12 research outputs found
FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) : Molecular clouds toward W33 ; possible evidence for a cloud-cloud collision triggering O star formation
We observed molecular clouds in the W33 high-mass star-forming region
associated with compact and extended HII regions using the NANTEN2 telescope as
well as the Nobeyama 45-m telescope in the 1-0 transitions of CO,
CO, and CO as a part of the FOREST Unbiased Galactic plane
Imaging survey with the Nobeyama 45-m telescope (FUGIN) legacy survey. We
detected three velocity components at 35 km s, 45 km s, and 58 km
s. The 35 km s and 58 km s clouds are likely to be
physically associated with W33 because of the enhanced CO 3-2 to
1-0 intensity ratio as due to the ultraviolet
irradiation by OB stars, and morphological correspondence between the
distributions of molecular gas and the infrared and radio continuum emissions
excited by high-mass stars. The two clouds show complementary distributions
around W33. The velocity separation is too large to be gravitationally bound,
and yet not explained by expanding motion by stellar feedback. Therefore, we
discuss that a cloud-cloud collision scenario likely explains the high-mass
star formation in W33.Comment: 29 pages, 18 figures, 3 tables, accepted for publication in PAS
FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) 2: Possible evidence for formation of NGC~6618 cluster in M17 by cloud-cloud collision
We present CO 1--0, CO 1--0 and CO 1--0
images of the M17 giant molecular clouds obtained as part of FUGIN (FOREST
Ultra-wide Galactic Plane Survey InNobeyama) project. The observations cover
the entire area of M17 SW and M17 N clouds at the highest angular resolution
(19) to date which corresponds to 0.15 pc at the distance of
2.0 kpc. We find that the region consists of four different velocity
components: very low velocity (VLV) clump, low velocity component (LVC), main
velocity component (MVC), and high velocity component (HVC). The LVC and the
HVC have cavities. UV photons radiated from NGC 6618 cluster penetrate into the
N cloud up to 5 pc through the cavities and interact with molecular gas.
This interaction is correlated with the distribution of YSOs in the N cloud.
The LVC and the HVC are distributed complementary after that the HVC is
displaced by 0.8 pc toward the east-southeast direction, suggesting that
collision of the LVC and the HVC create the cavities in both clouds. The
collision velocity and timescale are estimated to be 9.9 km s and yr, respectively. The high collision velocity can provide the
mass accretion rate up to 10 M_{\solar} yr, and the high column
density ( cm) might result in massive cluster
formation. The scenario of cloud-cloud collision likely well explains the
stellar population and its formation history of NGC 6618 cluster proposed by
Hoffmeister et al. (2008).Comment: 24 pages, 13 figures, submitted to PAS
Massive star formation in W51A triggered by cloud-cloud collisions
W51A is one of the most active star-forming region in our Galaxy, which
contains giant molecular clouds with a total mass of 10^6 Msun. The molecular
clouds have multiple velocity components over ~20 km/s, and interactions
between these components have been discussed as the mechanism which triggered
the massive star formation in W51A. In this paper, we report an observational
study of the molecular clouds in W51A using the new 12CO, 13CO, and C18O
(J=1-0) data covering a 1.4x1.0 degree region of W51A obtained with the
Nobeyama 45-m telescope at 20" resolution. Our CO data resolved the four
discrete velocity clouds at 50, 56, 60, and 68 km/s with sizes and masses of
~30 pc and 1.0-1.9x10^5 Msun. Toward the central part of the HII region complex
G49.5-0.4, we identified four C18O clumps having sizes of ~1 pc and column
densities of higher than 10^23 cm^-3, which are each embedded within the four
velocity clouds. These four clumps are distributed close to each others within
a small distance of 5 pc, showing a complementary distribution on the sky. In
the position-velocity diagram, these clumps are connected with each others by
bridge features with intermediate intensities. The high intensity ratios of
13CO (J=3-2/J=1-0) also indicates that these four clouds are associated with
the HII regions. We also found these features in other HII regions in W51A. The
timescales of the collisions are estimated to be several 0.1 Myrs as a crossing
time of the clouds, which are consistent with the ages of the HII regions
measured from the size of the HII regions in the 21 cm continuum emissions. We
discuss the cloud-cloud collision scenario and massive star formation in W51A
by comparing with the recent observational and theoretical studies of
cloud-cloud collision.Comment: 53 pages, 23 figure
FOREST unbiased Galactic plane imaging survey with the Nobeyama 45 m telescope (FUGIN): Possible evidence of cloud-cloud collisions triggering high-mass star formation in the giant molecular cloud M16 (Eagle Nebula)
M16, the Eagle Nebula, is an outstanding \HII \ region which exhibits
extensive high-mass star formation and hosts remarkable "pillars". We herein
obtained new CO 1-0 data for the region observed with NANTEN2, which
were combined with the CO 1-0 data obtained using FUGIN survey.
