Journal Club
December 16th, 2011
“Observations of Arp 220 Using
Herschel-SPIRE: An unprecedented view
of the molecular gas in an extreme star
formation environment”
Rangwala et al. 2011, ApJ, 743, 94
Reporter:
K. Kohno (IoA)
Multi-wavelengths view of Arp 220
L(FIR) ～ 2×1012 Lo
Scoville et al. 1998,
ApJ, 492, L107
See also Sakamoto+
For double nuclei
⇒ Numerous SNe ?
Genzel & Tacconi, 1998, Nature, 395, 859
Downes & Solomon
1998, ApJ, 507, 615
Sub-arcsec CO(3-2) in Arp 220
SMA
Sakamoto
et al. 2008
ApJ, 684, 957
3
Highest resolution
CO(3-2) &
continuum images
of Arp 220
Sakamoto
et al. 2008
ApJ, 684, 957
Counter rotation
4
Between E and W !
SPIRE-FTS 190 – 670 μm spectrum
• df FWHM ~ 1.44 GHz or dV = 280 km/s – 950 km/s
across the spectral range, beam size = 17” – 40”
• Total on-source: 10455 sec = 2.9 hours
– Deep dark sky observations (13320 sec = 3.7 hours) for the
emission from the telescope
Conclusions (1)
• Complete spectral view of Arp220
– 190 – 670μm using SPIRE-FTS
– Continuous coverage  eliminating systematics due to
cross calib. using multiple instruments over different
atmospheric windows
• Detection of many spectral features
– Emission: High-J CO, HCN, [CI], [NII], water and water
related species
– Absorption: high-J HCN, OH+, H2O+, CH+, HF and
several nitrogen hydrides
– H2O+ (742GHz and 746 GHz)  1st detection
• Good measurements of continuum also
SPIRE-FTS spectrum of Arp 220 (1)
• Emission lines from mid-J CO, HCN,
water related molecules
• Absorption: CH+, OH+
SPIRE-FTS spectrum of Arp 220 (2)
• High-J CO; numerous water & water related
lines; absorption in high-J HCN, water, etc..
Mkn 231 Herschel/SPIRE-FTS
Van der Werf et al. 2010, A&A, 518, L42
~ 1200 GHz coverage !!!
R ~ 400 - 1200
• Very high-J CO lines up
to J=13-12 are still well
excited !!!
• Very rich in species;
many bright H2O, H2O+,
OH+ lines
SPIRE-FTS spectrum of M82, a pure starburst
galaxy, is dominated by CO, no H2O
Panuzzo et al. 2010
PDR vs XDR: 4 major differences
• X-ray penetrate much larger column densities
than UV photons
• Gas heating efficiency in XDRs is very high (1050 %), compared to PDRs (< 1%)
• Dust heating much more efficient in PDRs
than in XDRs
• High ionization levels in XDRs drive ionmolecule chemistry over large column density
Heating source modeling: XDR vs PDR
• XDRs produce larger column
densities of warmer gas
• Identical incident energy
densities give very different
CO spectra
• Very high-J CO lines are
excellent XDR tracers
• Need good coverage of CO
ladder
Spaans & Meijerink 2008
Conclusions (2)
1. Modeling the continuum
– Dust is warm: T = 66 K
– Unusually high optical depth: τ_d ~ 5 @100μm
– Dust mass: 10^8 Mo
– Total hydrogen column density: 1025 cm-2 (factor of 3
uncertainty;  dust cross section)
2. Warm molecular gas
– Extinction corrected CO luminosity: dominated by midJ to high-J. peak = CO(6-5)
– Non-LTE radiative transfer modeling: mid-J to high-J
line are tracing (very) warm gas: Tkin ~ 1350 K (!)
– Low-J transitions: “cold” gas, Tkin ~ 50 K
Modeling dust
in Arp220
Modified black body model:
where
T = 66 K
β = 1.84
Large optical depth in Arp 220
• Dust optical depth: ~1 @240μm, ~5 @100μm
– ~1 even @860μm (Sakamoto et al. 2008)
• Dust mass: 10^8 Mo  Mgas/Mdust ~ 100
• Optically thin, two temperature model gives
unrealistic results!
– 24K + 46K, beta=2.0  Mdust = 10^9 Mo,
Mgas/Mdust ~ 10(!) … unrealistic!
• CO line luminosities are also affected by large
dust optical depths: I = I0(1-exp(-τλ)/τλ
– Correction : I0/I = 1.08 @450GHz, 1.95 @1.6THz
Conclusions (3)
2. Warm molecular gas (cont.)
– Inferred temperature for the warm & cold components
are much lower if CO line fluxes are not corrected for
dust extinction (!)
