Unveiling Dusty Starbursts by ngVLA Observations of Radio Recombination Lines
Tomonari Michiyama1 , Kouichiro Nakanishi2 , Daisuke Iono2
1

Kavli Institute for Astronomy and Astrophysics, Peking University, 5 Yiheyuan Road, Haidian District, Beijing 100871, P.R.China
2
National Astoronomical Observatory of Japan, 2–21–1 Osawa, Mitaka-shi, Tokyo 181–8588
t.michiyama.astr@gmail.com

Abstract
The observation of radio recombination lines (RRLs) is one of the most powerful tools to investigate extremely
dusty starburst activities in nearby galaxies because RRLs are extinction free direct probes of ionizing photons thatare produced by young massive stars. The benefit of sensitive observations with high angular resolution of RRLs by
ngVLA are as follows, we can (1) identify dusty starburst regions where optical observations such as Hα and Hβ
are not detected, (2) constrain the timescale of starbursts, (3) measure the properties of HII regions (i.e., electron
temperature). The ngVLA will detect ∼ 1 − 10 pc scale star forming clumps (with a SFR of ∼ 0.02 M yr−1 )in a central molecular zone of active galaxies such as luminous infrared galaxies that often have extremely dusty
starburst regions (e.g., NGC 3256 at 40 Mpc) as well as the Milky Way and nearby starburst galaxies (e.g., NGC 253
at 3 Mpc). This project can answer whether the star formation process on ∼ 1 − 10 pc scale clumps is uniform innormal star forming and extreme starburst galaxies or not.
Key words: starburst galaxy — radio recombination line — U/LIRG

1.

Introduction

For 13.8 billion years after the big bang, individual galaxies have formed stars with unique histories that depend on e.g.,their mass, morphology, and environment. One of the mostimportant achievements of past years is mapping the historyof star formation in the universe. Madau et al. (1996) showthat the galaxies were more actively forming stars at z ∼ 2 − 3(called Cosmic Noon), suggesting that a significant fractionof stars formed in short episodic events. This implies thatthe number of galaxies that are experiencing a period of intense star formation called “starburst galaxies” are common at
Cosmic Noon. The open question is that “How are starbursts
triggered and regulated?”. In order to answer this question,
it is necessary to link star formation processes on small scale
clouds with the size of 1 − 10 pc and global galaxy properties.
For this purpose, the immediate issue that ngVLA can challenge is measuring the properties of 1 − 10 pc clumps in very
active galaxies. The goal of our proposing ngVLA science is toanswer whether the star formation process on a small scale isuniform in normal star forming and extreme starburst galaxies
or not.
The most important parameter to explain starburst activities is the number of newborn stars per year, called star formation rate (SFR in the unit of M yr−1 ). In the case of
the Milky Way, SFR is measured for 0.68-1.45 M yr−1 bycounting the number of young stellar objects (YSOs) throughe.g., Spitzer/IRAC survey of the Galactic plane (Robitaille &Whitney 2010). Besides, SFR = 0.08 M yr−1 is measured in
the central molecular zone (CMZ) of the Milky Way (Immer
et al. 2012), where CMZ is the central 100×500 pc2 region.
In the case of other galaxies, it is impossible to directly countthe number of YSOs. Instead, we can predict SFR from thenumber of UV photons that are produced by YSOs in galaxies. We can also use hydrogen recombination lines such as Hαand Hβ because YSOs ionize gas around. However, interstellar dust often veils the information of UV photons and opticallines in intense starburst galaxies. When we measure SFR in
dusty starburst galaxies, we often use infrared (IR) luminosity
because interstellar dust absorbs starlight and re-emits it in the
IR. The details on the SFR measurements are reviewed by e.g.,
Kennicutt & Evans (2012); Calzetti et al. (2012).
Using these techniques, astronomers found nearby starburst
galaxies that are the main targets in this project. Especially, infrared method is important to find starburst galaxies. For example, in the case of one galaxy NGC 253, SFR = 3.5 M yr−1 in
CMZ (Melo et al. 2002), suggesting ten times more intense starformation activities than CMZ of the Milky Way. Therefore,
NGC 253 is referred to as a starburst galaxy. One of the mostimportant achievements by infrared telescopes such as Infrared
Astronomical Satellite (IRAS) is the discovery of luminous infrared galaxies (LIRGs: 1011 L < LFIR < 1012 L ) andultra-luminous infrared galaxies (ULIRGs: 1012 L < LFIR <1013 L ) whose SFR is 10-1000 M yr−1 . It is said that dust
obscured star formation activities (e.g., in U/LIRGs) dominate
the star formation budget at Cosmic Noon (Zavala et al. 2021).
The main goal of this project is to comprehensively understand the star formation process in normal star forming galaxies like the Milky Way and intense dusty starburst galaxies like
U/LIRGs, i.e., how to trigger and quench starbursts. In orderto achieve this goal, it is necessary to access a small scale because the global SFR in galaxies is determined by the ensembleof each star forming clouds. However, UV and optical telescopes are not powerful in this purpose due to dust extinction.
While infrared telescopes are very powerful to identify dusty
starburst galaxies, it is difficult to achieve higher angular resolution (than optical telescopes and radio interferometers) due
to diffraction limits. Therefore, radio (including millimeter)interferometers are important for our study.
One of the most powerful methods to investigate dusty in-2[Vol. ,

