Active galaxies illuminating their companions – Galaxy Zoo identifies cross-ionization

One of the most enduring serendipitous finds of the original Galaxy Zoo was a category of giant gas clouds shining from the energy input of active galactic nuclei (AGN) which have since faded (being a little cavalier here with time and verb tenses, since we can’t get news faster than light travels). The most famous of the is of course Hanny’s Voorwerp, whose discovery led to subprojects which turned up many more (“Voorwerpjes”). We have new results now on a related project going back to the Galaxy Zoo Forum, where we searched for gas in companions to active galaxies which is ionized by the AGN, and therefore gives us one more way to learn about how bright the AGN was tens of thousands of years before our direct view. This fills a gap in our knowledge of the feeding history of the mammoth black holes driving this activity – astronomers have been able to follow some for decades (in the best cases, a bit over century using archive photographs), and we can make inferences about their behavior of hundreds of millions of years from statistics in merging galaxies, but timespans of 1-100 millennia were new and interesting for exploring the environments of the black holes.

As a start, Zooites helped compile an initial list of active galaxies with potential companions for further study. (Major roles in this step were played by c_cld, Zutopian, and Graham Mitchell). Then we could take a closer look, make sure the list object really has a spectroscopic AGN, and rank these objects by likelihood of showing the effect we sought, named cross-ionization. Colleagues Vardha Bennert and Anna Pancoast, along with several California students, then measured spectra of the most promising examples, using the 3-meter Shane telescope at Lick Observatory. (The Kast double spectrograoh on that telescope is very flexible and efficient, and I would still say that even had it not been designed by my graduate advisor Joe Miller). In a series of observations over two years, the spectra showed ten strong cases for cross-ionization, as revealed by ratios of the emission lines from gas in the companion galaxies. While the paper reporting the results was being revised after initial submission, the opportunity arose to get new data on two especially interesting objects, which proved to be just as interesting. Spectra from the Russian 6-meter telescope by Alexei Moiseev showed that the distant lumps and filaments of gas around the “Telephone” galaxy pair glow due to energy input from AGN (whether from one or two in this dual-AGN system is hard to say). And in some of the first data from a tunable filter on the new 2.5-meter telescope of the Caucasus Mountain Observatory, we see that the dual AGN system UGC 6081 has vast clouds of ionized gas (so both these pairs rate as Voorwerpjes by our size definition, extending more than 10 kiloparsecs from the nuclei).

The paper reporting these results (including the additional data along the way) was just accepted for publication in the Monthly Notices of the Royal Astronomical Society. The accepted version is available on the arXiv preprint server. As a whole, the results fit with AGN varying dramatically over millennia, so that the companion galaxies where we would otherwise expect to see cross-ionization aren’t always the ones where it is actually seen. (The connects to analogous conclusions from other groups  based on how powerful quasars do and do not influence the gas in their environments).


The galaxy pair NGC 5278/9 (or to use a nickname which takes more and more explaining, Vorontsov-Velyaminov’s “Telephone” galaxy pair). The green image shows [O III] emission, from the SARA 1-m telescope in Arizona, with blue and red showing the broadband starlight from a Hubble archival image. The outer filaments and emission blobs were first seen in H-alpha some years ago, but only with the new spectra contributed by Alexei Moiseev can we show that their gas is ionized by an active galactic nucleus (or maybe two, since both galaxies show modest AGN).


The dual-AGN galaxy pair UGC 6081, comparing starlight (blue) with H-alpha emission in red and [O III] emission in green. The data come from the new tunable-filter instrument on the also-new 2.5-meter telescope of the Caucasus Mountain Observatory operated by Moscow State University. Highly-ionized gas appears in clouds and filaments extending more than 45,000 light-years from the core of each galaxy.

This won’t be the end of this approach – now we know which galaxy pairs will reward still more detailed study, and there are integral-field spectrographs just calling out to have proposals written… Thanks to all the Zooites who participated, and keep your eyes open.

One response to “Active galaxies illuminating their companions – Galaxy Zoo identifies cross-ionization”

  1. zutopian says :

    Dear Mr Keel,

    I am somehow not pleased about being acknowledged in the blog post/ paper, because I got banned from Galaxy Zoo/ the Zooniverse half a year ago, as you know!

    Best regards

    PS: It is a ban by the PI of Galaxy Zoo/ the Zooniverse aka zookeeper Chris, who is one of the co-authors in the paper.

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