Table Of ContentMolecular Distribution in the Spiral Arm of M51
Yoshimasa Watanabe1, Nami Sakai1,2, Kazuo Sorai3, Junko Ueda4,5
6
and
1
0
2
Satoshi Yamamoto1
n
[email protected]
a
J
6
2
] ABSTRACT
A
Molecular line images of 13CO, C18O, CN, CS, CH OH, and HNCO are obtained towardthe
G 3
spiral arm of M51 at a 7′′×6′′ resolution with the Combined Array for Research in Millimeter-
.
h wave Astronomy (CARMA). Distributions of the molecules averaged over a 300 pc scale are
p found to be almost similar to one another and to essentially trace the spiral arm. However, the
- principal component analysis shows a slight difference of distributions among molecular species
o
r particularlyfor CH3OH andHNCO. These two species do notcorrelate wellwith star-formation
st rate,implying that they are not enhancedby localstar-formationactivities but by galactic-scale
a phenomenasuchasspiralshocks. Furthermore,thedistributionofHNCOandCH OHarefound
3
[
to be slightly different, whose origin deserves further investigation. The present results provide
1 us with an important clue to understanding the 300 pc scale chemical composition in the spiral
v arm and its relation to galactic-scale dynamics.
4
7 Subject headings: astrochemistry — galaxies: individual(M51) — galaxies:ISM — galaxies: spiral —
8 ISM
6
0 1. Introduction clear regions, which usually give bright molec-
1. ular emissions. Spectral line surveys have ex-
Recently, various molecular species have read-
0 tensively been conducted in such regions to
6 ily been detected in nearby galaxies owing to characterize chemical nature of molecular gas
1 rapid improvements in sensitivities of radioastro-
(e.g. Mart´ın et al. 2006; Aladro et al. 2011, 2013,
: nomical observations, and chemistry of molecu-
v 2015; Costagliola et al. 2011; Nakajima et al.
i lar gas in external galaxies has attracted more 2011; Snell et al. 2011). Distributions of vari-
X
attention of astronomers than before. So far,
ous molecular species around the nuclear regions
r about 60 molecular species have been identified
a have been revealed with radio interferometers
in external galaxies. Chemical studies for ex-
such as ALMA, and they are used to investi-
ternal galaxies have mostly been focused on nu-
gate peculiar physical states of the nuclear re-
gions(e.g.Izumi et al.2013;Sakamoto et al.2014;
1DepartmentofPhysics,TheUniversityofTokyo,7-3-1 Takano et al.2014;Meier et al.2015;Mart´ın et al.
Hongo,Bunkyo-ku,Tokyo, 113-0033,Japan
2015; Nakajima et al. 2015).
2RIKEN, 2-1, Hirosawa, Wako, Saitama 351-0198,
Japan On the other hand, there are relatively few
3DepartmentofPhysics/DepartmentofCosmoscience, chemical studies in disk regions of external galax-
Hokkaido University, Kita 10, Nishi 8, Kita-ku, Sapporo, ies, because intensities of molecular line emissions
Hokkaido,060-0810, Japan
are usually much weaker there than in the galac-
4National Astronomical Observatory of Japan, 2-21-1
tic nuclei. Meier & Turner (2005, 2012) imaged
Osawa,Mitaka,Tokyo181-8588, Japan
5Harvard-SmithsonianCenterforAstrophysics,60Gar- distributions of about 10 molecular species in the
denStreet, Cambridge,MA02138, USA nuclear and bar regions of IC 342 and Maffei 2
1
with the OVRO and BIMA, and revealed signifi- P1 position, 15 molecular species have been iden-
cantchemicaldifferentiationamonggiantmolecu- tified. The spectrum pattern is much different
larclouds(GMCs). Forinstance,CH OH,HNCO, from that reported for high-mass star forming re-
3
and SiO are enhanced in the bar region. They gions such as Orion KL (e.g. Tercero et al. 2010;
concluded that large-scale shocks induced by gas Watanabe et al. 2015), indicating that the ob-