tense starbursts is hydrogen radio recombination lines (RRLs)
because RRLs are extinction free direct probes of ionizing photons that are produced by young massive stars. Since RRLs are
faint (e.g., 0.05 times fainter than Hydrogen cyanide (HCN)
for RRLs at ∼100 GHz), only tentative detections in nearby
galaxies were achieved by VLA so far (Section 2.1). Now,
Atacama Large Millimeter/submillimeter Array (ALMA) can
detect RRLs in high signal to noise ratio in nearby galaxies(Section 2.2). However, ALMA cannot investigate the very
active galaxies such as U/LIRGs that are in a relatively distant universe due to angular resolution and sensitivity limits.
Therefore, ngVLA is the only telescope to detect RRLs from∼pc scale clumps in the CMZ of nearby active dusty galaxies
such as U/LIRGs.
In Section 2, we briefly summarize the basic information andobservations of RRLs in nearby galaxies by VLA and ALMA.
In Section 3, we explain our targets (NGC 253 and NGC 3256).
In Section 4, we explain the science goals. Section 5 is a summary of this article.
2.

Basic information

2.1.

What are RRLs?

Radio spectrometers enable us to observe spectral lines fromvarious molecules and atoms. In this article, we focus on spectral lines emitted when electrons and ions combine. Except for
the early universe, ionized gas is produced by the ionizationof neutral atoms, so it is called “recombination” in the sensethat these atoms recombine to return to neutral. Recombinationlines have been observed in interstellar space, including hydrogen, helium, and carbon. In the case of hydrogen atoms, theLyman series, which transitions to the energy level of principal quantum number n = 1, is emitted in the ultraviolet region,
while the Balmer series, which transitions to the level of n = 2,
is mainly emitted in the visible light region. But when n transitions to the tens of levels, it is emitted in the radio region andcalled radio recombination lines (RRLs). The frequency ν ofthe lines in these series can be represented by a simple formula
of


