The James Webb Space Telescope's infrared vision has transformed our view of a large, barred spiral galaxy, revealing its skeleton of dust illuminated by the glow of young stars.
Visible-light images of NGC 1559, such as those taken by the Hubble Space Telescope, show a glowing whirlpool of light with bright, young star clusters scattered across spiral arms laced with lanes of black dust.
The JWST has now peered past the glare, its infrared vision revealing the galaxy's innards. The JWST's Near-Infrared Camera (NIRCam) sees starlight filtered through the obscuring dust as well as the glow of ionized hydrogen gas in star-forming regions. Meanwhile, the space telescope's Mid-Infrared Instrument (MIRI) was able to observe the dust directly, capturing clouds of tiny particulate matter produced by past generations of stars and tracing out NGC 1559's spiral structure.
The JWST's new image of NGC 1559 isn't just exquisite in its beauty. It is also scientifically valuable, produced as part of the PHANGS (Physics at High Angular resolution in Nearby GalaxieS) project to better understand how stars are born, how they live and how they die in all manner of galaxies across the universe. PHANGS is also interested in learning more about the interplay between those stars and the gas and dust clouds found in a galaxy, as well as how they together impact a galaxy's overall, large-scale structure. The project, led by an international team of astronomers, is mapping these galaxies across the electromagnetic spectrum, using not only the JWST, but also a wealth of other powerful observatories. The list includes the Hubble Space Telescope, the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Compact Array (ACA) in Chile, the Very Large Array (VLA) of radio telescopes in the United States, the MeerKAT radio telescope in South Africa and the Northern Extended Millimeter Array (NOEMA) in France.
NGC 1559, however, stands out among the galaxies that PHANGS observes because it is situated in a lonely patch of space. It doesn't have any close galactic neighbors, and it lies in the southern hemisphere constellation of Reticulum, the Reticule. NGC 1559 is also home to four supernovas discovered over the past 40 years. The first three — SN 1984J, SN 1986L and SN 2005df — were all discovered by Australian amateur-astronomer Robert Evans. That was back in the day when amateurs conducting supernova patrols found the bulk of exploding stars, before professional automated telescopic surveys came online. A case in point is the fourth supernova to be witnessed in NGC 1559, namely SN 2009ib, which was discovered by CHASE, the Chilean Automatic Supernova Search at the Cerro Tololo Inter-American Observatory.
The last two supernovas — SN 2005df and SN 2009ib — are particularly important because they've enabled astronomers to calculate the distance to NGC 1559 and therefore the galaxy's place on the cosmic distance ladder. This could aid in calculating the expansion rate of the universe.
SN 2009ib is what's known as a Type-II-P supernova. It represents the cataclysmic explosion of a massive star, but its light curve — how the supernova's brightness changed over time — stayed flat, or plateaued, for 130 days after reaching peak brightness. The plateau is caused by the hydrogen gas in the supernova's debris being rendered opaque when ionized by the supernova shockwave. Particular characteristics of these plateau supernovas allow for a measurement of each supernova's (and hence galaxy's) distance from us. In 2009, astronomers led by Katalin Takáts of the Universidad Andrés Bello in Chile used SN 2009ib to calculate the distance to NGC 1559, yielding an answer of 19.8 megaparsecs, or 64.57 million light-years.
Meanwhile, SN 2005df is a type 1a supernova, which signals the destruction of a white dwarf. Type 1a supernovae have standardizable brightnesses. The farther away they are, the fainter they seem, but if we know what their true, standardizable luminosity is, we can figure out exactly how far away they must be to appear as faint as they do. Hence, they can be used to ascertain cosmic distances. For this reason, they're also considered "standard candles." In 2019, astronomers led by Caroline Huang and Adam Riess of Johns Hopkins University used SN 2005df, in conjunction with two other types of standard candles, namely Cepheid and Mira-type variable stars, to confirm the distance to NGC 1559. They came up with an answer of 19.8 megaparsecs, in excellent agreement with the previous measurement from SN 2009ib.
Partly based on this distance measurement, Huang and Riess's team were able to calibrate the brightnesses of Type 1a supernovas in galaxies farther away to more accurately measure their distances. Then, they compared these distances to their redshift to calculate the Hubble constant, a measure of the universe's expansion, to be 73.3 kilometers per second per megaparsec.
This calculation is an interesting one; it has added more fuel to the cosmological paradox known as "Hubble tension," in which measurements of the expansion rate using Type 1a supernovas provide a different answer to measurements of the expansion rate based on the study of the cosmic microwave background, which yields a Hubble constant of 67.8 kilometers per second per megaparsec instead.
It remains a mystery why these measurements are different when, by all rights, they should be the same.
NGC 1559 also has one other notable feature. In 2023, Taiwanese astronomers used data from NASA's Chandra X-ray Observatory to find eight ultra-luminous X-ray sources (ULXs) in NGC 1559. ULXs are mysterious objects that unleash torrents of extremely high-energy X-rays in greater amounts than can be explained by any known processes in stars. The suspicion is that these phenomena involve compact objects such as neutron stars and black holes.
One of the eight ULXs in NGC 1559 stood out above the others. Designated X-24, its X-ray emission varies with a periodicity of 7,500 seconds (two hours and five minutes). It is suspected that this periodicity is related to the orbital period of an object, probably a star, moving around a stellar-mass black hole with a gravitational force strong enough to rip material from the orbiting object and consume it. If so, it would be the first compact binary ULX to be discovered.
For a loosely wound but pretty spiral galaxy positioned in the middle of nowhere with few other galaxies close to it, NGC 1559 manages to hold an important place for astronomers in the study of stars, galaxies and the universe at large.
