A Spiral Galaxy’s Invisible, Opposing Arms

by Anashe Bandari

NGC 7479 – also known as Caldwell 44 – is a barred spiral galaxy, with a bar-shaped center filled with stars, as is characteristic of the majority of spiral galaxies, and S-shaped arms. But looking at features of NGC 7479 that are hidden to the naked eye reveals another pair of S-shaped arms, bending in the opposite direction to the visible galaxy.

Radio wavelength emissions from these small, so-called “counter-arms” have been observed before, but with the help of SOFIA – along with observations by ALMA and archival data from a number of other observatories – their presence has now been confirmed by X-ray, ionized carbon, and carbon monoxide emissions as well. SOFIA’s new observations of the counter-arms can help reveal their origin.

Hubble Space Telescope image of NGC 7479 with 20 cm radio continuum contours in yellow. The boxes highlight the ends of the lower and upper counter-arms; expanded versions of these regions are shown in the left and right panels where the circles depict the aperture of SOFIA’s FIFI-LS instrument.
Middle panel: Hubble Space Telescope image of NGC 7479 created from observations at visible and near-infrared wavelengths with 20 cm radio continuum contours in yellow. The boxes highlight the ends of the lower and upper counter-arms; expanded versions of these regions are shown in the left and right panels where the circles depict the aperture of SOFIA’s FIFI-LS instrument. Credit: ESA/Hubble & NASA

“The really important thing in this galaxy are the two little counter-arms that go in the opposite direction of the optical arms that are seen in radio, but nobody had seen them in the X-ray,” said Dario Fadda, lead author on a recent paper describing the analysis. “Seeing them in X-ray is important because it shows there’s energy coming out of the nucleus, something that comes out in jets that originate in the nucleus.”

The fact that these jets originate at the galaxy’s center implies the galaxy harbors an active nucleus – a supermassive black hole.

As the jet approaches the dense molecular clouds along the bar, some of its momentum is absorbed by the clouds, causing the jet to bend in the direction opposite to the rotation of the galaxy. This process is responsible for the orientation of the counter-arms.

By comparing the X-ray emissions of the jet to the ratio of ionized carbon and carbon dioxide emissions from the same area – both of which are considered indicators of star formation – the researchers discovered an anomaly. Certain hotspots within the counter-arms have too much ionized carbon, meaning the X-ray emission cannot entirely be explained by star formation.

“We knew about these counter-arms and tried to observe with SOFIA if ionized carbon is actually produced by star formation, or if there’s some extra component that can come from the energy injected by the active galactic nucleus,” said Fadda.

This calls into question the relationship between ionized carbon and star formation, and can have implications on the study of galaxies that are more distant than NGC 7479.

“This is where SOFIA becomes uniquely useful: Studying these cases of galaxies close to us to have an idea of what to encounter when we go to higher redshift to study galaxies and the farther universe,” Fadda said.

SOFIA’s role in these observations pushes the limits of its capabilities. Primarily suited for studying objects fairly close to our home galaxy, SOFIA’s spatial and spectral resolution were just enough to distinguish ionized carbon in NGC 7479’s region of interest. Specifically, SOFIA’s Far Infrared Field-Imaging Line Spectrometer (FIFI-LS) was used to map the ionized carbon in the area.

SOFIA is a joint project of NASA and the German Space Agency at DLR. DLR provides the telescope, scheduled aircraft maintenance, and other support for the mission. NASA’s Ames Research Center in California’s Silicon Valley manages the SOFIA program, science, and mission operations in cooperation with the Universities Space Research Association, headquartered in Columbia, Maryland, and the German SOFIA Institute at the University of Stuttgart. The aircraft is maintained and operated by NASA’s Armstrong Flight Research Center Building 703, in Palmdale, California.

SOFIA Observes Star Formation Near the Galactic Center

Looking at the ionized carbon emission from Sagittarius B provides critical information about star formation in our own galaxy and beyond.

Sagittarius, or Sgr B, a cloud of gas and dust near the center of the Milky Way is one of the brightest sources in the Central Molecular Zone – a massive, dense area of gas in the center of our galaxy, home to very high star formation rates and turbulent molecular gas clouds. At less than 27,000 light-years away, Sgr B is a relatively close neighbor, making it a useful region to study, both as a proxy for understanding other galaxies throughout the universe and also for understanding our own galactic center.

FORCAST image of the galactic center next to  the same area showingi onized carbon intensity contours of the Sagittarius B region
Left: An image of the Sagittarius B region in the galactic center taken by SOFIA’s FORCAST instrument, combined with images from NASA’s Spitzer Space Telescope and the Herschel Space Observatory of the European Space Agency. Right: Ionized carbon intensity contours of the Sagittarius B region. The striped pattern is a scanning artifact due to the motion of the telescope. In both panels, crosses indicate the locations of the three star-forming cores of Sagittarius B2. Credit: Left: NASA/SOFIA/JPL-Caltech/ESA/Herschel; Right: Harris et al., 2021

In particular, observing the concentration of ionized carbon in a molecular cloud like Sgr B is a powerful method for probing the properties of the system, including its level of star formation.

Using SOFIA’s upgraded German Receiver for Astronomy at Terahertz Frequencies, or upGREAT, a team of researchers imaged the ionized carbon characteristics of Sgr B. GREAT has ample spectral resolution to study Sgr B in detail at scales ranging from small clouds to star formation regions, allowing the scientists to understand the dynamics of gas within our galactic center. UpGREAT’s rapid imaging capabilities and detailed velocity resolution were crucial for enabling the study, which is part of a much larger scan of the area.

Among a number of findings, astronomers noted the steady carbon emission from Sgr B implies the entire region is physically connected, making it one continuous structure spanning about 34 by 15 parsecs, or about 111 by 49 light-years. It is spatially complex, comprised of arcs and ridges undergoing large-scale, turbulent motion.

By comparing the brightness of different emission lines, the group obtained an estimate of the ratio of ionized carbon emission coming from regions dominated by ionized hydrogen compared to emission from photodissociation regions, which are created by far-UV photons from massive stars.

Notably, the three star-forming cores of Sagittarius B2, within Sgr B, exhibit no ionized carbon emission, which is atypical of extreme star forming regions. They appear to be within a dark, narrow lane of dust which appears to be slightly physically distanced and in front of the rest of the region – though they remain, for the most part, dynamically related. This may answer the debate about the origin of star formation in Sgr B — dark dust lanes have been associated with cloud-cloud collisions and are a common sign of a shock-induced star formation trigger. This possibility is also consistent with the fact that multiple star formation stages coexist within Sgr B, as a recent burst of star formation within Sgr B indicates some sort of trigger has likely occurred.

“The nuclear regions of galaxies are fascinating places, and our relatively nearby galactic center lets us explore its gas clouds, stars, and black hole in far more detail than we can get in any other galaxy,” said Andrew Harris, astronomer at the University of Maryland and lead author on the paper. “The SOFIA results we found in our US-German project join those made at wavelengths across the electromagnetic spectrum made from telescopes all over the world and in space, allowing us to better understand not only our galaxy but others as well.”

SOFIA is a joint project of NASA and the German Space Agency at DLR. DLR provides the telescope, scheduled aircraft maintenance, and other support for the mission. NASA’s Ames Research Center in California’s Silicon Valley manages the SOFIA program, science, and mission operations in cooperation with the Universities Space Research Association, headquartered in Columbia, Maryland, and the German SOFIA Institute at the University of Stuttgart. The aircraft is maintained and operated by NASA’s Armstrong Flight Research Center Building 703, in Palmdale, California.