1
1
νn2 →n1 = Rc−,
(1)
n1 2 n2 2
where n1 and n2 are positive integers (n2 > n1 > 1 > 0), c
is the speed of light, and constant R is called as the Rydberg
constant. For example, in the case of n1 = 40 and n2 = 41, thefrequency is 99.0 GHz (represented as H40α). The lines thatngVLA can observe are shown in Figure 1. The ngVLA canobserve RRLs that ALMA does not cover (e.g., n >∼ 60).
According to Gordon & Sorochenko (2002), the first detection of RRLs was succeeded by Pulkovo radio astronomers in
1964. They detected H104α at 5.76 GHz in the Omega andOrion nebula. In 1977, the detections of RRLs in extragalactic
galaxies were reported e.g., in M82 (Shaver et al. 1977; Bell& Seaquist 1977; Chaisson & Rodriguez 1977) and NGC 253(Seaquist & Bell 1977). In the 1990s, radio interferometerssuch as VLA and millimeter single dish telescopes such as
Nobeyama (NRO45m) detected RRLs in other galaxies such
as NGC 2146, NGC 1365, NGC 3638, IC694 NGC 3690,
Arp 220, M83, and NGC 660 (see references in Gordon &Fig. 1. The black dots indicate the frequency of Hnα emission.
The horizontal shades indicate the frequency coverage of ngVLA(red) and ALMA (blue for receivers in operation and cyan isfor receivers in development).
Thanks to the wide frequency
coverage of ngVLA, we can observe RRLs in any receiver.

Sorochenko 2002). However, due to limited sensitivity andangular resolution, we cannot understand star formation processes in galaxies based on these observations.
2.2.

Observations of RRLs by ALMA

In the ALMA era, RRLs are observed in various nearby
galaxies, e.g., NGC 253 (Bendo et al. 2015), NGC 4945
(Bendo et al. 2016). These studies demonstrated that RRLs
observations by ALMA are necessary to understand starburst
activities at dusty regions (AV > 10) by cross-checking SFR
from RRLs and the other wavelengths. Recently Michiyama
et al. (2020) observed RRLs to investigate dusty starbursts in a
merging galaxy NGC 3256 which is a LIRG at relatively distant universe (DL ∼ 40 Mpc). In this galaxy, H40α and H42α
were detected by ALMA, identifying nuclear starbursts whereHβ emissions are undetected due to dust extinction (AV ∼25) even by deep observations of Multi Unit Spectroscopic
Explorer at the Very Large Telescope of the European Southern
Observatory (MUSE). In the case of a nearby galaxy, ALMA
identifies compact (∼ a few pc) RRLs sources. For example,
Emig et al. (2020) detected RRLs from compact sources withthe sizes of 1.4-4.0 pc in NGC 4945 (DL = 3.8 Mpc).
2.3.

Conversion from hydrogen recombination lines to SFR

The SFR can be predicted by ionizing photon rate Q [s−1 ].
For example,


SFR
Q
−54
= 5.41 × 10
(2)
M yr−1s−1No. ]
is used, where the conversion coefficient depends on the shapeand mass range of the initial mass function (IMF) of the stars,
and timescale (τ ) over which star formation needs to remain
constant, and on stellar rotation effects (see Bendo et al. 2015
and Bendo et al. 2016, for more details). The ionizing photon
rate Q [s−1 ] can be estimated byh αi  EM 
Q
B
=
,
(3)
s−1
cm3 s−1 cm−3
where αB is the total recombination coefficient and emission
measure (EM ) is ne np V (ne , np , and V are ionized electronvolume density, proton volume density, and volume of ionized
H II region), respectively. The EM can be predicted by recombination line luminosity (LRL )


−1
EM
LRL

,
(4)
=
cm−3
erg s−1 erg s−1 cm−3
where  is the emissivity per unit of ne np . By using equation
2, 3, and 4, SFR can be estimated as
SFRRL= 5.41 × 10−54
M yr−1

−1 h



αB i LRL×.(5)erg s−1 cm−3
cm3 s−1 erg s−1
The equations are based on Michiyama et al. (2020); Bendo
et al. (2015); Bendo et al. (2016).
3.

