[N II] 1.46 THz Line
Galactic Plane Survey
Umemoto, T. (NRO)
南極天文ワークショップ（極地研）

2018年9月12日

warm, neutral, and atomic hydrogen clouds (WNM). The WNM clouds are converted to cool
HI clouds (CNM) through a thermal instability and phase transition. These clouds are mostoften observed through the collisionally excited, hyperfine, spin-flip transition of hydrogenat 1420 MHz (λ ~ 21 cm). The clouds also radiate away energy by emission in the ionizedcarbon fine structure line ([CII]). At densities >105 cm−3, gas thermal energy can also be

Lifecycle of the ISM(N II)(C II)(C II)

Warm neutral
and ionized gas

Formation ofcold HI clouds

Disruption ofmolecular clouds(C II)(C II)

(C I)

(C I)

CO

CO

Formation ofmolecular clouds

Stellarevolution
Star formation

HD
CO

FIGURE 1.7(C II)O

Walker (2016)

Lifecycle of the ISM. Starting at the top, overlapping supernovae shock waves sweep up

HERTZ SPECTROSCOPY FOR ASTRONOMY: FROM COMETS TO COSMOLOGY

Atomic and molecular lines at THz

KULESA: TERAHERTZ SPECTROSCOPY FOR ASTRONOMY: FROM COMETS TO COSMOLOGY

233

233

TABLE IIMPORTANT
THz LINESSAMPLE OF ITABLESAMPLE OF IMPORTANT THz LINESInterstellar cloud ~30K

SOFIA

Observability from ground based sites and airborne conditions. Current
ground-based facility conditions such as APEX are assumed to host a
median value of 500 microns of precipitable water vapor (PWV); future
ground-based sites assume best quartile winter conditions of 75 micrometers
precipitable water vapor (PWV) on the high Antarctic plateau (equivalently
110 micrometers
exceptional
dayssitesat the andChajnantor/Sairecabur
summit).Current
Observability
fromongroundbased
airborne conditions.
Dome AThe airborne facility (SOFIA) assumes 8 micrometers at an elevation of 13
ground-basedsuch asAPEX throughout.
are assumed
tolabelhost akilometers.facility
A zenithconditions
angle of 30 degrees
is assumed
A “Y”
medianimplies
value25%of transmission
500 micronsof precipitable
vapor(PWV); futureor greater,“M” implieswater
marginaltransmission
Fig. 1. Schematic of the spectral characteristics observed toward an interstellarof 5–25%,sites
whereas“N” implies
typical transmissionof less thanof
5%.
ground-based
assumebest quartile
winter conditions
75 micrometers
cloud, featuring a 30 K continuum, an assortment of selected atomic andAtmospherictransmission
computed
from
the
am
model
[2].
molecular spectral lines, underlined by exceptional atmospheric transmission

Kulesa (2011)

Atomic and molecular lines at THz

Mauna Kea 1.5mm
Chanantor 0.60mm
Dome Fuji 0.14mm
Dome Fuji 0.10mm

Kuno 資料より

mitting clouds in velocity space. A sample [CII] spectrum taken along one line ofgh the Milky Way with the heterodyne instrument for the far-infrared (HIFI) (seeaboard the 3.5 m Herschel space observatory is shown in Figure 1.12. As part of the

COBE/FIRAS maps of [C II] & [N II]
158µm (CII) emission• All sky survey with a special
resolution of 7 & a velocity
resolution of 1000km/s
3 25 70–1120• [C II] line is the dominantcooling line of the ISM at~0.3% of infrared continuum
205µm (NII) emission• [N II] & [C I] are less intense
by a factor of 10 & 100–0.2

0.3 2.5 7

12• [N II] line will appear atstrongly ionized regions -->by comparing [C II] &[N II],
determine if [C II] is arising
Fixsen, Bennett, & Mather (1999)
COBE FIRAS images of Milky Way [CII] 158 μm emission (top) and [NII] 205 μm emisfrom ionized or neutral gasm). Images were made at 7° angular and ~1000 km/s velocity resolution. Black swaths

[C II] Galactic Plane Survey by BICE

• Balloon-borne Infrared Carbon Explorer
PLATE 32

• 15ʼ angular resolution & 175 km/s velocity resolution

PLATE 33

Nakagawa et al. (1998)
FIG. 8.ÈFar-infrared [C II] line intensity contour map obtained by BICE with a spatial resolution of 15@. Contour levels are 0.3, 0.6, 1, 1.5, 2, 3, 4, 6, and 9 ] 10~4 ergs s~1 cm~2 sr~1. The
shading shows the observed area. Representative bright sources are labeled.
NAKAGAWA ET AL. (see 115, 264)

Herschel [C II] Galactic plane survey• Galactic Observations of Terahertz C+

and N+ (right) line intensities. The low velocity resolutionsr(GOT C+)

h
t
!
m
y
s
n
e
+
s
r
e
r
d Figure 2:GOTC+ observations along line of sights• [C II] survey by Herschel(3.5m)/
HIFI with 12” angular resolution& 0.1 km/s velocity resolution• 452 LOSs volume-weighted
sample of the Galactic plane• Every 0.87 (|l|<60 ), 1.3 (30 <|l| <60 ), 4.5 (60 < |l| <90 ), and
4.5 to 13.5 (|l| >90 )• b=0 , 0.5 , and 1.0 , 2.0 (|l|>90 )resulted in an under-sampled survey (from Pineda etal 2010)Langer et al. (2010), Pineda et al. (2013), Langer et al. (2014)2

Herschel [C II] Galactic plane survey
A&A 554, A103 (2013)

HI• P-V maps at b=0

HI&CO

HI

HI&[CII]All

All+13COA&A 554, A103 (2013)

HI

[CII]

Fig. 8. Position velocity maps of the diﬀerent types of regions defined in Sect. 4.1 at b = 0◦ . Mask 0 (grey) represents velocity components with
only H I detected, Mask 1 (white) are components with only H I and CO detected, Mask 2 (blue) components with only H I and [C II], Mask 3(black) components with H I, [C II], and CO, and Mask 4 (red) components with H I, [C II], 12 CO, and 13 CO.
Table 1. Definition of mask regions.
Mask
0
1
2
3
4

HI
!
!
!
!
!

[C II]
×
×
!
!
!

12

CO
×
!
×
!
!

13

CO
×
!×
×
×
!

Notes. !: Emission is detected in individual spaxels. ×: Emission is notdetected. !×: Emission is either detected or not detected.

