular and ionized hydrogen gas, reaching a line-of-sight velocity of up to 100 km s
−1
. We conclude that the winds or shocked gas from the central AGN may expel the low-density molecular gas and diffuse ionized gas on the surface of the rotating disk.
We analyze the CO-to-H2 conversion factor (αCO) in the nearby barred spiral galaxy M83. We present new H i observations from the VLA and single-dish GBT in the disk of the galaxy, and combine them ...with maps of CO(1-0) integrated intensity and dust surface density from the literature. αCO and the gas-to-dust ratio (δGDR) are simultaneously derived in annuli of 2 kpc width from R = 1–7 kpc. We find that αCO and δGDR both increase radially, by a factor of ∼2–3 from the center to the outskirts of the disk. The luminosity-weighted averages over the disk are αCO = 3.14 (2.06, 4.96) M⊙pc−2Kkms−1−1 and δGDR = 137 (111, 182) at the 68% (1σ) confidence level. These are consistent with the αCO and δGDR values measured in the Milky Way. In addition to possible variations of αCO due to the radial metallicity gradient, we test the possibility of variations in αCO due to changes in the underlying cloud populations, as a function of galactic radius. Using a truncated power-law molecular cloud CO luminosity function and an empirical power-law relation for cloud mass and luminosity, we show that the changes in the underlying cloud population may account for a factor of ∼1.5–2.0 radial change in αCO.
Abstract
We report high-resolution (2″ ∼ 200 pc) mappings of the central region of the nearby barred spiral galaxy NGC 1365 in the CO(1–0) and CO(2–1) emission lines. The 2–1/1–0 ratio of integrated ...intensities shows a large scatter (0.15) with a median value of 0.67. We also calculate the ratio of velocity dispersions and peak temperatures and find that in most cases the velocity dispersion ratio is close to unity and thus the peak temperature ratio is comparable to the integrated intensity ratio. This result indicates that both CO(1–0) and CO(2–1) lines trace similar components of molecular gas, with their integrated intensity (or peak temperature) ratios reflecting the gas density and/or temperature. Similar to recent kiloparsec-scale studies, these ratios show a positive correlation with a star formation rate indicator (here we use an extinction-corrected H
α
map), suggesting that molecular gas associated with recent star formation is denser and/or warmer. We also find that some CO spectra show two peaks owing to complicated kinematics, and such two components likely trace molecular gas at different conditions. This result demonstrates the importance of spectral fitting to measure integrated intensities and their ratios more accurately.
ABSTRACT
We investigate dynamical states of grand-design spiral arms in three local galaxies: M51, NGC 3627, and NGC 628. Based on linear perturbation analysis considering multiple components in the ...galaxies, we compute instability parameters of the spiral arms using their observational data and argue whether the arms will fragment by their self-gravity. Our analysis utilizes observations of carbon monoxide (CO), 21-cm line emission, and multiband photometric images for molecular gas, atomic gas, and stellar components in the arms, respectively. We find that the grand-design arms of these galaxies indicate marginally stable states, and hence they are not on the way to fragment. We consider this to be consistent with the commonness of spiral galaxies and the relative rarity of fragmented discs at low redshifts. In the analysis, molecular gas is the dominant component to determine the (in)stability of the arms, whereas atomic gas and stars are far less important. Therefore, the results of our analysis are sensitive to an assumed CO-to-H2 conversion factor. If we assume a typical scatter of the measurements and admit nearly twice as large a conversion factor as our fiducial value, our analysis results in predicting the instability for the spiral arms. More sophisticated determination of the conversion factor is required for more accurate analysis for the (in)stability of spiral arms.
Abstract We report the detection of the CO(2–1) emission line with a spatial resolution of 0.″9 (3.5 kpc) from the host galaxy of the fast radio burst (FRB), FRB 20191001A at z = 0.2340, using the ...Atacama Large Millimeter/submillimeter Array. This is the first detection of spatially resolved CO emission from the host galaxy of an FRB at a cosmological distance. The inferred molecular gas mass of the host galaxy is (2.3 ± 0.4) × 10 10 M ⊙ , indicating that it is gas-rich, as evidenced by the measured molecular gas fraction μ gas = 0.50 ± 0.22. This molecular gas mass and the star formation rate of the host, SFR = (8.06 ± 2.42) M ⊙ yr −1 , differ from those observed in the other FRB host galaxies with the average M gas = 9.6 × 10 8 M ⊙ and SFR = 0.90 M ⊙ yr −1 . This lends further credibility to the hypothesis that FRBs may originate from single or multiple progenitors across a diverse range of galaxy environments. Based on the observed velocity field modeling, we find that the molecular gas disk is dominated by an ordered circular rotation, despite the fact that the host galaxy has a gas-rich companion galaxy with a projected separation of ∼25 kpc. The formation of the FRB’s progenitor might not have been triggered by this interaction. We derive the 3 σ upper limit of the molecular gas column density at the FRB detection site to be <2.1 × 10 21 cm −2 with a 3 σ upper limit.