dynamics specific to the bar structure are re- served chemical composition cannot simply be in-
sponsible to the enhancements. However, chem- terpretedin terms ofa composite ofcontributions
ical compositions observedtowardexternalgalax- from embedded star forming cores. The GMC-
ies are those averaged over one or more GMCs scale distribution of molecules would mainly con-
within a telescope beam. Therefore, we need to tribute to the spectrum. Indeed, we detected the
pay particular attention to this point, when we colddense gas tracerN H+ in our observationto-
2
discuss chemical compositions of external galax- ward M51, which is relatively weak in high-mass
ies on the basis of astrochemical concepts estab- star forming regions (Watanabe et al. 2015). As-
lishedfor sub-pc scaleregionsin Galactic molecu- suming optically thin and local thermodynamic
larclouds. Forinstance, chemicalevolutionaryef- equilibrium (LTE) conditions,the excitationtem-
fects(e.g.Suzuki et al.1992),whichareimportant peratures of CS, HNCO, and CH OH are esti-
3
forsub-pcscaleclouds,maynotbeveryimportant mated to be less than 10 K in M51 P1. This
in the GMC-scale chemical compositions,because result indicates that most of detected molecules
thetimescaleofthechemicalequilibrium(106 yr) reside in a cold (∼10 K) and widespread molecu-
is much shorter than the sound crossing time of lar gas,although a part of the molecular emission
GMCs (> 107 yr). Hence, it seems likely that may also come from the hot molecular gas, e.g.
the GMC-scale chemical compositions are more hot cores affected by the feedbacks from star for-
or less in chemical equilibrium, and mainly de- mation activities.
pend on structures and environmental conditions However, the resolution of our previous line-
of GMCs. For better understanding of chemical survey observation is about 30′′, which corre-
effects in starbursts, AGNs, and shocks in spiral sponds to the linear scale of 1 kpc. High angu-
arms and bar regions, we need to establish the lar resolution observations are needed to explore
‘standard’ chemical composition of GMCs with- the origin and the nature of the molecular emis-
out or almost without these specific effects. With sion in more detail. In this study, we imaged the
this motivation, we have been studying the chem- 6 molecular species in the spiral arm of M51 with
ical composition in the spiral arm region of M51 the Combined Array for Research in Millimeter-
(Watanabe et al. 2014). wave Astronomy (CARMA).
M51 is a grand-design spiral galaxy located at
the distance of 8.4 Mpc (Feldmeier et al. 1997; 2. Observations with CARMA
Vink´o et al. 2012). The distribution and dynam-
The observations were carried out with the
ics of molecular gas in this galaxy have exten-
CARMA in May and June 2014. It consists of
sively been studied using both single dish tele-
six 10.4 m and nine 6.1 m antennas. The pri-
scopes (Nakai et al. 1994; Schuster et al. 2007;
mary beams of the 10.4 m and 6.1 m anten-
Kuno et al.2007;Miyamoto et al.2014)andinter-
nas at 110 GHz are about 60′′ and 100′′, respec-
ferometers (Aalto et al. 1999; Helfer et al. 2003;
tively. The phase-center coordinate is : (R. A.,
Koda et al. 2009; Schinnerer et al. 2013). Spec-
Dec.) = (13:29:50.8, +47:11:19.5) in J2000 (Fig-
tral line survey observations have been conducted
ure 1). Six molecular species, 13CO, C18O, CN,
towardtwopositionsinthespiralarminthe2mm
CH OH, HNCO, and CS (Table 1), were simul-
and3mmbands(Watanabe et al.2014)aswellas 3
taneously observed in the array configuration of
thenuclearregioninthe3mmband(Aladro et al.
