Mapping the starburst in blue compact dwarf galaxies^*

PMAS integral field spectroscopy of Mrk 1418

¹ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: [luzma;pweilbacher;mmroth]@aip.de
² Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Canary Islands, Spain e-mail: [nicola.caon;czurita]@iac.es
³ University of Michigan Department of Astronomy 830 Dennison Bldg, 500 Church St, Ann Arbor, MI 48109-1042, USA e-mail: kehrig@umich.edu

Received: 23 December 2008
Accepted: 5 August 2009

Abstract

Aims. By means of optical integral field spectroscopy observations, we aim to differentiate and characterize the starburst component in the blue compact dwarf (BCD) galaxy Mrk 1418. In particular we propose to study the stellar and ionized gas morphology, to investigate the ionization mechanism(s) acting in the interstellar medium, and to derive the physical parameters and abundances of the ionized gas.

Methods. Integral field spectroscopy observations of Mrk 1418 were carried out with the Potsdam multi-aperture spectrophotometer (PMAS) at the 3.5 m telescope at Calar Alto Observatory. The central 16″16″ (1.14  1.14 kpc² at the distance of Mrk 1418) were mapped with a spatial sampling of 1″; we took data in the 3590-6996 Å spectral range, with a linear dispersion of 3.2 Å per pixel. The seeing was about 15. From these data we built maps of the most prominent emission lines, namely [O ii], Hβ, [O iii], Hα, [N ii], and [S ii], as well as of several continuum bands, plus maps of the main line ratios: [O iii]/Hβ, [N ii]/Hα, [S ii]/Hα, and Hα/Hβ, and derived the physical parameters and gaseous metal abundances of the different star-forming regions detected in the field of view.

Results. Mrk 1418 shows a distorted morphology both in the continuum and in the ionized gas maps; the current star- formation episode is taking place in five knots, distributed around the nucleus of the galaxy. The interstellar medium surrounding these knots is photo-ionized by stars, with no clear evidence for other excitation mechanisms. The galaxy displays an inhomogeneous dust distribution, with the high Hα/Hβ ratio in the central areas indicating a large amount of dust. The oxygen abundances derived for the individual star-forming knots are very similar, suggesting that the ionized interstellar medium is chemically homogeneous in O/H over spatial scales of hundreds of parsecs. This abundance (Z ≈ 0.4  from the empirical calibrations) places Mrk 1418 among the high metallicity BCD group.

Conclusions. These findings show the advantages of IFS when investigating the properties of such complex objects as BCDs, with an asymmetric star-forming component. Only a bidimensional mapping of their central regions allows approaching such questions as the star formation processes in BCDs, the star-forming history of the individual starburst knots, or the abundance gradients.