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

Abstract

Context. Age, metallicity, and spatial distribution of globular clusters (GCs) provide a powerful tool for reconstructing major starformation 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 V −I colour, we used WHT/LIRIS K S -band photometry and derived photometric ages and metallicities by comparison with SSP models in the V, I, K s colour space. Results. IKN GCs' V IK s 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 +0.6 −0.2 dex). Their photometric mass range (0.5 < M GC < 4 × 10 5 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.

Stellar systems in the direction of the Hickson Compact Group 44

“…In the latter case, they would show an assymetric distribution, similar to those found in other dSph galaxies (e.g. Tudorica et al 2015;). …”

“…However, if dSph1 and dSph2 were low-mass galaxies at the distances of NGC 3193 and/or HCG 44, it would not be expected a high number of GCs belonging to them. If the unresolved objects were indeed GCs, and dSph1 and dSph2 were at such distances, dSph1 might contain massive nuclear star clusters like those reported by Georgiev et al (2009) in several dSph galaxies, and the distribution of the GCs of dSph2 would be assymetric like that shown by the IKN dwarf in the M 81 group (Tudorica et al 2015). …”

KDG218, a nearby ultra-diffuse galaxy

“…′ 9 or 8.6 kpc. Georgiev et al [26] and Tudorica et al [27] have detected six globular clusters in IKN, the half of which are concentrated in a 3.4 kpc-diameter circle. Taking these data into consideration and using the photometric profile of IKN on the southern side of the galaxy from the images obtained with the 6-m telescope [28] we estimated the effective diameter of IKN equal to 3.15 kpc.…”

“…Substitution of the photographic plates on CCD detectors resulted in detection of an enormous number of new extremely low-surfacebrightness dwarf galaxies in the range of SB ≃ [25][26][27] m /⊓ ⊔ ′′ . The typical average surface brightness in the nearest dwarf systems resolved into stars reaches approximately [28][29][30] m /⊓ ⊔ ′′ .…”

Massive Star Clusters in Dwarf Galaxies

“…The total integrated luminosity of the GCs corresponds to M V = −9.3, so the GCs then account for about 12% of the total luminosity of IKN. From photometry of RGB stars, the field star metallicity distribution appears similar to that in Fornax, with a broad peak around [Fe/H] ∼ −1 and only a few stars below [Fe/H] ∼ −2 (Lianou et al 2010;Tudorica et al 2015), so the ratio of metal-poor GC stars vs. field stars at low metallicities in IKN is likely to be even higher than in Fornax. It is possible, however, that the luminosity of the IKN galaxy has been significantly underestimated as deep images show it to be very extended (Okamoto et al 2015).…”