D and E. These configurations cover a baseline
2015). The spectral pattern observed in the two
range of ∼ 2−55 kλ. The system noise temper-
positions (P1 and P2) were very similar to each
atures were about 160–380 K. The CH OH and
other indicating similar chemical compositions, 3
CS lines and the C18O, HNCO, 13CO, and CN
although the one (P1) shows higher star forma-
lines were observed in the lower sideband and the
tion activity than the other (P2). Toward the
2
upper sideband, respectively. We employed 6 cor- CO isotopologue lines. The CN emission is de-
relators for the 6 molecular lines. The bandwidth tected along the spiral arm and shows a strong
of each correlator is 125 MHz with a frequency peak at the P1 position. The CN distribution is
resolution of 0.781 MHz. The bandpass calibra- extended from the CN peak to the northern di-
tion was done with the radio sources 3C273 and rection. The CSemissionis alsodistributedalong
3C279. 1419+543 was observed for 3 minutes ev- the spiral arm, and extends to the northern di-
ery15minutesasaphaseandgaincalibrator. The rection from the peak at the P1 position. On
absolute flux of 1419+543 was measured by com- the other hand, CH OH shows a rather compact
3
paring with the flux of Mars. The uncertainty of distribution without any extended structures to-
flux calibration is 20 %. The data reduction and ward the northern direction from the peak at the
analysis were done by using the MIRIAD pack- P1. HNCOshowsasinglepeakattheP1position
age. The synthesized beam size and the root- probably due to an insufficient sensitivity of this
mean-square(r.m.s.) noiselevelofeachmolecule observation. The HNCO peak coincides with the
are summarized in Table 1. In the imaging pro- CH OH peak. Although the distributions of all
3
cedure, the spectral channels were regrided. The the molecules are along the spiral arm, we recog-
final velocity resolutions are summarized in Ta- nize a slight variation among molecular species.
ble 1.
3.2. Missing flux
3. Results
Missing fluxes in the CARMA observationsare
3.1. Overview of molecules evaluated by comparing with the fluxes obtained
with the IRAM 30 m telescope (Watanabe et al.
The 13CO (J = 1 − 0), C18O (J = 1 − 0), 2014). The angular resolution of the CARMA
CN (N = 1 − 0), CH3OH(J = 2k − 1k), maps are adjusted by convolving the Gaussian
HNCO(505 − 404), and CS(J = 2 − 1) lines beam to match the beam size of the IRAM 30 m
were successfully detected. Figure 2 shows inte- telescope. The convolution is carried out by us-
grated intensity maps of the 6 molecular species. ing a task convol of the MIRIAD software pack-
A primary beam correction was applied to these age. The angular resolution of the IRAM 30 m
maps. The 13CO emission is found to be widely observation for each molecular line is summa-
distributed along the spiral arm, nuclear bar, rized in Table 2. Figure 3 shows the spectra of
and molecular ring regions, which are defined by CH OH, CS, C18O, HNCO, 13CO, and CN to-
3
Hughes et al. (2013). Our 13CO(J = 1−0) map wardM51P1preparedbyconvolvingtheCARMA
is similar to the 13CO(J = 1−0) map observed dataandthe correspondingspectraobtainedwith
with the Owens Valley Radio Observatory mm- the IRAM 30 m telescope. The spectral profile
interferometer (Schinnerer et al. 2010), although and intensity of the convolved spectrum is simi-
the synthesized beam size of our map is coarser lar to the single dish spectrum for each molecule,
than their map (2.9′′). Since the intensity of the although the spectral lines of HNCO and CN ob-
13CO line is the strongest among the 6 molecules tainedwithCARMAaresignificantlyweakerthan
andthe criticaldensityisaslowas103 cm−3,this those with the IRAM 30 m telescope. The in-
linewouldtraceanentiredistributionofmolecular tegrated intensities of the both observations are
gas including relatively diffuse gas which cannot showninTable 2. If uncertaintiesof intensity cal-
be traced by the other lines with higher criti- ibrations (20 % both for the IRAM 30 m obser-
cal densities. The distribution of C18O is more vations and the CARMA observations) are taken
clumpy than that of 13CO. The C18O emission into account, most of fluxes are recovered in the
traces higher column-density regions, because the CARMA observation, except for CN and HNCO.
opticaldepthofthe C18Oline is generallythinner As for HNCO, the missing flux is probably due to
than the 13CO line. the poor S/N ratio in the CARMA observation.
