We present the results of ALMA observations in 12CO(J = 2 − 1), 13CO(J = 2 − 1), and C18O(J = 2 − 1) lines and 1.3 mm continuum emission toward a massive (∼106 M ) giant molecular cloud associated ...with the giant H ii region NGC 604 in one of the nearest spiral galaxies, M33, at an angular resolution of 0 44 × 0 27 (1.8 pc × 1.1 pc). The 12CO and 13CO images show highly complicated molecular structures composed of a lot of filaments and shells whose lengths are 5-20 pc. We found three 1.3 mm continuum sources to be dense clumps at the edges of two shells and also at the intersection of several filaments. We examined the velocity structures of the 12CO(J = 2 − 1) emission in the shells and filaments containing dense clumps, and concluded that expansion of the H ii regions cannot explain the formation of such dense cores. Alternatively, we suggest that cloud-cloud collisions induced by an external H i gas flow and the galaxy's rotation compressed the molecular material into dense filaments/shells which are ongoing high-mass star formation sites. We propose that multiple gas converging/colliding events with a velocity of a few tens of kilometers per second are necessary to build up NGC 604, the most significant cluster-forming complex in the Local Group of galaxies.
The Serpens South infrared dark cloud consists of several filamentary ridges, some of which fragment into dense clumps. On the basis of CCS (J{sub N} = 4{sub 3}-3{sub 2}), HC{sub 3}N (J = 5-4), N{sub ...2}H{sup +} (J = 1-0), and SiO (J = 2-1, v = 0) observations, we investigated the kinematics and chemical evolution of these filamentary ridges. We find that CCS is extremely abundant along the main filament in the protocluster clump. We emphasize that Serpens South is the first cluster-forming region where extremely strong CCS emission is detected. The CCS-to-N{sub 2}H{sup +} abundance ratio is estimated to be about 0.5 toward the protocluster clump, whereas it is about 3 in the other parts of the main filament. We identify six dense ridges with different V {sub LSR}. These ridges appear to converge toward the protocluster clump, suggesting that the collisions of these ridges may have triggered cluster formation. The collisions presumably happened within a few × 10{sup 5} yr because CCS is abundant only for a short time. The short lifetime agrees with the fact that the number fraction of Class I objects, whose typical lifetime is 0.4 × 10{sup 5} yr, is extremely high, about 70% in the protocluster clump. In the northern part, two ridges appear to have partially collided, forming a V-shape clump. In addition, we detected strong bipolar SiO emission that is due to the molecular outflow blowing out of the protostellar clump, as well as extended weak SiO emission that may originate from the filament collisions.
Abstract
Atomic carbon (C i) has been proposed to be a global tracer of the molecular gas as a substitute for CO, however, its utility remains unproven. To evaluate the suitability of C i as the ...tracer, we performed C i(3P1–3P0) hereinafter C i(1–0) mapping observations of the northern part of the nearby spiral galaxy M 83 with the Atacama Submillimeter Telescope Experiment (ASTE) telescope and compared the distributions of C i(1–0) with CO lines CO(1–0), CO(3–2), and 13CO(1–0), H i, and infrared (IR) emission (70, 160, and 250 μm). The C i(1–0) distribution in the central region is similar to that of the CO lines, whereas C i(1–0) in the arm region is distributed outside the CO. We examined the dust temperature, Tdust, and dust mass surface density, Σdust, by fitting the IR continuum-spectrum distribution with a single-temperature modified blackbody. The distribution of Σdust shows a much better consistency with the integrated intensity of CO(1–0) than with that of C i(1–0), indicating that CO(1–0) is a good tracer of the cold molecular gas. The spatial distribution of the C i excitation temperature, Tex, was examined using the intensity ratio of the two C i transitions. An appropriate Tex at the central, bar, arm, and inter-arm regions yields a constant C$/$H2 abundance ratio of ∼7 × 10−5 within a range of 0.1 dex in all regions. We successfully detected weak C i(1–0) emission, even in the inter-arm region, in addition to the central, arm, and bar regions, using spectral stacking analysis. The stacked intensity of C i(1–0) is found to be strongly correlated with Tdust. Our results indicate that the atomic carbon is a photodissociation product of CO, and consequently, compared to CO(1–0), C i(1–0) is less reliable in tracing the bulk of “cold” molecular gas in the galactic disk.
