ASTE CO(3-2) Observations of the Barred Spiral Galaxy M 83: I. Correlation between CO(3-2)/CO(1-0) Ratios and Star Formation Efficiencies

0.57 MB·English

by  Kazuyuki Muraoka

PASJ:Publ.Astron.Soc.Japan,1–??,
(cid:13)c 2008.AstronomicalSocietyofJapan.
ASTE CO(3-2) Observations of the Barred Spiral Galaxy M 83:
I. Correlation between CO(3-2)/CO(1-0) Ratios and
Star Formation Efficiencies
Kazuyuki Muraoka,1 Kotaro Kohno,1 Tomoka Tosaki,2 Nario Kuno,2
Kouichiro Nakanishi,2 Kazuo Sorai,3 Takeshi Okuda,1 Seiichi Sakamoto,4
Akira Endo,1,4 Bunyo Hatsukade,1 Kazuhisa Kamegai,1 Kunihiko Tanaka,1
Juan Cortes,4,5 Hajime Ezawa,4 Nobuyuki Yamaguchi,4 Takeshi Sakai,2
and Ryohei Kawabe4
1Institute of Astronomy, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015
7
[email protected]
0
0 2Nobeyama Radio Observatory, Minamimaki, Minamisaku, Nagano 384-1305
2 3Division of Physics, Graduate School of Science, Hokkaido University, Sapporo 060-0810
4National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588
n
a 5Departmento de Astronomia, Universidad de Chile, Casilla 36-D, Santiago, Chile
J
0 (Received ;accepted )
3
Abstract

3 WepresentCO(J=3−2)emissionobservationswiththeAtacamaSubmillimeterTelescopeExperiment
v (ASTE) toward the 5′×5′ (or 6.6 × 6.6 kpc at the distance D = 4.5 Mpc) region of the nearby barred
9
spiral galaxy M 83. We successfully resolved the major structures, i.e., the nuclear starburst region, bar,
5
and inner spiral arms in CO(J=3−2) emissionat a resolutionof 22′′ (or 480pc), showing a good spatial
7
1 coincidence between CO(J =3−2) and 6 cm continuum emissions. We found a global CO(J =3−2)
0 luminosity L′ of 5.1×108 K km s−1 pc2 within the observedregion. We alsofound L′ in the
CO(3−2) CO(3−2)
7 disk region (0.5<r<3.5 kpc) of 4.2×108 K km s−1 pc2, indicating that CO(J =3−2) emission in the
0 disk region significantly contributes to the global L′ . From a comparison of a CO(J =3−2) data
/ CO(3−2)
h with CO(J =1−0) intensities measured with Nobeyama 45-m telescope, we found that the radial profile
p of CO(J =3−2)/CO(J =1−0) integrated intensity ratio R is almost unity in the central region
- 3−2/1−0
o (r<0.25 kpc), whereas it drops to a constant value, 0.6–0.7, in the disk region. The radial profile of star
r formation efficiencies (SFEs), determined from 6 cm radio continuum and CO(J =1−0) emission, shows
t
s the same trend as that of R . At the bar-end (r∼2.4 kpc), the amounts of molecular gas and the
3−2/1−0
a massivestarsareenhancedwhencomparedwithotherdiskregions,whereasthereisnoexcessofR
: 3−2/1−0
v and SFE in that region. This means that a simple summation of the star forming regions at the bar-end
i and the disk cannot reproduce the nuclear starburst of M 83, implying that the spatial variation of the
X
dense gas fraction traced by R governs the spatial variation of SFE in M 83.
3−2/1−0
r
a Key words: galaxies: ISM—galaxies: starburst—galaxies: individual (M 83; NGC 5236)
1. Introduction 2001)? Sincestarsareformedbythecontractionofmolec-
ular gas, which would be initiated by gravitational insta-
In the central regions of disk galaxies,we often find in- bility and/or cloud-cloud collision of molecular clouds, it
tensestarformationactivities(e.g.,Hoetal.1997)where can be easily expected that some properties of molecular
notonlythestarformationrates(SFRs)butalsothestar gas play a key role in controlling the variations of SFEs.
formation efficiencies (SFEs), defined as the fraction of Investigations of molecular gas are widely performed us-
the SFR surface density in the surface mass density of ing CO(J =1−0) emission, which traces the total mass
molecular gas, are also enhanced. These star formation ofmoleculargas. Inparticular,extensivesurveysormap-
activities are referred to as starburst phenomena, which pings of CO(J =1−0) emission toward galaxies are ex-
are different from the star formations in the disk regions amined (Young et al. 1995, Kuno et al. 2000, Sorai et al.
of galaxies (Kennicutt 1998b). The starburst phenom- 2000a, Nishiyama & Nakai 2001, Nishiyama et al. 2001,
ena areprominentnotonly because they showhighSFRs Kuno et al. 2007). However, CO(J =1−0) emission si-
but also because they show elevated SFEs. What causes multaneously traces both diffuse/cold molecular gas and
these “high SFE” star formations? In other words, why dense/warmmoleculargas,whichisdirectlylinkedtostar
do SFEs differ among galaxies (e.g., Young et al. 1996) formation. Therefore,CO(J=1−0) emissionis useful to
or within locations/regions in a galaxy (e.g., Gao et al. determinethetotalamountofmoleculargas,butitisdif-
2 K. Muraoka et al. [Vol. ,
ficult to compare with SFEs quantitatively. Thus, it is found to be comparable to the CO(J =1−0) intensities.
essential to study dense molecular gas, because stars are These results clearly indicate a potential for the use of
formed from the dense cores of molecular clouds. CO(J =3−2) emission as a dense gas tracer in the disk
In fact, recent studies of the dense molecular medium regions of galaxies.