[CII]

CO

result happens with 13 CO in Mask 3. The detection of emission
after averaging the data in azimuth shows that our classification
into masks depends on the sensitivity of our observations. We
will study the [C II] emission in Mask 0 and 1 and the 13 CO emission in Mask 3 separately as they represent a diﬀerent populationof clouds that are weak in [C II] and 13 CO, respectively.
4.2. Atomic gas

We first assumed that all the observed [C II] emission arises froma purely atomic gas at a kinetic temperature T kin = 100 K. For
optically thin emission, the [C II] intensity (in units of K km s−1 )is related to the C+ column density, NC+ (cm−2 ), and volume
density of the collisional partner, n (e− , H, or H2 ; cm−3 ), as (seee.g. Goldsmith et al. 2012),
!
"
"
#$−1
#
Aul 91.21/T kin
I[CII] = NC+ 3.05 × 1015 1 + 0.5 1 +
, (1)
e
Rul n

arrangement indicates that Mask 3 and Mask 2 represent the envelopes of the active star-forming regions traced by Mask 4.
There is a significant fraction of spaxels in Mask 1, particularly in the molecular ring and local arms. Mask 0 represents thelargest fraction of spaxels.
Figure 9 presents the intensity distribution of H I, [C II], where Aul = 2.3 × 10−6 s−1 is the Einstein spontaneous decay
12CO, and 13 CO for the diﬀerent mask regions defined above rate and Rul is the collisional de-excitation rate coeﬃcient at afor b = 0◦ . Although by definition
Masks 0 and 1 have no kinetic temperature T kin . We used a value of Rul◦ for collisions
Fig. 2. Position velocity
maps inof aHgiven
I, [Cspaxel,II], andits12emission
CO J =can1→
forseenthe observed
LOSs= −100100◦ &
at b = 0◦ . W
still0 be= 100
K ofbetween
8.1 × 10l−10
s−1 cm−3and(Launay[C II] emission
with H atGOT
T kin C+
overlaid the Scutum-Crux,
Sagittarius
Carina,Perseus,Norma-Cygnus
spiral1977).
arms In
projected
ontothin
the limit
position-velocity
maps
when the data
is averaged
over thediﬀerentand
rings.
The same Roueﬀ
the optically
we can estimate
the (see Sect.

Pineda et al. (2013)

A103, page 8 of 28

A103, page 4 of 28

Herschel [C II] Galactic plane survey• ISM components in the Galactic plane• Combining HI, [C II], and CO data
• [C II] emission comes from PDRs(~47%), “CO-dark” cloud(~28%), cold atomic gas(~21%) and ionized gas(~4%)
inner Galaxy, but WNM in the outer• HI is mostly CNM in the
A&A 521, L17 (2010)
GalaxyFig. 1. [CII] spectra obtained with Herschel/HIFI for the GOT C+ program at l = 345.65◦ and b = 0◦ , along with the CO data from the M
Langeral.of (2010),
Pineda(2013),
Langeret HI
al.and(2014)
telescope and HI surveys. Manydiﬀerentet
types
interstellar clouds
can beetseenal.
in this
LOS; diﬀuse
clouds with
C+ emission, but wit
or barely any, CO; strong [CII] emission towards dense molecular clouds, as indicated by the presence of 13 CO and in some cases C18 O82.0!"#$%"&'("

G013.90+0.0

G000.0 +0.0

7

[C II] and [N II] by Herschel/HIFI
1.5

6

5

)*%&)"&'("

3

1.0• [N II] emission to the GOT C+
T(K)

T(K)

4

0.5

W. D. Langer et al.: [C ii] and [N ii] from d• Can separate the contributions from highly ionized and2

Table 2. [C ii] emission and absorption components.

1

weakly ionized gas to the [C II] emission
0.0

0

•

[C ii] sourcecomponent
Itot (⌧)(K km s 1 )
103.5
33.6
88.5
42.3

LOS
# T
I /I
I /I
T
V
[C II] emission arising from strong sources
of
[N
II]
emissionlabel(K)
observed corrected (K) (kmG031.3 3 6.3
0.93
0.56
2.9
1
G049.1
2
3.7
0.55
0.34
1.5
V (km/s)
V (km/s)
is frequently
absorbed by foregroundgasG316.6--> 1underestimate
5.1
0.94
0.58
1.4
–
G337.0 1 4.3
2.37
1.37
1.0
–
Langer et al.: [C ii] and [N ii] from dense highly
ionized regions ionized gas by only [C II] observations
-1
-200

-150

s

-100

-50

0

50

100

150

-0.5
-40 -30 -20 -10 0

200

10 20 30 40 50 60 70 80 90 100
lsr

lsr

1.5

6 2.0

5
1.5

[C II]

5

1.0

4

1.0

T(K)

T(K)

3

T(K)
T(K)

3

0.5

2

2
0.5

3.1.3. F[N II]

1

1

0.0
0.0

0

50

100

150

200

o

G316.6+0.0

4)"&'("

ion tot[C ii] em[N ii] em
region b
Gaussian ﬁt of [CtheII]fract
four soufrom [N II] emissioncles, anabsorpti
have thereducedhas a fra
nations,[C ii] lin

6
G049.10+0.0

G031.3+0.0
G013.90+0.0

G000.0 +0.0

ion obs0-1-0.5
0 -40 -30
20 -20 -10
40 0

-0.5
60 20 30
80 40 50
100 60 70
120 80 90
14010010

V V(km/s)(km/s)

m/s)

lsr lsr

0

10

20

30

40

50

60

V (km/s)
lsr

70

80

90

100

-1
-100

-75

-50

-25

0

25

V (km/s)lsr

Assumin[C ii] ingroundItot ([C iifunctionof the reT s is theand T o iglect [Cis likely
re-emissT s and T
the abso
The colu

Langer, Goldsmith & Pineda (2016)

Fig. 1. Main beam temperature versus velocity for [C ii] (blue solid spectra) and [N ii] (red solid spectra) taken with HIFI – see Table Fig.
1 for6.some
A Gaussian fit (black dash) to the line wings of the Vlsr ⇠of the line parameters, and 13 CO(1 0) (black dashed line) from Pineda et al. (2013). Each LOS is labeled with the GOT C+ LOS label.
Some
47 km
s 1 [C ii] component (blue) in G316.6+0.0 using the Vlsr andfeatures that are indicative of [C ii] absorption are highlighted with arrows.
line width of the Gaussian fit (not shown) to the [N ii] emission (red).
1.5

[N II] Galactic Plane
Surveyl=198l=236

FUGIN Project(Umemoto et al. 2017 PASJ, 69,78)• Using mulL-beam receiver FOREST,OTF mapping of the GalacLc planein 12CO, 13CO, C18O(J=1-0),
simultaneously

Mapping area:
inner disk: l = 10°〜50° |b|≦1°

l=50

Spiral arms, interarm, bar/barend,outer disk: l = 198°〜236° |b|≦1°Comparison with inner disk

l=10
NASA/R. Hurt

# 01 Molecular Cloud Identification
Identified Strunctures by Dendrogram (Fujita et al. in prep.)

(D) Sagittarius
Arm(C) Inter arm/Aquila spur(B) Norma Arm/Scutum Arm(A) Bar/Far-3kpc Arm

