Mapping NH3 in nearby U/LIRGs using the ngVLA
Junko Ueda, 1 Daisuke Iono, 1,2 Shuro Takano, 3
1

National Astronomical Observatory of Japan,
2–21–1 Osawa, Mitaka, Tokyo 181–8588
2
Department of Astronomy, School of Science, SOKENDAI (The Graduate University for Advanced Studies),
Osawa, Mitaka, Tokyo 181-8588, Japan3
Department of Physics, General Studies, College of Engineering, Nihon University,
Tamuramachi, Koriyama, Fukushima 963-8642

Abstract
The ammonia molecule (NH3 ) is a powerful tracer of the gas kinetic temperature in the interstellar
medium, with the main emission lines seen in the 20–30 GHz range (Band 4 of ngVLA). We propose to
map the NH3 line at 300-400 pc resolution in 100 nearby Ultra/Luminous Infrared Galaxies in order toquantify the physical conditions of the star-forming molecular gas in these extreme starbust galaxies. A
sensitive and spatially resolved temperature map can provide a better understanding of how galaxy merger
event is linked to the nuclear starburst and increased black hole activity. We further comment on thepossibility of detecting ammonia from intermediate redshift galaxies using the ngVLA.
Key words: ngVLA1 — Ammonia2 — ULIRG3
1. Introduction
The ammonia molecule (NH3 ) is a powerful tracer of thegas kinetic temperature in the interstellar medium (ISM),
with the main emission lines seen in the 20-30 GHz range(Table 1). The quantum energy level is represented by
two numbers (J, K), where J is the total angular momentum and K is the projection along the molecular axis.
The spontaneous decay time of the metastable state, thatis J = K, is long (∼ 109 sec), and thus the populations between the metastable states are mainly governed by collisions with H2 . In starless dark clouds, the (1,1) transition
is mainly observed. On the other hand, the higher excitedtransitions, such as (2,2), (3,3), are also observed in star
forming molecular clouds including U/LIRG. In environments where the background infrared radiation is significant, the non-metastable states (J > K), particularly the(2,1) transition can be observed (e.g. Morris et al. (1973)).
Because collision of molecules is the dominant mechanismthat dictates the population in the different quantum levels, the relative intensity between different transitions issensitive to the kinetic temperature of the ISM.
Surveys of the ammonia molecule in the galactic molecular clouds had been carried out since the early days ofradio astronomy (Ho & Townes 1983). However, due
to the limited sensitivity of the past instruments, mostof the extragalactic detections have been limited to ahand-full of bright nearby star forming galaxies (e.g.
Mauersberger et al. (2003); Takano et al. (2005); Lebron etal. (2011); Ott et al. (2011); Mangum et al. (2013); Takano
et al. (2013); Zschaechner et al. (2016)), starting with a detection of the (1,1) line in IC 342 and NGC 253 (Martin &Ho 1979) using the 100-m telescope at the MPIfR. Despite1

https://splatalogue.online//index.php

Table 1. NH3 lines (J = K) in the 20-30 GHz band

(J, K)
(1,1)
(2,2)
(3,3)
(4,4)
(5,5)
(6,6)
(7,7)
(8,8)
(9,9)
(10,10)Frequency [GHz]23.69449550
23.72263330
23.87012920
24.13941630
24.53298870
25.05602500
25.71518200
26.51898100
27.47794300
28.60473700

E [K]22.127763.3096122.3935
199.3624
294.1934
406.8566
537.3166
685.5317
851.4542
1035.0308

The frequency and energies of the ammonia transitions available
in Band 4 of ngVLA (from Splatalogue[1])the limitations, these studies have demonstrated nicelythat the observations of multiple ammonia lines can bea powerful way to quantify the physical properties of theISM. The ”Boltzmann diagram” (the plot of column density vs. rotational energy level) has been used to derive
the rotational temperature (and the kinetic temperatureassuming the Local Thermal Equilibrium). By observing
two or more rotational levels and a linear fit to the column densities, the slope of the corresponding fit yields
the rotational temperature. Radiative transfer modelingis used concurrently to derive the physical parameters of
the ISM.
The order of magnitude increase in sensitivity offered by
the next generation Very Large Array (ngVLA) provides
a unique opportunity to investigate the detailed distribution and kinematics of the ammonia molecule, yielding the
distribution of the temperature of the ISM which is notpossible to map with the current instruments. We propose

