Editor’s note: A version of this has been previously published on the author’s website.
A beautiful spiral galaxy hides a mystery. The star city is NGC925, which is in a good location this season for astronomy, as seen from our hemisphere.
NGC925, in the constellation Triangulum, will remain nicely positioned through January. In early November it is about 30 degrees above the east-northeast horizon at the end of dusk. (An easy way to measure degrees is to hold your fist at arm’s length, the inside of your hand facing yourself; from top to bottom is 10 degrees.) The galaxy continues to rise, reaching its greatest altitude shortly before midnight at 83.5 degrees, and then begins to sink. It remains a fist and a half above the west-northwest horizon at sunrise.
Fairly large as seen from Earth, at magnitude 11.5 it has low surface brightness; the best way to enjoy it is by taking and studying astrophotos. NGC925 is a barred spiral galaxy, a thick band of stars cutting across its diameter with the spiral arms curving out from the band’s ends. Inclined about 57 degrees so that it isn’t face-on, it looks like a fat backwards S. At nearly 28 million light-years away, it’s one of the closer galaxies.
I tried to photograph it twice, first in the fall of 2010 and then five years later.
On Nov. 10, 2010, I drove to the Bureau of Land Management’s Knolls Off-Highway Vehicles Special Recreation Management Center, a site 80 miles west of Salt Lake City in Tooele County — far enough from city lights. I planned to set up just outside the management center, as inside, the risk was too great of an ORV barrelling over a sand dune and into me.
But a couple of people, with their cars, were already at the place I’d chosen. After talking to them, they were planning to camp for the night and build a fire.
The other spot was miles down a rutted, overgrown road. I found an unused-looking concrete driveway in front of a fiber optic relay station. I set up there and prepared to photograph.
When dusk came, a fierce yellow light automatically turned on above the station’s door. “The yellow light did severely damage my astrophotos, I guess because its aura lit up the air above the telescope even when the light didn’t reflect on it,” reads my journal notation of Nov. 4, 2010. “I rigged a bit of a rubbery mat material to block the direct glare but the results were still bad. ”
The second attempt was successful. On Oct. 11, 2015, my friend Jay Eads, a friend of and I set up near Vernon, Tooele County. The following is from my journal entry of the next day.
”The night was exceptional, with clear, mostly steady air,” reads my journal notation from that night. “The Milky Way stretched across most of the heavens, only ending near the modest light dome from Salt Lake City. Headlight of cars on Utah 36 showed up sometimes, although they weren’t a problem. Jay said the next site a short distance down the dirt road is hidden from the highway and maybe I’ll go there next time.
“By about 2 a.m. (I think) I was able to start imaging.” The result was a nice view of NGC925.
European astronomers published a study in 2018 showing that two startling objects lurk within the galaxy. Their paper, “The two Ultraluminous X-ray sources in the galaxy NGC 925” was published as an online preprint at https://arxiv.org/pdf/1806.11444.pdf in June and by The Monthly Notices of the Royal Astronomical Society, Oxford, England, in September online at academic.oup.com/.
An ultraluminous X-ray source, abbreviated ULX, is a powerful, rare critter. The Smithsonian Astrophysical Observatory noted in 2014, “ Most galaxies (including our own Milky Way) have none, and those galaxies that do host a ULX usually contain only one. ULXs are also mysterious objects.” The observatory says they “are point sources in the sky that are so bright in X-rays that each emits more radiation than a million suns emit at all wavelengths.”
According to a news item published in physics.org, “ULXs .... are less luminous than active galactic nuclei, but more consistently luminous than any known stellar process. Although numerous studies of ULXs have been conducted, the basic nature of these sources still remains unsolved.
”Usually there is one ULX per galaxy in galaxies which host them, however some galaxies were found to contain many such sources.”
A 2012 NASA news release about the discovery of a second ULX in the nearby Andromeda Galaxy says, “Scientists have long debated the nature of these super-bright X-ray sources, and Chandra’s (orbiting X-ray telescope) 2009 discovery of a nearby ULX in Andromeda sparked intense interest.
”Why are these sources so bright in X-rays? Are they normal stellar-mass black holes gorging on unusually large amounts of gas? Or are they long-sought ‘intermediate mass’ black holes, dozens of times more massive than their stellar counterparts but smaller than the monster black holes found in the centers of most galaxies?”
The European paper about those in NGC925 analyzed data from two X-ray detecting satellites, the European Space Agency’s XMM Newton and NASA’s orbiting Nuclear Spectroscopic Telescope Array, NuSTAR. They dubbed the objects in that galaxy NGC925 ULX-1 and ULX-2.
At first glance, ULX-1 looked like X-rays released by an intermediate mass black hole, the report says.
Until recently, no good evidence had been discovered for this sort of black hole. But a survey that used NASA’s Chandra X-Ray Observatory orbiting telescope, the agency’s Spitzer Space Telescope and other powerful instruments “provided strong evidence” for them, according to an Aug. 9, 2018, NASA release.
Further examination of the NGC925 data led the scientists to conclude that ULX-1 doesn’t represent X-rays streaming from an intermediate black hole, but that they come from a regular old stellar-mass black hole. Perhaps the emissions appear unusual because they are seen at a “small angle” to our viewpoint, “possibly through the evacuated cone of a powerful wind” of swirling material originating in the black hole’s accretion disc.
ULX-2 is another story. Its spectral properties don’t match those of an intermediate black hole. “Unfortunately, we cannot draw firm conclusions on the nature of the compact object in ULX-2,” says the study, whose main author is Pintore. It adds that optical observations show the object is surrounded by a region of diffuse emissions in hydrogen alpha light — that is, deeply red light — “whose origin is not clear.”
The report concludes that “further and deeper X-ray and optical observations of this source are strongly needed to better constrain its nature.”
Nothing is as tantalizing as the mysterious unknown. What’s the nature of UXL-2 in that lovely galaxy? Why are such X-ray sources so blindingly powerful — if they really are? Would X-radiation more energetic than the entire outputs of a million stars wipe out life on nearby planets?
Or is it possible ULXs are not as powerful as believed? Presumably, calculations of their output assume they are radiating in all directions. But what if ultraluminous X-ray sources are something like pulsars — narrow jets that look extremely powerful just because those we can detect happen to be aimed in our direction?
As mentioned in an earlier essay, high energy X-rays are extremely hard to focus, their particles blasting out in a straight line so strongly that they tend to go through obstacles. But instruments like NuSTAR, control them with sets of carefully shaped mirrors that bend the particle streams a little at a time, gradually bringing them to a focus. Since black holes and other special objects like neutron stars generate extreme conditions, could some unknown mechanism there channel high-energy X-rays into narrow beams, giving the appearance of ultraluminosity?
Joe Bauman, a former Deseret News science reporter, writes an astronomy blog at the-nightly-news.com and is an avid amateur astronomer. His email is email@example.com.