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Fumi Egusa, Yoshiaki Sofue, Hiroyuki Nakanishi, Offsets between Hα and CO Arms of a Spiral Galaxy, NGC 4254: A New Method for Determining the Pattern Speed of Spiral Galaxies, Publications of the Astronomical Society of Japan, Volume 56, Issue 6, 25 December 2004, Pages L45–L48, https://doi.org/10.1093/pasj/56.6.L45
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Abstract
We examined the offsets between H II regions and molecular clouds belonging to spiral arms of a late-type spiral galaxy, NGC 4254 (M99). We used a high-resolution |${{12\atop} \mathrm{CO}}$| (|$J = 1 \hbox{--} 0$|) image obtained by Nobeyama Millimeter Array (NMA) and an |$\mathrm{H}\alpha$| image. We derived angular offsets (|$\theta$|) in the galactic disk, and found that they show a linear dependence on the angular rotation velocity of the gas (|$\Omega_{\mathrm{G}}$|). This linear relation can be expressed by the equation |$\theta = (\Omega_{\mathrm{G}}-\Omega_{\mathrm{P}}) \cdot t_{\mathrm{H}\alpha}$|, where |$\Omega_{\mathrm{P}}$| and |$t_{\rm H\alpha}$| are constant. Here, |$\Omega_{\mathrm{P}}$| corresponds to the pattern speed of the spiral arms and |$t_{\mathrm{H}\alpha}$| is interpreted as being the timescale between the peak compression of the molecular gas in the spiral arms and the peak of massive star formation. We could thus determine |$\Omega_{\mathrm{P}}$| and |$t_{\mathrm{H}\alpha}$| simultaneously by fitting a line to our |$\theta \hbox{--} \Omega_{\mathrm{G}}$| plot, if we assume they are constant. From the plot for NGC4254, we obtained |$t_{\mathrm{H}\alpha} = (4.8 \pm 1.2) \times 10^6 \,\mathrm{yr}$| and |$\Omega_{\mathrm{P}} = 26^{+10}_{-6} \,\mathrm{km} \,\mathrm{s}^{-1} \,\mathrm{kpc}^{-1}$|, which are consistent with previous studies. We suggest that this |$\theta \hbox{--} \Omega_{\mathrm{G}}$| plot can be a new tool to determine the pattern speed and the typical timescale needed for star formations.