We present a giant molecular cloud (GMC) catalog of M33, containing 71 GMCs in total, based on wide-field and high-sensitivity CO(J = 3-2) observations with a spatial resolution of 100 pc using the ...ASTE 10 m telescope. Employing archival optical data, we identify 75 young stellar groups (YSGs) from the excess of the surface stellar density, and estimate their ages by comparing with stellar evolution models. A spatial comparison among the GMCs, YSGs, and H II regions enable us to classify GMCs into four categories: Type A, showing no sign of massive star formation (SF); Type B, being associated only with H II regions; Type C, with both H II regions and <10 Myr old YSGs; and Type D, with both H II regions and 10-30 Myr YSGs. Out of 65 GMCs (discarding those at the edges of the observed fields), 1 (1%), 13 (20%), 29 (45%), and 22 (34%) are Types A, B, C, and D, respectively. We interpret these categories as stages in a GMC evolutionary sequence. Assuming that the timescale for each evolutionary stage is proportional to the number of GMCs, the lifetime of a GMC with a mass >10 super(5) M sub(middot in circle) is estimated to be 20-40 Myr. In addition, we find that the dense gas fraction as traced by the CO(J = 3-2)/CO(J = 1-0) ratio is enhanced around SF regions. This confirms a scenario where dense gas is preferentially formed around previously generated stars, and will be the fuel for the next stellar generation. In this way, massive SF gradually propagates in a GMC until gas is exhausted.
Abstract
We present the results of ALMA-ACA 7 m array observations in
12
CO(
J
= 2–1),
13
CO(
J
= 2–1), and C
18
O(
J
= 2–1) line emission toward the molecular-gas disk in the Local Group spiral ...galaxy M33 at an angular resolution of 7.″31 × 6.″50 (30 × 26 pc). We combined the ACA 7 m array
12
CO(
J
= 2–1) data with the IRAM 30 m data to compensate for emission from diffuse molecular-gas components. The ACA+IRAM combined
12
CO(
J
= 2–1) map clearly depicts the cloud-scale molecular-gas structure over the M33 disk. Based on the ACA+IRAM
12
CO(
J
= 2–1) cube data, we cataloged 848 molecular clouds with a mass range from 10
3
–10
6
M
⊙
. We found that high-mass clouds (≥10
5
M
⊙
) tend to associate with the 8
μ
m bright sources in the spiral arm region, while low-mass clouds (<10
5
M
⊙
) tend to be apart from such 8
μ
m bright sources and to exist in the inter-arm region. We compared the cataloged clouds with GMCs observed by the IRAM 30 m telescope at 49 pc resolution (IRAM GMC), and found that a small IRAM GMC is likely to be identified as a single molecular cloud even in ACA+IRAM CO data, while a large IRAM GMC can be resolved into multiple ACA+IRAM clouds. The velocity dispersion of a large IRAM GMC is mainly dominated by the line-of-sight velocity difference between small clouds inside the GMC rather than the internal cloud velocity broadening.
