An Extragalactic Widefield Search for Technosignatures with the Murchison Widefield Array

Draft version August 21, 2024
Typeset using L
A
TEX twocolumn style in AASTeX631
An Extragalactic Widefield Search for Technosignatures with the Murchison Widefield Array
C.D. Tremblay 1, 2
and S.J. Tingay 3
1SETI Institute, 339 Bernardo Ave, Suite 200, Mountain View, CA 94043, USA
2Berkeley SETI Research Center, University of California, Berkeley, CA 94720, USA
3International Centre for Radio Astronomy Research, Curtin University, Bentley, WA 6102, Australia
ABSTRACT
It is common for surveys that are designed to find artificial signals generated by distant civilizations to
focus on galactic sources. Recently, researchers have started focusing on searching for all other sources
within the field observed, including the vast population of background galaxies. Toward a population
of galaxies in the background toward the Vela supernova remnant, we search for technosignatures,
spectral and temporal features consistent with our understanding of technology. We set transmitter
power limits for the detection of signals in a population of over 1,300 galaxies within a single field of
view observed with the Murchison Widefield Array.
Keywords: planets and satellites: detection – radio lines: planetary systems – instrumentation: inter-
ferometers – techniques: spectroscopic
1. INTRODUCTION
When we consider the search for intelligent life beyond
Earth, we often consider the age and advancement of
technology that may produce a signal detectable by our
telescopes. In popular culture, advanced civilizations
are portrayed as having interstellar spacecraft and com-
munications. Nikolai Kardashev (Kardashev 1964, 1985,
1986), and later modified by Gray (2020), proposed the
Kardashev scale to quantify the degree of technologi-
cal advancement of intelligent life beyond Earth. The
Kardashev scale has three levels: Type I civilizations
are capable of accessing all the energy available on their
planet (upward of 1016
W); Type II civilizations can di-
rectly consume a star’s energy (upwards of 1026
W); and
Type III civilizations can capture all the energy emitted
by their galaxy (upward of 1036
W).
Civilizations on the upper end of the Kardashev scale
could produce large quantities of electromagnetic ra-
diation detectable at galactic distances. Some of the
ideas explored in the past involve harnessing their galax-
ies’ starlight (Annis 1999), colonizing their solar sys-
tem (Gaviraghi & Caminoa 2016), or using pulsars as
communication networks (Chennamangalam et al. 2015;
Corresponding author: Chenoa Tremblay
ctremblay@seti.org
Haliki 2019). The ability for radio waves to permeate
space over long distances and through planetary atmo-
spheres (e.g. Tremblay et al. 2022) made them a practi-
cal method for searching for interstellar communication
since Cocconi & Morrison (1959) first proposed the con-
cept.
Uno et al. (2023) estimated the prevalence of ex-
tragalactic civilizations possessing a radio transmitter
based on past Breakthrough Listen1
observational data
from Isaacson et al. (2017) and Price et al. (2020). They
determine the transmitter rate (TR) for four of the large
surveys using the Parkes 64 m (Murriyang) Telescope
in New South Wales and the Robert C. Byrd Green
Bank Telescope in West Virginia. Based on an esti-
mate of background galaxies in each field of view (FoV)
searched they set upper limits of log(TR) ranging from
13.41 to 14.46, representing thousands of observational
fields. Overall, they concluded that less than one in
hundreds of trillions of extragalactic civilizations within
969 Mpc possess a radio transmitter above 7.7 × 1026
W
of power, assuming one civilization per one-solar-mass
stellar system.
Garrett & Siemion (2023) and Choza et al. (2024)
represent other dedicated or opportunistic extragalac-
tic searches with modern telescopes. Similar to Uno
1 https://breakthroughinitiatives.org
arXiv:2408.10372v1
[astro-ph.GA]
19
Aug
2024

2
et al. (2023), Garrett & Siemion (2023) searched for
background galaxies within previously published fields
from the Green Bank telescope. For this work, they re-
ported a log(TR) of ∼10, with a sample of over 143,024
objects from 469 fields based on searches using NED
and SIMBAD2
(Wenger et al. 2000). Their limits for the
equivalent isotropic radiative power ranged from ∼1023
to ∼1026
depending on the object class.