Request Observations

As shown in Section 1, CMZ is the place for massive starformation and dusty intense starburst activities. The goal ofthis project is to answer “whether the star formation process
is uniform in the CMZ of normal star forming and extreme
starburst galaxies”. The ngVLA can distinguish two possible
scenarios to answer why global SFR in CMZ is different amonggalaxies as follows.
• The local star formation process is uniform among thetree galaxies. This suggests that the number of star forming regions is varied among galaxies.
• The properties of small clumps are different among thethree, suggesting small scale star formation process isvaried among galaxies.
In order to make the observation strategy, we choose CMZ
in the Milky Way, NGC 253, and NGC 3256 as an example.
Figure 2 is a summary of carbon monoxide (CO) and RRLs
observations of these galaxies and Table 1 is a summary of thebasic information.
In the case of CMZ of the Milky Way, single dish telescopes achieve ∼pc scale spatial resolution. For example,
CO (1–0) survey by NRO45m achieves the angular resolution of ∼ 1700 that corresponds to ∼0.7 pc. Jones et al.
(2011) detected H51α−H58α emission at Sagittarius B2 (SgrB2). Based on Figure 2(a3), the total integrated H51α intensity is 426 [K km s−1 ], which corresponds to LH51α =5.7 × 1031 [erg s−1 ] within the aperture radius of 14000 (∼6 pc). Assuming the electron temperature (Te ) of 5000 K
and electron density (ne ) of 103 cm−3 , the expected SFR is3∼ 0.01 M yr−1 . We note that RRLs mapping of the entire
CMZ is difficult by single dish even for the Milky Way (seeJones et al. 2012 and MALT90 project1 ). The RRLs mappingof the Milky Way CMZ should be also an important missionthat single dish mode and compact array of ngVLA will complete.
In the CMZ of NGC 253, Bendo et al. (2015) identified threeblobs i.e., C, W, E in Figure 2(b2), with the ∼ 1.00 8 (∼ 30 pc)
resolution H40α map. The SFR is 0.63 ± 0.06, 0.29 ± 0.02,
and 0.17 ± 0.02 M yr−1 for C, W, E, respectively. Althoughhigh angular resolution observations by VLA and ALMA havealready demonstrated that detecting RRL in ∼pc scale is feasible (e.g., Mohan et al. 2005 for VLA; Nakanishi+ in prep. for
ALMA), only small numbers of ∼pc scale star-forming clumps
were detected due to the limited sensitivity. The ngVLA will
enable us to identify many faint and small clumps that VLA
and ALMA are missing.
In the case of merging starburst galaxy NGC 3256, H40αemission indicates that SFR is 9.8 ± 0.5 M yr−1 and 6.8 ±0.5 M yr−1 at the northern and southern nucleus, respectively. The achieved angular resolution is 1.500 (∼ 300 pc),
which means that we can only access global SFR at CMZ in
NGC 3256 (Michiyama et al. 2020). What ALMA told us isthat the SFR in CMZ of NGC 3256 is ∼ 10 times larger than
CMZ of NGC 253 and ∼ 100 times larger than CMZ of the
Milky Way. This ALMA project demonstrated that only RRLs
can investigate the starburst activities in extremely dusty regions. When we use ngVLA, we can observe RRLs at starburst
clumps like SgrB2. For example, ngVLA can achieve 10 mas(∼ 2 pc) resolution at H40α. Assuming 1h integration with thevelocity resolution of 10 km s−1 , we can achieve the sensitivity of 42.3 µJy. This sensitivity corresponds 3σ upper limit of
SFR=0.02 M yr−1 assuming that velocity width of the clump
is 30 km s−1 . This means that ngVLA can detect 1 − 10 pc
clumps like SgrB2 at NGC3256.
In summary, single dish telescopes detected RRLs at 1-10 pc
clumps such as SgrB2, and ALMA has been detected same size
clumps in very nearby starburst galaxy such as NGC 253, andngVLA will detect the small clumps in extremely dusty regions
such as CMZ of NGC 3256.
4.
4.1.