These observations revealed that a giant molecular cloud (GMC) of \Msun \ is associated with M16, which is elongated by over 30 pc
and is perpendicular to the galactic plane, at a distance of 1.8 kpc. This GMC
can be divided into the northern (N) cloud, the eastern (E) filament, the
southeast (SE) cloud, the southeast (SE) filament, and the southern (S) cloud.
We also found two velocity components (blue and red shifted component) in the N
cloud. The blue-shifted component shows a ring-like structure, as well as the
red-shifted component coincides with the intensity depression of the ring-like
structure. The position-velocity diagram of the components showed a V-shaped
velocity feature. The spatial and velocity structures of the cloud indicated
that two different velocity components collided with each other at a relative
velocity of 11.6 \kms. The timescale of the collision was estimated to be yr. The collision event reasonably explains the formation of the
O9V star ALS15348, as well as the shape of the Spitzer bubble N19. A similar
velocity structure was found in the SE cloud, which is associated with the
O7.5V star HD168504. In addition, the complementary distributions of the two
velocity components found in the entire GMC suggested that the collision event
occurred globally. On the basis of the above results, we herein propose a
hypothesis that the collision between the two components occurred sequentially
over the last several yr and triggered the formation of O-type stars
in the NGC6611 cluster.Comment: Accepted for publication on PAS
ALMA Observations of Giant Molecular Clouds in M33. II. Triggered High-mass Star Formation by Multiple Gas Colliding Events at the NGC 604 Complex
We present the results of ALMA observations in CO(),
CO(), and CO() lines and 1.3 mm continuum emission
toward a massive () giant molecular cloud associated with
the giant H II region NGC 604 in one of the nearest spiral galaxy M33 at an
angular resolution of 0''.44 0''.27 (1.8 pc 1.1 pc). The
CO and CO 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 as dense clumps at edges of two shells and
also at an intersection of several filaments. We examined the velocity
structures of CO() 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
galactic rotation compressed the molecular material into dense filaments/shells
as ongoing high-mass star formation sites. We propose that multiple gas
converging/colliding events with a velocity of a few tens km s are
necessary to build up NGC 604, the most significant cluster-forming complex in
the Local Group of galaxies.Comment: 15 pages, 8 figures, accepted for publication in The Astrophysical
Journa
ALMA Observations of Giant Molecular Clouds in M33 I: Resolving Star Formation Activities in the Giant Molecular Filaments Possibly Formed by a Spiral Shock
We report molecular line and continuum observations toward one of the most
massive giant molecular clouds (GMCs), GMC-16, in M33 using ALMA with an
angular resolution of 044 027 (2 pc 1 pc). We
have found that the GMC is composed of several filamentary structures in
CO and CO ( = 2-1). The typical length, width, and total mass
are 50-70 pc, 5-6 pc, and 10 , respectively,
which are consistent with those of giant molecular filaments (GMFs) as seen in
the Galactic GMCs. The elongations of the GMFs are roughly perpendicular to the
direction of the galaxy's rotation, and several H{\sc ii} regions are
located at the downstream side relative to the filaments with an offset of
10-20 pc. These observational results indicate that the GMFs are
considered to be produced by a galactic spiral shock. The 1.3 mm continuum and
CO ( = 2-1) observations detected a dense clump with the size of
2 pc at the intersection of several filamentary clouds, which is referred
to as the hub filament, possibly formed by a cloud-cloud collision. A
strong candidate for protostellar outflow in M33 has also been identified at
the center of the clump. We have successfully resolved the parsec-scale local
star formation activity in which the galactic scale kinematics may induce the
formation of the parental filamentary clouds.Comment: 13 pages, 5 figures, Accepted for publication in Ap
ALMA Observations of Giant Molecular Clouds in M33 III: Spatially Resolved Features of the Star-Formation Inactive Million-solar-mass Cloud
We present CO ( = 2-1), CO ( = 2-1), and CO ( =
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 1pc). The earlier studies revealed that its high-mass star
formation is inactive in spite of a sufficient molecular reservoir with the
total mass of 10 .