(a)
12

CO

13

CO

C18O

W51

W43

(b)

Torii et al. 2018

#2 Catalog of HI shells and associated CO clouds

HI and H2 mass
• FUGIN CO clouds identified with the DENDROGRAM
ID=4

M(HI) ~ 40,000 Mo

M(H2) ~ 2,500 Mo

M(HI) ~ 100,000 Mo
M(H2) ~ 170,000 Mo

Herschel-SPIRE of DR21

• Herschel-SPIRE spectroscopy•• [N II] 205µm <--> [C II] : n = 46 cm , Te=8000K• [N II] emission to the east coincides with a hole in the excited
196-671µm (CO, 13CO, HCO+, H2O, [C I], [NII])λ/Δλ=1000
157

crit

-3

518, L114(2010)
H2, indicate a A&A
cavityof ionizedgas
G. J. White et al.: Herschel-SPIRE spectroscopy of DR21

26:00.0

CO J=3-2

DR21FIR1

25:00.0

24:00.0

Declination

23:00.0

DR21OH

22:00.0

21:00.0

42:20:00.0

19:00.0

DR21

Fig. 4. CO and 13 CO line rotational temperature diagram.
18:00.0

17:00.0
20.0

10.0

20:39:00.0

38:50.0

Right ascension

Fig. 3. CO J = 3–2 integrated emission map of DR21 between velociFig. 2. The SPIRE maps of selected species toward DR21. Since theties of –40− ± 40 km s−1 . The lowest contour is drawn at 1 K km s−1 (5σ−1
as observed in a 61 kHz (∼0.05 km s ) spectral channel), and the condata points do not fully sample the mapped area, this map was con−1
tour intervals are incremented in steps of 50 K km s . The peak antenna
structed by interpolating between the data points. The (0,0) position istemperature and velocity integrated temperatures at the DR21 peak are

White et al. (2010)

The Astrophysical Journal, 814:133 (21pp), 2015 December 1

Goldsmith et al.
Table 3
(Continued)

Herschel [N II] Galactic plane survey
LOS LabelLongitudeOBSID[N II] 122 μm

The Astrophysical Journal, 814:133 (21pp), 2015RDecember
1(a )Intensity±(Err)[×10−8](W m−2 sr−1)[N II] 205 μm

σ[×10−8]R (a )

Intensity±(Err)[×10−8](W m−2 sr−1)n(e)±(Err)

N(N+)±(Err)(cm−3)

Goldsmith[×1016] et al.
(cm−2)

G344.8+0.0
344.7830
1342267185(2)
1.37±0.020
0.42(3)0.80±0.056
36.4±4.3
1.3±0.02
G345.7+0.0
345.6520
1342267869(2)
12.60±0.044
7.55(2)
5.44±0.081
59.6±1.57.7±0.03
G346.5+0.0
346.5220
1342267178(2)2.90±0.020
0.77(2)
1.71±0.059
36.0±2.1
2.8±0.02
G347.4+0.0
347.3910
1342264238(2)
1.92±0.013
0.54(3)
1.32±0.049
27.5±1.8
2.4±0.02
G348.3+0.0
348.2610
1342264237(2)
3.02±0.020
0.90(2)2.07±0.064
27.7±1.5
3.7±0.02
G349.1+0.0
349.1300
1342267870(2)
21.31±0.096
4.41(2)
8.67±0.057
65.7±0.812.1±0.05
G350.0+0.0
350.0000
1342265935(3)
0.36±0.008
0.23(1)<0.22±...
...±...
...±...
G350.9+0.0
350.8700
1342265684(2)
1.42±0.013
0.41(2)
1.22±0.069
18.1±2.1
2.8±0.03
G351.7+0.0
351.7390
1342265683(2)
1.61±0.025
0.48(3)
1.68±0.055
12.1±1.0
4.9±0.07
G352.6+0.0
352.6090
1342265682(2)
1.47±0.021
0.36(3)
1.22±0.055
19.6±1.8
2.6±0.04
G353.5+0.0
353.4780
1342265681(2)
3.68±0.027
0.62(2)
2.95±0.066
20.9±0.9
6.1±0.04
G354.3+0.0
354.3480
1342265680(2)2.33±0.020
0.65(2)
1.53±0.048
29.9±1.7
2.7±0.02
G355.2+0.0
355.2170
1342267874(2)
5.22±0.017
1.34(2)
1.87±0.060
80.5±4.2
2.6±0.01
G356.1+0.0
356.0870
1342265934(2)
2.59±0.020
0.31(3)
1.63±0.038
32.3±1.4
2.8±0.02
G357.0+0.0
356.9570
1342265933(3)
0.35±0.010
0.21(1)<0.18±...
...±...
...±...
G357.8+0.0
357.8260
1342265932(2)
0.82±0.020
0.31(1)<0.19±...
...±...
...±...
G358.7+0.0
358.6960
1342267875(0)...±...
...
(0)...±...
...±...
...±...
Figure 19. Observed
intensity1342265931
ratio of the two (2)[N II] ﬁne 17.30±0.102
structure lines and derived
are plotted as a functionof Galactic longitude.
G359.5+0.0
359.5000
1.69 electron(2)densities 9.16±0.095
42.9±0.814.1±0.08• [N II] at 122 & 205 µm with PACS(5x5)• 149 LOSs selected from GOT C+, 10”(122um),15”(205µm)• Bothe lines are detected in the range -60 ≤ l ≤60• [N II] emission highly correlated with that of [C II]• High electron density--> extended envelopes of H II regions,and low-ﬁlling factor high-density ﬂuctuations of WIM

Figure 5. HIFI [N II] spectrum before baseline removal (with positive
continuum offset; shown in red), with continuum offset removed (lower
spectrum; shown in blue), and with third order baseline removed (lower
spectrum; shown in green). There is only a minimal difference between the
spectra with zeroth and third order baselines removed in this case.
Figure 4. Illustration of effect of polynomial baseline removal from PACS
spectra toward G024.3+0.0. A DC offset had previously been removed to yield
Figure 20. Comparison of the intensities of the [N II] of 122 and 205 μm lines.
the 25 spectra displayed as the red lines in this 122 μm observation. The
Thehigher-order
blue squarespolynomial
are the LOS
in the shown
range −30°<l< 30°,and theﬁtting
triangles(spectrain black after
baselineandare removal)
the LOS modestly
positions reducesin the region–60°<l<−30°
and30°<l<60°.
any residual offset in the vicinity of the line as The
well
black
equal
intensities(1:1 ofratio),
andII]theblue line
emission.
as dashed
reducingline
thecorresponds
amplitude oftothe
featureto the rightthe [Nspaxels. It is thus reasonable to consider the [N II] emission to