– These two components are not in pressure equibrium;
line widths are different by a factor of ~ 1.5
– Warm gas mass is ~ 10% of cold molecular gas, but
luminosity and cooling are dominated by warm CO.
– L_CO/L_FIR ~ 10-4 (L_CO: total CO luminosity)
– Warm molecular gas temperature: excellent
agreement with H2 rotational lines from Spitzer 
high-J CO is still a good tracer of H2 at these high T.
Observed CO ladder
Brightness (= const if thermalized)
∝ Flux （∝Ｊ＾２ ｉｆ ｔｈｅｒｍａｌｉｚｅｄ）
• CO(10-9) is blended with a water line.
CO and HCN
ladders:
model fit
results
Radiative transfer modeling of CO
and HCN
※ IR pumping is not included in this modeling !!!
Conclusions (4)
2. Warm molecualr gas (cont.)
– Cooling of ISM: at 1350K, H2 dominates the cooling of
ISM over CO
– Contribution of dense gas to the observed CO: small
• Possible sources of this warm molecular gas
– PDR, XDR, cosmic rays  ruled out (!)
– The mechanical energy from supernova and stellar
winds  can satisfy the energy budget required to
heat this gas (but exact mechanism is ???)
– Such warm molecular gas has been confirmed in only 2
galaxies so far (!): M82 and Arp 220  need more
SPIRE-FTS observations of galaxies !
Conclusions (5)
3. Very high-J HCN: seen in absorption
– The transition from emission to absorption takes
places somewhere between J=4-5 and J=12-11
– These high-J lines are populated via IR pumping of
photons at 14μm
– The condition for IR pumping to populate J=17-16 level
 intense radiation field with T>350 K
4. Massive molecular outflow
– P Cygni profiles of OH+, H2O+, and H2O
– Major molecules involved in the ion-neutral chemistry
producing water in the ISM
– Outflow mass: 107Mo (!), velocity < 250 km/s
HCN ladder: transition from
emission to absorption
Transition?
Wλ: equivalent width
Nj: column density at J level
f: oscillator strength
Molecular P Cyg profiles in Arp 220
• asdf
Conclusions (6)
4. Massive molecular outflow (cont.)
– It is massive, but bound (!) because its velocity is less
than the espace velocity of the Arp 220 nuclei.
– ALMA high resolution imaging & HIFI high spectral
resolution spectroscopy are required !!
– 3 massive molecular outflows so far: Mrk 231, NGC
1266, and Arp 220
5. AGN in Arp220? Long debated
– Significant evidence for an AGN in Arp220 (!)
– The large observed column densities in OH+, H2O+,
and H2O can ONLY be produced by a luminous XDR
with LX = 1044 erg/sec. outflow: from AGN? SB?
AGN in Arp220 west !?
• 0.1-0.3 arcsec
resolution 1.3 mm
continuum imaging
with PdBI
• Compact (~35pc) and
hot (~90K) dust
continuum source 
suggesting the
presence of AGN (at
least for Arp 220
West) ???
Downes & Eckart, 2007,
A&A, 468, L57
Continuum sources in Arp 220
• Beam-averaged brightness temperature: > 50 K
• Dust emission  Arp220Eでは、radio SNRの分布と似ている
• しかし、Arp220Wでは、SNRの分布よりもcompact？
Arp220E
Arp220W
Sakamoto
et al. 2008
ApJ, 684, 957
27
6. Atomic lines
Conclusions (7)
– [NII] 205μm line: a deficit relative to FIR continuum
• Similar to the [CII] deficit found in ULIRGs
– This is surprising ! Because [NII] arises in the HII
regions, not PDRs
– This deficit is consistent with recent results from PACS
atomic line surveys of LIRGs/ULIRGs ([NII] 122μm
deficits were found for several ULIRGs)
– The line deficits in HII regions: result of higher
ionization parameter in ULIRGs compared to more
quiescent systems.
– Observed [CII]/[NII] line ratio  [CII] in PDRs/in HII
regions = 75-85%
Conclusions (8)
7. Carbon vs CO
– NC/NCO ratio: close to unity
• Higher compared to the Galactic PDRs
• But consistent with the correlation between [CI]/CO line
strength and FIR lumiosity among several nearby luminous
galaxies with AGN and SB.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
• Comprehensive picture of the state of molecular
gas in Arp 220, for the first time !