With a large FOV, the Murchison Widefield Array
(MWA; Tingay et al. 2013; Wayth et al. 2018) in West-
ern Australia provides a unique dataset to study galac-
tic (Tingay et al. 2016, 2018; Tremblay & Tingay 2020;
Tremblay et al. 2022)) and extragalactic communica-
tion. Currently, these frequencies (80–300 MHz) are rel-
atively unexplored for technosignatures, and at such a
sensitivity that a Kardashev II or III civilization can be
detected. As discussed by Garrett et al. (2017), aper-
ture arrays like the MWA provide simultaneous coverage
across up to 900 square degrees, containing thousands
of galaxies and millions of stars. Using examples of low-
frequency transmitters on Earth, we can consider the
Air Force Space Surveillance System, known as ‘Space
Fence,’ which operated up until 2013. This system was
a 1 MW continuous wave (0.1 Hz bandwidth) system op-
erating at 216 MHz. These factors add up to a powerful
motivation for a study of the extragalactic population
for technosignatures in this frequency range.
In this paper, we search for signals at a 10 kHz
spectral resolution originating from galaxies within a
400 square degree FOV toward the Vela supernova rem-
nant. This is the same field that was searched for signals
from known stars and published by Tremblay & Tingay
(2020). Here we explain the sample selection process
and the results of this study, and discuss the results in
the context of extragalactic communication and other
searches of a similar nature.
2. OBSERVATIONS
The data for this analysis were obtained and processed
as part of a molecular line survey as described by Trem-
blay et al. (2020), but we provide some detail here. The
data were collected between 2018 January 5 and 2018
January 23 using the MWA in a frequency range of 98–
128 MHz. For this dataset, 91 tiles (of 16 dipoles each)
were online and baselines extended to 6 km length. This
resulted in a synthesized beam of 1 arc minute full width
at half maximum.
Each night of data was calibrated using observations
of Hydra A and the calibration was further refined with
2 https://simbad.u-strasbg.fr/simbad/
phase-based self-calibration. Imaging of the data was
done using WSClean (Offringa & Smirnov 2017) and
flagged using aoflagger (Offringa et al. 2015a,b).
After correction for ionospheric distortion and
flux density scaling, each 5-minute observation was
reference-frame corrected to the local-standard of rest.
All the 5-minute observations that passed quality con-
trol tests were stacked together using inverse variance
weighting for a total of 17 hours of integrated data time.
The resultant data were a series of 24 three-dimensional
cubes (each cube covering 1.28 MHz of bandwidth) with
10 kHz frequency resolution and a median root mean
squared (RMS) noise of 0.035 Jy beam−1
for each 10
kHz channel, although the noise varied depending on the
sky position and frequency. The largest impact of the
frequency-dependent noise fluctuations was due to radio
frequency interference (RFI) flagging in some observa-
tions compared to others. The sky-position dependence
is a joint effect of the primary beam sensitivity pattern
and positions of the sky where multiple processed fields
had more data stacked3
See Figure 3 in Tremblay et al.
(2020) for an example of the sensitivity pattern on the
sky.
3. SOURCE SELECTION
To complete a search for technosignatures from galax-
ies, we used the NASA/IPAC Extragalactic Database
(NED) and searched for sources where the pretype was
listed as ‘G’ for galaxy. Out of millions of objects cata-
loged in the 400 square degree field of view, there were
a total of 2880 known galaxies from the full collection
of surveys available in NED, including the Two Mi-
cron All Sky Survey (2MASS; Skrutskie et al. 2006), HI
Parkes All Sky Survey (HIPASS; (Meyer et al. 2004)),
Williams et al. (2014), and Staveley-Smith et al. (2016)
surveys (Figure 1). Of these galaxies, we focus on the
1317 sources that have known redshifts since these can
be used to estimate distances, and hence transmitter
power requirements. Fewer galaxies with known red-
shifts are found in regions where the Galactic plane ex-
tends through the field, as shown in the right-hand plot
in Figure 1.
4. RESULTS
Each of the 24 data cubes were searched blindly for
any signal with a 6σ value over local noise using Aegean
and with RMS maps created using BANE (The Back-
3 Even though the MWA can observe the entire sky, we only process
a small segment of the field due to computational constraints.
When imaging, some of the observations had slightly different
phase centers, changing the sensitivity across the sky when all
observations were stacked.

3
Figure 1. Left: The distribution of the 2,880 galaxies found by searching through NED using the table access protocol (TAP)
services available within TopCat (Taylor 2005). Right: The distribution of 1,317 galaxies with known red shifts.
ground and Noise Estimation tool) (Hancock et al.
