CO(4-3) & [CI] Observationsin Merging ULIRGs with ASTE
Misaki Ando (SOKENDAI/NAOJ), D. Iono, T. Michiyama(SOKENDAI/NAOJ), T. Saito(MPIA), J. Ueda(CfA)

Abstract
Galaxy mergers play an important role in the evolutionary process of galaxies, as it changes the physical andchemical condition of the ISM, and intensities the starburst activity as seen in the frequent occurrence of mergers inUltra/Luminous Infrared Galaxies (U/LIRGs). Observations of diﬀuse and dense gas traces will allow us to investigate
the physical condition of the ISM and star formation properties triggered during mergers. We present here thepreliminary results of CO(4-3) and [CI](1-0) observations obtained toward merging U/LIRGs with ASTE. Using PDR
models, we have constrained the gas density and UV radiation ﬁeld of systems to be higher than 1.6 104 cm-3 and3.1 104, suggesting the highly condensed and energized gas conditions in merging ULIRGs.

Introduction & Observations
It is known that interaction between gas-rich galaxies caneﬀectively remove the angular momentum of gas, allowing largeamounts of gas fall into the nucleus of galaxies. As aconsequence, the gas density increases, triggering intense starformation (starburst; SB) and/or active galactic nucleus (AGN)
from massive accretion of gas to the supper massive black hole.
During the process, the physical properties of gas are changed
by various large scale processes such as heating, compressingand shocks. It is therefore important to quantify the physicalproperties of gas in such merging galaxies in order to understandthe relation between large scale processes and ISM condition.

Fig. 1: (upper) HST images of threetarget ULIRGs. The orange circles
represents beam size of ASTE Band 8.
(lower) The CO(1-0) images with ALMA.
Telescope

ASTEReceiver

Band8Spectrometer WHSF
Obs. Mode

On The Fly

Lines

CO(4-3), [CI](1-0)

Sources

IRASF 17207-0014
IRASF 05189-2524
ESO 286-IG019
2016 Sep. 29 ‒ Nov. 15
2017 Jul. 08 ‒ Sep. 01.

Obs. Date

In order to study the condition of gas in merging galaxies, our group have been surveying several molecular gas tracers inmerging Ultra Luminous Infrared Galaxies (ULIRGs). As one part of this survey, six ULIRGs have been observed with ALMA. In thisposter, We present the preliminary results of ASTE observations in three of them. We observed observe CO(4-3) and [CI](1-0)
emission lines. CO(4-3) can be used for a tracer of high density gas. On the other hand, [CI] is known as a low density gas tracer
like as CO(1-0) line, and also known as a photodissociation region (PDR) tracer. In this study, we estimate the gas density and thestrength of radiation ﬁeld.

Results & DiscussionGas density and radiation ﬁeld from PDR model
We constrain the gas densities and the strengths of far-UV radiationﬁeld of systems globally, by comparing the luminosity ratios to thevalues from PDR model by Kaufman et al. (1999). The estimatedvalues for three merging ULIRGs are similar:• n > 1.6-7.9 104 [cm-3]• G0 > 3.1-7.1 104※G0: the UV radiation ﬁeld in the Milky Way
in Habing units (1.6 10-3 erg s-1 cm-2).

Fig. 2: The CO(4-3) (solid line)
and [CI](1-0) (sparse line)
spectrum in IRASF 17207-0014(upper-left), IRASF 05189-2524(upper-right) and ESO 286IG019 (lower-left). The verticallines represent the systematic
velocity of systems.
LʼCO(4-3) [K km/s pc2] Lʼ[CI](1-0) [K km/s pc2] FIR [L¤]
IRASF 17207-0014
IRASF 05189-2524

5.0( 0.3) 109
3.6( 0.2) 109

7.9( 0.5) 108
7.7( 0.4) 108

2.5 1012
1.3 1012

ESO 286-IG019

2.1( 0.1) 109

9.4( 0.5) 108

1.0 1012

CO(4-3)/[CI](1-0) ﬂux ratio
as an indicator of mergers/disk galaxiesThe LCO(4-3)/L[CI](1-0) can be used as a tracer of dense
gas mass fraction, and it is known that this ratio issystematically higher in merger driven systems.
Papadopoulos+12 ﬁnd that the LʼCO(4-3)/Lʼ[CI](1-0) ratio(in units of K km/s pc2) is 4.55
1.5 for the merger-driven
ULIRGs and 0.45-1.3 for the disk-dominated galaxies.
We ﬁnd LʼCO(4-3)/Lʼ[CI](1-0) of 6.3 0.8, 4.6 0.5 and 2.2 0.2
for IRASF 17207-0014, IRASF 05189-2524 and ESO
286-IG019 respectively. These results are consistent
with Papadopoulos+12ʼs ﬁndings and thus suggest thedominance of dense gas in these merging ULIRGs.

These properties are comparable tothose of the local ULIRGs determinedby using other lines. Both n and G0
are higher than those of normal starforming and starburst galaxies.
Alaghband-Zadeh+13 conducted thesame analysis for z > 2 sub-millimeter
galaxies, ﬁnding n of 1.3-3.2 104[cm-3] and G0 of 0.3-1.2 104. We
therefore derive higher gas density
and radiation ﬁeld in merging ULIRGs
than in the high-z SMG through thesame analysis using CO(4-3), [CI](1-0)
and the luminosity of far infrared.

Fig. 3: The contours represent L[CI]/LCO(4-3) (red line) and L[CI]/LFIR (black line)(in units of L¤) at various levels of gas
density (n) and FUV radiation ﬁeld (G0)from PDR models of Kaufman+99. The
pink circle, orange triangle and green
square shows n and G0 in IRASF
17207-0014, IRASF 05189-2524 andESO 286-IG019 respectively.

Fig. 4: The far-UV radiation ﬁeld G0
against the gas density n from
Alaghband-Zadeh+13. The pink circle,orange triangle and green square
represents IRASF 17207-0014, IRASF
05189-2524 and ESO 286-IG019
respectively. The other points are fromthe previous studies.