Figure 21. Intensities of [N II] 122 and 205 μm transitions compared with thosebe spatially extended and fairly uniform in each pointingof [C II] 158 μm. The red and blue solid lines are power law ﬁts of thedirection (;1[Narcmin
in size). Given that the PACS footprint
II] lines vs. [C II]. The black dashed line is a slope of unity,
corresponding
size is 47″
corresponds
pc at a with
distancetransitions
and alsoto[N1.5
II] measured
HIFI asofwellindicating
thatandboth that[N II]1′

Goldsmith et al. (2015)

[N II] 1.46 THz Galactic plane survey• [N II] emission line

• [N II] line will appear at strongly ionized regions --> bycomparing [C II] &[N II] maps, it is possible to determine if [CII] is arising from ionized or neutral gas• [C II] emission is frequently absorbed by foreground gas -->•underestimate highly ionized gas by only [C II] observations
An excellent probe of star formation rate (SFR) andinfrared dust luminosity (LIR) (Zhao et al. 2013) because the[N II] is less contaminated from the emission of older star
due to an ionization potential higher than hydrogen• [N II] 1.46 THz Galactic plane survey S.P.
@

• Wide area mapping of the Milky Way --> evolutionary

process of interstellar medium not only neutral gas but
also ionized gas

THz Telescopes on the ground• Receiver Lab Telescope (80cm) (SAO)• Ground-base at 5525m , 40 km north of ALMA
• 1~2 THz ( CO, CO, [C I] 809GHz, [NII] 1.46THz)• did not detect [N II] emission at Ori KL<--> 5K expected from12

13

[C II]/[N II] by COBE--> diﬀerent ﬁlling factor of two lines trace gas•
The Receiver Lab Telescope

16th International Symposium on SpacR. Blundell, S. Paine, D. Marrone, E. Tong, D. C. Papa, T. Hunter, M. Smith, R. Plante, J. Battat, S. Leiker,
4. [N II] in Orion-KL, with CO(0.881 THz) overplotted
T.K. Sridharan (CfA); J. Kawamura, J. Pearson, J. Stern, H. Yorke, I. Mehdi, J. Ward,Fig.
S. Lordas a roughvelocityreference. The spectral rms is shown in the lower left.
(JPL/Caltech), J. May, L. Bronfman, D. Luhr, C. Barrientos, W. Moerback (U. Chile); H. Gibson(RPG);
B.
Voronov, G. Goltsman (MSPU); M. Diaz (BU); D. Loudkov (Delft), D.Meledin (Chalmers), F. Bensch

Marrone et al. (2005)

aver
case
[20]
meas
this
over
[C I
lines
be p
[C I
foun
smal
at lo
the
bette
be m
sour
in th

Detection of 205µm [N II] from Ground• AST/RO at South Pole (1.7m)• The ﬁrst detection of 205µm [N II] line from ground-base• [N II] emission reveals the fraction of [C II] emission arises
from the ionized gas and the neutral ISM
L128

OBERST ET AL.

• 27% of [C II] arises from the low-density ionized gas but[N ii] IN CARINANEBULA
73% from the neutral
ISM!

No. 2, 2006

L127
4.1. The [N ii] Line Ratio[N II] emission from Carina nebular

Fig. 1.—[N ii] 205 mm spectrum obtained using SPIFI on AST/RO. These
data were obtained at the peak of the radio continuum contours of the Carina IIwith velocity
km/s
H ii region (Retallack1983). Velocityresolution
resolution is 60 kmof
s!1.60The dashedlineshows the intrinsic instrumental profile. The data were smoothed with a Hann

The [N ii] lines provide a distinct probe of the ionization
structure and gas density in H ii regions. Formation of O""
requires energetic 35.1 eV photons, so that the [O iii] lines trace
H ii regions formed by very early-type (O7) stars. Lower energy
photons (14.53 eV) can form N" so that these lines arise from
H ii regions formed by the softer UV radiation of late O-type
or early B-type stars, or from the lower ionization “outskirts” of
H ii regions formed by earlier type stars. Since O"" and N""
have similar ionization potentials (35.1 and 29.6 eV, respectively), an O"" zone contains mainly N"" rather than N". The
twin [N ii] lines have significantly lower critical densities(ncrit ∼ 44 and 293 cm!3 (8000 K) for the 205 and 122 mm lines,
respectively) for thermalization than the [O iii] lines (ncrit ∼510 and 3600 cm!3 (8000 K) for the 88 and 52 mm lines, respectively), so that they make better probes of lower density gas.
Unfortunately, the undersampled ISO LWS mapping of Mizutani et al. (2002) does not directly sample the position where
we obtained our spectrum displayed in Figure 1. The nearest pos
sition is at l p 287!. 550, b p !0!. 583 [R.A. p 10 h 44 m 54.1,′′′
decl. p !59!37 17 (J2000.0)], or about 1".27 (more than a fullbeam) offset from our peak line flux position. However, this position is covered by other pixels in the array, for which we obtainan integrated line flux of 21.4 K km s!1, or (3.6 " 0.7) #10!15 W m!2 in our 54! beam. Using the ISO data archive, we
obtain a 122 mm [N ii] line flux of 1.66 # 10!14 W m!2, or making
the extended source correction, a flux of 1.12 # 10!14 W m!2 in
Fig. detector
2.—[N ii]
122(Gry
mm–to–205mm lineintensity ratio as a function of ionized
the 78!.2 LW2 ISO LWS
beam
et al. 2003).
Assuming
gasdensity,n(solidline).
The
open
circleour observed line ratio ofa uniform intensity source, ande correcting for the differencemarks
in
1.5
!
0.64
.
The
ratio
of
the
[C
!15
!2ii] 158 mm to [N ii] 205 mm lines as a function
beam size, the ISO flux is 5.33 # 10 W m referred to ourof ne (dashed line). The filled circle marks the observed line ratio of 9.2 ! 3.9.

Oberst et al. (2006)

and dips to be
the upper J p
ulated at dens
From the I
10!13 W m!2.
responds to 5.
LW4 beam. A
our 54! beam
30% calibrati
205 mm line r
gas density of
we would exp
an upper bou
observed [C iremaining 73%
results supporof the observe
regions, the G
in warm densThis work
OPP-0338149
NNG05GK70
P. A. R. Ade(C. A. Allen,
for their exce
local oscillato
grateful to K.
the TREND l2.2 Specifications of 10‐m Telescop
Small Telescope Designed as Survey• Diameter:
Diameter：>
10m
3-6 m
•
•
Surfaceaccuracy：<
20μSurfaceaccuracy:<
20µm•
the detection of 205µm [N II]•• ForFrequency：200GHz‐1.3Tline from ground-base, < 10µm?
•
Field
of
view：1°Wideﬁeldheterodyne•
camera• receiver
Pointing
accuracy：2″
For wide area mapping, 100-250•• multi-beam
Tracking
accuracy：0.5″
receiverAngular resolution
GHz

