[Paper Review] Constraints on extragalactic transmitters via Breakthrough Listen

M. A. Garrett, A. Siemion|arXiv (Cornell University)|Sep 16, 2022

Space Science and Extraterrestrial Life4 citations

TL;DR

This paper demonstrates that Breakthrough Listen's radio surveys of nearby stars also constrain extragalactic transmitters by serendipitously observing distant galaxies, galaxy groups, and active galactic nuclei. By analyzing 469 fields, it places tighter limits on powerful extraterrestrial transmitters than previously recognized, extending SETI's reach into the distant universe and identifying a new 'terra incognita' for future surveys.

ABSTRACT

The Breakthrough Listen Initiative has embarked on a comprehensive SETI survey of nearby stars in the Milky Way that is vastly superior to previous efforts as measured by a wide range of different metrics. SETI surveys traditionally ignore the fact that they are sensitive to many background objects, in addition to the foreground target star. In order to better appreciate and exploit the presence of extragalactic objects in the field of view, the Aladin sky atlas and NED were employed to make a rudimentary census of extragalactic objects that were serendipitously observed with the 100-m Greenbank telescope observing at 1.1-1.9 GHz. For 469 target fields (assuming a FWHM radial field-of-view of 4.2 arcminutes), NED identified a grand total of 143024 extragalactic objects, including various astrophysical exotica e.g. AGN of various types, radio galaxies, interacting galaxies, and one confirmed gravitational lens system. Several nearby galaxies, galaxy groups and galaxy clusters are identified, permitting the parameter space probed by SETI surveys to be significantly extended. Constraints are placed on the luminosity function of potential extraterrestrial transmitters assuming it follows a simple power law and limits on the prevalence of very powerful extraterrestrial transmitters associated with these vast stellar systems are also determined. It is demonstrated that the recent Breakthrough Listen Initiative, and indeed many previous SETI radio surveys, place stronger limits on the prevalence of extraterrestrial intelligence in the distant Universe than is often fully appreciated.

Motivation & Objective

• To assess the serendipitous detection of extragalactic sources during Breakthrough Listen's targeted searches of nearby stars.
• To quantify the number and types of extragalactic objects (e.g., AGN, radio galaxies, galaxy clusters) within the field of view of the Green Bank Telescope.
• To extend SETI parameter space by placing constraints on the luminosity function of hypothetical extraterrestrial transmitters in distant stellar systems.
• To evaluate the sensitivity of current SETI surveys to very powerful, rare transmitters in massive extragalactic systems.
• To identify new observational strategies—such as deep, wide-field observations of galaxy groups—for future SETI surveys to explore previously uncharted parameter space.

Proposed method

• Utilized the Aladin sky atlas and NASA/IPAC Extragalactic Database (NED) to identify extragalactic sources within the 4.2 arcminute FWHM beam of Breakthrough Listen observations.
• Cataloged 143,024 extragalactic objects across 469 target fields, including AGN, radio galaxies, interacting systems, and a confirmed gravitational lens.
• Assessed the sensitivity of Breakthrough Listen to continuous-wave transmitters from these distant systems, assuming a power-law luminosity function.
• Calculated minimum effective isotropic radiated power (EIRP) limits for transmitters based on survey sensitivity and source distance.
• Compared constraints from this work with prior studies (e.g., Wlodarczyk-Sroka et al. 2020) to define a new region of parameter space—'terra incognita'—for future SETI surveys.
• Evaluated the feasibility of high-EIRP transmitters (up to ~10^20 W) using phased-array technology, as proposed by Benford et al. (2010).

Experimental results

Research questions

• RQ1 How many extragalactic sources are serendipitously observed in Breakthrough Listen's fields of view during targeted searches of nearby stars?
• RQ2 To what extent do current SETI surveys constrain the presence of powerful, continuous-wave transmitters in distant galaxies and galaxy groups?
• RQ3 What are the limits on the luminosity function of extraterrestrial transmitters when assuming a power-law distribution, based on observations of background extragalactic systems?
• RQ4 How do the constraints from this study compare to those from previous SETI surveys, particularly in terms of EIRP and transmission rate?
• RQ5 What new observational strategies can be derived to explore the 'terra incognita' region of parameter space where transmitters may be rare but extremely powerful?

Key findings

• Breakthrough Listen observations of 469 target fields serendipitously detected 143,024 extragalactic sources, including 14,000+ AGN and multiple galaxy groups and clusters.
• The survey places stronger constraints on the prevalence of very powerful extraterrestrial transmitters (EIRP > 10^20 W) than previously recognized, due to sensitivity to distant, massive stellar systems.
• Constraints extend to sources at distances up to 30 Mpc, including nearby galaxies like NGC 7714/NGC 7715 and the 3C402 galaxy group.
• The study identifies a new 'terra incognita' region in the EIRP-transmission rate plane, defined by lower transmission rates and higher EIRP, which future surveys should target.
• The analysis confirms that SETI surveys using large single-dish telescopes are sensitive to transmitters in distant galaxies, significantly expanding the parameter space explored.
• The results suggest that future surveys should prioritize deep, long integrations and wide-field observations of galaxy groups and clusters to improve sensitivity to rare, high-power transmitters.