Star formation in gaseous galaxy halos

Free access article

Issue		A&A Volume 412, Number 1, December II 2003


Page(s)		69 - 80
Section		Galactic structure and dynamics
DOI		http://dx.doi.org/10.1051/0004-6361:20031521

A&A 412, 69-80 (2003)
DOI: 10.1051/0004-6361:20031521

VLT-spectroscopy of extraplanar H II-regions in NGC 55

R. Tüllmann¹, M. R. Rosa², T. Elwert¹, D. J. Bomans¹, A. M. N. Ferguson³ and R.-J. Dettmar¹

¹ Astronomisches Institut, Ruhr-Universität Bochum, Universitätsstr. 150, 44780 Bochum, Germany
² Space Telescope European Coordinating Facility, c/o European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
³ Max-Planck Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany

(Received 4 March 2003 / Accepted 22 September 2003 )

Abstract
We present first deep VLT-spectra of a new class of compact extraplanar objects located in the disk-halo interface of the edge-on spiral galaxy NGC 55. Their spectra reveal continuum emission from stars and show typical emission-lines as observed in ordinary disk $\ion{H}{ii}$ -regions. With respect to emission-line fluxes, these spectra are very similar to those obtained for the diffuse ionized gas (DIG), except [ $\ion{O}{iii}$ ] $\lambda 5007$ which is strongly decreased by more than a factor of 3. Similar to the DIG the prominent ionization stage of oxygen is O ⁺, whereas the corresponding one for low metallicity $\ion{H}{ii}$ -regions is O ⁺⁺. A comparison with CLOUDY model simulations reveals that the ionization mechanism of these compact objects is most likely photoionization by late OB stars (O9.5 to B0). Further analysis of diagnostic diagrams unambiguously confirms the $\ion{H}{ii}$ -region character. This raises the question whether these extraplanar $\ion{H}{ii}$ -regions (EHRs) originated from the prominent extraplanar gas of this galaxy or have just been expelled from the disk into the halo. From hydrodynamical considerations ejection from the disk can be ruled out. Therefore, these objects must have formed within the halo. Compared to the average abundance of the central disk $\ion{H}{ii}$ -region ( $45\%\,Z_{\odot}$ ) both EHRs reveal substantially lower [O/H] abundances of about $10\%\,Z_{\odot}$ . We could establish for the first time strong differences in the metal content along the minor axis of this galaxy. Oxygen appears to be less abundant in the halo by about a factor of 4. Since both EHRs are located above the central part of NGC 55, it appears likely that their formation was triggered by star formation activity in the disk below. In this environment the molecular gas clouds out of which EHRs have formed can survive and collapse only in the period between two successive bursts of star formation.

Key words: stars: formation -- ISM: abundances -- $\ion{H}{ii}$ -regions -- galaxies: evolution -- galaxies: individual: NGC 55

Offprint request: R. Tüllmann, tullmann@astro.rub.de

SIMBAD Objects

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.