Angularresolution（10m）
3m
4m
6m

460

55”

850

29”

200GHzNANTEN II (4m)

37″ 17”

1500

41”

27”

22”

15”

800GHz

1.3T

9.3″8.5”

5.8

13”

l=221l=236（注 2: Confusion limit は Bla

l=198

Visible Sky（注 3: ホーンを使用予定）(2) スペクトル線観測（冬季 50%レベル＠新ドームふじ）
周波数帯

周波数範囲

感度(5σ

τ=60seMapping area:(220 GHz)
210-275
0.096 K
Inner disk: l = 221°〜25°|b|≦1° 0.081
350
275-373450

385-5000.15

850787-950

0.34

1000

1010-1060

0.86

1500

1440-1540

1.05

650
600-710G.C.0.21
Spiral arms, interarm,bar,Partially overlap
area
1300 FUGIN
1260-1380
0.92
232 < l < 14 (EL >30 ), 221 < l < 25 (EL>20 )＊ON 点（注 1: 感度は 1 τ ∆V に比例（注 2: 多素子受信機を搭載）

l=50（注 3: 分光電波カメラ[R~10〇観測可能天域（＠新ドームふじ）

l=25
l=10

仰角(EL)赤緯(Decl.)

> 5°

<＋8°

>10°

<＋3°

>20°

<－7°

NASA/R. Hurt

Survey with Small Telescope• [N II] 1.46THz Galactic plane survey• [N II] emission reveals the fraction of [C II] emission arises
from the ionized gas and the neutral ISM• [C I] 809GHz observation when the weather is not good
• Mapping area: l = 221 - 25 (EL>20 ), |b| ≤1 (|b| ≤2 )• 20” grid, ΔT(5σ)=0.76K, 10x10 beams ̶> 20,000h (OTF)>> more low noise receiver and more beams (or smaller D)
Freq. band
(GHz)

Freq. range
(GHz)

Lines

Beam

460

385-540

CO (J=4-3), [C I] 3P1-3P0

250

0.054 K

41.3”

650

575-735

HCl, D2H+

250

0.092 K

28.5”

850

775-965

CO (J=7-6), [C I] 3P2-3P1

250

0.14 K

21.7”

1000

1000-1060

CO (J=8-7), NH+

100

0.35 K

18.7’

1300

1250-1380

CO (J=11-10), H2D+

100

0.47 K

14.2”

1500

1450-1550[N II]

100

0.76 K*

12.7”

Sensi>vity (5σ)
Angular Resolu>on(τ=10min, Δv=1km/s)(D=4m)(*:Tsys=6000K) From Kunoʼs document

Survey with Small Telescope• [N II] 1.46THz Galactic plane survey• [N II] emission reveals the fraction of [C II] emission arises
from the ionized gas and the neutral ISM• [C I] 809GHz observation when the weather is not good
• Mapping area: l = 221 - 25 (EL>20 ) some strip scans• 20” grid, ΔT(5σ)=0.76K, 10x10 beams ̶> 55h (OTF)/strip>> some strip scans at latitude b
Freq. band
(GHz)

Freq. range
(GHz)

Lines

Beam

460

385-540

CO (J=4-3), [C I] 3P1-3P0

250

0.054 K

41.3”

650

575-735

HCl, D2H+

250

0.092 K

28.5”

850

775-965

CO (J=7-6), [C I] 3P2-3P1

250

0.14 K

21.7”

1000

1000-1060

CO (J=8-7), NH+

100

0.35 K

18.7’

1300

1250-1380

CO (J=11-10), H2D+

100

0.47 K

14.2”

1500

1450-1550[N II]

100

0.76 K*

12.7”

Sensi>vity (5σ)
Angular Resolu>on(τ=10min, Δv=1km/s)(D=4m)(*:Tsys=6000K) From Kunoʼs document

THz Survey Telescopes• FIRSPEX (~1m)

On Oct. 15, 2009 STO had its test flight from Ft. Sumner, NM. During its ~12 hours at float altitude (~126,000ft.) key components of STO were tested to help ensure the system would meet the objectives of the upcoming
science flight. STO consists of 3 major components; a gondola, an 80 cm telescope, and a THz heterodyne
receiver system. The gondola and telescope have been refurbished from the successful Flare Genesis
Experiment. The gondola was upgraded by APL to use 3 gyroscopes for inertial guidance and an optical tracker
for absolute pointing. The telescope was light-weighted and its primary and secondary mirrors re-aluminized. A
room temperature receiver system and a cryostat were constructed and flown on the test flight. A computercontrolled, sliding weight was added to dynamically compensate for cryogen evaporation during flight.

(Rigopoulou et al. 2015)• Small satellite from LEO by ESA and CAS(China)• [C II] 1.9THz, [NII] 1.46THz, [C I] 809GHz, CO(6-5) 690

2.0 TEST FLIGHT INSTRUMENTGHz• STO (80cm)

The STO flight from New Mexico was designed to provide a faithful test of the Antarctic gondola, telescope
and key elements of the science instrumentation.

(Walker et al. 2016)• The Stratospheric TeraHertz Observatory by Balloon• [C II] 1.9THz and [NII] 1.46THz at 1 arcmin. angularresolution• SOFIA(2.5m)/GREAT

Figure 1: STO test-flight telescope and instrument package configuration, as conceptualized(left) and being integrated at Fort Sumner in early October, 2009 (right).

(Young et al. 2012)

Antarctic ready versions of the telescope/gondola systems, instrument electronics, computing, control, and dat
storage systems were flown. To faithfully characterize the performance of the telescope drives and pointin
system, the test flight instrument was designed with the same CG and weight as the full Antarctica fligh
instrument. The STO test flight carried a test cryostat and an un-cooled Schottky receiver. The receiver wa
used to point on the 13CO J=3-2 line. The dewar was used to test the ability of a sliding weight to compensat
for the loss of cryogens during flight.

• GREAT: 60‒240µm (Heyminck et al. 2012)• 1.25~5 THz ([N II] 1.46THz, [C II] 1.90THz, [O I] 2.06,
4.74THz)

Figure 1 shows the STO telescope and instrument package as it was configured for the test flight. Th
instrument flight system as deployed contained the following subsystems:

Hα, [C II}], CO(3-2)

Summary• To understand the evolutionary process of interstellar
medium not only neutral gas but also ionized gas,
the observations of atomic lines at THz are critical.

• [N II] is an excellent probe of star formation rate(SFR) and infrared dust luminosity (LIR), and [N II]
emission reveals the fraction of [C II] emission arises
from the ionized gas and the neutral ISM.

• So I would like to propose the [N II] 1.46THz Galactic
plane survey with the heterodyne receiver camera atSouth Pole. There is diﬃculty to survey entire theMilky Way, but it is good to observe the limited area
(strips) or the latitude direction.

