February 2-5, 2021

Oral Presentation

The Blue Spiral at the Center of NGC 1275, the Bright Cluster Galaxy of the Perseus Clusters of Galaxies: Yet Another Merging Satellite

Author(s): Michael Yeung (Department of Physics, The Unversity of Hong Kong), Youichi Ohyama (Academia Sinica, Institute of Astronomy and Astrophysics), Jeremy Lim (Department of Physics, The Unversity of Hong Kong)

Presenter: Michael Yeung (The University of Hong Kong)

We performed detailed spatially resolved optical spectral analysis of the stellar population and the velocity field of the blue several kpc-scale central region with spiral-like arm structures around the nucleus of NGC 1275, the brightest cluster galaxy of the Perseus cluster. We found that the velocity field shows significant structure, showing large (projected) maximum velocity (up to $\pm250$\,km\,s$^{-1}$), a decline of the velocity back to the systemic velocity over a few to several kpc scale from the nucleus, as well as its asymmetry around the nucleus.
We also measured spectral indices of higher-order Balmer lines (H$\eta$ and H$\zeta$) and metal absorption lines (Ca K, G4300, Mg2, and $\langle$Fe$\rangle$), and found that the blue spiral is distinguished by very deep Balmer and shallow Ca K absorption lines. We modelled the observed spectral indices based on single stellar population (SSP) and two-SSP composite models. Although the single SSP model over-predicts Ca K if the age is optimized for both deep H$\zeta$ and moderate Mg2 and $\langle$Fe$\rangle$, the composite model (0.15\,Gyr old post-starburst on top of the older 2.5\,Gyr old host galaxy with assumed Solar metallicity) can reproduce all spectral features simultaneously. We argue that this blue spiral is likely formed as a result of recent (within a few to several 0.1\,Gyrs) merging of a satellite galaxy and the resultant induced starburst because most of its distinct characteristics (peculiar velocity field, almost uniform blue color, short burst duration expected for very deep Balmer absorption, as well as its morphology showing narrow arms) are naturally explained. If this is the case, the blue spiral is the second confirmed infalling satellite in NGC 1275 system, after the well-studied one known as the high-velocity system.

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