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propose in this document is to observe and map multiple
transitions of the ammonia line using band 4 of ngVLA
toward ∼ 100 U/LIRGs in the local universe.
3. Feasibility of a U/LIRG Survey

Fig. 1. Optical images of U/LIRGs obtained with the Hubble
Space Telescope/Wide Field and Planetary Camera 2[2].

a science case to observe the ammonia molecule in ∼100
Ultra/Luminous Infrared Galaxies (U/LIRGs) in the local universe, with the main goal to characterise the spatial
distribution of the temperature and study its relation to
the starburst and Active Galactic Nucleus (AGN) activities traced in optical/IR images. See Wong et al. (2018)
for the utility of the ammonia line in nearby galaxies.
2. U/LIRG Sample
U/LIRGs are sources which emit the bulk of the emission in the far-infrared (FIR) bands with luminosities of1011−12 L⊙ . Morphological studies suggest that interactions and mergers of gas rich galaxies are key dynamicalmechanisms which often trigger high degree of starbursts
and AGN in the galaxies (Figure 1). The ensemble ofdust can be heated to ∼40 K in part due to the starburst
activity (Clements et al. 2018), but note that dust temperature could vary significantly depending on the exact
bands used for the SED fit (Mangum et al. 2013).
A wealth of complementary multi-wavelength data exists for 100s of U/LIRGs in the local universe. The Great
Observatories All-sky LIRG Survey (GOALS) (Armus etal. 2009) project provides a comprehensive data set using Spitzer, Hubble, Chandra, and GALEX. Some of thesources were followed up in molecular gas using the ALMA
telescope (e.g. Ueda et al. (2014)). Out of the sample
sources compiled by the GOALS project, ∼ 130 sources
are observable from the northern hemisphere with goodvisibility (dec > 10 deg). The ngVLA science case we2

Image credits: NASA, Kirk Borne (Raytheon and NASA
Goddard Space Flight Center, Greenbelt, Md.), Luis Colina(Instituto de Fisica de Cantabria, Spain), and Howard Bushouse
and Ray Lucas (Space Telescope Science Institute, Baltimore,
Md.

We use the NH3 (1,1) line detection toward NGC 253(DL = 3.44 Mpc; Mangum et al. (2013)) with the National
Radio Astronomy Observatory Green Bank Telescope(GBT) and the properties of Arp 220 (DL = 83.1 Mpc(1′′ = 388 pc)) as a template of ULIRGs for our basis of
the feasibility calculation. The target angular resolution
is set to 1′′ to match the resolution of optical/IR images.
We scale the NH3 (1,1) line strength of NGC 253 to match
the predicted line strength of ULIRGs. The peak emission toward NGC 253 is Tmb = 72 mK ≃ 300 mJy beam−1
observed with the 30” beam of the GBT (30′′ ≃ 510 pc
at the distance of NGC 253). Assuming that (1) thestrength of the NH3 (1,1) line emission per unit area in
our target is the same as that of NGC 253 and (2) the
uniform distribution of the NH3 (1,1) emission within the
GBT beam, the observed peak emission toward our target is expected to be 300 µJy beam−1 . According to thengVLA Performance Estimates, an rms noise (on-source
time = 1 hr) is 24.1 µJy in 10 km s−1 channel. This would
give >10 sigma detection at the peak with a conservative
assumption of uniform distribution. The emission will bemuch stronger in a more realistic case where the filling
factor < 1. According to these estimates, the higher transition lines, even the (9,9) emission which could be 10
times fainter than the (1,1) transition, can be observedwith high significance. Lower filling factor is predicted forthe transitions with high excitation energies, increasing
the chances of detection (and mapping) even further.
4. What can be learned?
As a first step, the spatial distribution of the different transitions of ammonia will be compared against eachother. In general, higher transition lines trace higher temperature gas, and we expect those lines to be more compact, identifying the regions of elevated star formation
activity. The ratio maps (channel maps if sufficient S/N
is obtained) along with the help of radiative transfer modeling will allow us to characterize the temperature distribution within each U/LIRG. Lines which are excited atlower and higher energies can be used as tracers of different temperature components of the ISM. For example,
as seen in Figure 2, the ratio of the lower transition lines
can be a good tracer of the relatively low temperaturegas (< 50 K), whereas incorporating the pair of higher
transition lines will allow us to probe the warmer component of the gas. One caveat is that multiple temperature components can lie along the same line of sight, inwhich case one can carefully investigate the distribution
in each 10 km s−1 channel and apply different temperature components for different gas clumps. The ratio of
the ortho-lines (i.e. (3,3), (6,6)) can be used as an independent check of the temperature. In addition, detection

