SETI: Going Deep with the Data Search

by | Sep 3, 2020 | Astrobiology and SETI | 48 comments

What Breakthrough Listen is calling the most comprehensive SETI search to date is now in the books, or at least, the journals, with results accepted and in process at Monthly Notices of the Royal Astronomical Society. Here we are in the realm of data reanalysis, using previously acquired results to serve as a matrix for re-calculation, with the catalog produced by the European Space Agency’s Gaia spacecraft as the key that turns the lock.

No signatures of extraterrestrial technology were detected in the two analyses produced by Breakthrough Listen in 2017 and 2020. The data for these efforts come largely from the Green Bank Telescope (GBT) in West Virginia and the CSIRO Parkes Radio Telescope in Australia, with a focus on 1327 individual stars. Results were published by the Breakthrough Listen science team at UC-Berkeley, and the choice of targets was telling. The search homed in on relatively nearby stars within about 160 light years of the Sun, under the assumption that less powerful transmitters would be detectable the closer they are to the Earth.

The new analysis of these results has been produced by Bart Wlodarczyk-Sroka, a masters student at the University of Manchester (UK) and his advisor Michael Garrett, working with Berkeley SETI director Andrew Siemion. The Manchester duo realized that when one of the large telescopes was pointed at an individual target, the observation also took in a wide range of background stars. This fact meant that stars much further away could be considered if we could make a determination about their distance.

The Gaia catalog measures the distances to over a billion stars. Wlodarczyk-Sroka and Garrett realized that Gaia now gave them measured parallaxes and inferred distances to stars that were found in the full width half-maximum (FWHM) of the main beam of the telescopes used for the Breakthrough Listen observations. FWHM specifies the angular width of the main beam — think of it as the width of the frequency range where less than half the signal’s power is attenuated. Although not targets of the earlier campaign, these numerous stars were available in the data, previously ignored because their distances from Earth were at the time unknown.

Image: This is Figure 1 from the paper. Caption: . An optical colour image of the stellar field centred on HIP109427 from the Pan-STARRS DR1 z and g broadband filters, showing the extent of the FWHM for the GBT L-band and GBT S-band receivers, circled in red and white respectively. 46 sources with geometric distances calculated from Gaia parallax data are marked with green crosses. Credit: Wlodarczyk-Sroka. Garrett & Siemion.

The Gaia information allowed the researchers to select targets out to 33,000 light years, all found within the original observations, thus expanding the number of stars examined from 1327 to 288,315. Their distance would mean that as the range increased, only more powerful transmitters would be visible to the telescopes. The sample takes in not only many main-sequence stars but extends to giant stars and white dwarfs as well.

Andrew Siemion comments on the significance of the effort:

“This work shows the value of combining data from different telescopes. Expanding our observations to cover almost 220 times more stars would have required a significant investment of our telescope time, not to mention the computing resources to perform the analysis. By taking advantage of the fact that we already had radio scans of stars in the background of our primary targets, and by reading their positions and distances from the Gaia catalog, Bart’s analysis has extracted additional information from the existing dataset. Work like this gets us one step closer to the goal of knowing the answer to humanity’s most profound question: Are we alone?”

Given that the only qualifying criteria for the stars in the new study is that they were within the view of the original observations (i.e., within the FWHM of the telescope beam), the range of stellar types is broad, and this marks the first time scientists have been able to place limits on the prevalence of continuous extraterrestrial transmitters on the basis of spectral type.

In earlier studies, no evidence was found of continuous transmitters associated with stars systems within 50 parsecs of the Sun, given power constraints as explained in the paper:

Both Enriquez et al. (2017) and Price et al. (2020) find no evidence for continuous (100% duty cycle) transmitters associated with the nearby (d < pc) star systems observed. This includes directional transmitters (e.g. radio beacons) directed at the Earth with a power output equal to or greater than the brightest human-made transmitters (e.g. a canonical Arecibo planetary radar-like system with a gain of 70 dB and a transmitter power of ? 1 MW). To detect a non-directional isotropically radiating antenna, the transmitter power must be ? 1013 W (around the current energy consumption of our own civilisation).

We don’t know if any civilizations in this range are broadcasting at all, but the data are consistent with the statement that fewer than ? 0.1% of the stellar systems within 50 pc are using these types of transmitters to contact us. The new work now moves well past the nearby sampling of stars, while factoring in the decrease in sensitivity at larger distances. At 100 to 200 parsecs, for instance, fewer than 0.061 such transmitters can be present, given the same power constraint. We have no candidate signals, but we have hugely widened the scope of the search and tightened the numbers. Wlodarczyk-Sroka comments:

“Our results help to put meaningful limits on the prevalence of transmitters comparable to what we ourselves can build using 21st century technology. We now know that fewer than one in 1600 stars closer than about 330 light years host transmitters just a few times more powerful than the strongest radar we have here on Earth. Inhabited worlds with much more powerful transmitters than we can currently produce must be rarer still.”

Setting constraints is not glamorous work, but it’s how we go about building information. We’ve seen the same phenomenon in exoplanet studies. At Proxima Centauri, scientists progressively drilled down by radial velocity research, first eliminating large gas giants and then progressively smaller worlds as possibilities until finally uncovering Proxima Centauri b, at about Earth size. All the patient data analysis builds the structure needed to arrive at eventual conclusions. When it comes to SETI, we’re learning, bit by bit, what is not there, and also clarifying how much remains to be explored.

As just one case in point: What if the beam is not continuous? Should we expect it to be? See SETI: Figuring Out the Beacon Builders for more on ‘Benford Beacons,’ a topic of frequent discussion in these pages.