Critical densities of the CN, CH OH, HNCO, On the other hand, the missing flux of CN, which
3
and CS lines (∼ 105 cm−3) are much higher than is estimated to be 42 %, is puzzling. Since the
thoseofthe13COandC18Olines,andhence,these missing flux of 13CO is small, this implies that
lines tend to trace denser molecular gas than the CN is more extended than 13CO. Weak CN emis-
3
sion extended outside the arm region may be re- sis of the single dish result for a few molecules
solved out in the CARMA observation. Although (Watanabe et al. 2014). Since the beam sizes of
wecannotinprincipleruleoutthepossibilitythat allmoleculardataarethesameasthatofCH OH
3
the missingflux ofCNjustoriginatesfromimper- (seeSection3.3),acorrectionofabeamfillingfac-
fectcalibration,thispossibilityisunlikely,because tor is not applied for each molecular line. Hence,
the missing flux of most of molecules are negligi- these column densities are ones averagedover the
ble. In any case, the CN abundances estimated 6.0′′×6.9′′ scale. The resultis summarisedin Ta-
in the latter sections tend to be lower than that ble 5, and the errors are estimated from the rms
estimated in Watanabe et al. (2014). noise of the spectrum and the calibration uncer-
tainty (20 %). The column densities are sensitive
3.3. Spectra at C18O peaks to the assumed rotation temperature. When we
assumetherotationtemperatureof5Kand20K,
Spectrallineparametersarederivedforthefive
the derived column densities vary by factor of 3
peaks of the C18O integrated intensity (Figure 4
and 2, respectively, for the largest cases. In the
and Table 3), because the C18O peaks coincide
followinganalyses,wediscussthefractionalabun-
with most of the other molecular line peaks and
dances relative to H instead of the column den-
likely represent association of GMCs. In order to 2
sities in order to mitigate the systematic errors
compare the spectra at the same angular resolu-
caused by the assumption of the rotation temper-
tion, the 13CO, C18O, CN, CS, and HNCO maps
ature.
are convolved with the Gaussian beam for the
CH OH map (6.9′′×5.5′′) by using the MIRIAD Fractional abundances relative to H2 are de-
3
task convol. The angular resolution of the final rived for the C18O peak positions by using H2
column densities estimated from C18O, where
mapsisthusthesameasthatofCH OH.Figure5
showsthespectraobtainedatthefiv3eC18Opeaks. the [H2]/[C18O] ratio of 2.9 × 106 is assumed
(Meier & Turner 2005). The fractional abun-
Table4summarizespeakintensities,integratedin-
dances are summarised in Table 6. They are
tensities,line-of-sightvelocities,andlinewidthsof
less affectedby the assumedrotationtemperature
the spectra. In the case of non-detection, the 3σ
than the column densities. Indeed, a difference
upperlimitstothepeaktemperatureandtheinte-
of the fractional abundance is estimated to be at
gratedintensity are given. The line profilesof CN
mostwithinafactorof2fortherotationtempera-
aredifferentfromthoseoftheothermolecules,be-
ture rangefrom5K to20K,wherethe maximum
cause four hyperfine lines are blended. Therefore,
case is for HNCO.
the linewidths and the line-of-sight velocities are
not evaluated for the CN lines.
4. Discussion
3.4. Molecular abundances
4.1. Principal component analysis
Beam-averaged column densities of 13CO,
We conduct a principal-component analysis
C18O,CN,CH OH,HNCO,andCSareevaluated
3
(PCA)toevaluatesimilarityanddifferenceamong
for the C18O peak positions (Table 3), under the
maps of different molecular lines quantitatively.