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Table 2. Notable Molecular Lines in the 20–30 GHz band

Molecule
HC5 N (J = 8-7)
CCS (JN = 21 -10 )HC5 N (J = 9-8)
HC5 N (J = 10-9)
HC3 N (J = 3-2)
HC5 N (J = 11–10)

Frequency [GHz]21.301252
22.344033
23.963910
26.626547
27.294327
29.289159

Molecules with Ta∗ > 1 K in TMC-1 (Kaifu et al. 2004).

molecule, and others such as carbon-chain molecules (e.g.,
CCS, HC3 N, and HC5 N; see Table 2 for details) can fall
into the same IF band coverage. There have been efforts to detect these molecules in nearby galaxies, mainly
in millimeter band (e.g., Costagliola et al. 2015). The
detection of HC3 N and HC5 N suggests an early, embedded stage of massive star formation (e.g., Rathborne etal. 2008, Taniguchi et al. 2018). The NH3 /CCS ratio has
been used as an indicator of chemical evolutionary stage ofdark cloud cores in the Galaxy (Suzuki et al. 1992; Hirota
et al. 2009). It is possible that utilizing multi-line data offer clues for the starburst phases.
5. Redshifted ammonia to trace the cosmic evolution of
the physical condition of the ISM

Fig. 2. RADEX calculations of the NH3 (2,2)/(1,1) and NH3(4,4)/(2,2) line ratios as functions of the kinetic temperature
Tkin (top) and the H2 density nH2 (bottom). The line widthis fixed to 10 km s−1 . The line ratios do not mostly change
depending on the column density of NH3 , so that the solidline (NNH3 = 1014 cm−2 ) and dashed line (NNH3 = 1013
cm−2 ) overlap.

of both ortho- and para- lines will allow us to derive thespin temperature of ammonia. While ammonia is a good
tracer of the temperature, its utility as a density tracer appears relatively insignificant, as seen in the bottom panelof Figure 2. However, when used concurrently with othermolecular lines, radiative transfer modeling can allow us
to derive the gas density. Absorption may be significant
especially at the centers of the U/LIRGs, in which casethe temperature of the intervening molecular clouds will
be derived. Finally, the temperature map will be compared with maps obtained at similar angular resolution
through the archival GOALS data and obtained at theexisting facilities such as ALMA, Subaru, etc.
We note that the lines which can be detected in
the Band 4 of ngVLA are not limited to the ammonia

The cosmic history of star formation rate is quantified
and reasonably well constrained up to redshifts 2-4, and
it is anticipated that ngVLA will be able to measure thehistory of the gas density up to redshift 4 using the CO (1–0) line as a probe of H2 molecule (Decarli et al. 2018). A
detailed quantification of the gas temperature evolution
across the history of the universe will be a highly complementary probe of the evolution of the physical conditionsof the global ISM as a function of cosmic time. The (1,1)
ammonia line from galaxies at z > 6 is redshifted to Band1-2 of ngVLA. The ammonia line, up to the (10,10) transition, has been detected in absorption from lensed sources(e.g. PKS1830-211 (z=0.9): Henkel et al. 2008), but studies in the literature are limited and a systematic surveyof the ammonia line both in absorption and emission will
be an interesting way to utilize the superb sensitivities
offered by the ngVLA.
6. Summary
The ngVLA will allow us to observe and map the ammonia molecule (NH3 ) in 100s of local U/LIRGs at unprecedented quality. The spatially resolved temperaturemaps will provide us with better understanding of how
galaxy merger event is linked to the nuclear starburst andincreased black hole activity. Other important molecules
will be observed simultaneously in the same band, allowing us to probe the phase of the starburst activities inthese merger induced U/LIRGs.

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