in galaxies based on the observations of HCN(J =1−0) Spatially-resolvedwide-areaCO(J=3−2)observations
emission,adensemoleculargastracer(n > afew ×104 of nearby galaxies are also essential for the appropriate
H2
cm−3)due toitslargedipolemoment(µ =3.0Debye, interpretationsofhigh-J COobservationsofhighredshift
HCN
whereasµ =0.1Debye),demonstratetheintimatecon- objects (see a review by Solomon & Vanden Bout 2005),
CO
nection between dense molecular gas and massive star because most of these high-J CO observations of distant
formation in galaxies. For example, after the first re- objects are spatially unresolved.
portbySolomonetal.(1992),atightcorrelationbetween Previous CO(J =3−2) observations of nearby galax-
HCN(J=1−0)andFIRcontinuumluminositieshasbeen ies, however, were almost limited to their central regions
shownamongmanygalaxiesincludingnearbyones(Gao& or to the whole regions of galaxies without resolving the
Solomon2004a,Gao&Solomon2004b),andhighredshift structures using a large observing beam (e.g. Wild et al.
quasar host galaxies (Carilli et al. 2005). This suggests 1992,Hurtetal.1993,Devereuxetal.1994,Mauersberger
that SFEs are governed by the fraction of dense molecu- et al. 1996, Wielebinski et al. 1999, Nieten et al. 1999,
lar gas to the total amount of molecular contents traced Mauersberger et al. 1999,Harrison et al. 1999, Sandqvist
bytheHCN(J=1−0)/CO(J=1−0)intensityratios(e.g., 1999, Dumke et al. 2001, Liszt 2001, Meier et al. 2001,
Solomon et al. 1992, Kohno et al. 2002, Shibatsuka et al. Wiedner et al. 2002, Vila-vilar´o et al. 2003, Yao et al.
2003, Gao & Solomon 2004a). A spatial coincidence be- 2003, Hafok & Stutzki 2003, Matsushita et al. 2004, Iono
tween dense molecular gas tracedby HCN(J=1−0)and etal.2004,Narayananetal.2005,Weiß etal.2005,Israel
massivestar forming regionswas also reported(Kohno et 2005, Israel et al. 2006).
al. 1999, Shibatsuka et al. 2003). In this paper, we present wide-area observations
However, HCN(J =1−0) emission is often too weak of CO(J = 3−2) line emission using the Atacama
to trace a wide-area map in the disk regions of galaxies. SubmillimeterTelescopeExperiment(ASTE;Ezawaetal.
HCN(J =1−0) intensity is typically 10–20 times lower 2004, Kohno 2005) towards the whole inner disk of M 83
thantheCO(J=1−0)intensityinthediskregionsofthe (NGC 5236). M 83 is a nearby, face-on, barred, grand-
Galaxy (Helfer & Blitz 1997b) and galaxies (e.g., Kohno design spiral galaxy hosting an intense starburst at its
etal.1996,Helfer&Blitz1997a,Curranetal.2001,Sorai center. The distance to M 83 is estimated to be 4.5 Mpc
etal.2002). Therefore,HCN(J=1−0)emissionismainly (Thim et al. 2003); therefore, 1′′ corresponds to 22 pc.
usedtomeasuretheamountofdensemoleculargasinthe The inclination of M 83 is 24◦ (Comte 1981). Its proxim-
central regions of galaxies or to obtain information inte- ity and face-on view enable us to resolve its major struc-
gratedoverthewholeregionsofgalaxiesduetoinadequate tures such as the nuclear starburst region, bar, and inner
sensitivity of current observing facilities (e.g. Nguyen-Q- spiralarms even by single-dishobservations atmillimeter
Rieu et al. 1989, Nguyen-Q-Rieu et al. 1992, Henkel et or submillimeter wavelength. For these reasons, M 83 is
al. 1990, Solomon et al. 1992, Israel 1992, Helfer & Blitz thebesttargettoinvestigatethespatialvariationsofSFE
1993, Aalto et al. 1995, Curran et al. 2000, Kohno et al. and the physical state of the molecular gas between the
2003). nuclear region and the disk region. The CO observations
Instead, CO(J =3−2) emission is a promising probe toward M 83 were reported for various transitions (see a
of dense molecular clouds even in the disk regions of comprehensivesummaryinLundgrenetal.2004). Handa
galaxies. The critical density for CO(J =3−2) transi- et al. (1990) present a CO(J =1−0) map of the nucleus
tion is a few × 104 cm−3. Therefore, the CO(J =3−2) and the bar at a resolution of 16′′ using a Nobeyama 45-
emission can also trace the dense component of molec- m telescope. A new wider CO(J =1−0) map using the
ular clouds as well as HCN(J =1−0). In fact, in the Nobeyama 45-m telescope is now available (Kuno et al.
central regions of nearby star forming galaxies, a tight 2007). Moreover,Crosthwaiteet al. (2002)andLundgren
correlation between CO(J =3−2) and Hα luminosities, etal.(2004)presentCO(J=1−0)andCO(J=2−1)maps
whichisbetterthanthecorrelationbetweenCO(J=1−0) covering the entire optical disk. However, the high-J CO
and Hα luminosities, has been reported (Komugi et al. line observations such as CO(J =3−2) were still limited
2007). In addition, Yao et al. (2003) discovered a trend tothenuclearregionandthebar(Petitpas&Wilson1998,
that CO(J = 3−2)/CO(J = 1−0) ratios, hereafter re- Israel & Baas 2001, Dumke et al. 2001, Sakamoto et al.
ferred to as R , increase with the SFE in infrared- 2004, Bayet et al. 2006).