Expected Outcome
Gas depletion time scale of 1 − 10 pc clumps

Thanks to the wide instantaneous bandwidth and the long
baseline that ngVLA will provide, RRLs and molecular lines
such as CO and HCN can be simultaneously observed withhigh angular resolution. This means that we can simultaneously access the information of both SFR and molecular gas
mass (MH2 ) of 1 − 10 pc clumps. We can investigate thetime scale in terms of gas depletion, called gas depletion time
(τgas = MH2 /SFR or ΣMH2 /ΣSFR , where ΣMH2 and ΣSFR
is the surface density of molecular gas and SFR). It is said thatτgas is much longer in the normal star forming galaxies than active galaxies like U/LIRGs (Daddi et al. 2010). In most cases,
especially in radio astronomy, the conversion factor (from CO
luminosity to molecular gas mass) is highlighted when we in1

http://malt90.bu.edu

4[Vol. ,

Fig. 2. (a1) Night landscape photograph including Galactic center taken by Ko Hosokawa.
(a2) CO (1–0) map of CMZ
in the Milky Way obtained by NRO45m (Oka et al. 1998).
The data was downloaded from NRO45m Data Release pagehttps://www.nro.nao.ac.jp/˜nro45mrt/html/results/data.html.
The angular resolution is 1700 (∼ 0.67 pc).
(a3)
H58α map of SgrB2 obtained by Mopra (Jones et al. 2011). The data was downloaded from Mopra Central Molecular Zone Survey
webpage https://atoa.atnf.csiro.au/CMZ. The angular resolution is 7500 (∼ 3 pc). (b1) Optical image of NGC 253 downloaded from NASA website (https://svs.gsfc.nasa.gov/30980).
(b2) The CO (2–1) map of NGC 253 obtained by ALMA.
The data was downloaded from Japanese Virtual Observatory (JVO, http://jvo.nao.ac.jp/portal/v2/) with the fits name of
member.uid___A001_X1284_Xf59.ngc253_sci.spw29.cube.I.pbcor.fits. The black ellipticals indicates the region where H40α is
detected. The angular resolution is 1.00 2 × 0.00 9 (∼ 20 × 16 pc). (b3) The H40α map of NGC 253 obtained by ALMA (Nakanishi et al., in prep). The data
was based on the project of 2016.1.00732.S. The angular size is smoothed in 100 (∼ 17 pc). (c1) Optical image of NGC 3256 downloaded from ESA website(https://esahubble.org/images/heic0810ar/). (c2) The CO (2–1) map of NGC 3256 obtained by ALMA. The data was downloaded from
JVO with the fits name of member.uid___A001_X2d6_Xb0.ari_l.NGC_3256_sci.spw0_227531MHz.12m.cube.I.pbcor.fitsandmember.uid___A001_X2d6_Xb0.ari_l.NGC_3256_sci.spw1_229289MHz.12m.cube.I.pbcor.fits.
The
an00 86 × 0.
00 59 (∼ 15 × 10 pc).
gular resolution is 0.
The map is not entirely the full field of view (zooming up to the southern nucleus).
(c3) The H40α map of NGC 3256 obtained by ALMA (Michiyama et al. 2020).
The angular resolu00 5 × 1.
00 3 (∼ 26 × 23 pc).
tion is 1.
In each panel, the grey elliptical indicates the 500 pc × 100 pc CMZ region.

Name
(1)
Milky WayNGC 253
NGC 3256

DL

1 pc

(2)
8 kpc
3 Mpc
40 Mpc(3)
2500
58 mas
5 mas

SFRCMZ
M yr−1(4)0.08
1.73±0.12
6.8±0.3

SFRtotal
M yr−1(5)
0.68-1.45
4.2
49±2

Σ3σ
SFR
M yr−1 pc−1
(6)−5.0e-06
1.6e-04

Table 1. Information of Milky Way, NGC 253, and NGC 3256. (1) Name. (2) Distance. (3) The angular scale corresponding to 1 pc. (4)SFR in central molecular zone. The references are Immer et al. (2012), Melo et al. (2002), and Michiyama et al. (2020) for the Milky Way,NGC 253, and NGC 3256, respectively. In the case of NGC 3256, the value is for the southern nucleus. (5) The total SFR. The references
are Robitaille & Whitney (2010), Sanders et al. (2003), and Michiyama et al. (2020) for the Milky Way, NGC 253, and NGC 3256, respectively. (6) The 3σ upper limits for high resolution. For NGC 253, we assume 1h. integration assuming 100 mas angular resolution and 10
km s−1 spectral resolution (rms=49.7µJy). We assume FWHM=30 km s−1 and electron temperature of 4000 K to calculated the 3σ upper
limit of SFR. For NGC 3256, we assume 1h. integration assuming 10 mas angular resolution and 10 km s−1 spectral resolution (rms=42.3µJy).

