Investigating the Galactic Evolution of M101 with Deep Narrowband Imaging

Image credit: AstroBackyard


In this thesis, I present a series of in-depth studies on the nearby spiral galaxy M101 and its group environment. The M101 Group is a dynamic group, and thus it contains features both secular and tidal in origin: M101 is believed to have undergone an interaction with its most massive satellite NGC 5474 approximately 300 Myr ago. Each study utilized deep, wide-field, narrowband imaging from the Burrell Schmidt telescope targeting the emission lines of Hα, Hβ, [OIII]λλ4959,5007, and [OII]λλ3726,3729.

Searching for collisional evidence of this interaction, I present a study of the group environment of M101 and its satellites. I find that there is no large population of outlying, intragroup HII regions down to extremely low Hα flux levels. Only two sources, one associated with NGC 5474 and another associated with the background galaxy NGC 5486 were found. The former is likely an outlying HII region, while the latter is likely a background dwarf galaxy.

Turning towards the disks of the M101 Group galaxies, I analyze the oxygen abundances of M101 and its two primary satellites NGC 5477 and NGC 5474. M101 shows a strong abundance gradient, while the two satellites present little or no gradient. There is some evidence of a flat gradient in M101 beyond R~15 kpc as well as azimuthal abundance variations to the west and southwest. These are likely caused by the tidal interaction in combination with the internal dynamical effects of the corotation barrier.

Finally, using absorption signatures in the same narrowband images, I present constraints on the stellar ages of M101’s disk and interpret these in the context of spiral wave patterns. In the inner disk, I find that stellar ages get progressively older with distance through a spiral arm, consistent with a quasi-steady spiral pattern. In the outer disk, there is a significantly young stellar population, likely the outcome of the tidal interaction with NGC 5474.

In total, this thesis shows that weak tidal interactions in low-density groups have sizable impacts on the galaxies involved. Additionally, this thesis presents several new techniques for the analysis of narrowband images on par with spectroscopic information.

PhD Thesis
Ray Garner
Ray Garner
TAMU Astronomy Postdoctoral Researcher

I’m a scientist, Star Wars fan, and amateur photographer raised in Georgia. My research interests include galaxy evolution, star formation, satellite galaxies, and nebular diagnostics.