Examining Star Formation in the Evolution of Elliptical Galaxies

by Max Watzky (V), Milenka Men (V), Inimai Subramanian1, Juliana Karp1

1Astronomy Camp, University of Arizona, Tucson, AZ

Abstract The predominant theory of galaxy evolution holds that elliptical galaxies are formed when two or more spiral galaxies merge with one another. We hypothesized that if elliptical galaxies do indeed form in this way, they would exhibit a low star formation rate (SFR) compared to other types of galaxies, as they had exhausted their reserves of star-forming gas in violent collisions. In order to test this hypothesis, we used a hydrogen-alpha (H-alpha) filter, which isolates light from the ionized hydrogen found in stellar nurseries, to image star-forming regions in multiple galaxies at different stages in the galaxy merger process. Although we are still in the process of drawing quantitative measurements from the data, we speculate that star formation in the sample elliptical galaxy is less pronounced than in the sample spiral and merging galaxies, indicating that it may have been formed in a galaxy merger. Introduction According to Hubble’s Galaxy Classification System, there are four major types of galaxies: spiral, barred spiral, elliptical, and irregular. Spiral and barred spiral galaxies like the Milky Way were formed by collapsed clouds of gas that clumped together into larger hierarchical structures. Many spiral galaxies exist in structures known as galaxy clusters, which consist of several galaxies gravitationally bound to one another. Within such clusters, spiral galaxies frequently collide and merge with one another. As galaxy mergers take place, high-density regions of interstellar gas collide, creating an environment of enhanced star formation. In these violent events, the merging galaxies burn through their reserves of star-forming gas, meaning that the resulting galaxies would then exhibit relatively little active star formation. Elliptical galaxies, which have well-established

Figure 3: Stephan’s Quintet in all visible light Figure 4: Stephan’s Quintet in H-alpha

low SFRs, match this description, leading some to believe that they are formed in galaxy mergers. Alternative hypotheses attribute ellipticals’ low SFR to ram pressure stripping and thermal evaporation, processes by which spiral galaxies in galaxy clusters and groups lose star-forming gas through interactions with the surrounding medium. The merger theory of elliptical evolution is supported by the fact that relatively few ellipticals are spotted billions of light years away, indicating that they had not yet had time to form via collision in the early universe. In an effort to confirm or refute this theory, we decided to compare the SFRs of spiral and merging galaxies to that of ellipticals, a proven technique which has been used by several prior studies to investigate elliptical evolution. Our comparison was performed by imaging samples of the different types of galaxies using a filter that isolates light in the H-alpha wavelength. Hydrogen-alpha is a spectral line emitted when electrons in hydrogen atoms fall from the third energy level to the second. This energy level jump is indicative of hydrogen ionization, which sometimes occurs when hydrogen is exposed to newly-formed stars. Thus, by imaging the galaxies with a H-alpha filter, we were able to detect star formation. Using this kind of filter, we took data from the spiral galaxy M106, a group of merging galaxies in Stephan’s Quintet, and the elliptical galaxy NGC 5813. In order to take baseline measurements of the galaxies’ luminosity, we also imaged M106, Stephan’s Quintet, and NGC 5813 with filters that allowed all light from the visible spectrum to pass through.

Results

In the full visible spectrum image of the spiral galaxy M106, a central nucleus, an inner disc containing two prominent spiral arms, and two faint outer arms are visible (Fig. 1). By comparison, only the central nucleus and two inner spiral arms are discernible in the H-alpha image (Fig. 2). In both the H-alpha image and the full visible spectrum image of Stephan’s Quintet (Fig. 3, 4), two galaxies with spiral arms, NGC 7318a and NGC 7318b, can be seen undergoing collision at the center. However, the spiral arms of another galaxy above and to the left of the colliding pair, NGC 7319, are only clearly visible in the full spectrum image. A fourth galaxy, NGC 7320, is visible below NGC 7319, but this galaxy is actually hundreds of millions of light-years away from the others, and not involved in the merger. In the full visible spectrum image of the elliptical galaxy NGC 5813, a bright core surrounded by a faint yellow disc can be seen (Fig 5.), but these features are considerably dimmer in the H-alpha image (Fig 6.). Discussion

Due to the use of various telescopes and instruments in this project, our images are of differing quality, meaning that reliably interpreting the data has been difficult, and the effort to produce quantitative measurements of

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star formation is still ongoing. Nonetheless, a few aspects of the results stand out to us. Only some of the features detected in the full visible spectrum image of the spiral galaxy M106 are defined in the H-alpha data (Fig. 1, 2), indicating that star formation is localized to certain regions of the galaxy. By contrast, the features of the colliding galaxies in Stephan’s Quintet, NGC 7318a and NGC 7318b, are well defined in both images (Fig. 3, 4), suggesting a higher SFR in the merging pair than in M106. The fuzzy disc and central core of the elliptical galaxy NGC 5813 are much dimmer in the H-alpha image than in the full visible spectrum image, which we speculate reflects a low SFR compared to the spiral and merging galaxies (Fig. 5,6). If correct, this observation indicates that NGC 5813 may be the result of a galaxy merger. However, because NGC 5813 is located in a small galaxy group, ram pressure stripping or thermal evaporation may have played a role in its low SFR, although the extent of these influences is unclear. All things considered, our results’ qualitative nature, the plausibility of alternative explanations for the data, and the small scope of our project make it difficult to draw a meaningful conclusion. We hope that future quantitative analysis of our data and the conduction of broader surveys will offer greater insight into the evolution of elliptical galaxies.

Methods

In order to produce the hydrogen-alpha images of M106, Stephan’s Quintet, and NGC 5813, we took between 10 and 20 two-minute exposures of each target using the 24” Mt. Lemmon SkyCenter telescope, a 16 megapixel CCD camera, and a narrowband H-alpha filter. The images of each galaxy were then stacked together in the Maxim DL astronomy image processing software. We also took images of NGC 5813 and M106 using the 32” Schulman telescope, a 16.78 megapixel CCD, and red, green, and blue filters, which we stacked together to create full color images of the galaxies, which contain light from the entire visible spectrum. The same effect was achieved for Stephan’s Quintet by taking an image using a clear filter, which also allows light from the entire visible spectrum to pass through.

Acknowledgements

Thank you to the University of Arizona’s Mt. Lemmon SkyCenter Observatory for access to the telescopes, as well as Don McCarthy and all camp counselors at the University of Arizona Advanced Astronomy Camp 2021 for their guidance.