vestigate the different modes (i.e., the variation of τgas ) of
star formation activities among normal and starburst galaxies.
However, the method to measure SFR is also an issue, and measuring τgas by the same SFR calibration is important to directlycompare star formation processes in the Milky Way and active
galaxies. Figure 3 is the relation between ΣMH2 and ΣSFR for

SgrB2, NGC 253, and NGC 3256 using CO and RRLs, showing τgas ∼ 4 Gyr in SgrB2, τgas < 0.1 Gyr at small clumps in
NGC 253, and τgas ∼ 0.1 Gyr in NGC 3256. This suggests the
rapid mode in starbursts and normal mode in SgrB2 although
the number of detected clouds are limited. The current important issue in this plot is that the τgas is measured by different

No. ]

5

scales in SgrB2 and NGC 253 and NGC 3256. The ngVLA
will detect 1 − 10 pc clumps even in NGC 3256 and enable us
to understand whether (1) 1 − 10 pc scale clumps in the Milky
Way and extremely dusty starburst regions in NGC 3256 havesame τgas or not and (2) the τgas of 1 − 10 pc clumps are uniform or not in a galaxy (e.g., northern and southern nucleus in
NGC 3256).

by assuming constant star formation and a Kroupa IMF in thestellar mass range of 0.1–100 M (Michiyama et al. 2020).
In this analysis, the global RRL luminosity is enough and we
do not need to access the spatially resolved maps. Therefore,
we can investigate very active galaxies like ULIRGs that is ina relatively distant Universe. Figure 4 shows the expected peak
flux of (a) Arp 220 and (b) a ULIRG who has SFR=300 M
yr−1 at z = 0.1. Because a large number of RRLs are populated
in ngVLA frequency coverage (Figure 1), stacking methods arealso very powerful to detect RRLs from galaxies at a cosmological distance (Emig et al. 2019). The starburst timescalemeasured by SFRRL /LTIR ratio should be compared with theother timescales such as τgas and merger timescale. The detail modeling to convert SFRRL /LTIR to starburst timescale
should be developed before the ngVLA era. This goal will beachieved thanks to thr very high sensitivity and wide frequency
coverage achieved by ngVLA.

Fig. 3. The relation between ΣMH2 and ΣSFR investigated by RRLs.
The yellow circle indicates SgrB2 that has the τgas ∼ 4 Gyr. The grey
circles indicates the clumps identified in H40α map in Figure 2(b3)that has τgas ∼ 0.02 − 0.04 Gyr. The red and blue circles indicate the CMZ of merging galaxy NGC 3256 (Michiyama et al.
2020). Current ALMA angular resolution and sensitivity cannot
investigate small (a few pc) clumps in NGC 3256. The ngVLA
will enable us to investigate small clumps (like SrgB2) even in
the intense starburst galaxies such as NGC 3256. For each circle, the size of the markers corresponds to the size of clumps.

4.2.

Starburst timescaleFig. 4. (a) The red and green dots indicate the expected peak line
flux for each RRL assuming Te = 1000 K and 5000 K, respectively. The  and αB are calculated by Storey & Hummer (1995)
and Prozesky & Smits (2018). The yellow star indicates the previous observation of H92α by VLA (Anantharamaiah et al. 2000).
The grey line indicates the naturally weighted ngVLA sensitivity for 1h integration with the velocity resolution of 10 km s−1(https://ngvla.nrao.edu/page/performance). (b) The
same plot but we assume a ULIRGs with SFR=300 M yr−1 atz = 0.1. The 100 GHz is the most sensitive in this project.