LTE assumption by using the following formula:
The PCA has been used as a common analyt-
8π3Sµ2νN exp(hν/kT )−1 E ical technique to quantify morphological correla-
Wν = 3kU(T0 ) (cid:26)1− exp(hν/kTrot)−1(cid:27)exp(cid:18)−kTu (cid:19)tio,nofmoleculardistributionsinmolecularclouds
rot bg rot
and externalgalaxies (e.g. Ungerechts et al. 1997;
(1)
Meier & Turner 2005). In addition to the molec-
where W , S, µ , ν, N, k, T , U(T ), h, T ,
ν 0 rot rot bg
ular distributions obtained with the CARMA, a
and E are integrated intensity, line strength,
u
surface density distribution of the star-formation
dipolemoment,transitionfrequency,totalcolumn
rate(SFR)isincludedinthePCA.TheSFRises-
density,theBoltzmannconstant,rotationtemper-
timated from the intensities of the Hα and 24 µm
ature,partitionfunction,thePlanckconstant,the
emission observed by Kennicutt et al. (2003). In
cosmic microwave background temperature, and
order to compare the SFR map with our molecu-
upper state energy, respectively. The rotation
lar maps, the angular resolutions of the Hα and
temperature is assumed to be 10 K on the ba-
4
24 µm maps are convolved to be the resolution 4.2. Radial distribution of abundance
of the CH OH map. After the convolution, the
3 Figure 9 shows the fractional abundances of
SFR is evaluated by using the method given by
13CO,CN,CS,CH OH,andHNCOrelativetoH ,
Calzetti et al. (2007), as shown in Figure 6. We 3 2
andthestarformationefficiencies(SFE)whichare
used SFR(M⊙yr−1) = 5.3 × 10−42[L(Hα)obs + derived by dividing the SFR by the H gas mass,
(0.031±0.006)×L(24µm)], where L(Hα) and 2
obs at the C18O peak positions. They are indicated
L(24µm) are observed luminosities of Hα and
asafunctionofthe galactocentricdistance,where
24 µm in erg s−1, respectively.
the galactocentric distances are estimated by as-
The PCA is conducted in the same way as
suming the distance to M51 of 8.4 Mpc, an incli-
Meier & Turner (2005) reported. The molecular nation angle of 22◦, and a position angle of 173◦
line maps are convolved and re-gridded to the
(Colombo et al.2014). Thefractionalabundances
same angular resolution for the CH3OH emission. of 13CO and CS are found to be almost constant
Then, the pixel values of the molecular maps and
overtheobservedregionsalongthespiralarm. On
the SFR are normalized and mean-centered (Ap-
the other hand, the fractional abundances of the
pendix A). After these preprocesses,a correlation
other molecules and the SFE slightly depends on
matrix is calculated from the preprocessed maps,
positions. The fractional abundances of CN and
and its eigenvalues and eigenvectors are then cal-
the SFE are higher at Positions A, B, and C than
culated by diagonalizing the correlation matrix.
at D and E by factor of 2–1.5. The fractional
Tables 7 and 8 show the correlation matrix and
abundance of CN and the SFE may be correlated
the eigenvectors, respectively. Figure 7 shows the
with each other along the spiral arm, as the PCA
maps of the first, second and third principal com-
showsa hintofcorrelationbetweenthe integrated
ponents.
intensitymapofCNandthe SFRdistribution. In
Figure 8 shows projection of each species on contrast, the fractional abundance of CH OH is
3
the first, second, and third PC axes. On the first loweratPositionAthanattheotherpositions. It
principal component (PC1) axis, all the species apparently differs from the trend in the SFE.
have positive projections with a similar magni-
tude (0.24–0.45). Hence, the PC1 represents an 4.3. CH OH
3
averageddistributionofmoleculesandSFR,which
In this study, we find that CH OH has a rel-
essentially traces the spiral arm structure (Fig- 3
atively high fractional abundance (∼ 6 ×10−9).
ure 7). On the second and third principal com-
In addition, we also recognize that CH OH is
ponent (PC2 and PC3, respectively) axes, 13CO, 3
distributed differently from the other molecules.
