Featured Image: A Survey of Disk Galaxy Halos

The 12 galaxies in the sample, ordered from high to low stellar mass. Click for high-resolution version.
Credit: Gilhuly et al. 2022

Hi-res Image

Studying galaxy halos is key to understanding how galaxies form and evolve. These diffuse, extended regions contain clues to a galaxy’s past interactions, such as elongated streams of stars that mark the capture of globular clusters or satellite galaxies. However, because halos are faint and can spread a great distance beyond the luminous disk of a galaxy, observing them can be challenging. A team led by Colleen Gilhuly (University of Toronto, Canada) used the Dragonfly Telephoto Array to survey a dozen nearby edge-on galaxies, pictured above and to the right, and measure the starlight coming from each galaxy’s halo — and, by extension, estimate the mass of the halo stars. Gilhuly and collaborators found that the stellar halo mass fractions (the mass of stars in the halo compared to the mass of stars in the galaxy as a whole) varied widely among the galaxies in their sample, but the overall mass of stars in these galaxies was correlated with the masses of their stellar halos. To learn more about this survey of nearby galaxies, be sure to check out the full article below!

By Kerry Hensley

Citation

“Stellar Halos from the Dragonfly Edge-on Galaxies Survey,” Colleen Gilhuly et al 2022 ApJ 932 44. doi:10.3847/1538-4357/ac6750

Source:American Astronomical Society - AAS/Nova

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Glowing gas and dark dust in a side-on spiral

NGC 4634
Credit: ESA/Hubble & NASA

The NASA/ESA Hubble Space Telescope has produced a sharp image of NGC 4634, a spiral galaxy seen exactly side-on. Its disc is slightly warped by ongoing interactions with a nearby galaxy, and it is crisscrossed by clearly defined dust lanes and bright nebulae.

NGC 4634, which lies around 70 million light-years from Earth in the constellation of Coma Berenices, is one of a pair of interacting galaxies. Its neighbour, NGC 4633, lies just outside the upper right corner of the frame, and is visible in wide-field views of the galaxy. While it may be out of sight, it is not out of mind: its subtle effects on NGC 4634 are easy to see to a well-trained eye.

Gravitational interactions pull the neat spiral forms of galaxies out of shape as they get closer to each other, and the disruption to gas clouds triggers vigorous episodes of star formation. While this galaxy’s spiral pattern is not directly visible thanks to our side-on perspective, its disc is slightly warped, and there is clear evidence of star formation.

Along the full length of the galaxy, and scattered around parts of its halo, are bright pink nebulae. Similar to the Orion Nebula in the Milky Way, these are clouds of gas that are gradually coalescing into stars. The powerful radiation from the stars excites the gas and makes it light up, much like a fluorescent sign. The large number of these star formation regions is a telltale sign of gravitational interaction.

The dark filamentary structures that are scattered along the length of the galaxy are caused by cold interstellar dust blocking some of the starlight.

Hubble’s image is a combination of exposures in visible light produced by Hubble’s Advanced Camera for Surveys and the Wide Field and Planetary Camera 2.

Source:  ESA/Hubble - Space Telescope