Abstract
The barred spiral galaxy NGC 613 has a star-forming ring in the center, and near-infrared observations have previously shown that the star formation activity on the eastern and western sides ...of the ring is asymmetric. We examined the dynamics and physical state of the molecular gas in the ring using high-resolution (∼15 pc) 12CO(1–0), 12CO(3–2), and 13CO(1–0) observations with ALMA. Using a dendrogram, we identified 111 molecular clouds in the bar and ring, and found that the virial parameter (αvir) gradually decreases (αvir < 2) toward the confluence of the northern bar and eastern ring, while the virial parameter is large (αvir > 2) around the corresponding confluence in the western side of the ring. A non-LTE analysis using RADEX showed that the temperature and density of the molecular gas increase downstream of the eastern point of confluence, whereas they change irregularly on the western side. The star formation efficiency is (1.7 ± 0.2) × 10−8 yr−1 in the eastern side of the ring, which is substantially higher than the (0.9 ± 0.3) × 10−8 yr−1 for the western side of the ring. Position–velocity diagrams show that the relative velocity of the gas between the bar and the ring is ∼70 km s−1 in the east, while it reaches ∼170 km s−1 in the west. We suggest that the star formation activity in the central region of NGC 613 depends strongly on the relative velocity of the gas between the bar and the ring.
Abstract
W$\, 51\,$A is one of the most active star-forming regions in the Milky Way, and includes copious amounts of molecular gas with a total mass of ${\sim }6\times 10^{5}\, M_{\odot }$. The ...molecular gas has multiple velocity components over ∼20 km s−1, and interactions between these components have been discussed as the mechanism that triggered the massive star formation in W$\, 51\,$A. In this paper, we report on an observational study of the molecular gas in W$\, 51\,$A using the new 12CO, 13CO, and C18O (J = 1–0) data covering a 1${^{\circ}_{.}}$4 × 1${^{\circ}_{.}}$0 area of W$\, 51\,$A obtained with the Nobeyama 45 m telescope at 20′ resolution. Our CO data resolved four discrete velocity clouds with sizes and masses of ∼30 pc and 1.0–$1.9\times 10^{5}\, M_{\odot }$ around radial velocities of 50, 56, 60, and 68 km s−1. Toward the central part of the Hii region complex G49.5−0.4 in W$\, 51\,$A, in which the bright stellar clusters IRS 1 and IRS 2 are located, we identified four C18O clumps having sizes of ∼1 pc and column densities of higher than 1023 cm−2, which are each embedded within the four velocity clouds. These four clumps are concentrated within a small area of 5 pc, but show a complementary distribution on the sky. In the position–velocity diagram, these clumps are connected with each other by bridge features having weak intensities. The high intensity ratios of 13CO (J = 3–2)$/$(J = 1–0) also indicate that these four clouds are associated with the Hii regions, including IRS 1 and IRS 2. We also reveal that, in the other bright Hii region complex G49.4−0.3, the 50, 60, and 68 km s−1 clouds show a complementary distribution, with two bridge features connecting between the 50 and 60 km s−1 clouds and the 60 and 68 km s−1 clouds. An isolated compact Hii region G49.57−0.27 located ∼15 pc north of G49.5−0.4 also shows a complementary distribution and a bridge feature. The complementary distribution on the sky and the broad bridge feature in the position–velocity diagram suggest collisional interactions among the four velocity clouds in W$\, 51\,$A. The timescales of the collisions can be estimated to be several 0.1 Myr as crossing times of the collisions, which are consistent with the ages of the Hii regions measured from the sizes of the Hii regions with the 21 cm continuum data. We discuss a scenario of cloud–cloud collisions and massive star formation in W$\, 51\,$A by comparing these with recent observational and theoretical studies of cloud–cloud collision.
Abstract
We present
12
CO (
J
= 2–1),
13
CO (
J
= 2–1), and C
18
O (
J
= 2–1) observations toward GMC-8, one of the most massive giant molecular clouds (GMCs) in M33 using ALMA with an angular ...resolution of 0.″44 × 0.″27 (∼2 pc × 1 pc). The earlier studies revealed that its high-mass star formation is inactive in spite of a sufficient molecular reservoir with a total mass of ∼10
6
M
⊙
. The high-angular resolution data enable us to resolve this peculiar source down to a molecular clump scale. One of the GMC’s remarkable features is that a round-shaped gas structure (the “Main cloud”) extends over the ∼50 pc scale, which is quite different from the other two active star-forming GMCs dominated by remarkable filaments/shells obtained by our series of studies in M33. The fraction of the relatively dense gas traced by the
13
CO data with respect to the total molecular mass is only ∼2%, suggesting that their spatial structure and the density are not well developed to reach an active star formation. The CO velocity analysis shows that the GMC is composed of a single component as a whole, but we found some local velocity fluctuations in the Main cloud and extra blueshifted components at the outer regions. Comparing the CO with previously published large-scale H
i
data, we suggest that an external atomic gas flow supplied a sufficient amount of material to grow the GMC up to ∼10
6
M
⊙
.