C18O, CS, and CN have relatively smaller projec-
CH OH is thought to be formed on the cold
tion than the other molecules. The PC2 mainly 3
dust mantle through hydrogenation of CO, since
highlights the CH OH, HNCO, and SFR distri-
3
the CH OH formation in the gas phase is not
butions, which is characteristic in distributions 3
efficient (Garrod et al. 2007). Some liberation
around the peak C of C18O (PC2 in Figure 7).
mechanisms are necessary for CH OH to be de-
The SFRhas anopposite signto those ofCH OH 3
3
tected in the gas phase. In the Galactic ob-
and HNCO on the PC2 axis, indicating that the
jects,theenhancementofCH OHisusuallyfound
distributions of the SFR and the CH OH–HNCO 3
3
in hot cores and hot corinos in star-forming re-
grouptendto be anticorrelated. The PC3charac-
gions (e.g. Jørgensenet al. 2005; Bisschop et al.
terizesthedifferentdistributionsbetweenCH OH
3
2007; Bachiller & P´erez Guti´errez 1997). Radia-
and HNCO. The PC2-PC3 plane shows that CN
tive heatings or/and shocks induced by star for-
is the only molecule which has same direction of
mation activities such as protostellar outflows are
SFR, although the magnitude of CN is relatively
thought to be main desorption mechanisms of
small. In this way, the PCA sensitively shows the
CH OH. However, the observed CH OH abun-
slight difference among the distributions of the 3 3
dance is not clearly correlated but slightly anti-
molecules and the SFR at a 300 pc scale in the
correlatedwith the SFE in the spiralarmof M51.
spiral arm.
Thisresultisconsistentwiththeresultofourpre-
vious observation with the IRAM 30 m telescope
5
at a 1 kpc scale (Watanabe et al. 2014). No rele- Above all, the characteristic distribution of
vance of the CH OH enhancement to star forma- CH OH would be originated from a combination
3 3
tionactivitiesataGMC-scalewasfoundinthebar of evaporation mechanisms and formation mech-
regions of IC 342 and Maffei 2 (Meier & Turner anisms of CH OH. For further understanding of
3
2005, 2012), either. Hence, it is most likely that CH OH in the spiral arm, sensitive multi-line ob-
3
the star formation feedback does not contribute servations of CH OH in other regions including
3
to the gas-phase CH OH abundance in M51 at a inter-arm regions where the spiral shock does not
3
300 pc scale. occur, as well as detailed analyses of kinemat-
Meier & Turner (2005, 2012) concluded that ics and physical conditions of molecular gas, are
the CH OH distribution traces large-scale shocks necessary. These are left for future studies.
3
induced by gas orbital resonance in the bar re-
4.4. Other molecules
gions. Indeed, other shock tracers such as SiO
are found to be enhanced in the bar regions
In this study, we find a hint that the fractional
(Usero et al. 2006; Meier & Turner 2012). Al-
abundanceofCNiscorrelatedwiththeSFE.Ifso,
though our observed position of M51 is not the
the CN production may be related to photodis-
bar region, spiral shocks may occur in the spiral
sociation of HCN by UV photons from star for-
arm (e.g. Fujimoto 1968; Roberts 1969; Shu et al.
mation activities or from the nuclear region (e.g.
1972). Suchaspiralshockcouldberesponsiblefor
Ginard et al. 2015). In this case, enhancement of
evaporation of CH OH. If the spiral shock causes
3 othermoleculesrelatedtothePDR(photodissoci-
the enhancementofCH3OH,the non-uniformdis- ationregion),suchasCCHandCO+,couldbeex-
tributionofthe CH OHwouldbe originatedfrom
3 pected(e.g.Ginard et al.2015;Fuente et al.2006;
variation of shock effects in the spiral arm (i.e.
Pety et al. 2005). Sensitive observations of these
variation of shock strength and/or internal struc-
molecules are thus interesting.
ture of the arm). Another possible mechanism of
Meier & Turner (2005, 2012) suggested that
the CH OH liberation is non-thermal desorption
3 HNCO comes from grain mantle by the shock
such as cosmic-ray induced UV photon, as dis-
evaporation, because the distribution of HNCO
cussedinWatanabe et al.(2014). The abundance
is similar to that of CH OH in IC 342 and Maf-
3
of CH OH estimated in this observation is on
3 fei 2. The fractional abundance of HNCO in
the same order of that in the cold quiescent core M51 is similar to that in IC 342 ((1 − 3) ×
TMC-1 (∼ 10−9), where the non-thermal desorp- 10−9) (Meier & Turner 2005). The peak position
tion is thought to be responsible for the widely
of HNCO almost coincides with that of CH OH
3
distributed CH OH (Soma et al. 2015). Hence,
3 in M51, and hence, HNCO in our observation
the non-thermaldesorptioncould alsoexplain the
may also come from grain mantle. On the other
observed CH OH abundance in the spiral arm of
3 hand, our PCA suggests that the distribution of
M51.
