The effect of spatial resolution on optical and near-IR studies of stellar clusters: implications for the origin of the red excess

Abstract

Recent ground based near-IR studies of stellar clusters in nearby galaxies have suggested that young clusters remain embedded for 7 − 10 Myr in their progenitor molecular cloud, in conflict with optical based studies which find that clusters are exposed after 1 − 3 Myr. Here, we investigate the role that spatial resolution plays in this apparent conflict. We use a recent catalogue of young (< 10 Myr) massive (> 5000 M ⊙ ) clusters in the nearby spiral galaxy, M83, along with Hubble Space Telescope (HST) imaging in the optical and near-IR, and ground based near-IR imaging, to see how the colours (and hence estimated properties such as age and extinction) are affected by the aperture size employed, in order to simulate studies of differing resolution. We find that the near-IR is heavily affected by the resolution, and when aperture sizes > 40 pc are used, all young/blue clusters move red-ward in colour space, which results in their appearance as heavily extincted clusters. However, this is due to contamination from nearby sources and nebular emission, and is not an extinction effect. Optical colours are much less affected by resolution. Due to the larger affect of contamination in the near-IR, we find that, in some cases, clusters will appear to show near-IR excess when large (> 20 pc) apertures are used. Our results explain why few young (< 6 Myr), low extinction (A V < 1 mag) clusters have been found in recent ground based near-IR studies of cluster populations, while many such clusters have been found in higher resolution HST based studies. Additionally, resolution effects appear to (at least partially) explain the origin of the near-IR excess that has been found in a number of extragalactic YMCs.

Studying the YMC population of M83: how long clusters remain embedded, their interaction with the ISM and implications for GC formation theories

“…If the clusters are exposed but there is still Hα emission within the aperture from the nearby gas, a fit using Hα may still be more accurate than without. Bastian et al (2014a) and Bastian, Hollyhead & Cabrera-Ziri (2014b) found a similar effect in the nebular continuum in the near-IR when studying YMCs.…”

“…These clusters should be visible in the near-IR, however, studies that attempted to use IR data, e.g. Grosbøl & Dottori (2013), using aperture photometry as with optical data, have proven unreliable due to spatial resolution effects (Bastian et al 2014a). Despite this, space-based instruments with high spatial resolution [e.g.…”

Massive star cluster formation and evolution in tidal dwarf galaxies

“…However, the coarser spatial resolution of the F160W might lead to an over-estimation of the photometry in this band (see e.g. Bastian et al 2014, , but with aperture photometry). This effect will be discussed in section 5.1.…”

“…Some F160W flux measurements might have been contaminated by regions that are very close to the clusters and that are not included in the other bands, which might affect the derivation of the physical quantities (e.g. Bastian et al 2014, in the case of aperture photometry).…”

Star Clusters in a Nuclear Star Forming Ring: The Disappearing String of Pearls

“…However, with the small aperture sizes used, r ≈ 9 pc, crowding effects are not expected to significantly bias the statistical results (Randriamanakoto et al 2013). The reasonably small apertures also help in mitigating filter-dependent resolution effects (Bastian et al 2014). Figure 2a shows the luminosity function of all our YMCs fitted with Φ(L)dL ∝ L −α dL.…”