3−2/1−0
luminousgalaxies,whichisverysimilartoafindingbased The goals of this paper are (1) to depict the globaldis-
on HCN(J =1−0)/CO(J =1−0) ratios (Solomon et al. tribution of CO(J =3−2) line emission or dense molec-
1992,Gao&Solomon2004a). Moreover,thereseemtobe ular gas in the disk region of M 83, (2) to investigate the
detectable amounts of CO(J =3−2) emitting dense gas spatial variations of R within the disk of M 83,
3−2/1−0
in the disk regions of galaxies such as M 51 (Wielebinski by computing the radial profiles of CO(J = 3−2) and
etal.1999,Dumkeetal.2001),M31(Tosakietal.2007), CO(J =1−0) intensities, and (3) to examine the corre-
NGC 6946 (Walsh et al. 2002), and Antennae galaxies lation of the R values and SFEs through a com-
3−2/1−0
(Zhu et al. 2003),where the CO(J=3−2)intensities are parison of the radial profiles. In particular, we focus on
No. ] ASTE CO(3-2) Observations of M 83 3
doubler, two frequency doublers, and a frequency tripler,
providedbyVirginiaDiodeInc. Asourcesignalgenerated
by a GPS-locked frequency synthesizer with a frequency
range of 13.5 GHz to 15.5 GHz is multiplied by 24 us-
ing the multiplier chain system. Both the attenuation of
the LO signal power and the change in the frequency set-
ting without a phase lock loop of Gunn oscillator easily
facilitate fully remote operations of the receiver system.
The resultantLOfrequencyrangeis324to 372GHz, and
the typical receiver noise temperature is about 100 K (in
double side-band) for this frequency range. The average
systemtemperaturewas200K(indoubleside-band)dur-
ing our observations of M 83. The temperature scale was
calibrated by the chopper-wheel method, using a room
temperature single absorber in front of the receiver. The
observations were made remotely from the ASTE oper-
ation rooms of the Institute of Astronomy (IoA), Univ.
of Tokyo, NAOJ, and the Nobeyama Radio Observatory
(NRO) using a network observationsystem N-COSMOS3
Fig. 1. Points observed (crosses) using the ASTE super- developedatNAOJ(Kamazakietal.2005). Theskyemis-
posed on a V-band image of M 83 obtained with VLT
sion was subtracted by position switching with two off-
(Comero´n 2001). The grid points are spaced by 11′′ at the
center, bar, and along the spiral arms and by 22′′ in the in- sourcepositionsatoffsetsinanazimuthof±10′ fromthe
ter-arm. central position, which was taken at α=13h37m00s.48,
δ =−29◦51′56′′.5 (J2000). This position corresponds to
the difference of properties of the nuclear starburst and the radio continuum center (Sukumar et al. 1987). The
star formation in the disk region. Further analysis of our spectra ofthe CO(J=3−2)emissionwereobtainedwith
CO(J=3−2)dataofM83willbepresentedinforthcom- four units of XF-type digital spectrometers, MAC (Sorai
ing papers. et al. 2000b), with a band width of 512 MHz and 1024
channels, corresponding to a velocity coverage of 440 km
2. Observations and Data Reduction s−1 for each unit. The absolute pointing of the antenna
was checked every hour using Jupiter and the accuracy
CO(J = 3−2) observations towards M 83 were per- was found to be better than ∼2′′ rms.
formed using the ASTE from December 2004 to January We observedthe CO(J=3−2)emissionofIRC+10216
2005, and June 2005. The size of the CO(J =3−2) map twice a day to verify the stability of the main beam
is about5′×5′ (6.6×6.6kpc), including the nuclearstar- efficiency (ηMB) of the ASTE 10-m dish, and we have
burst region, the bar, and the inner spiral arms. We set compared our data with the CO(J =3−2) emission of
the grid spacing to 11′′, except for the inter-arm region IRC+10216 obtained by CSO observations (Wang et al.
where the grid spacing was set to 22′′. We set the posi- 1994), which are carried out using an SSB-filter. For the
tion angle of the major axis to 45◦ (Comte 1981). The CSO observations, the line peak temperature of CO(J =
total number of points observed is 419, and the observed 3−2) emission of IRC+10216 in TMB was 32.5 ± 0.5 K.
positions are indicated in figure 1. The full-half power Our CO(J =3−2) data was calibrated using this value.
beam width (HPBW) is 22′′, which corresponds to about The resultant ηMB obtained by the calibration of the line
480 pc at a distance of 4.5 Mpc. temperaturewas0.63±0.06. Inadditiontothis,wehave
We used a cartridge-type cooled SIS mixer receiver, also calculated ηMB using the brightness temperature of
SC345, developed by collaboration among the University Jupiter recorded during the pointing observation. We as-
of Tokyo, the Osaka Pref.University, the Nagoya sumed that the brightness temperature of Jupiter at 345
University, and the National Astronomical Observatory GHz is 174 K (Mangum 1993). The resultant ηMB de-
Japan (NAOJ). The receiver SC345 was plugged into the duced from the Jupiter measurements was 0.62 ± 0.03.
Single Cartridge Dewar of the ASTE, which is based on These ηMB values obtained by the two methods are con-
the design of the ALMA test cryostat (Sekimoto et al. sistent, indicating that the side-band gain ratio of the re-
2003,Sugimotoet al.2003,Yokogawaet al.2003)using a ceiver SC345 was almost 1.
3-stage Gifford McMahon cryocooler with an He pot for Data reduction were carried out using NEWSTAR and
temperaturestabilization. ANb-basedparallel-connected GBASE, tools for single-dish data analysis developed by
twin-junction-type mixer is mounted on the 4 K stage of NAOJ. The linear baselines were removed from most of
the dewarwitha corrugatedfeedhornanda localoscilla- thespectra. Webinnedtheadjacentchannelstoavelocity
tor(LO)signalcouplertoinjectradio-frequency(RF)and resolutionof5kms−1 forthe CO(J=3−2)spectra. The
LO signals into the receiver. The intermediate-frequency resultantrmsnoiselevelwastypicallyintherangeof40to
(IF) range is 4.5 to 7.0 GHz. An LO system is com- 70 mK in the TMB scale. The observation parameters for
posed of a chain of multipliers, i.e., an active frequency M 83 are listed in table 1. Also, the adopted parameters
4 K. Muraoka et al. [Vol. ,
Table 1. Observation parameters of M 83.