HNCO is slightly differentfromthose ofthe other
In addition to the liberation mechanisms, the
molecules including CH OH, although sensitivity
3
efficiency of the CH OH formation on grain man-
3 of the HNCO observation is not good. Indeed,
tle would also affect the CH OH distribution in
3 the HNCO is detected only in one of five po-
the spiral arm. For dust temperature higher
sitions (C) although the signal-to-noise ratio is
than 30 K, the CH OH formation is inefficient
3 3.2. From observations of the Galactic GMCs,
on grain mantle, because CO depletion does
HNCO has been detected with relatively strong
not occur above 20 K (e.g. Aikawa et al. 2008).
intensities in the GMCs near the Galactic cen-
Schinnerer et al.(2010)foundthatthe gaskinetic
ter(e.g.Jackson et al.1984;Armstrong & Barrett
temperaturedecreaseswiththegalactocentricdis-
1985;Cummins et al.1986). The fractionalabun-
tance in M51 on the basis of the LVG analysis of dance toward the hot core of Sgr B2(N) is 1.6×
their CO observation. In our observation, the 10−7 (Marcelino et al. 2010), while that toward
CH3OH abundance is relatively low in Position the hot core of Orion KL is much lower (9 ×
A. This position is the closest position to the nu- 10−9) (Zinchenko et al. 2000) than Sgr B2(N).
clear region of M51, where the temperature may
The HNCO abundance is also known to be en-
be higher than the other positions. hanced in the shocked region L1157 B1 ((0.4 −
6
1.8)×10−8), whereHNCO is liberatedfromgrain James S. McDonnell Foundation, the Associates
mantle(Rodr´ıguez-Ferna´ndez et al.2010). Onthe of the California Institute of Technology,the Uni-
other hand, the HNCO abundance is as low as versityofChicago,thestatesofCalifornia,Illinois,
(1 − 5) × 10−10 in cold clouds (Marcelino et al. and Maryland, and the National Science Founda-
2009), where the gas phase production is consid- tion. CARMA development and operations are
eredtobedominant. Thefractionalabundancesin supportedbytheNationalScienceFoundationun-
M51is muchlowerthanabundancesinSgrB2(N) dera cooperativeagreement,andbythe CARMA
andL1157B1,whileitisslightlyhigherthanthat partner universities. This study is supported by
in cold clouds. Hence, evaporation of grain man- a Grant-in-Aid from the Ministry of Education,
tleseemstocontributetothegasphaseabundance Culture,Sports,Science,andTechnologyofJapan
of HNCO. Considering poor correlation with the (No. 21224002,21740132,and 25108005).
SFR, a galactic shock mayplay an importantrole
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This2-columnpreprintwaspreparedwiththeAASLATEX
macrosv5.2.
9
A. Preprocessing Procedure in the PCA
In order to compare different physical quantities of the integrated intensities of molecules and the star
formation rate, the pixel values of a map are normalized and mean-centered as follows:
I −I
ij i
I = , (A1)
ij
σ
i
e
where I , i, j, I , I , and σ , are a processed pixel value, an index of molecules or star formation rate,
ij ij i i
an index of the pixel, a pixel value of the original map, a mean pixel value, and an unbiased estimate of
e
variance of the pixel value, respectively. The mean pixel value is given as:
n
1
I = I . (A2)
i ij
n
Xj=1
where n is the number of pixel. The unbiased estimate of variance of the pixel value is denoted as:
n
1
σ2 = (I −I )2. (A3)
i n−1 ij i
Xj=1
10
