The ionized gas in the central region of NGC 5253

2D mapping of the physical and chemical properties^⋆

¹ Instituto de Astrofísica de Andalucía (CSIC), C/ Camino Bajo de Huétor, 50, 18008 Granada, Spain
e-mail: ami@iaa.es
² European Southern Observatory, Karl-Schwarzschild Strasse 2, 85748 Garching bei München, Germany

Received: 4 May 2012
Accepted: 8 June 2012

Abstract

Context. Blue compact dwarf (BCD) galaxies constitute the ideal laboratories to test the interplay between massive star formation and the surrounding gas. As one of the nearest BCD galaxies, NGC 5253 was previously studied with the aim to elucidate in detail the starburst interaction processes. Some open issues regarding the properties of its ionized gas still remain to be addressed.

Aims. The 2D structure of the main physical and chemical properties of the ionized gas in the core of NGC 5253 has been studied.

Methods. Optical integral field spectroscopy (IFS) data has been obtained with FLAMES Argus and lower resolution gratings of the Giraffe spectrograph.

Results. We derived 2D maps for different tracers of electron density (n_e), electron temperature (T_e) and ionization degree. The maps for n_e as traced by [O ii], [S ii], [Fe iii], and [Ar iv]  line ratios are compatible with a 3D stratified view of the nebula with the highest n_e in the innermost layers and a decrease of n_e outwards. 2D maps of T_e were measured from [O iii] and [S ii] line ratios; to our knowledge, this is the first time that a T_e map based on [S ii] lines for an extragalactic object has been presented. The joint interpretation of the T_e([S ii]) and T_e([O iii]) maps is consistent with a T_e structure in 3D with higher temperatures close to the main ionizing source surrounded by a colder and more diffuse component. The highest ionization degree is found at the peak of emission for the gas with relatively high ionization in the main Giant H ii Region and lower ionization degree delineating the more extended diffuse component. We derived abundances of oxygen, neon, argon, and nitrogen. Abundances for O, Ne and Ar are constant over the mapped area within  ≲0.1 dex. The mean 12 + log (O/H) is 8.26 ± 0.04 while the relative abundances of log (N/O), log (Ne/O) and log (Ar/O) were  ~−1.32 ± 0.05,  −0.65 ± 0.03 and  −2.33 ± 0.06, respectively. There are two locations with enhanced N/O. The first (log (N/O) ~  −0.95) occupies an area of about 80 pc  ×  35 pc and is associated to two super star clusters. The second (log (N/O) ~  −1.17), reported here for the first time, is associated to two moderately massive (2–4 × 10⁴   M_⊙) and relatively old (~10 Myr) clusters. A comparison of the N/O map with those produced by strong line methods supports the use of N2O2 over N2S2 in the search for chemical inhomogeneities within a galaxy. The results on the localized nitrogen enhancement were used to compile and discuss the factors that affect the complex relationship between Wolf-Rayet stars and N/O excess.