The Breakthrough Listen Search for Intelligent Life: Technosignature Search of 97 Nearby Galaxies

Brief

Choza et al. (2023) present the largest extragalactic narrowband radio technosignature search to date, using 459 ABACAD cadences (139.7 TB, 229 hours) collected at the 100-m Green Bank Telescope between 2018 and 2022. The turboSETI pipeline searched 97 galaxies spanning 60 kpc (Ursa Minor Dwarf) to 29.2 Mpc (NGC 5813) across four receiver bands (L, S, C, X: 1.10–11.20 GHz), yielding 6,034,313 hits reduced to 1,519 events after RFI excision; none survived visual inspection as credible technosignature candidates. The null result places an upper limit of 3.7% on the fraction of galaxies harboring an active transmitter detectable at minimum EIRPs of 2.31–3.46 × 10²⁴ W (band-dependent), with the most distant targets requiring transmitters approaching Kardashev Type II output (~10²⁶ W) for detection.

Metadata

Category

Search

Venue

The Astronomical Journal

Type

Peer-reviewed

Year

2023

Authors

Carmen Choza, Daniel Bautista, Steve Croft, Bryan Brzycki, Andrew Siemion

DOI

10.3847/1538-3881/acf576

arXiv

2312.03943

Access

Open access

Length

7.3 M

Programs

Breakthrough Listen

Instruments

Robert C. Byrd Green Bank Telescope (GBT)

Data sources

Breakthrough Listen GBT backend (139.7 TB, HDF5), NASA Extragalactic Database, turboSETI pipeline, seticore, setigen

Tags

SETI, technosignature, radio-astronomy, narrowband-search, Kardashev-scale, extragalactic

Key points

• 97 nearby galaxies searched for narrowband Doppler-drifting radio signals at 1.10–11.20 GHz using the GBT; galaxy distances span 60 kpc to 29.2 Mpc, a five-order-of-magnitude range in minimum detectable EIRP.p.2
• 459 total cadences analyzed, representing 139.7 TB of fine-frequency-resolution spectrograms (spectral resolution ~2.79 Hz, time resolution ~18.25 s); total observation time 229 hours across five years (2018–2022).p.6
• 6,034,313 raw hits reduced to 1,519 events after on/off cadence RFI filtering (ABACAD pattern); zero-drift hits discarded as likely terrestrial RFI; no events survived visual inspection as technosignature candidates.p.4
• Transmitter upper limit: ≤3.7% of galaxies host an active transmitter at or above the EIRP detection threshold in each of the four frequency bands, derived from 97 trials with zero detections.p.4
• Minimum detectable EIRPs range from 2.31 × 10²⁴ W (L-band) to 3.46 × 10²⁴ W (X-band); constraints on the most distant targets correspond to Kardashev Type II output (~10²⁶ W).p.4
• Signal injection and recovery tests using setigen quantified turboSETI detection efficiency as a function of S/N and drift rate; efficiency decreases as 1/N for signals drifting faster than the unit drift rate (~0.153 Hz s⁻¹).p.7
• Drift rate range of ±4 Hz s⁻¹ covers ≥99% of drift rates expected from known NASA Exoplanet Archive targets at L, S, and C bands, and 70% of that range at X-band, per Li et al. (2023).p.7
• Complete X-band tiling of all 97 galaxies would require 6,161 GBT pointings; M31 alone demands 3,029 pointings, making full coverage impractical under standard observing procedures.p.3

Verbatim

• “We performed a narrowband Doppler drift search using the turboSETI pipeline with a minimum signal- to-noise parameter threshold of 10, across a drift rate range of ± 4 Hz s − 1 , with a spectral resolution of 3 Hz and a time resolution of ∼ 18 . 25 s.”

  p.1

• “At present, humankind has yet to reach a ranking of Kardashev Type I (Zhang et al. 2023).”

  p.2

• “The sample spans five years from early 2018 to late 2022, totalling 229 h of observation time, and includes both spliced and unspliced data.”

  p.6

Most interesting

• A radiator capable of dissipating 1 solar luminosity of waste heat at 300 K would need a surface area of ~8.23 × 10¹⁷ km², comparable in size to the surface of the Sun itself, the paper's estimate of the engineering challenge facing any transmitting Kardashev Type II civilization.
• The L-band receiver's half-power beam covers nearly 40 times more sky area than the X-band receiver; 90 of 97 galaxies fit within a single L-band pointing, but M31 alone would require 3,029 X-band pointings for complete coverage.
• The minimum detectable EIRP varies by five orders of magnitude across the sample, a transmitter in the Ursa Minor Dwarf (60 kpc) requires vastly less power than one in NGC 5813 (29.2 Mpc) to clear the detection threshold.
• The turboSETI noise calculation and setigen's sigma-clipping method disagree on signal S/N by a factor whose mode is 3.25 across X-band coarse channels, a previously underappreciated pipeline discrepancy exposed by the injection-recovery tests.
• A beamed transmitter targeting only the inner 10 kpc of the Milky Way from 10 Mpc distance would need an actual radiated power of just ~1.9 × 10²⁰ W, roughly four million times less than its EIRP, because tight beam focus dramatically reduces power requirements.
• Gray & Mooley (2017) sampled ~10¹² stars by observing M31 and M33 at 21 cm with the Jansky VLA, while Uno et al. (2023) reached over 10¹³ stars in serendipitous GBT extragalactic fields, providing the prior extragalactic SETI benchmark that this survey builds upon.