L for54galaxiesofwhichL isintherangeof109L
FIR FIR ⊙
to 1012L . L of M 83 is ∼9×109L (Mauersberger
Telescope, Receiver ASTE 10-m, SC345 ⊙ FIR ⊙
et al. 1999), and the average L′ in Yao’s sample is
Spectrometer 512 MHz, 1024 channel CO(3−2)
Velocity coverage 440 km s−1 ∼104.5 Kkms−1 Mpc2 Ωb atLFIR=9×109L⊙. Because
Map description 419 positions, 5′×5′ 104.5 K km s−1 Mpc2 Ωb in Yao et al. (2003)corresponds
rms noise level 40–70 mK (in T scale) to2×108 Kkms−1 pc2,L′ ofM83determinedby
MB CO(3−2)
Beam size 22′′ our wide-area CO(J =3−2) observations is comparable
Main-beam efficiency∗ 0.62 ± 0.06 ± 0.03 to that of Yao’s sample.
∗ Forthemain-beamefficiency,thefirsterrorindicates The peak temperature and the integrated intensity at
the nucleus (r < 250 pc) are 1.51 ± 0.04 K and 161 ±
systematicerrorandthesecond,randomerror.
4 K km s−1, respectively, in the T scale. This inte-
MB
grated central CO value is in excellent agreement with
Table 2. Adopted parameters of M 83.
167 ± 15 K km s−1 for 21′′ beam reported by Israel &
Map centera: Baas(2001). Note that Mauersbergeret al.(1999)report
R.A. (J2000) 13h37m00s.48 the CO(J =3−2) integrated intensity at the nucleus of
Decl. (J2000) −29◦51′56′′.5 M83of126Kkms−1,whereasDumkeetal.(2001)found
Distanceb 4.5 Mpc that the line intensity was 234 ± 14 K km s−1, although
Linear scale 22 pc arcsec−1 they used the same 10-m telescope, the Heinrich-Hertz-
Systemic velocity (LSR)c 510 km s−1 Telescope. Our measured CO(J =3−2) integrated in-
Position angled 45◦ tensity lies between these two values. It is unclear what
Inclinationd 24◦ causes such a significant discrepancy. There is slight dif-
ferenceinthe positionofthe “center”,afew arcsec,yetit
Reference—. aSukumaretal.(1987), bThimetal.(2003), is clearly insufficient to accountfor the large discrepancy.
cthispaper,dComte(1981) At the bar-end (110′′ or 2.4 kpc away from the center
to the X direction of M 83; Kuno et al. 2007), where sec-
ondary intensity peaks exist (see also figure 4 and figure
of M 83 are listed in table 2.
7a),thepeaktemperaturesareintherangeof0.5Kto0.9
K.Thetypicallinewidthisabout50kms−1. Inthespiral
3. Results
arm region, the line temperature is in the range of 0.3 to
3.1. CO(J =3−2) Spectra 0.5 K. In the spectra of M 51 (Wielebinski et al. 1999),
the peak temperatures of the CO(J =3−2) emission in
Figure2showsall419spectraoftheCO(J=3−2)emis-
the spiral arms are 0.3–0.4 K, which are comparable to
sion of M 83. The X and Y axes were set parallel to the
those of M 83.
major and minor axes of the galaxy, respectively. In ad-
dition, figure 3 shows the spectra in the central44′′×44′′ 3.2. Global Distributions of the CO(J=3−2) Emission
region. Significant CO(J =3−2) emission for which the
Figure 4 shows an integratedintensity map and a peak
signaltonoiseratioexceeds4isdetectedfor275pointsout
temperature map of the CO(J =3−2) emission of M 83.
ofthe419observedpoints. Strongemissionisobservedat
Extended CO(J = 3−2) emission along the bar can
the nucleus and the leading edges of the bar. Moreover,
be clearly seen as well as a prominent concentration of
theemissionlineissignificantlydetectedfrommostofthe
CO(J=3−2)emissiontowardthenucleus. Theextended
pointsalongtheinnerspiralarms. Nosignificantemission
components are in the leading sides of the bar; they are
lines are detected in the inter-arm regions.
often referred to as “molecular offset ridges,” which are
The physical properties of the observed CO(J =3−2)
visible in high resolution CO(J =1−0) maps of barred
emissionlineofM83aresummarizedintable3. Velocity-
galaxies. A strong CO(J =3−2) emission is also evident
integrated intensities, I , were calculated from the
CO(3−2) along the spiral arms; in particular, we can clearly trace
COemissionwithina velocityrangeof380to 620kms−1
two “CO(J =3−2) arms” in the peak temperature map.
of spectra. In addition, we calculated the luminosity of
Note that they arenotso clearinthe integratedintensity
CO(J =3−2), L′ . We found that L′ was
CO(3−2) CO(3−2) mapdue to the narrowline width andweakerintensity of
9.3×107 Kkms−1 pc2 inthecentralregion(r <0.5kpc) CO in the spiral arms.
and 4.2×108 K km s−1 pc2 in the disk region(0.5 < r < PreviousCO(J=3−2)observationsof M83 werecon-
3.5kpc),respectively. TheresultantglobalCOluminosity finedtoasmallportion(∼1′−2′)ofthe galaxy(Petitpas
wasfoundto be5.1×108 Kkms−1 pc2. Thismeansthat & Wilson 1998,Israel& Baas2001). Dumke et al.(2001)
L′CO(3−2) inthediskregionsignificantlycontributestothe mapped a wider region (∼ 2′×2′); however, detected
global L′ . Note that the global L′ is still CO(J = 3−2) emission remained confined within a re-
CO(3−2) CO(3−2)
the lower limit because we did not observe CO(J=3−2) gionalongthebar. Thus,thisisthefirstmapthatclearly
emission toward the outer disk region (r > 3.5 kpc). depicts the distribution of warmand dense molecular gas
Now, we compare L′ of M 83 to those of other clouds in the disk region of M 83 including spiral arms.