Radial Distribution of ISM

A Herschel [C II] Galactic plane survey. I.

emissivity
ved LOSs

gittariust 8.7 kpc)
brightest
Scutum-A&A 554, A103 (2013)

L. Pineda et al.: A Herschel
Pineda et J.al.
(2011)

Radial distribution of CNM and WNM A&A 554, A103 (2013)
Fig. 13. Radial distribution of the mass fraction of the atomic gas in

et [Cal.:IIat
AH
Herschel[C II12.
] Galactic
survey.of
I. the azimuthally averaged total H I colRadialplane
distribution
Radialdistributionof azimuthally[C II],J.HL.I, Pineda
and 12CO
b=0
Fig. 7. Radial
distributionof theaveraged],I, and Fig.
12◦
umn
densityintheGalacticplane. We also include the estimated raCO emissivities at b = 0 . The discontinuity in the H I emissivity
distribution at ∼9 kpc is discussed in Sect. 5.1. Typical 1σ uncertainties dial distribution of the cold neutral medium (CNM) and warm neutralin the [C II], H I, and 12 CO emissivities (before scaling) are 0.02, 0.6, medium (WNM) gas. Typical uncertainties of the H I column densities
are 1018 cm−2 kpc−1 . The vertical line shows the Solar radius. At 9 kpc
and 0.1 K km s−1 kpc−1 , respectively.
there is a discontinuity in the total and WNM N(H I) distribution which
is attributed to the abrupt change between the number of rings sampled in the inner and outer Galaxy. We therefore also show lines that
4. The origin of the [C II] emission in the Galaxysmoothly connect the distributions in the inner and outer Galaxy.

4.1. Velocity components

the cold neutral medium (CNM). The horizontal line shows the local
Table
2. Mass of Milky
Wayderived
ISM components.
mass fraction
the CNMby Heiles & Troland (2003). The
vertical line denotes the solar radius. The typical uncertainty in the ratiois ∼0.002.
Component
Mass
109 M⊙
H2WNM
from CO
1.9inner Galaxy is similarcolumn density of thegas in theH
CO-dark
0.7 density required to reto what we estimated 2for the H I column
H2 Total
2.6
produce the [C II] emission resulting from the azimuthal average
H I CNM
0.9
in Mask 0. This result
indicates that evena larger fraction of the
H I WNM
1.6
atomic gas in the inner
Galaxy might 2.5
be in the form of CNM.
H I Total
CNM clouds may still
exist
in
the
outer
Total H I + H2
5.1 Galaxy, as molecular

In the following, we associate the [C II] emission with othertracers in order to separate the contribution from diﬀerent ISM can be detected in the region where only H I is detected inphases to the observed emission. We define 5 mask regions the pointed observations (Mask 0). The average H I column for
20−2−1
represent diﬀerent stages of the ISM evolution depending Rgal = 3−8 kpc (∼1 × 10 cm kpc ), would imply a [C II] clouds and star formation are present, and the reduction of its
II] emis- thatwhether H I, [C II], 12 CO, and 13 CO are detected in an individual emissivity of about 0.17 K km s−1 kpc−1 in the case that all gas is contribution in the outer Galaxy might be a result of a reduced◦, and forpixel in the position-velocity map (spaxel). We define Mask 0 CNM and 0.006 K km s−1 kpc−1 in the case that all gas is WNM. filling factor of the CNM gas.
from theThe surface density distribution and mass of the atomic
as those spaxels in which H I emission is detected but [C II], CO, Thus, the emission detected in Mask 0 can only be associated 5.5. Thereis a discontinuity in the total H I column density distrintensities
and molecular gas in the Galactic planeand 13 CO not. These regions likely represent low-volume density with CNM gas. Note that for the inner galaxy, when we asso- bution
ataround9 kpc, which may be an artifact of the assumed
intensity
atomic gas (see Sect. 5.1). Mask 1 includes spaxels in which H I ciate H I with [C II] and/or 12 CO emission in individual spaxels, In
flattherotationcurvewith 21,
purelycircularAn alternative
top panelof Fig.
we showthemotions.
vertical surface
densityak of theand 12 CO (or 13 CO) are detected but [C II] is not. These spax- we are assuming that the H I and [C II] and/or 12 CO emission distribution
explanation
is
shown
in
Fig.
A.2
in
Appendix
A,
which
shows
of the total hydrogen gas in the Galactic plane.
We
cal strucels trace dense molecular gas that is too cold for [C II] to be arises from both the near and far side of the Galaxy. We could the number of ring samples as a function of Galactocentric disalso show the relative contributions from atomic and molecularobserved
detectable at the sensitivity limit of the observations. Mask 2 in principle overestimate the H I column density associated with tance. Because a LOS samples a ring in the inner Galaxy twicefrom thegas.
WeRadial
calculatedthesurfacedensities
assuming
aofGaussian
21. Toppanel:
radialdistributionof the density
surfacedensityhydrodistributionof surfaceof ahydrogenare spaxels where H I and [C II] are detected but 12 CO is not. the CNM in the case that the [C II] and/or 12 CO emission arisesFig. but
a
ring
in
the
outer
Galaxy
only
once,
there
is
discontinuityar side ofvertical
distribution
of
the
gas.
For
the
moleculargas
we asin the Galactic
plane. Weplanealso show the contribution of the atomic
These regions are likely larger volume density atomic clouds from either the near or far side of the galaxy, but H I from bothgen inin the Galactic
thenumberofsamples
between
the
inner
and
outer
Galaxy.
same sita constantFWHM
diskpanel:
thickness
130 pc (Sect.
3.2).
molecularhydrogengas. Lower
radial of
distributionof the difRadial
distribution
CO-to-H2
confession
distributionof theof
CO-to-Hfactor.factor
We showandsume
that have Radial
insuﬃcient
FUV-photon
shieldingto allow the build- Fig. 20. Radial
distribution
H2 column
density
2 conversion
We
considerthissituation
unlikely,
however,aswe
exThisdiscontinuity
will
thereforeaﬀect
theazimuthally
averaged
GaussianFig. 18. Radial distribution of the H2 column density in the plane of the thesides.
Fig.
19.
Radialdistributionofthefraction
of
the
total
H
column
den2
In
the
case
of
the
atomic
gas,
the
disk
thicknessisobserved
ferentISM
components,
including
the
cold
and
warm
neutralmediumdistribution for values estimated considering the CO-traced H2 gas
up of CO and thereforethe
inrelative
which mostof the gas-phasecarbonis12
contributions
to the total
H2 colsity thatcomprisedby the CO-dark
H gas.
pect
the physicalconditions
within a galactocentric ring to(atomic
ICE (seeMilky Way. We also show
emissivity
eﬀect
on[CHII2we]gas
andemisgas), thedistributions.
CO-dark
H2 Thecomponent,
andthethethatCOis tracedtoincreasewith
Galactocentric
distance,
and
used
the
expo1213in the form of C+ . Mask 3 includes
spaxels
in which H I, [C II], as well as that including the CO-dark2 H2 gas component.