Abstract
We present Atacama Large Millimeter/submillimeter Array (ALMA) imaging of molecular gas across the full star-forming disk of the barred spiral galaxy M83 in CO(
J
= 1–0). We jointly ...deconvolve the data from ALMA’s 12 m, 7 m, and Total Power arrays using the MIRIAD package. The data have a mass sensitivity and resolution of 10
4
M
⊙
(3
σ
) and 40 pc—sufficient to detect and resolve a typical molecular cloud in the Milky Way with a mass and diameter of 4 × 10
5
M
⊙
and 40 pc, respectively. The full disk coverage shows that the characteristics of molecular gas change radially from the center to outer disk, with the locally measured brightness temperature, velocity dispersion, and integrated intensity (surface density) decreasing outward. The molecular gas distribution shows coherent large-scale structures in the inner part, including the central concentration, offset ridges along the bar, and prominent molecular spiral arms. However, while the arms are still present in the outer disk, they appear less spatially coherent, and even flocculent. Massive filamentary gas concentrations are abundant even in the interarm regions. Building up these structures in the interarm regions would require a very long time (≳100 Myr). Instead, they must have formed within stellar spiral arms and been released into the interarm regions. For such structures to survive through the dynamical processes, the lifetimes of these structures and their constituent molecules and molecular clouds must be long (≳100 Myr). These interarm structures host little or no star formation traced by H
α
. The new map also shows extended CO emission, which likely represents an ensemble of unresolved molecular clouds.
Abstract
We present integral field, far-infrared (FIR) spectroscopy of Mrk 54, a local Lyman continuum emitter, obtained with FIFI-LS on the Stratospheric Observatory for Infrared Astronomy. This is ...only the second time, after Haro 11, that C
ii
158
μ
m and O
iii
88
μ
m spectroscopy of the known LCEs have been obtained. We find that Mrk 54 has a strong C
ii
emission that accounts for ∼1% of the total FIR luminosity, whereas it has only moderate O
iii
emission, resulting in the low O
iii
/C
ii
luminosity ratio of 0.22 ± 0.06. In order to investigate whether O
iii
/C
ii
is a useful tracer of
f
esc
(LyC escape fraction), we examine the correlations of O
iii
/C
ii
and (i) the optical line ratio of O
32
≡ O
iii
5007 Å/O
ii
3727 Å, (ii) specific star formation rate, (iii) O
iii
88
μ
m/O
i
63
μ
m ratio, (iv) gas-phase metallicity, and (v) dust temperature based on a combined sample of Mrk 54 and the literature data from the Herschel Dwarf Galaxy Survey and the LITTLE THINGS Survey. We find that galaxies with high O
iii
/C
ii
luminosity ratios could be the result of high ionization (traced by O
32
), bursty star formation, high ionized-to-neutral gas volume filling factors (traced by O
iii
88
μ
m/O
i
63
μ
m), and low gas-phase metallicities, which is in agreement with theoretical predictions. We present an empirical relation between the O
iii
/C
ii
ratio and
f
esc
based on the combination of the O
iii
/C
ii
and O
32
correlation, and the known relation between O
32
and
f
esc
. The relation implies that high-redshift galaxies with high O
iii
/C
ii
ratios revealed by the Atacama Large Millimeter/submillimeter Array may have
f
esc
≳ 0.1, significantly contributing to the cosmic reionization.
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