CO(3−2)
galaxies. Yao et al. (2003) gave the plot of L′ vs. Basically, the overall appearance of our CO(J = 3−2)
CO(3−2)
No. ] ASTE CO(3-2) Observations of M 83 5
Fig. 2. Allmeasured spectra of CO(J=3−2) emissionof M 83. The scale of the spectra is indicated by the smallbox inserted
in the lower right corner. Significant emission is detected not only at the nucleus but also at the bar and inner spiral arms. The
central5×5spectraaremagnifiedandshowninfigure3.
Table 3. Physical properties of observed CO(J =3−2) emission line of M 83.
Region I L′ R
CO(3−2) CO(3−2) 3−2/1−0
[K km s−1] [K km s−1 pc2]
M 83 center (r < 0.5 kpc) 120 9.3 × 107 0.97 ± 0.10
disk (0.5 < r < 3.5 kpc) 10 4.2 × 108 0.65 ± 0.07
global 12 5.1 × 108 0.69 ± 0.07
6 K. Muraoka et al. [Vol. ,
observedpoint. To matchthe beam sizeof CO(J=1−0)
+30″ to that ofCO(J=3−2),we convolvedthe CO(J=1−0)
data of 16′′ resolution to 22′′ resolution. The red circles
indicatethe observedpoints inthecentralregion(r <0.5
kpc) of M 83 and the black ones, the inner disk region
(0.5 < r < 3.5 kpc). This plot excludes the data points
wheretheCO(J=1−0)intensitiesarelowerthan8Kkm
s−1, corresponding to a typical 3 σ rms. At the center of
Y M83,theCO(J=3−2)intensityisclosetoorgreaterthan
+00″ the CO(J =1−0) intensity, whereas the CO(J =3−2)
intensitiesareweakerthantheCO(J=1−0)intensityfor
most of the data points in the disk region.
K] 1.5 The azimuthally averaged radial profiles of CO(J =
1.0
[B 0.5 3−2) and CO(J=1−0) integratedintensities are shown
TM 0.0 in figure 7a. The radialprofile ofCO(J=3−2)is similar
V 400 [k 6m0/0s] to that of CO(J=1−0); both of them have strongpeaks
LSR
atthenucleusandasecondarypeakatthebar-end(∼2.4
+30″ +00″ −30″
kpc), where the molecular gas is accumulated. Such a ra-
X
dial CO profile is often observed in barred spiral galaxies
(Nishiyama et al. 2001). Note that the radial profile of
1F1i′g′.. 3. The central 5×5 spectra of M 83. Grid spacing is CO(J =3−2) integrated intensities for larger radii (1 <
r < 3.5 kpc) are biased toward the intensites in the bar
and the inner spiralarms since the observedpoints ofthe
maps is similar to those of previous CO(J =1−0) and
CO(J = 3−2) emission are biased toward the bar and
CO(J = 2−1) maps presented by Handa et al. (1990),
the spiral arms. The same bias exists in the radial pro-
Crosthwaite et al. (2002), Lundgren et al. (2004), Kuno
file of R , which will be described in the following
et al. (2007). Our high-quality data set allows us to 3−2/1−0
subsection.
see the molecular gas structures more clearly through
CO(J =3−2) emission. 3.3. CO(J = 3 − 2)/CO(J = 1− 0) Intensity Ratio
In figure 5, we see a striking similarity between the R
3−2/1−0
contour map of the CO(J = 3−2) peak temperature
We have obtained R for each observed point.
and a 6 cm radio continuum map obtained with the 3−2/1−0
The absolute error of the CO(J = 3−2) and CO(J =
VLA (Ondrechen 1985, Neininger et al. 1993), which
is convolved to 22′′ resolution to compare it with our 1−0)intensitiesareestimatedtoabout15%,respectively.
Thus, the absolute error of R is estimated to be
CO(J =3−2) image. The centimeter continuum emis- 3−2/1−0
about 30%.
sionismainlydominatedbynonthermalsynchrotronradi-
The central R at a resolution of 22′′ is 1.1. For
ationfromsupernovaremnantsinM83(Ondrechen1985, 3−2/1−0
thecenterofM83,Dumkeetal.(2001)reportedR
Turner & Ho 1994). The observed radio intensity is pro- 3−2/1−0
ataresolutionof22′′ is ∼1.4. This discrepancyis mainly
portional to a recent star formation rate. We find that
due totheir highCO(J=3−2)integratedintensityvalue
these maps are well correlated, indicating a good spa-
than our CO(J =3−2) data (see section 3.1).
tialcorrelationbetweenstarformationandthe dense and
NotethatKrameretal.(2005)reportedthatthecentral
warm gas traced by the CO(J =3−2) emission.
R is 0.6 at a resolution of 80′′. Unfortunately,
Then, we examine a comparison of CO(J =3−2) in- 3−2/1−0
we cannot convolved our CO(J =3−2) data to 80′′ due
tensitywithCO(J=1−0)intensity. WeusenewCO(J=
to imperfect sampling in the disk region. Therefore, we
1−0) data obtained using the Nobeyama 45-m telescope
cannotcompareR ataresolutionof80′′ withthat
(Kuno et al. 2007). The HPBW of the CO(J = 1−0) 3−2/1−0
observations with the 45-m telescope is ∼ 16′′, and the of Kramer et al. (2005).
central CO(J =1−0) intensity is 170 K km s−1 in the A radial profile of R3−2/1−0 is displayed in figure 7b.
