CHEMICAL EVOLUTIONS OF GMCS
WHILE CROSSING THE SPIRAL ARM IN THE NEARBY GALAXY M 83
Taku Sugiuchi, Kazuyuki Muraoka, Ryouhei Harada, Shota Takada, Aya Homma (Osaka Pref. Univ), Kazuki Tokuda (Osaka Pref. Univ. /NAOJ), Tomoka Tosaki (Jouetsu Univ.
of Education), Junichi Baba , Akihiro Hirota , Fumi Egusa (NAOJ), Kotaro Kohno (IoA/UT), Kouichiro Nakanishi(NAOJ), Nario Kuno (Univ. of Tsukuba)

ALMA observations

Abstract

We observed molecular lines including 13CO(1-0), C18O(1-0), CS(2-1), and CH3OH(2-1)
Target : M 83
simultaneously toward the eastern spiral arm of M 83 in order to study the evolution of GMCs
while crossing the spiral arm in terms of chemical properties in the ALMA Cycle-4. The fieldcoverage is ~107”×70”. 13CO(1-0) emission revealed the global distribution of molecular gas, and ・grand-design barred spiral galaxy
C18O(1-0), CS(2-1), and CH3OH(2-1) emissions are found in local peaks of 13CO(1-0). C18O/13CO,・D = 4.5 Mpc (1′′ = 22 pc ; Thim et al. 2003)
CS/13CO ratios tend to increase in the downstream side of the spiral arm. We have identified 32
clumps as GMCs based on dendrogram algorithm. We examined the correlations between size
・face-on : i = 22
and velocity widths and between molecular gas mass (LTE mass) and virial mass for identified
GMCs. These correlations are well consistent with those in earlier studies (e.g., Tosaki et al. 2017).

Introduction

Fig. 1 : The target field(107” × 70” rectangle) for
ALMA cycle-4 superposedon the 12CO(1-0) intensity
map.

Observation parameters

Since Giant Molecular Clouds (GMCs) are major sites of massive star formation, it isimportant to study the evolution of GMCs in galaxies.
So far, GMC evolutions while crossing spiral arms have been studied by many
investigators. Based on mm-wave interferometric observations, Egusa et al. (2011) andHirota et al.(2011) revealed that massive clouds tend to be located in downstream side of
the spiral arm and are virialized, associated with HII regions, while smaller clouds tends tobe in upstream side and are less massive and unvirialized.
However, these studies are mostly based on the limited molecule, such as CO. It is also
important to observe other molecular lines (e.g., Tosaki et al. 2017). For example, C18O(1-0)
and CS(2-1) emission are enhanced in dense gas/massive star forming regions while
CH3OH(2-1) is enhanced in spiral shock wave and is dissociated by intense UV radiationfrom massive stars.
Examining spatial variations in C18O/13CO, CS/13CO and CH3OH/13CO ratios in the spiralarm enables us to investigate physical/chemical evolutions of GMCs in detail.

Results & Discussion

・Telescopes : ALMA 12-m Array
・Observed area : 107”×70” rectangle (see Fig.1)
・Observed lines

Cloud Identification We applied the dendrogram algorithm

13CO(1-0) integrated intensities (contours)We
compareIntegrated Intensity map
with other emission (color).
The strong Hα emission is observed in the downstream side of 13CO(1-0) arm rather than theupstream side. IRAC 8μm emission is well spatially coincident with the 13CO(1-0) intensity.

(Rosolowsky et al. 2008) to the 13CO(1-0) data cube in order to decompose
the 13CO emission into discrete clouds and to measure their physicalproperties. This algorithm makes a tree that represents the hierarchy ofthe structures in the data. For example, large clouds are trunks, and small
ones are leaves.
We have identified 32 clumps as GMCs(Fig. 5). For identified GMCs, we
examined the size-velocity width relation of identified GMCs (Fig. 6) andcorrelation between LTE mass and virial mass of them(Fig. 7), using thefollowing equations.
𝑀𝑉𝐼𝑅 = 210𝑅 ∆𝑣 2[M⦿ ]NH2[pc][km s-1]

= 2.41 × 105 ×5.29ൗ
𝑇𝑒𝑥𝑒H2× 𝐼13𝐶𝑂
13CO 𝑒 5.29/𝑇𝑒𝑥 − 1

cm−1

K km s −1

MLTE = NH2 × 𝜇 × 𝑝𝑖𝑥 2 × 𝑀⦿ /𝑀𝐻2
Fig. 3.1 : 13CO(1-0)

Fig. 3.2 : HαFig. 3.3 : IRAC 8μm

[M⦿ ]

cm−1

kg

[cm]

[M⦿ kg −1 ]H
COassuming 13 2 = 5.0 × 105 Dickman et al. 1978 ,Excitation temperature ∶ 𝑇𝑒𝑥 = 20 K
mean molecular weight per H2 : μ = 2.8 kg
length per 1 pixel : pix = 1.6264 ×1019 cm

Fig. 3.4 : C18O(1-0)

Fig. 3.5 : CS(2-1)

Fig. 5 : GMCs identified bydendrogram. Each red
enclosure corresponds to aGMC.

Fig. 3.6 : CH3OH (2(0,2)-1(0,1))

Fig. 3 : Integrated intensity maps of 13CO(1-0) emission (contours) superposed on other emission (color).
The pixel size is 0”.5. The contour levels are 60, 200, 400 and 700 σ where 1 σ = 0.43 mJy beam-1.
18O(1-0)/13CO(1-0),
We
examined
spatialvariations
inC
Intensity Ratio Map
CS(2-1)/13CO(1-0), and CH3OH(2-1)/13CO(1-0) ratios.
C18O/13CO, CS/13CO ratios tend to increase in the downstream side of the spiral arm.

Fig. 6 : Correlation between size andvelocity width of the identified GMCsin M83. The diagonal line indicates
Larson’s Law, σv∝R0.5(Larson 1981).

Fig. 7 : Correlation between molecular
gas mass (LTE mass) and virial mass of
the identified GMCs in M83. The
diagonal line shows MLTE = MVIR

Future Works• Deriving C18O/13CO, CS/13CO and CH3OH/13CO ratios and virial parameters
for each identified GMC.
• Investigating their variations according to upstream/downstream side ofthe spiral arm.
• Unveiling physical/chemical evolution of GMCs while crossing spiral arms.

References
Fig. 4.1 : C18O/13CO

Fig. 4.2 : CS/13CO

Fig. 4.3 : CH3OH/13CO・Egusa et al. 2011 Apj, 726, 85 ・Hirota et al. 2011, ApJ, 737, 40・Tosaki et al. 2017 PASJ, 69
18 ・Thim et al. 2003, ApJ, 590, 256 ・Rosolowsky et al. 2008, ApJ, 679, 1338-1351・Dickman, R. L.
1978, ApJS, 37, 407・Larson, R. B. 1981, MNRAS, 194, 809