Abstract
The centers of starburst galaxies may be characterized by a specific gas and ice chemistry due to their gas dynamics and the presence of various ice desorption mechanisms. This may result in ...a peculiar observable composition. We analyse the abundances of CO
2
, a reliable tracer of ice chemistry, from data collected as part of the Atacama Large Millimeter/submillimeter Array large program ALCHEMI, a wide-frequency spectral scan toward the starburst galaxy NGC 253 with an angular resolution of 1.″6. We constrain the CO
2
abundances in the gas phase using its protonated form HOCO
+
. The distribution of HOCO
+
is similar to that of methanol, which suggests that HOCO
+
is indeed produced from the protonation of CO
2
sublimated from ice. The HOCO
+
fractional abundances are found to be (1–2) × 10
−9
at the outer part of the central molecular zone (CMZ), while they are lower (∼10
−10
) near the kinematic center. This peak fractional abundance at the outer CMZ is comparable to that in the Milky Way CMZ, and orders of magnitude higher than that in Galactic disk, star-forming regions. From the range of HOCO
+
/CO
2
ratios suggested from chemical models, the gas-phase CO
2
fractional abundance is estimated to be (1–20) × 10
−7
at the outer CMZ, and orders of magnitude lower near the center. We estimate the CO
2
ice fractional abundances at the outer CMZ to be (2–5) × 10
−6
from the literature. A comparison between the ice and gas CO
2
abundances suggests an efficient sublimation mechanism. This sublimation is attributed to large-scale shocks at the orbital intersections of the bar and CMZ.
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.
Herein, we present 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 Spitzer bubble ...N4. We observed clouds at three discrete velocities: 16, 19, and 25 km s−1. Their masses were 0.1 × 104 M , 0.3 × 104 M , and 1.4 × 104 M , respectively. The distribution of the 25 km s−1 cloud likely traces the ring-like structure observed at mid-infrared wavelength. The 16 and 19 km s−1 clouds have not been recognized in previous observations of molecular lines. 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 (CCC), between the 16 and 25 km s−1 clouds. We proposed a possible scenario wherein the formation of a massive star in N4 was triggered by a collision between the two clouds. The timescale of collision is estimated to be 0.2-0.3 Myr, which is comparable to the estimated dynamical age of the H ii 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 CCC. Thus, we also proposed a possible scenario in which massive- or intermediate-mass star formation was triggered via a CCC in N4W.
We report the statistical physical properties of the C18O(J = 1-0) clumps present in a prominent cluster-forming region, Cygnus X, using the data set obtained by the Nobeyama 45 m radio telescope. ...This survey covers 9 deg2 of the northern and southern regions of Cygnus X, and, in total, 174 C18O clumps are identified using the dendrogram method. Assuming a distance of 1.4 kpc, these clumps have radii of 0.2-1 pc, velocity dispersions of <2.2 km s−1, gas masses of 30-3000 M , and H2 densities of (0.2-5.5) × 104 cm−3. We confirm that the C18O clumps in the northern region have a higher H2 density than those in the southern region, supporting the existence of a difference in the evolutionary stages, consistent with the star-formation activity of these regions. The difference in the clump properties of the star-forming and starless clumps is also confirmed by the radius, velocity dispersion, gas mass, and H2 density. The average virial ratio of 0.3 supports that these clumps are gravitationally bound. The C18O clump mass function shows two spectral index components, = −1.4 in 55-140 M and = −2.1 in >140 M , which are consistent with the low- and intermediate-mass parts of the Kroupa's initial mass function. The spectral index of the star-forming clumps >140 M is consistent with that of the starless clumps ranging from 55-140 M , suggesting that the latter will evolve into star-forming clumps while retaining the gas accretion. Assuming a typical star-formation efficiency of molecular clumps (10%), about 10 C18O clumps having a gas mass of >103 M will evolve into open clusters containing one or more OB stars.