R is almost unity in the nuclear region (r < 0.25
T scale. Here,wecomparethe newCO(J=1−0)data 3−2/1−0
MB
kpc), whereas it drops to a constant value in the regions
with previous CO(J =1−0) data presented by Handa et
with larger radii (1 < r < 3.5 kpc). The mean value of
al. (1990). According to Handa et al. (1990), the central
CO(J=1−0)intensity ata resolutionof16′′ was ∼80 K the ratio is 0.65 ± 0.07 within the inner disk region.
km s−1 in the T∗ scale. Since the beam efficiency of their There is a discrepancy between this radial profile of
A the R and the CO(J=2−1)/CO(J=1−0) inte-
observationswas0.45,theresultantcentralCO(J=1−0) 3−2/1−0
intensity in the T scale was 180 K km s−1. Therefore, gratedintensity ratioR2−1/1−0 presentedby Lundgrenet
MB
al. (2004). Their R radial profile has a peak not
thenewCO(J=1−0)dataisinexcellentagreementwith 2−1/1−0
at the nucleus but at the radius of 1 kpc, corresponding
the previous CO(J =1−0) data.
to the bar region. The reason for this discrepancy is un-
Figure 6 displays a plot of CO(J = 1−0) integrated
clear,althoughthe differenceinthe observingbeams(our
intensity vs. CO(J =3−2) integrated intensity for each
R is obtained at a 22′′ resolution, whereas their
3−2/1−0
No. ] ASTE CO(3-2) Observations of M 83 7
Fig. 4. (left) A contour mapof the CO(J=3−2) integrated intensity of M 83. The central cross indicates the central reference
position of the map. Contours are at 5, 10, 15, 20, 25, 30, 40, 50, 60, 80, 100, 120, 140, and 160 Kkms−1, and the peak is 161.2
Kkms−1. The1σ noiselevel ofthemapis2Kkms−1. (right) Acontour mapoftheCO(J=3−2)peakbrightness temperature.
Contours are at0.2, 0.3, 0.4, 0.5, 0.7, 0.9, 1.1, 1.3, and 1.5 K, andthe peak is 1.53K. Majorstructures, i.e.,the nuclear starburst
region,bar,andinnerspiralarmsareclearlyresolved.
R is at a 49′′ resolution) may partly account for
2−1/1−0
this.
In other galaxies, the trend of the spatial variations of
R is similar to that of R in M 83. For
2−1/1−0 3−2/1−0
example, the reported R in the central 900 pc of
2−1/1−0
the Galaxy is 0.96 (Oka et al. 1998, Sawada et al. 2001),
which is higher than the typical value of R in the
2−1/1−0
Galacticdisk,0.6–0.7(Sakamotoetal.1995,Sakamotoet
al. 1997). Further, for the Large Magellanic Cloud, Sorai
et al. (2001) reported that there existed a radial gradient
of R of 0.94 in the inner region (<2 kpc from the
2−1/1−0
kinematic center) and 0.69 in the outer region.
We divided the R values of M 83 into two
3−2/1−0
groups, i.e., the central region (r < 0.5 kpc) and the
disk region (0.5 < r < 3.5 kpc). The intensity-weighted
(summation of the intensity in each R range) his-
3−2/1−0
tograms of R for these two groups are shown in
3−2/1−0
figure8. Thesehistogramsincludeonlythedataforwhich
signaltonoiseratiooftheCO(J=3−2)emissionexceeds
5. The number of observed points used to make this his-
togram is 11 for the center and 219 for the disk.
In the central region (r < 0.5 kpc) of M 83, R
Fig. 5. The CO(J =3−2) peak temperature map (white 3−2/1−0
is in the rangeof0.7to 1.2,andthe averagevalue is 0.97.
contour)superposedonthe6cmradiocontinuummap(grey
scale) obtained withthe VLA (Ondrechen 1985). These two This is comparable with the averagevalue of R3−2/1−0 in
mapscorrelatewellspatially. nearby starburst galaxies, ∼0.9 (Devereux et al. 1994),
andslightlyhigherthantheaveragevaluesofR in
3−2/1−0
compact galaxies, ∼0.7 (Israel 2005); infrared luminous
galaxies,∼0.6(Yaoetal.2003);dwarfstarburstgalaxies,
∼0.6 (Meier et al. 2001); and early type galaxies, ∼0.4
(Vila-vilar´o et al. 2003). This could be partly due to the
8 K. Muraoka et al. [Vol. ,
180
Distance from center
center (r < 0.5 kpc)
160 disk (0.5 < r < 3.5 kpc) ) 200 20" 40" 60" 80" 100" 120"140"160"
s )-1 -1m s (a) ICO(3-2)
km 140 K k 100 ICO(1-0)
y (K 120 R3-2/1-0 = 0.97 sity ( 50
nsit 100 nten
nte 80 R = 0.65 ed I 20
3-2) I 60 3-2/1-0 ntegrat 10
( I
O 40 O
C C 5
20 (b)
1

0 20 40 60 80 100 120 140 160 180
CO(1-0) Intensity (K km s -1 ) 0
1-
3-2/ 0.5
R
Fig. 6. A plot of the CO(J = 1−0) intensity vs.
CO(J=3−2)intensityforeachobservedpointofM83. Data
pointswhereCO(J=1−0)intensitiesarelowerthan8Kkm
s−1 are excluded. The red circles indicate the points in the 0.3
centralregion(r<0.5kpc)andtheblackones,theinnerdisk
region(0.5< r <3.5kpc). Inthisplot, the errororiginated
fromthenoiseisonlyincluded,buterrorsfromuncertainties 0 0.5 1 1.5 2 2.5 3 3.5
of baselines and calibrations are not included. The red line Distance from center (kpc)
represents the average R3−2/1−0 in the central region, 0.97,
while the black line represents that in the disk region, 0.65.
AtthecenterofM83,theCO(J=3−2)intensitiesareclose Fig. 7. (a) CO(J =3−2) and CO(J =1−0) line intensi-
toorgreaterthanCO(J=1−0),whereastheCO(J=3−2) ties as afunction of thegalactocentric radiusof M 83. Both
intensitiesareweakerthanCO(J=1−0)formostofthedata profilesshowstrongpeaksinthecentralregionaswellassec-
pointsinthediskregion. ondarypeaks atthe bar-end(r∼2.4kpc). (b) R3−2/1−0 as
a function of the galactocentric radius of M 83. R3−2/1−0
is almost unity at the nucleus (r < 0.25 kpc) and drops to
difference in the aperture sizes where the R val-
3−2/1−0 a constant value, 0.6–0.7, in the disk region (0.5 < r < 3.5
ues were measured (e.g., Meier et al. 2001). Moreover,
kpc),showingnosecondarypeakatthebar-end.
our R in the central region of M 83, 0.97, ap-
3−2/1−0
pears to be slightly higher than the average values of
arm of the Galaxy is ∼0.5, but there exist locations at
R in nearby galaxies, ∼0.6 (Mauersberger et al.