2018). No such signals were detected at this level or
above. To determine the limits on transmitter power,
we estimate the distance of the sources to the parsec
scale from the red-shifts (z) reported for the galaxies in
NED,
D = (c × z)/Ho , (1)
where D is the distance in parsecs, c is the speed
of light, and Ho is the Hubble constant. However, we
acknowledge that this might not be a definitive mea-
sure of distance as nearby galaxy red shifts will be
dominated by random velocities rather than the Hub-
ble flow (e.g. Karachentsev et al. 2009 determined
a value of (78±2) kms−1
Mpc−1
for galaxies less than
3 Mpc from the Milky Way). We used the value of
67.4±0.5 km s−1
Mpc−1
(Planck Collaboration 2020) for
Ho for the entire sample. The distances, shown in Figure
2, range in values from 2.43×101
Mpc to 1.01×103
Mpc.
To determine the minimum detectable equivalent
isotropic radiative power (EIRPmin) we use Equation
1 from Tremblay & Tingay (2020),
EIRPmin < 1.12 × 1012
Srms R2
, (2)
where Srms is the local image noise in Jy beam−1
and
R2
is the square of the distance in pc. For the near-
est galaxies, we obtain an upper limit for EIRPmin of
7.07×1022
W with a maximum value of 4.35× 1028
W.
The mean value for the survey is 4.40× 1027
W. We
added the details regarding the closest 15 galaxies in
Table 1.
To compare these results to the surveys by (Garrett &
Siemion 2023) and (Uno et al. 2023), we use the trans-
mitter rate calculation as per Figure 5 of (Price et al.
2020):
TR = (Nstar(
νc
νtot
))−1
, (3)
where Nstar is the number of stars observed, νc is the
central frequency, and νtot is the total bandwidth. In
this survey we cover a bandwidth of 30.72 MHz, a central
frequency of 113.28 MHz, and 2,880 known galaxies. To
determine the number of stars per galaxy we use the
assumption made by Garrett & Siemion (2023) that all
galaxies contain the same number of stars at a value
of ∼1011
. The log(TR) for our single FOV with the
MWA is 15.06 if all galaxies are considered, or 14.69 if
we determine the value for only those galaxies in which
a distance was determined.
5. DISCUSSION
Although no signals were detected, we have placed
stringent limits on the transmitter power over the 1 317
galaxies on the order of 1022
W. Kardashev (1985) sug-
gests that large-scale technology would have a lot of
mass, a large energy potential, and a high information
volume. With the sensitivity of modern radio telescopes,
it may be possible to detect radiation coming from such
a supercivilization even at galactic distances.
Gaviraghi & Caminoa (2016) suggest methods in
which a Kardashev Type II or Type III civilization
may use significant power loads to colonize an entire
solar system. They explore the ideas for strategies that
range from noncrewed and crewed preparation phases
or by sending mind-uploaded machines and the energy
required for these ideas. Overall, they compute that the
emitting power would be upwards of 1024
W, which is
within the detection limits of this work.

4
Figure 2. Left:A histogram of the distances for the 1,317 galaxies with known red shifts in our sample. Right: A plot of the
distribution of galaxies where the color represents the calculation of distance from Equation 1 converted to Mpc.
Table 1. The nearest 15 galaxies and the isotropic limits.
Source Name RA (deg) Dec (deg) Type z z reference EIRPmin (W) Distance (pc)
WISEA J091547.16-435623.7 138.947 -43.940 G 2.880E-04 Wen et al. (2021) 7.068E+22 1.289E+06
WISEA J090611.07-422218.5 136.546 -42.372 G -4.640E-04 Wen et al. (2021) 1.835E+23 -2.076E+06
It is generally agreed upon that it would be feasible for
supercivilizations to harness the energy from natural ob-
jects and transmit signals over large distances (Ćirković
2015). Haliki (2019) and Chennamangalam et al. (2015)
created a model where pulsars, rapidly spinning neutron
stars, could be used as modulated signal beacons to con-
tact other civilizations. Their recommendation was that
Type III civilizations could not only use the beacons of
light and change the modulation for communication but
could potentially create an entire communication net-
work. As modern radio telescopes can detect pulsars in
galaxies external to our own, it offers another potential
mechanism for generating emission, although broadband
(frequency width over MHz) in nature; we could poten-
tially detect the resultant energy from the technology.