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 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 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 .Comment: 16 pages, 9 figures, accepted for publication in The Astrophysical
Journa
FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN). VI. Dense gas and mini-starbursts in the W43 giant molecular cloud complex
We performed new large-scale CO, CO, and CO 1--0
observations of the W43 giant molecular cloud complex in the tangential
direction of the Scutum arm () as a part of the FUGIN
project. The low-density gas traced by CO is distributed over 150 pc
100 pc (), and has a large velocity dispersion (20-30 km
s). However, the dense gas traced by CO is localized in the W43
Main, G30.5, and W43 South (G29.96-0.02) high-mass star-forming regions in the
W43 GMC complex, which have clumpy structures. We found at least two clouds
with a velocity difference of 10-20 km s, both of which are
likely to be physically associated with these high-mass star-forming regions
based on the results of high CO 3-2 to 1-0 intensity ratio
and morphological correspondence with the infrared dust emission. The velocity
separation of these clouds in W43 Main, G30.5, and W43 South is too large for
each cloud to be gravitationally bound. We also revealed that the dense gas in
the W43 GMC has a high local column density, while "the current SFE" of entire
the GMC is low () compared with the W51 and M17 GMC. We argue that
the supersonic cloud-cloud collision hypothesis can explain the origin of the
local mini-starbursts and dense gas formation in the W43 GMC complex.Comment: 51 pages, 34 figures, 6 tables, accepted for publication in PAS
Molecular gas in a Spitzer bubble N4: possible evidence for cloud-cloud collisions as a trigger of massive star formation
Herein, we present the 12CO (J=1-0) and 13CO (J=1-0) emission line
observations via the FOREST Unbiased Galactic plane Imaging survey with the
Nobeyama 45-m telescope (FUGIN) toward a Spitzer bubble N4. We observed clouds
of three discrete velocities: 16, 19, and 25 km/s. Their masses were 0.1x10^4
Msun, 0.3x10^4 Msun, and 1.4x10^4 Msun, respectively. The distribution of the
25-km/s cloud likely traces the ring-like structure observed at mid-infrared
wavelength. We could not find clear expanding motion of the molecular gas in
N4. On the contrary, we found a bridge feature and a complementary
distribution, which are discussed as observational signatures of a cloud-cloud
collision, between the 16- and 25-km/s clouds. We proposed a possible scenario
wherein the formation of a massive star in N4 was triggered by a collision
between the two clouds; however whereas the 19-km/s cloud is possibly not a
part of the interaction with N4. The time scale of collision is estimated to be
0.2-0.3 Myr, which is comparable to the estimated dynamical age of the HII
region of ~0.4 Myr. In N4W, a star-forming clump located west of N4, we
observed molecular outflows from young stellar objects and the observational
signature of a cloud-cloud collision. Thus, we also proposed a possible
scenario in which massive- or intermediate-mass star formation was triggered
via a cloud-cloud collision in N4W
FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) I: Project Overview and Initial Results
The FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m
telescope (FUGIN) project is one of the legacy projects using the new
multi-beam FOREST receiver installed on the Nobeyama 45-m telescope. This
project aims to investigate the distribution, kinematics, and physical
properties of both diffuse and dense molecular gas in the Galaxy at once by
observing 12CO, 13CO, and C18O J=1-0 lines simultaneously. The mapping regions
are a part of the 1st quadrant (10d < l < 50d, |b| < 1d) and the 3rd quadrant
(198d < l <236d, |b| < 1d) of the Galaxy, where spiral arms, bar structure, and
the molecular gas ring are included. This survey achieves the highest angular
resolution to date (~20") for the Galactic plane survey in the CO J=1-0 lines,
which makes it possible to find dense clumps located farther away than the
previous surveys. FUGIN will provide us with an invaluable dataset for
investigating the physics of the galactic interstellar medium (ISM),
particularly the evolution of interstellar gas covering galactic scale
structures to the internal structures of giant molecular clouds, such as small
filament/clump/core. We present an overview of the FUGIN project, observation
plan, and initial results, which reveal wide-field and detailed structures of
molecular clouds, such as entangled filaments that have not been obvious in
previous surveys, and large-scale kinematics of molecular gas such as spiral
arms.Comment: 19 pages, 14 figures, accepted for publication in PAS