Deciphering a Rosetta stone: The nebula MA 1 in M33

Donald R. Garnett, Stephen C. Odewahn, Evan D. Skillman

Research output: Contribution to journalArticlepeer-review

11 Scopus citations


We present a new longslit CCD spectrum and Hα imaging of the nebula MA 1 in the Local Group spiral galaxy M33, one of McCall's four "Rosetta Stone" extragalactic H II regions. Our Hα image shows MA 1 to be a complex of arcs, loops, and filaments having a diameter of about 45 arcsec. Our spectrum does not confirm McCall's suggestion that MA 1 is a matter-bounded nebula; rather, the spatial profile of the emission lines shows that [O II] is quite weak near the central stellar object, but becomes more nearly normal in strength in the outer parts of the nebula. The integrated line intensities we obtain for MA 1 are consistent with those observed in relatively metal-poor H II regions found in dwarf galaxies. A tentative detection of [O III] λ4363 yields a rough estimate of 14 000 K for the electron temperature; if this result holds, MA 1 has an oxygen abundance of approximately one-tenth the solar value, which is consistent with that expected from extrapolation of the known abundance gradient in M33. This makes MA 1 one of the most oxygen-poor H II regions known within a spiral galaxy. The central stellar object in the nebula shows He II λ4686 emission and Balmer absorption lines, but also Ca H and K absorption lines. The origin of the Ca lines is uncertain, but may be due to interstellar absorption.

Original languageEnglish (US)
Pages (from-to)1714-1720
Number of pages7
JournalAstronomical Journal
Issue number5
StatePublished - Nov 1992


Dive into the research topics of 'Deciphering a Rosetta stone: The nebula MA 1 in M33'. Together they form a unique fingerprint.

Cite this