Abstract
CO(2–1) emission is often used as a tracer of giant molecular clouds (GMCs) as an alternative to CO(1–0) emission in recent years. Therefore, understanding the environmental dependence of ...the line ratio of CO(2–1)/CO(1–0),
R
21
, on the GMC scale is important to accurately estimate the mass of GMCs. We thus measured
R
21
in the strongly barred galaxy NGC 1300, where star formation activity strongly depends on galactic structure, on a ∼100 pc scale. CO images were obtained from the Atacama Large Millimeter/submillimeter Array and the Nobeyama 45 m telescope. The resultant typical
R
21
in NGC 1300 is 0.57 ± 0.06. We find environmental variations in
R
21
: it is the highest in the bar-end region (0.72 ± 0.08), followed by arm (0.60 ± 0.07) and bar regions (0.50 ± 0.06). GMCs with H
α
emission show a systematically higher ratio (0.67 ± 0.07) than those without H
α
(0.47 ± 0.05). In the bar region, where massive star formation is suppressed, H
α
emission is not associated with most GMCs, resulting in the lowest
R
21
. These results raise a possibility that properties of GMCs derived from CO(2–1) observations with the assumption of a constant
R
21
are different from those derived from CO(1–0) observations. Furthermore, we find the
R
21
measured on the kiloparsec scale tends to be lower than that of the GMCs, probably due to the presence of an extended diffuse molecular gas in NGC 1300.
Molecular Gas Properties in M83 from CO PDFs Egusa, Fumi; Hirota, Akihiko; Baba, Junichi ...
Astrophysical journal/The Astrophysical journal,
02/2018, Letnik:
854, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We have obtained 12CO(1-0) data of the nearby barred spiral galaxy M83 from Atacama Large Millimeter/submillimeter Array and Nobeyama 45 m observations. By combining these two data sets, the total CO ...flux has been recovered, and a high angular resolution ( corresponding to ∼40 pc at the distance of M83) has been achieved. The field of view is corresponding to ∼3.4 kpc and covers the galactic center, bar, and spiral arm regions. In order to investigate how these galactic structures affect gas properties, we have created a probability distribution function (PDF) of the CO integrated intensity ( ), peak temperature, and velocity dispersion for a region with each structure. We find that the PDF for the bar shows a bright-end tail while that for the arm does not. As the star formation efficiency is lower in the bar, this difference in PDF shape is contrary to the trend in Milky Way studies where the bright-end tail is found for star-forming molecular clouds. While the peak temperature PDFs are similar for the bar and arm regions, velocity dispersion in the bar is systematically larger than in the arm. This large velocity dispersion is likely a major cause of the bright-end tail and of suppressed star formation. We also investigate an effect of stellar feedback to PDF profiles and find that the different PDFs between bar and arm regions cannot be explained by the feedback effect, at least at the current spatial scale.
We resolve 182 individual giant molecular clouds (GMCs) larger than 2.5 x 10 super(5) M sub(middot in circle) in the inner disks of 5 large nearby spiral galaxies (NGC 2403, NGC 3031, NGC 4736, NGC ...4826, and NGC 6946) to create the largest such sample of extragalactic GMCs within galaxies analogous to the Milky Way. Using a conservatively chosen sample of GMCs most likely to adhere to the virial assumption, we measure cloud sizes, velocity dispersions, and super(12)CO (J = 1-0) luminosities and calculate cloud virial masses. The average conversion factor from CO flux to H sub(2) mass (or X sub(CO)) for each galaxy is 1-2 x 10 super(20) cm super(-2) (K km s super(-1)) super(-1), all within a factor of two of the Milky Way disk value (~2 x 10 super(20) cm super(-2) (K km s super(-1)) super(-1)). We find GMCs to be generally consistent within our errors between the galaxies and with Milky Way disk GMCs; the intrinsic scatter between clouds is of order a factor of two. Consistent with previous studies in the Local Group, we find a linear relationship between cloud virial mass and CO luminosity, supporting the assumption that the clouds in this GMC sample are gravitationally bound. We do not detect a significant population of GMCs with elevated velocity dispersions for their sizes, as has been detected in the Galactic center. Though the range of metallicities probed in this study is narrow, the average conversion factors of these galaxies will serve to anchor the high metallicity end of metallicity-X sub(CO) trends measured using conversion factors in resolved clouds; this has been previously possible primarily with Milky Way measurements.
Abstract
We present Karl G. Jansky Very Large Array
K
-band (19 GHz) observations of the redshifted CO(1–0) line emission toward the radio galaxy TN J0924−2201 at
z
= 5.2, which is one of the most ...distant radio galaxies with CO detected. With an angular resolution of ∼2″, the CO(1–0) line emission is resolved into three clumps, within ±500 km s
−1
relative to its redshift, which is determined by Ly
α
. We find that the clumps are located off-center and 12–33 kpc away from the center of the host galaxy, which has counterparts in the Hubble Space Telescope
i
band, Spitzer/IRAC, and the Atacama Large Millimeter/submillimeter Array (ALMA) band 6 (230 GHz; 1.3 mm). With the ALMA detection, we estimate the infrared luminosity
L
IR
and the star formation rate (SFR) of the host galaxy to be (9.3 ± 1.7) × 10
11
L
⊙
and 110 ± 20
M
⊙
yr
−1
, respectively. We also derive the 3
σ
upper limit of
M
H
2
<
1.3
×
10
10
M
⊙
at the host galaxy. The detected CO(1–0) line luminosities of the three clumps,
L
′
CO
(
1
−
0
)
= (3.2–4.7) × 10
10
K km s
−1
pc
2
, indicate the presence of three massive molecular gas reservoirs, with
M
H
2
= (2.5–3.7) × 10
10
M
⊙
, assuming a CO-to-H
2
conversion factor of
α
CO
= 0.8
M
⊙
(
K
km
s
−
1
pc
2
)
−
1
, although the SFR is not elevated due to the nondetection of the ALMA 1.3 mm continuum (SFR < 40
M
⊙
yr
−1
). From the host galaxy, the nearest molecular gas clump, labeled “clump A,” is apparently in alignment with the radio jet axis, showing radio–CO alignment. The origins of these three clumps around TN J0924–2201 can possibly be interpreted as outflows, mergers, or jet-induced metal enrichment.
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