The poststarburst galactic nucleus of NGC 4736

Yoshiaki Taniguchi, Youichi Ohyama, Toru Yamada, Hideaki Mouri, Michitoshi Yoshida

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24 Citations (Scopus)


We present optical spectroscopic analysis of the nuclear region of the nearby Sab galaxy NGC 4736 in which the strong Balmer absorption lines are observed, providing evidence for a past starburst event. In order to study both the present-day stellar populations and the emission-line activity in the nucleus, we have made the stellar population synthesis and found that a luminous starburst occurred 1 × 109 years ago in the nuclear region. Correcting for the effect of the underlying stellar absorption features, we find that the ionized gas in the nuclear region shows the very marginal nature between an ordinary H II region and a low-ionization nuclear emission-line region (LINER). The absorption-corrected Hα luminosity amounts only to 1.3 × 1038 ergs s-1, which is lower than that of the Galactic H II region W49. If we consider a case of the photoionization by massive stars, only six O6 stars are responsible for the ionization in the central 37 pc × 60 pc region. We also study the emission-line ratio properties of the classical LINERs in the literature. The LINERs tend to show a sequence in the excitation diagram: the positive correlation between log [O III] λ5007/Hβ and log [N II] λ6583/Hα. This correlation may be attributed to a scatter of the relative strength of stellar Balmer absorption to nebular Balmer emission among the sample. Therefore, we call attention to the possibility that some LINERs may in fact be post-starburst galaxies rather than genuine active galactic nuclei.

Original languageEnglish
Pages (from-to)215-220
Number of pages6
JournalAstrophysical Journal
Issue number1 PART I
Publication statusPublished - 1996


  • Galaxies: individual (NGC 4736)
  • Galaxies: nuclei
  • Galaxies: starburst
  • Galaxies: stellar content

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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