Diﬀuse Gas Density• [N II]

122µm/[N II]205µm line ratio• [C II] 158µm/[N II] 205µm line ratio indicates [C II] emission
arising in ionized gas (insensitive gas density)• [N II] 122µm/[N II] 205µm can be used as an eﬀective

density
tracerbetweenand 2000cm(Stacey
the inaugural issue of IEEE
Transactions
on Terahertz
Science 20
and Technology,
April 5, -3
2011
2 2011)d. Furthermore, the energy levels
n a few hundred K of the groundcollisionally excited by e-1, H, orstrophysical environments. The
THz lines important coolants ford excellent probes of the physicaland probes of the sources of heat,om nearby stars or AGN, cosmic
ocks.

d THz Spectral Linesm) Q (GHz) A (s-1) ncrit (cm-3)2
18 4745 9.0u10-5 4.7u105(*).53 2060 1.7u10-5 9.4u104(*)
82 5786 9.8×10-5 3.6×103
36 3393 2.6u10-5510
3(*)-6 2.8u10

Fig. 1. Ratio of the [NII] 122 µm to 205 µm fine-structure lines as afunction of gas density(solid line).
ratio is a sensitive probe of gas
StaceyetTheal.line(2011)
density for HII regions with electron densities between 20 and 2000 cm -3.
Also shown is the ratio of the [CII] 158 µm/[NII] 205 µm lines as a functionof gas density. This line ratio can be used to discern the fraction of theThe hotter a star burns, the shorter will be its lifetime on the main sequence, τMS,

[N II] as Probe⎛ of
SFR
and
L
IR
M ⎞
−2.5

(1.22)
τ MS ≈ 1010 ⎜⎟M ⊙ ⎠ will be its lifetime on the main sequence, τMS,
The hotter a star burns, the⎝shorter• [N II] emission line•

−2.5

10 ⎛ M ⎞
≈
τ
10
MS
⎜⎝ M
⎟ (SFR) andAn excellent probe of star formation
rate
⊙⎠

where
τMS = stellarinfrared
lifetime on
the mainsequence(years).
dust
luminosity(LIR) (Zhao et al. 2013) because(1.22)

the [Nwhere
II] is less contaminated from the emission of older
Massive stars have theluminosities,
aresequence
the largest
contributors to the galactic
τMSgreatest= stellar lifetimeon the main
(years).
starthedue
to anlifetimes.
ionization
potential
thanofhydrogen.
UV flux, and haveshortest
Theirpresencehigher
is indicative
recent star-forming

Massive stars have the greatest luminosities, are the largest contributors to the galactic
activity and canbetracedby the may
PDRsbeandaatomic
finestructureline emission
they genTherefore,
more
accurate
indicator
ofofSFR[N II]
UV flux,Land
have the shortest
lifetimes.
Their presence
is indicative
recent star-forming
erate. In particular, the
[NII]
205
μmtracedline,bywith
an ionization
potentialjusthigherthanactivity
and
can
be
the
PDRs
and
atomicfine
structureline
emissionthey genthe
themoreconventionalL
IR -derived estimates.
hydrogen, has been demonstrated
to be
excellentof thestar formation
rate
erate. In particular,
thean
[NII]
205 μmprobe
line, with
an ionization
potential
just(SFR)
higher thanhydrogen, (LhasIRbeen
demonstrated
to be an excellent probe of the star formation rate (SFR)
and infrared dust luminosity) (Zhao
et al., 2013),•

and infrared dust luminosity (LIR) (Zhao et al., 2013),

log SFR = ( −5.31 log± 0SFR.32)=+(−(50.31.95± ±0.32
0.)05) log L .05] ) log L+ (0.95 ± [0NII[NII](1.23) (1.23)

logLIR) += (0
4..51
32.05
) + )(0log
.95 L
± [0
.05) log L[NII]
log LIR = (4.51 ± 0.3295± ±0.0
NII ](1.24) (1.24)

where
where
SFR = star formation rate (M⊙ yr −1)−1
SFR = star formationL[NII]
rate= (Mluminosity⊙ yr ) of [NII] line (L⊙)
L[NII] = luminosity ofL[NII]
line (L⊙of) dust in IR (L⊙)
IR = luminosity

LIR = luminosity of dust in IR (L⊙)

C. Neutral Gas LineslesserIn astrophysical environments, as one moves away from the mid-J
source of ionizing photons there is a transition from regions PDRs
where hydrogen is fully ionized (the HII region) to the neutral and 1
gas clouds beyond. While HII regions and neutral gas clouds of PD
have size-scales of the order parsecs (3.1×1018 cm), the lumintransition between the two is fairly abrupt. It is roughly given lumin
57µm
205µm
by the mean free path of a photon that is capable of ionizing H, propo
To probe the eﬀective orsurface
temperature,and spectral~0.01 pc for
typical interstellarTeﬀ
gas, densities.
Photons with the raenergies
less than 13.6 eV escape the HII region and penetrate strengtype of ionizing stars (B
2V -O7.5V)
the neutral gas clouds beyond where they can ionize elements (see [with ionization potentials less than 13.6 eV (e.g. C), and overphotodissociate molecules with dissociation energies less than contin
13.6 eV (e.g. CO, H2, H2O ) forming a photodissociation field
To constrain the spectral
type of ionizing start for wide range
region (PDR) (see [4]). The depth of the PDR into the cloud size o
Submitted for publicationin the inauguralissue of n
IEEE
Transactions
and Technology,
April 5, 2011A
candefined as on
theTerahertz
depth ofScience
penetrationof photons capableof electron
densities,
e beionizeof ionizing carbon. This depth is typically determinedcanbybextinction of these photons by dust, and is roughly given
samebvisual extinction, AV~3, which corresponds to a so
hydro
tha
21
-2
column density of 6×10 cm , or ~1 pc for a typicalonly-3
density of 2000 cm . Within the PDR gas is heated
HIIbyrphoto-electric (PE) effect – whereby hot electrons are
ejecstrengfrom grains by FUV photons (6< hQ < 13.6 eV) andarises
trantheir excess kinetic energy to the gas before recombining
constrw
grains. The efficiency of this heating mechanism depends
compthe grain charge. If the charge gets larger, more of the n
B2V
O7.5V
T
photon’s energy is expended in freeing the electron D.
frompositive potential of the grain, leaving less excess Th
kin
Teff (1000 K)
energy for heating the gas, thereby lowering gasgalaxheaFig. 2. Ratio of the [NIII]
57 µm to et[NII]al.
122 (2011)µm and 205 µm lines as a
Staceyefficiency. Therefore, the gas heating efficiency goes[CII]dowfunction of effective temperature of the ionizing star. This line ratio is a
Fig. 3. Ratio of the88 µmradiation
to [NII] 122µm strengthand 205 µm
linesup.
for However,threethethisga
the[OIII]
ambient
field
goes