Abstract
We present a new calibration for the second-order light contamination in the near-infrared grism spectroscopy with the Infrared Camera aboard AKARI, specifically for the post-cryogenic phase ...of the satellite (Phase 3). Following our previous work on the cryogenic phase (Phases 1 and 2), the wavelength and spectral response calibrations were revised. Unlike Phases 1 and 2, during Phase 3 the temperature of the instrument was not stable and gradually increased from 40 to 47 K. To assess the effect of the temperature increase, we divided Phase 3 into three sub-phases and performed the calibrations separately. As in Phases 1 and 2, we confirmed that there was contamination due to the wavelength dependence of the refractive index of the grism material in every sub-phase. The wavelength calibration curves for the three sub-phases coincided with each other and did not show any significant temperature dependence. The response decreased with temperature by ∼10% from the beginning to the end of Phase 3. We approximated the temperature dependence of the response at a linear relation and derived a correction factor as a function of temperature. The relative fraction of the second-order light contamination to the first-order light was found to be 25% smaller than that in Phases 1 and 2.
We have carried out a statistical study on the mid- and far-infrared (IR) properties of Galactic IR bubbles observed by Spitzer. Using the Spitzer 8 μm images, we estimated the radii and covering ...fractions of their shells, and categorized them into closed, broken, and unclassified bubbles with our data analysis method. Then, using the AKARI all-sky images at wavelengths of 9, 18, 65, 90, 140, and 160 μm, we obtained the spatial distributions and the luminosities of polycyclic aromatic hydrocarbon (PAH), warm, and cold dust components by decomposing six-band spectral energy distributions with model fitting. As a result, 180 sample bubbles show a wide range of total IR luminosities corresponding to the bolometric luminosities of a single B-type star to many O-type stars. For all the bubbles, we investigated relationships between the radius, luminosities, and luminosity ratios, and found that there are overall similarities in the IR properties among the bubbles regardless of their morphological types. In particular, they follow a power-law relation with an index of ∼3 between the total IR luminosity and radius, as expected from the conventional picture of the Strömgren sphere. The exceptions are large broken bubbles; they indicate higher total IR luminosities, lower fractional luminosities of the PAH emission, and dust heating sources located nearer to the shells. We discuss the implications of those differences for a massive star-formation scenario.
We present AKARI near- to far-infrared images of the nearby edge-on spiral galaxy NGC 3079 in 10 photometric bands. The spectral energy distribution consists of continuum emission from dust with a ...single temperature of 28–33 K together with strong mid-infrared emission features from polycyclic aromatic hydrocarbons (PAHs). We derived dust masses of 5.6
$\times$
10
$^{6}\ M$
$_{\odot}$
and 1.4
$\times$
10
$^7\ M$
$_{\odot}$
for the central 4 kpc region and the whole galaxy, respectively, and found that the gas-to-dust mass ratio is unusually high in the central region (
$\sim\ $
1100), and even for the whole galaxy (
$\sim\ $
860). The ratio of the surface brightness distribution at a wavelength of 7
$\ \mu$
m to that at 11
$\ \mu$
m suggests that the properties of PAHs have spatial variations. The emission from ionized and neutral PAHs is relatively strong in the center and the disk regions, respectively, suggesting a stronger radiation field, and thus relatively active star formation in the center. Yet, the total infrared luminosities of the galaxy indicate rather low star formation rates. These results suggest that NGC 3079 is in an early-phase starburst stage.
We present an AKARI mid-infrared diffuse map of an area of about 4
$^{\circ}$
$\times$
3
$^{\circ}$
near the Galactic center in the 9
$\mu$
m band. The band intensity is mostly attributed to aromatic ...hydrocarbon infrared emissions of carbonaceous grains at wavelengths of 6.2, 7,7, 8.6, and 11.3
$\mu$
m. We detected 9
$\mu$
m emission structures extending from the Galactic plane up to a latitude of
$\sim$
2
$^\circ\!\!\!.$
5, which have spatial correspondence to molecular loops revealed by NANTEN
$^{12}$
CO (
$J$
$=$
1–0) observations. We have found that the surface brightness at 9
$\mu$
m is suppressed near the foot points of the CO loops. The ratios of the 9
$\mu$
m to the IRAS 100
$\mu$
m brightness show significant depression near such bright regions in CO emission. With AKARI near-infrared 2.5–5
$\mu$
m spectroscopy, we find that the 3.3
$\mu$
m aromatic hydrocarbon emission is absent in the region associated with the loop, suggesting the processing and destruction of carbonaceous grains in the CO molecular loops.
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