George's Random Astronomical Object

Object 62: NGC 7457

Podcast release date: 13 December 2021

Right ascension: 23:00:59.9

Declination: +30:08:42

Epoch: J2000

Constellation: Pegasus

Corresponding Earth location: Just north of the Canary Islands in the Atlantic Ocean

Located at a distance of 39.4 million light years (12.1 Mpc) in the constellation Pegasus [1], NGC 7457 looks like a typical lenticular galaxy. Lenticular galaxies are a class of galaxies between elliptical and spiral galaxies that have a large spheroidal bulge of stars bisected by a smooth disk of stars. While NGC 7457 seems rather ordinary-looking, it's actually relatively close to Earth for a lenticular galaxy, which means that if astronomers want to understand lenticular galaxies in general, they may take a look at NGC 7457 first.

In fact, NGC 7457 was one of the very first things looked at by the Hubble Space Telescope. The galaxy was observed as part of science assessment observations that were performed within the first year of the telescope's operations, which started in 1990 [2]. This is so long ago that Czechoslovakia was still a country, and The Simpsons was still a funny TV show. It's also before the Hubble Space Telescope's optics were repaired to correct a critical defect in the primary mirror, so the images of NGC 7457 were relatively blurry.

Nonetheless, astronomers could use the Hubble Space Telescope images to study the center of NGC 7457. In the decades leading up to the launch of the Hubble Space Telescope, astronomers who studied nearby galaxies had been rather interested in understanding the central structures of these galaxies. In many nearby elliptical and lenticular galaxies, astronomers often found that the centers of the galaxies look brighter than what you would expect based on how the brightness of the stars varied with radius outside the central regions. The extra-bright central regions are called either cores or cusps. A very dense core of stars was indeed found in NGC 7457, although it was difficult to say exactly how large the core was because of the blurriness of the Hubble Space Telescope images [2]. Still, this was a useful demonstration of the telescope's capabilities, especially since no other telescope in existence at the time would have been able to even detect the presence of the core in this galaxy.

Having said all of this, the results from these relatively old Hubble Space Telescope observations of NGC 7457 are only a part of what makes this galaxy so interesting. Additional observations of the center of NGC 7457 were performed with a couple of different ground-based telescopes in 1999. Even though these telescopes could not image the dense stellar core of NGC 7457 like the out-of-focus Hubble Space Telescope could, they could still show that the stars in the very center were only about 2 to 2.5 billion years old, whereas the stars in the rest of galaxy's bulge were several billion years older [3]. They also demonstrated that the stars in the central region orbited the galaxy in the opposite direction as the rest of the galaxy's stars [3].

Meanwhile, astronomers had also become interested in the globular clusters orbiting NGC 7457, and a series of observations with both ground-based telescopes and the Hubble Space Telescope (with corrected optics) were performed in the late 1990s and early 2000s. Globular clusters are spheres of hundreds of thousands or million stars that are found within the halo of our galaxy and the haloes of other galaxies. Typically, the stars in globular clusters are very old. The globular clusters in our galaxy are generally more than 10 billion years old. In NGC 7457, however, some of the globular clusters looks much younger. In fact, many of the globular clusters in NGC 7457 seemed like they could be somewhere between 2 and 3 billion years old [4,5,6].

All of this evidence pointed to some sort of major event that took place between 2 and 3 billion years ago in NGC 7457, which would have been around about 2 to 3 billion years before the Hubble Space Telescope's optics were repaired in 1993 as well as 2 to 3 billion years before either Bjork released her first album, X-Files appeared on television, or Bill Clinton became president of the United States. It looks like two disk-like spiral galaxies with different sizes collided with each other and merged to form the lenticular galaxy that we see today [6,7]. Before this event, the stars in the two galaxies may have orbited their centers in nice circular orbits with the disks of each galaxy, but the collision would have scrambled the orbits of the stars so that they would instead move in random directions within the spheroidal bulge of the resulting lenticular galaxy. Tidal forces would have caused interstellar gas clouds in both galaxies to fall into the new galaxy's center, where the gas would have been compressed and would have formed new stars. Some of the gas from the smaller infalling galaxy probably fell into the center of the new lenticular galaxy in such a way that they ended up orbiting in the wrong direction, and this gas would have formed the stars that we now see orbiting the opposite direction from everything else. Additionally, some of the stars that formed would go on to become the relatively young globular clusters that we see today.

Astronomers actually think that a lot of other lenticular galaxies were also originally spiral galaxies that merged with smaller galaxies. As with NGC 7457, this process would have increased the size of the galaxies' bulges and formed a lot of new stars while depleting the galaxies of their interstellar gas, and the resulting galaxies would have large bulges and smooth disks of stars. Although lenticular galaxies are found all over the place, the fact that NGC 7457 is relatively close to Earth makes it a good object for understanding how galaxy collisions from a long time ago affected the galaxies that we see today.

References

[1] Tully, R. Brent et al., Cosmicflows-2: The Data, 2013, Astronomical Journal, 146, 86

[2] Lauer, Tod R. et al., The Core of the Nearby S0 Galaxy NGC 7457 Imaged with the HST Planetary Camera, 1991, Astrophysical Journal Letters, 369, L41

[3] Sil'chenko, O. K. et al., Young Stellar Nuclei in Lenticular Galaxies: NGC 5574 and NGC 7457, 2002, Astrophysical Journal, 577, 668

[4] Chapelon, S. et al., The globular cluster system around the low-luminosity S0 galaxy NGC 7457, 1999, Astronomy & Astrophysics, 346, 721

[5] Chomiuk, Laura et al., The Peculiar Globular Cluster System of the s0 Galaxy NGC 7457, 2008, Astronomical Journal, 136, 234

[6] Hargis, Jonathan R. et al., The Globular Cluster Population of NGC 7457: Clues to the Evolution of Field S0 Galaxies, 2011, Astrophysical Journal, 738, 113

[7] Molaeinezhad, A. et al., NGC 7457: evidence for merger-driven cylindrical rotation in disc galaxies, 2019, Monthly Notices of the Royal Astronomical Society, 488, 1012

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Last update: 25 April 2022