Spectral Classiﬁcation of Ionizing Star

• [N III]/[N II]

line ratio•• Multiple [O II]/[N II] line ratios

Line Ratio•

Ionized Gas in the Scutum Arm• [C II]

158µm & [N II]205µm obs. by SOFIA• Highly ionized gas of the WIM at the inner edge of the
Scutum arm tangency sampled along18 LOS (l=30.0-31.75 )• Strong [N II] emission throughout the Scutum tangency anddecreases
withlatitudea[Cscaleheight~55pc
W. D.exponentially
Langer et al.: Ionized gas in the
Scutum spiralarm as traced within [N ii] andii]
Primary Ref

Secondary Ref

Tertiary Ref

Fig. 13
4. The seven [C ii] spectra observed by upGREAT (black) and theFig. 5. Left: emission spectra of [C ii] (blue and black), [N ii] (red),central
and CO(green)for thetenobservedlines of sightthat aretowards
within b(l,=b)±0[N ii] spectrum(red)by GREAT=.25(30are.0, plotted
as T mb versus velocity. The central [C ii] pixel is plotted in blue and0theof thesevenshapes[C ii] pixels
black.theTheScutum
single and.0).average
While theoverallof the is[Cplottedii] linesinacrossspi-average
1 the footprint of the upGREAT array. The black[C ii] emission profiles are similar but not exactly identical owing tovariations
inemissionacrossral arm (Vlsr = 60 to 130 km s ) within each pixel are similar, the small
vertical dashed line marks the tangent velocity of the Scutum spiraldi↵erencesarm. Right:amongsame asin theleft panel foreightis linesof sight
b > 0 .25.
them
demonstrate
thatthethere
structure
duewith
to inThe downward black arrows mark a few of the clearly evident [C iidividual] absorption
features.
cloud components. The [N ii] emission in the central pixel isFig. 5. Left: emission spectra of [C ii] (blue and black), [N ii] (red),detected
and 13 COacross(green)the tenlines ofsightofthatb = ±0arm.
.25 are plotted
theforentirevelocityrange[C iiare] inwithin
the Scutum
as T mb versus velocity. The central [C ii] pixel is plotted in blue and the average of the seven [C ii] pixels is plotted in black. The single and average[C iiIn] emission
similar[Cbutii]notexactlyowing towevariations
emission across
footprintresolutionof the upGREATThe black
additionprofiles
to theare
SOFIA
and[N iiidentical] observations
4000 , inrespectively.
Thethespectralin thearray.
HI VGPSis
13 the tangent velocity of the Scutum spiral arm. Right: same as
1 in the left panel for the eight lines of sight
13 with b > 0 .25.
verticaldashedline
marks
use auxiliary H i and CO(1–0) data from public archives, [Nii] 1.56kmswith rms noise of reference
2K and inposition.
the COTheGRS thewithin0 .8) features.
primary/secondary/tertiary
1The
downward
black
arrows
mark
a
few
of
the
clearlyevident[Cii]
absorption
from
PACS
and HIFI
0].0)(31 .2766,.0) spectral
resolution
is at
0.21
s withpositions
rms of 2 were
K. thenFig. 3. The
18Herschellines of sightobservedin [C atii] (30
and.0,[N ii
areandindicatedby 0
calibrated
emission
spectrathekm
reference
white crosses
superimposedon a 21cm continuum
map andof the(Goldsmith
et al. 2015;
Langeret al. 2016),[CScutumii] with HIFI
Fig.incrementally
5. Left: emissionof [C ii] manner(blue andtoblack),[N ii] (red), and 13 CO (green) for th
addedin spectraa hierarchicalthe respective

Langer et al. (2017)

Ionized Gas in the Scutum Arm• There is highly ionized gas within the arm with 1-20 times
electron density of the interarm WIM• [N II] emission arises from shock compression layers of theWIM, accelerated A&A
by607,thegravitational potential of the arm
A59 (2017)

W. D. Langer et al.: Ionized gas in the Scutum spiral arm

Scutum spiral arm tangency: l = 30.0! to 31.75!

to the molecular spiral arm are also consistent with 13 CO in the(l–VLSR ) map shown in Fig. 2.
[CII]
[NII]
13CO
HI/50

GaussFit components
In Molecular Gas[CII] & [NII] Residual:
In WIM Gas

[CII]
[NII]
13CO

4. Discussion
Our SOFIA observations provide a rich data set of velocity resolved [C ii] and [N ii] emissions in the Scutum tangency. Our
results show widespread clear detections of [C ii] and [N ii]
along all LOSs, especially, at velocities VLSR = 60 to 125 km s 1
and at |b| < 0 .25. Although weaker at higher latitudes there is
detectable emission at least up to b = 1 .3. The unique combination of [C ii] and [N ii] provides a powerful tool to analyze theionized gas component in spiral arms and the ability to isolate[C ii] emission in the neutral and ionized gases.
In general the WIM is a highly ionized, low density, n(e) ⇠0.02 to 0.1 cm 3 , high temperature, T k = 6000 to 10 000 K
gas that fills a 2–3 kpc layer around the Galactic midplane (cf.
Ferrière 2001; Cox 2005; Ha↵ner et al. 2009). It is estimated tocontain about 90% of the ionized gas in the Galaxy and has a filling factor of order 0.2 to 0.4, which may depend on height above
the plane. At the midplane the filling factor may be as small
as 10%. It has been studied with pulsar dispersion measurements (Cordes & Lazio 2003), H↵ emission (Ha↵ner et al. 2009,2016), and nitrogen emission lines in the visible (Reynolds et al.
2001). More recently the WIM has been detected in absorption
with [N ii] (Persson et al. 2014). The [C ii] and [N ii] emission
lines can also be used to map the WIM, however, the low densities of the WIM make it difficult to detect their emission throughout most of the Galaxy. However, the spiral arm tangencies o↵er
a unique geometry that enables the detection of the WIM in [N ii]
and [C ii] because they correspond to regions with a very long
9.inAa schematicradialcrossdispersion.
section The
viewof the spiral arm perpenpathFig.
lengthrelatively narrowvelocityWIM
has dicularbeen detected[C ii] toemissionby Velusamy
et al.
(2012, showing the inner toto theinLOS
Scutumspiral armtangency2015) along the tangencies of the Perseus, Norma, Scutum, andLanger et al. (2017)

Fig. 6. Top: spectra for the atomic, molecular, and ionic gas tracers, H i

In Fig.
a function o(left panel)bols indica
includes thfunction of
GRS data
near b = 0 .
strong 13 CO
itude value
It is nearly
scale heighvelocity ran
molecularconsistentscale heighthe uncerta
have simila
the tangencof the gas dsion from tn(e)2 , andand [N ii] s