A Long-Slit Spectral Analysis of the Nuclear Region of M101

Abstract

Using long-slit spectra centered in wavelength about Hα, we have obtained a velocity map of a 20'' × 140'' east-west box centered on the nuclear region of M101. The velocity resolution is 2.5 km s-1, and the spatial resolution is approximately 1'' east-west and variable (3''–4'' on average) north-south. We have also obtained temperature, density, and shock maps of the same region using the [N II] λ6548, [N II] λ6583, Hα, [S II] λ6717, and [S II] λ6731 lines. The temperatures range from 4100 to 14,100 K, with an intensity-averaged temperature of 6200 K. The densities range from 6.6 to 36,600 cm-3, with an intensity-averaged density of 800 cm-3. Regions northeast and southwest of the nucleus have [N II] λ6583/Hα ratios 0.56 and [S II] λλ(6717 + 6731)/Hα ratios ≥0.4, which are indicative of shock-heated or power-law photoionized gas. The gas in an S-shaped region passing through the nucleus has [N II] λ6583/Hα ratios 0.56 and [S II] λλ(6717 + 6731)/Hα ratios of ≈0.1, which is representative of gas in H II regions. The velocity data are consistent with gas infalling toward the nucleus as part of streaming motions traveling west to east on the north side of the nucleus and east to west on the south side of the nucleus. However, there appears to be a local instability in which the motion southwest of the nucleus is moving west to east, creating a collision of the two opposing gas motions in a region of relatively high density and temperature. A blue arc emanating from an unresolved source on or near the nucleus shows evidence of collision in this same high-density, high-temperature region. Our data support a previous conclusion that M101 may be considered an intermediate galaxy with respect to activity, possibly in transition between a normal nucleus and a more active stage.