News and Views on "New estimates of scale heights and spiral structures for non-edge-on spiral galaxies"

(News & views on the paper by Hu & Peng, RAA, 2014, vol.14, 869-874)

Xi-Wei Liu

(Department of Physics, Huazhong Agricultural University)

Disk galactic diameters are far larger than their scale heights. Thus the scale height is usually omitted in studies and the galactic disk is assumed to be a 2-dimensional plane. Whether this assumption about 2-dimensions is good or not has been controversial for many years (Binney & Tremaine 1987; Peng 1988). The scale height is a key parameter for determining a model describing the 3-dimensional structure of spiral galaxies. Estimating the scale height would shed light on understanding the dynamics and evolution of galaxies. An estimate of the scale height of edge-on spiral galaxies has been well established (van der Kruit & Searle 1981a, 1981b; 1982a, 1982b). However, it has been challenging to measure the scale height of non-edge-on spiral galaxies for many years. Peng (1988) presented a mathematical method for determining the scale heights of non-edge-on spiral galaxies. This is the only method for determining scale heights of non-edge-on spiral galaxies until now.

Peng’s method is based on measuring parameters of spiral structure. The forbidden radius (i.e. spiral arms could not appear in the region r0, where the density wave could not exist) is one of the required parameters for determining scale heights in Peng’s method. However, the forbidden radius is hard to exactly determine due to the blurring of bulge brightness. This work improves estimates of scale heights by subtracting the surface-brightness distributions from images of the observed non-edge-on spiral galaxies. Two samples, i.e. PGC 24996 and M31, have been investigated using the improved method. The scale height of M31 obtained in this study is in good agreement with previous works.

References

  • Binney, J., Tremaine, S., 1987, Galactic Dynamics, Princeton University Press ADS
  • Peng Q. H., 1988, A&A, 206, 18 ADS
  • van der Kruit, P. C., Searle, L., 1981a, A&A, 95, 105 ADS
  • van der Kruit, P. C., Searle, L., 1981b, A&A, 95, 116 ADS
  • van der Kruit, P. C., Searle, L., 1982a, A&A, 110, 61 ADS
  • van der Kruit, P. C., Searle, L., 1982b, A&A, 110, 79 ADS