The paper is Wlodarczyk-Sroka et al., “Extending the Breakthrough Listen nearby star survey to other stellar objects in the field,” accepted at Monthly Notices of the Royal Astronomical Society (preprint).

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Andromeda’s Vast Halo Offers Clues about Galactic Evolution

by | Sep 2, 2020 | Deep Sky Astronomy & Telescopes | 11 comments

Wait long enough — something like 4.5 billion years — and we’ll have a huge elliptical galaxy resulting from the merger of our own Milky Way with Andromeda (M31). I’ve always been fascinated with Andromeda because being the nearest large galaxy, and a fine spiral at that, it gives us a look at how our own galaxy must appear from the outside. Its faintness to the naked eye belies its size, an object considerably larger than the Moon from our perspective, though best seen, of course, on a Moonless night. And now we learn it is even bigger than we thought.

The Absorption Map of Ionized Gas in Andromeda (Project AMIGA) is the source for this information. A new study coming out of this program uses Hubble data to map the vast gas envelope surrounding Andromeda, a diffuse halo of plasma extending 1.3 million light years from the galaxy and in some directions, as far as 2 million light years. To put this into perspective, Andromeda itself is 2.5 million light years away, meaning that our two galaxies may already be encountering each other as their two haloes nudge up against each other.

Now the reference to the Moon gives way to an even larger one. If we could see Andromeda along with its halo, we’d be dealing with an object the width of three Big Dippers. If the entire structure were visible, no other feature of the nighttime sky would be as large. The AMIGA study, led by Nicolas Lehner (University of Notre Dame) reveals the layered nature of this plasma halo, one that contains two nested and distinct shells of gas. Says Lehner:

“We find the inner shell that extends to about a half million light-years is far more complex and dynamic. The outer shell is smoother and hotter. This difference is a likely result from the impact of supernova activity in the galaxy’s disk more directly affecting the inner halo.”

Image: At a distance of 2.5 million light-years, the majestic spiral Andromeda galaxy is so close to us that it appears as a cigar-shaped smudge of light high in the autumn sky. If its gaseous halo could be seen with the naked eye, it would be about three times the width of the Big Dipper—easily the biggest feature on the nighttime sky. Credit: NASA, ESA, J. DePasquale and E. Wheatley (STScI) and Z. Levay.

Sometimes it’s necessary to step back from the minutiae of nearby exoplanet research to see the broader perspective afforded by these cities of stars. Lehner’s co-researcher Samantha Berek (Yale University) calls haloes like these “reservoir[s] of gas” that contain the stuff of future star formation, including the outflows from supernovae. That makes a galactic halo a laboratory for its future evolution, and given its size in our sky, there is no better place to study the phenomenon than Andromeda. Indeed, the team has already found the telltale signs of heavy elements in the halo here, the result of stellar explosions that will seed new worlds.

But despite its relative proximity, how do we go about studying something as diffuse as a galactic halo? AMIGA looks at the light of 43 quasars, those brilliant cores of active galaxies that can be seen in objects much further away than M31. The method: Work with background quasar light as filtered through the Andromeda halo and examine the patterns of absorption in different regions. The data come from Hubble’s Cosmic Origins Spectrograph (COS), working on quasar light in the ultraviolet, a wavelength absorbed by Earth’s atmosphere.

Image: This illustration shows the location of the 43 quasars scientists used to probe Andromeda’s gaseous halo. These quasars—the very distant, brilliant cores of active galaxies powered by black holes—are scattered far behind the halo, allowing scientists to probe multiple regions. Looking through the immense halo at the quasars’ light, the team observed how this light is absorbed by the halo and how that absorption changes in different regions. By tracing the absorption of light coming from the background quasars, scientists are able to probe the halo’s material. Credit: NASA, ESA, and E. Wheatley (STScI).

Ionized gas from carbon, silicon and oxygen turn up in these data, the signature of radiation stripping electrons from atoms. The new maps tune up work Lehner and colleagues performed in 2015, when the Andromeda halo’s complexity was still unknown. Fellow Notre Dame scientist Christopher Howk comments on the scope of the new study:

“Previously, there was very little information—only six quasars—within 1 million light-years of the galaxy. This new program provides much more information on this inner region of Andromeda’s halo. Probing gas within this radius is important, as it represents something of a gravitational sphere of influence for Andromeda.”

Image: This diagram shows the light from a background quasar passing through the vast, gaseous halo around the neighboring Andromeda galaxy (M31), as spectroscopically measured by the Hubble Space Telescope. The colored regions show absorption from two components that make up the halo. For ionized silicon, a significant absorption is shown in both plots. The more highly ionized carbon is absorbed by only one component. Astronomers can tell the two components apart because their line-of-sight motions, known as radial velocity, cause a Doppler shift that changes the wavelength of light being absorbed. Credit: NASA, ESA, and E. Wheatley (STScI).

The size and nearness of Andromeda thus pay off for the study of galactic haloes. Astronomers have examined them in more distant galaxies, but their distance means that there are far fewer background quasars that line up to allow analysis of the haloes. Here we have the kind of extensive sampling from which we can draw meaningful conclusions, using not just one or perhaps two sightlines but over 40, according to Lehner. Future space telescopes working in the ultraviolet will begin to extend such studies beyond the 30 galaxies of the Local Group.

The paper is Lehner et al., “Project AMIGA: The Circumgalactic Medium of Andromeda,” Astrophysical Journal, Volume 900, Number 1 (27 August 2020). Abstract.

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