The calibration constant between SFR and recombinationline luminosity is applicable if SFR is constant over a few Myr.
This is because the ionizing photon flux will decrease dramatically after the burst, e.g., by two orders of magnitude between
5-10 Myr (Calzetti et al. 2012). Therefore, recombination lines
can measure the “instantaneous” SFR. On the other hand, thecalibration constant between SFR and total infrared luminosity(LTIR ) changes depending on the starburst timescale because
old, long lived, low-mass stars contribute TIR luminosity as 4.3. Properties of H II region
well as young stellar populations. Therefore, by combining
As shown in equation (7), we need to calculate αB and both recombination line and infrared information, we can constrain the starburst time scale. For example, the ratio of SFR which depend on Te and ne (Storey & Hummer 1995). The
measured by recombination lines (SFRRL ) and LTIR is calcu- emission flux does not strongly depend on ne . However, as
shown in Figure (4), the RRL luminosity depends on Te . The
lated as
electron temperature can be estimated by comparing the ratio−44(τ = 10 Gyr)SF RRL [M yr−1 ]  1.6 × 10−44
between RRL and 3 mm continuum flux as follows.
2.8 × 10(τ = 100 Myr) (6)R= 3.7 × 10−44
LTIR [erg s−1 ]fνRL dvJy(τ = 10 Myr)R=fν cont
Jy km s−1

6[Vol. ,
= 4.38 × 1033 gff−1
erg s−1 cm−3



ν
GHz

−1 

0.5
Te
(7)
K

and

gff = 0.5535 ln

TeK1.5 hν i−1 −1
− 1.682.
Z
GHz

(8)

Here, we assume an ionic charge of Z = 1. We assume thatthe continuum can be produced by free-free emission from H II
regions as well as hydrogen recombination lines. We note thatthe contribution of dust and synchrotron emissions from supernovae and AGN should be carefully subtracted by continuum SED when we apply this method. The ngVLA’s wide
frequency enables us to investigate radio continuum SED efficiently. The determination of the electron temperature is one
reason why we need both RRL and continuum observations
for our purpose. In addition, the electron temperature itself
is an important parameter. For example, regions with lower
metallicity show higher electron temperature in the Milky Way
because the cooling is not efficient in low-metallicity regions(Pagel et al. 1979; Shaver et al. 1983). If the metallicity ismeasured by the other method, we can investigate whether the
relation between metallicity and electron temperature can be
reproduced in galaxies.
4.4.

OthersBecause RRLs are extinction free, the information can beused to investigate the properties of dust by comparing withrecombination lines in optical and infrared. For example, theratio between RRL, Brγ, Brδ, Paα, Paβ, Hα, Hβ represents thedust extinction curve. Based on the absolute extinction magnitude and the slope of the curve, we may investigate the properties of dust (Salim & Narayanan 2020). In addition, we canaccess the line profile in addition to the total RRL flux. Thisis an important advantage comparing with the method to measure SFR in continuum observations (for continuum observation, see Murphy et al. 2018). For example, a wing-like structure would be seen both in local (∼pc) and global (∼kpc) scale,
which should be important to understand supernovae and AGN
feedback processes. Finally, the detection of RRLs in AGN tori
is also an important science topics (Izumi et al. 2016).
5.

Summary

The ngVLA will provide an unparalleled opportunity forinvestigating starburst activities at a very dusty nucleus in
U/LIRGs by observing RRLs. For example, ngVLA will detect small (1-10 pc) clumps with the SFR of ∼ 0.02 M yr−1
(like SgrB2 of Milky Way) in a merging galaxy NGC 3256
(DL ∼ 40 Mpc). This will answer the fundamental questionof “whether the star formation process is uniform in normal
star forming and extreme starburst galaxies”, which will be an
important step to answer “How are starbursts triggered and regulated during a galaxy evolution?”.
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