Eight Blue E+A Galaxy Candidates Located inside a Large-scale Filament in the Coma Cluster

E+A galaxies are post-starburst galaxies that have undergone complete quenching of their star formation within the last ∼1 Gyr. This makes them a valuable source for studying the evolution of galaxies. We searched for E+A galaxies within the Coma cluster of galaxies (Abell 1656) by sampling a five-degree radius conic search of the Sloan Digital Sky Survey originating from the center of the cluster (J2000.0 R.A.: 194.96, decl.: 28.0). We further filtered our data set for background and foreground objects by only considering objects within z = 0.01 of the redshift of the cluster (z = 0.0231).

We then individually classified the 1,520 E+A galaxy candidates returned by our search, using the method described in Falcone (2020), and following E+A criteria of Greene et al. (2021). More specifically, a galaxy was determined to be an E+A candidate based on its intermediate optical color, strong Balmer absorption lines, strong D_n 4000, a bluer slope in the 4700–8000 Å range, and a lack of emission lines that would indicate ongoing star formation. We further classified E+A galaxies by their color, and sorted them into "green E+A candidate" and "blue E+A candidate" categories. Blue E+As are younger, bluer galaxies with strong Balmer absorption equivalent widths greater than 3 Å, while green E+As have an intermediate "green" color and have Balmer equivalent widths from 2–3 Å. Green E+A galaxy candidates have a u-r color range of 1.35–2.64, and blue E+A galaxy candidates have a u-r color range of 1.40–2.20. We call our E+A galaxies "candidates" because our method uses data from single-fiber spectra from central 3'' region of each galaxy. Greene et al. (2021) has shown that around 30% of galaxies with a single-fiber E+A spectrum are not fully quenched, and further analysis with integral field spectra often reveals regions with active star formation.

Of the 1520 SDSS galaxies in and around the Coma Cluster, we identified 117 green E+A candidates and 13 blue E+A candidates. E+A candidates consist of 8.6% of our galaxy sample, much higher than the typical prevalence in the field (Zabludoff et al. 1996).

We analyzed the spatial distribution of the galaxies by constructing a 3D virtual reality map of the Coma cluster (Liu & Liu 2021, in preparation) using R.A., decl., and redshift (velocity) as the axes. Examining the map, we observed that eight of the E+A galaxy candidates in the Coma cluster were located in a filament extending outward from the cluster center. This filament extends for eight degrees across the sky, from R.A. = 194°–202°, and has a redshift of z = 0.0234 ± 0.000408, as mapped by Malavasi et al. (2020). Figure 1 depicts the position of the E+A galaxy candidates over an X-ray map of the Coma Cluster, as mapped by Briel et al. (2000), and two examples of E+A galaxy candidates and their spectra for reference.

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Two of the blue E+A candidates in the filament appeared to have high H-gamma and H-delta Balmer equivalent widths, ranging from 6.3 to 5.9 Å (Hγ), and 6.9 to 6.7 Å (Hδ). These deeper Balmer lines imply that these galaxies have a very large number of A and B stars in their stellar populations.

The location of these blue E+A galaxy candidates in the filament suggests that there is likely a large number of blue E+A galaxy candidates elsewhere along the filament. Their location also provides a clue to how they were formed. E+A galaxies are generally thought to be formed through mergers, although ram pressure stripping is also a proposed mechanism. The dynamical activity expected of galaxies in a filament would imply that these galaxies were ram pressure stripped. However, the large number of blue E+A galaxy candidates in this filament could be attributed to mergers, as detailed by Bekki et al. (2005). This would simply be because of the relatively higher density of galaxies in filaments. We have not yet ruled out the possibility that these galaxies are formed through mergers.

Vulcani et al. (2020) suggested that E+A galaxies can also be formed when star formation has been quenched entirely in the galactic disk but is still in the process of being quenched in the nucleus. This model could explain the observed bimodality in stellar population age found in E+A galaxies. The blue E+A galaxy candidates that we observed seemed to be more distant from the nucleus of the cluster than the older, green E+A galaxy candidates. This could be because as galaxies fall in, they have gas stripped away from them through ram pressure, leading to the beginning of quenching in these galaxies. This could also explain why there are more green E+A galaxy candidates closer to the cluster center.

In the future, we plan to analyze these galaxies further to better understand how these galaxies were quenched, and why there is such a large number of blue E+A galaxy candidates in the filament. Unlocking these answers could help us further understand quenching mechanisms in the Coma Cluster.

We thank the other members of the Y/Dim Collaboration for their valuable assistance and discussions: Julia Falcone, Spencer Greenfield, Maya Joyce, Allen Liu, Mariarosa Marinelli, Rafid Quayum, and Rosemary Williams. This work was supported by the Alfred P. Sloan Foundation via the SDSS-IV Faculty and Student Team (FaST) initiative, ARC Agreement SSP483, and by NSF grants AST-1852355, 1852360, 1460939, and 1460860 to the American Museum of Natural History and CUNY College of Staten Island.