3−2/1−0 which R exceeds unity (T., Sawada. private com-
1999). However,aswepointedoutinsection3.1,I 3−2/1−0
CO(3−2) munication).
at the nucleus of M 83 reported by Mauersberger et al.
(1999) is about 30% lower than our result. Therefore, we
4. Discussion
cannot simply compare these two average values. The
reported R are also close to unity (∼ 0.7 to 1;
2−1/1−0 Thanks to our high-quality data set of CO(J =3−2)
Crosthwaite et al. 2002, Lundgren et al. 2004), although
emission, we have obtained the distribution of wide-
their aperture sizes are larger than ours by a factor of 2
ranging R , which can resolve the 500 pc scale
or more. 3−2/1−0
structure. As mentioned in section 3, R is almost
On the other hand, the ratio in the inner disk spreads 3−2/1−0
unity at the nucleus, and then drops to a constant value
over a wide range of 0.2 to 1.4, and the average value is
(0.6–0.7) in the disk regions.
0.65. More than half of the data points in the disk re-
Inthissection,wediscusstherelationbetweenthephys-
gionshowR valuesrangingfrom0.5to0.8. These
3−2/1−0 ical state of molecular gasand star-formationactivity us-
R valuesareslightlyhigherthantheaveragevalue
3−2/1−0 ing the data on R .
of cold giant molecular clouds (GMCs) in the Galactic 3−2/1−0
disk, 0.4 (Sanders et al. 1993). Note that there exist four 4.1. Interpretation of R3−2/1−0
data points for which R exceeds 1.2 even in the
3−2/1−0 To obtain the physical interpretation of the observed
disk region. These four points are in the spiral arm re-
R values,wehaveexaminedthedependenceofthe
gions but are not adjacent to each other, i.e. existing 3−2/1−0
R values on the kinetic temperature and gas den-
locally. A similar situation has been observed in a spiral 3−2/1−0
sity by employing the large velocity gradient (LVG) ap-
armoftheMilkyWaygalaxy. R intheSagittarius
3−2/1−0 proximation(Scoville&Solomon1974,Goldreich&Kwan
No. ] ASTE CO(3-2) Observations of M 83 9
1000 center AVG = 0.97 (a) Z( 1 2 CO) / dv/dr = 8×10-5
)
800 -3m 4
c
(
) 0.60
600 H 2 3 0.40 0.80
-1m s ) 400 nlog ( 2 0.100.20 0.020.0.4600
k
K 200 0.10
(
y
t 10 20 50 100
i
s
en 1000 disk AVG = 0.65 (b) Z( 1 2 CO) / dv/dr = 8×10-6
t )
-0) In 800 -3(cm 4 0.60
CO(1 600 n (H ) 2 3 0.100.400.20 010..8.0040.0060
g 0.20
400 o
l 2 0.10
200
10 20 50 100
0 0.2 0.4 0.6 0.8 1 .0 1.2 1.4 1.6 Kinetic temperature (K)
R
3-2/1-0
Fig. 9. CurvesofconstantR3−2/1−0 asfunctionsofkinetic
temperatureanddensity ofmoleculargas. (a) COfractional
Fig. 8. Intensity-weightedhistogramsofR3−2/1−0 ofM83. abundance per unit velocity gradient Z(12CO)/(dv/dr) was
(top)atthecenter(r<0.5kpc). (bottom)inthediskregion setto8×10−5, whereZ(12CO)isdefined as[CO]/[H2],and
(0.5<r <3.5kpc). the unit of dv/dr is km s−1 pc−1. Note that no contours
showing R3−2/1−0 of 1 are seen. (b) Z(12CO)/(dv/dr) was
1974). Figure9ashowstheresultoftheLVGcalculations, setto8×10−6.
assuming Z(12CO)/dv/dr =8×10−5, which would be a
typical value for galaxies. If we consider the one-zone seems to depend mainly on the density of the molecular
model to be valid and assume the kinetic temperature gas. Note that in the condition where R3−2/1−0 is close
Tkin∼20 K, which seems to be relevant for GMCs in the to unity, R3−2/1−0 also restricts the kinetic temperature
disk regions,itappearsthat R depends mainly on as well as the density of the molecular gas.
3−2/1−0
the density of gas rather than the kinetic temperature in Inthecaseofexternalgalaxies,theobservingbeamsare
the disk region where the R is in the range of 0.1 usually too large to distinguish the individual molecular
3−2/1−0
to 0.7. The H gas density is ∼102.5−3 cm−3 in the disk cloudstructures,andtheone-zoneassumptionisnolonger
2
region. However,in this assumption ofZ(12CO)/dv/dr = valid. Thus, R3−2/1−0 could be a rough measure of the
8×10−5, we cannot reproduce the high R3−2/1−0 values fraction of the dense molecular gas (nH2 > a few ×103
observedin the centralregion,whereR is close to cm−3) to the total (including diffuse) molecular clouds
3−2/1−0
or exceeds unity, suggesting a low opacity of CO emis- within the observing beam.
sion or difference in emitting regions of a clump between
4.2. Relation between the R and SFE
3−2/1−0
CO(J =3−2) and CO(J =1−0) due to external heat-
ing(Gierensetal.1992). Therefore,inthe centralregion, TheobservedradialvariationsofR3−2/1−0andtheLVG
we must assume another condition different from that in calculationsuggestthefractionofthedensemoleculargas
the disk region. Figure 9b shows the result of the LVG at the nucleus is higher than that in the disk region in
calculations,assumingZ(12CO)/dv/dr=8×10−6. Inthis M 83. Here we investigate the physical relationship be-
condition,thekinetictemperatureishigherthan60Kand tween the molecular gas properties traced by R3−2/1−0
theH gasdensityis∼1×104 cm−3 atthenucleuswhere andthe massivestarformation. The firststepis to quan-
2
the R exceeds unity. This is a fairly strong con- tify the star formation rates (SFRs) and star formation
3−2/1−0
straint, and the molecular gas is in a significantly warm efficiencies (SFEs) in M 83.