With motivation of why these powerful signals may ex-
ist from supercivilizations, Choza et al. (2024) presented
results from a targeted search toward 97 nearby galaxies
observed with the Green Bank Telescope. By comparing
Figure 3 in this work with Figure 2 from Choza et al.
(2024), we are up to 100 times less sensitive but cover a
sample size that is more than 10 times larger, and still
within the expected power limits of a Type II or Type
III civilization. With the large FOV of the MWA, we
also cover the entire galaxy with a single pointing and
all 2,880 galaxies are observed in a single observation.
The limits on the EIRPmin set for the MWA on this
field are about 10 times less sensitive than Garrett &
Siemion (2023), when similar distances are compared.
Several close (red shifts < 0.02) galaxies are included in

5
Figure 3. A histogram plot of the EIRPmin values calcu-
lated for this galaxy sample with the x-axis in Log (top) and
Linear (bottom) space.
their sample as well, where this sample extends to much
greater distances.
Although both Uno et al. (2023) and Garrett &
Siemion (2023) completed extragalactic searches in
archive data, Garrett et al. (2017) separated the lim-
its based on galaxy classifications. This adds an addi-
tional level of analysis to technosignature searches when,
as a community, we are trying to define where a signal
may present itself. However, in all three studies (Uno
et al. 2023, Garrett & Siemion 2023, and Choza et al.
2024), they use single-dish telescopes with poor point
source resolution. This MWA dataset has a resolution of
1 arc min compared to the Green Bank telescope which
has a 12 arc min resolution at 1 GHz. This means if a
signal was detected it could be challenging to locate the
exact source of emission.
6. CONCLUSION
This work represents the first low-frequency extra-
galactic technosignature search. Although the MWA is
less sensitive when compared to the single-dish exper-
iments, this work is important due to the unique fre-
quency range coverage, large instantaneous FOV, and
its point source resolution. Powerful emitters on Earth
and some of our earliest radio transmissions are at fre-
quencies less than 300 MHz but only a few searches have
been conducted.
We report EIRPmin transmitter limits for 1,317 galax-
ies and searched toward over 2,880 galaxies for tech-
nosignatures. By setting a limit of ∼1022
W on a trans-
mitter, we sit in a realm of possible detection based on
theoretical models (Gaviraghi & Caminoa 2016). Al-
though no artificial signals were detected, surveys of
this kind are important for narrowing down the cosmic
“haystack” (Wright et al. 2019).
One of the main challenges in searching for signs of
extraterrestrial intelligence is the speed at which we
can observe the sky. Even with the large FOV of-
fered by aperture arrays like the MWA, dedicated exper-
iments done on shared-use instruments limit how much
of the sky we cover and how often we cover the same
sources. This landscape is significantly changing due
to future and current commensal experiments on tele-
scopes, such as the Karl G. Jansky Very Large Array
(COSMIC; Tremblay et al. 2024) and the MeerKAT tele-
scope (BLUSE; Czech et al., in preparation). Continu-
ing to work together to cover the frequency space will
be crucial in the future.
C.D. Tremblay would like to thank Steve Croft and
Carmen Choza for discussions and suggestions regard-
ing this work. This research has made use of the
NASA/IPAC Extragalactic Database (NED), which is
operated by the Jet Propulsion Laboratory, California
Institute of Technology, under contract with the Na-
tional Aeronautics and Space Administration.
This scientific work makes use of the Murchison
Radio-astronomy Observatory, operated by CSIRO. We
acknowledge the Wajarri Yamatji people as the tradi-
tional owners of the Observatory site. Support for the
operation of the MWA is provided by the Australian
Government (NCRIS), under a contract to Curtin
University administered by Astronomy Australia Lim-
ited. Establishment of ASKAP, the Murchison Radio-
astronomy Observatory, and the Pawsey Supercomput-
ing Centre are initiatives of the Australian Government,
with support from the Government of Western Australia
and the Science and Industry Endowment Fund.
Facilities: MWA

6
Software: The following software was used in the cre-
ation of the data cubes and data analysis: aoflagger
and cotter – Offringa et al. (2015b); WSClean – Of-
fringa et al. (2014); Offringa & Smirnov (2017); Aegean
– Hancock et al. (2018); miriad – Sault et al. (1995);
TOPCAT – Taylor (2005); Python PANDAS – reb
(2020)
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