Optical-near-IR analysis of globular clusters in the IKN dwarf spheroidal: a complex star formation history

¹ Argelander Institut für Astronomie der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: alex.tudorica@gmail.com
² Max-Plank-Institute für Astronomie, Königsthul 17, 691717 Heidelberg, Germany
³ Departamento de Astronomia, Instituto de Física, Universidade Federal do Rio Grande do Sul, Caixa Postal 15051, Porto Alegre, RS, Brazil
⁴ Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, São Paulo, SP, 05508-090, Brazil

Received: 4 January 2015
Accepted: 4 June 2015

Abstract

Context. Age, metallicity, and spatial distribution of globular clusters (GCs) provide a powerful tool for reconstructing major star-formation episodes in galaxies. IKN is a faint dwarf spheroidal (dSph) in the M 81 group of galaxies. It contains five old GCs, which makes it the galaxy with the highest known specific frequency (S_N = 126).

Aims. We estimate the photometric age, metallicity, and spatial distribution of the poorly studied IKN GCs. We search SDSS for GC candidates beyond the HST/ACS field of view, which covers half of IKN.

Methods. To break the age-metallicity degeneracy in the colour, we used WHT/LIRIS K_S-band photometry and derived photometric ages and metallicities by comparison with SSP models in the V,I,Ks colour space.

Results. IKN GCs’ VIKs colours are consistent with old ages (≥8 Gyr) and a metallicity distribution with a higher mean than is typical for such a dSph ([Fe/H] ≃ -1.4_-0.2^+0.6 dex). Their photometric mass range (0.5 < ℳ_GC< 4 × 10⁵ M_⊙) implies an unusually high mass ratio between GCs and field stars, of 10.6%. Mixture model analysis of the RGB field stars’ metallicity suggests that 72% of the stars may have formed together with the GCs. Using the most massive GC−SFR relation, we calculated a star formation rate (SFR) of ~10 M_⊙/yr during its formation epoch. We note that the more massive GCs are closer to the galaxy photometric centre. IKN GCs also appear spatially aligned along a line close to the major axis of the IKN and nearly orthogonal to the plane of spatial distribution of galaxies in the M 81 group. We identify one new IKN GC candidate based on colour and the PSF analysis of the SDSS data.

Conclusions. The evidence of i) broad and high metallicity distribution of the field IKN RGB stars and its GCs, ii) high fraction, and iii) spatial alignment of IKN GCs supports a scenario for tidally triggered, complex IKN’s star formation history in the context of interactions with galaxies in the M 81 group.