and dense state. From these results of the LVG approxi-
mation,thevariationofR intherangeof0.4to1.0
3−2/1−0
10 K. Muraoka et al. [Vol. ,
4.2.1. Derivation of SFR and SFE
Distance from center
An SFE is expressedusing an SFR or a surface density
20" 40" 60" 80" 100"120"140" 160"
of SFR (Σ ) and a surface mass density of molecular −5
SFR
hydrogen (Σ ) as follows: (a)
H2
-1yr )−5.5
(cid:20)ySrF−E1(cid:21)=(cid:18)M⊙yΣrS−F1Rpc−2(cid:19)(cid:30)(cid:18)M⊙ΣpHc2−2(cid:19) (1) M-2 pc −6
maTpoocbatlaciunleadtewΣitShFtRh,ewVeLuAse(dOnad6recchmenra1d98io5)c,oanstisnhuouwmn Σ ( SFR−6.5
in figure 5. We derived a radial distribution of the 6 cm g
o −7
radio luminosity. Condon (1992) relates the nonthermal l
luminosity L and SFR as follows:
N
1.3
R
SFR(M ≥5M ) L (b) 3-2/1-0
⊙ ∼1.9×10−22 N SFE
(cid:20) M yr−1 (cid:21) (cid:18)WHz−1(cid:19) −8
⊙ 1.0
× ν α (2) -1r )
where α∼0.8 is a nonthermalspectral i(cid:16)nGdeHxza(cid:17)nd ν is the R3-2/1-0 0.7 −8.5SFE (y
ocmbserravdeido fcroenqtuinenucuym. BfluyxaossfuMmi8n3gotrhiagtinaaltletfhreomobnseornvtehder6- 0.5 −9 log
mal emission due to supernova remnants, we converted
the radial distribution of 6 cm radio luminosity into that
−9.5
of Σ by employing equation (2). This is shown in
SFR 0 0.5 1 1.5 2 2.5 3 3.5
figure 10a. It is similar to those of CO(J =3−2) and
Distance from center (kpc)
CO(J=1−0)integratedintensities;thereisastrongpeak
at the center. In addition, we can see a secondary peak
at the bar-end (r ∼ 2.4 kpc). The azimuthally averaged Fig. 10. (a)TheazimuthallyaveragedSFRasafunctionof
the galactocentric radius of M 83. The profileof SFRs show
SFRs are about 1×10−7 M yr−1pc−2 in the disk region
⊙ strongpeaksinthecentralregionandsecondarypeaksatthe
andabout3×10−6 M⊙yr−1pc−2 atthe nucleus ofM83, bar-end (r∼2.4 kpc) as well as CO intensities shown infig-
i.e., ΣSFR at the nucleus where a moderate starburst oc- ure6a. (b) Theazimuthallyaveraged SFEand R3−2/1−0 as
curs is enhanced by a factor of ∼30 as compared with a function of the galactocentric radius of M 83. Both pro-
filesshowastrongpeakatthecenter, whereasnosignificant
that in the disk region.
enhancement is visible at the bar-end, despite the fact that
Σ isestimatedfromtheCO(J=1−0)intensityusing
H2 thereisasecondarypeakintheCOintensitiesandSFRs.
a N / I conversion factor X . Here, we adopted
H2 CO CO
a Galactic X value of 1.8×1020 cm−2 (K km s−1)−1
CO from massive stars heavier than 5 M , the total (i.e., in-
⊙
(Dame et al. 2001). Note that the effect of the constant
cluding low-mass stars) SFE at the nucleus would be of
XCO will be discussed at the end of this subsection. ΣH2 an order of a few × 10−8 yr−1 (see Condon et al. 2002),
is calculated as
which is in fact within the range of circumnuclear star-
bursts (Kennicutt 1998a). The SFEs in the disk regionof
ΣH2 =2.89×cos(i) ICO M 83 are comparable to or slightly higher than those of
(cid:20)M pc−2(cid:21) (cid:18)Kkms−1(cid:19) nearby gas-rich spiral galaxies (e.g., Wong & Blitz 2002).
⊙
Note that we adopt a constant X value over the ob-
X CO
× CO (3) served region, but our conclusion that there is an excess
(cid:26)1.8×1020cm−2(Kkms−1)−1(cid:27)
of SFE at the center of M 83 is not weakened even after
where I is the CO(J = 1−0) intensity and i is the considering a possible variation of X within M 83. In
CO CO
inclination of the galaxy (i = 24◦ for M 83,Comte 1981). the central regions of various galaxies, X factors tend
CO
Figure 10b shows the resultant azimuthally averaged to show smaller values than those in the disk region by
SFEsasafunctionofthegalactocentricradiususingequa- a factor of 2–3 or more (e.g., Nakai & Kuno 1995, Regan
tion (1). In contrast with the radial profiles of CO and 2000),includingtheGalacticCenter(e.g.,Okaetal.1998,
Σ , the radial profile of SFE in figure 10b shows a re- Dahmen et al. 1998). A smaller X yields a smaller gas
SFR CO
markable difference; there is no enhancement of SFE at massinthecentralregion,providinghigherSFEthanthe
the bar-end (r ∼ 2.4 kpc). In the nuclear region, SFE is present one in figure 10b.
8.9×10−9 yr−1, whereas the average SFE is 2.2×10−9 4.2.2. Comparison of R with SFE
3−2/1−0
yr−1 in the disk region, i.e., four times lower than that Now we can compare the radial profile of R
3−2/1−0
in the nuclear region. Considering the fact that these de- with that of the SFE; figure 10b shows the results. Here,
rived SFR and SFE values are limited to contributions we find a remarkable similarity in the radial profiles of
R and SFEs. Both of them show a strong peak
3−2/1−0