No evidence for significant age spreads in young massive LMC clusters

Abstract

Recent discoveries have put the picture of stellar clusters being simple stellar populations into question. In particular, the colormagnitude diagrams of intermediate age (1-2 Gyr) massive clusters in the Large Magellanic Cloud (LMC) show features that could be interpreted as age spreads of 100-500 Myr. If multiple generations of stars are present in these clusters then, as a consequence, young (<1 Gyr) clusters with similar properties should have age spreads of the same order. In this paper we use archival Hubble Space Telescope (HST) data of eight young massive LMC clusters (NGC 1831, NGC 1847, NGC 1850, NGC 2004, NGC 2100, NGC 2136, NGC 2157 and NGC 2249 to test this hypothesis. We analyzed the color-magnitude diagrams of these clusters and fitted their star formation history to derive upper limits of potential age spreads. We find that none of the clusters analyzed in this work shows evidence for an extended star formation history that would be consistent with the age spreads proposed for intermediate age LMC clusters. Tests with artificial single age clusters show that the fitted age dispersion of the youngest clusters is consistent with spreads that are purely induced by photometric errors. As an additional result we determined a new age of NGC 1850 of ∼100 Myr, significantly higher than the commonly used value of about 30 Myr, although consistent with early HST estimates.

The morphology of the sub-giant branch and red clump reveal no sign of age spreads in intermediate-age clusters

“…While the current masses of the eMSTO clusters are similar to the most massive YMCs (e.g., NGC 1850, 1866and 1856-Niederhofer et al 2015, Goudrooij et al (2011; posit that the intermediate age clusters were much more massive (by a factor of ∼ 4) at birth (and also significantly denser). Based on size-of-sample effects (e.g., Gieles & Bastian 2008) we would expect that the SFR of the LMC was ∼ 4 times higher while the eMSTO clusters were forming (c.f., Maschberger & Kroupa 2011).…”

“…However, this result depends heavily on large "correction factors" to change the current observed escape velocities to the "initial ones". Without these correction factors many of the clusters that display the eMSTO feature would have lower escape velocities than young clusters that have been shown not to have age spreads within them (e.g., Bastian & Silva-Villa 2013;Niederhofer et al 2015). The applied "correction factors" are based on a number of extreme assumptions.…”

“…No age spreads were found, with upper limits of any potential spreads of ∼ 30 Myr. Niederhofer et al (2015) extended the Bastian & Silva-Villa survey to an additional eight LMC young massive clusters, including NGC 1850, a ∼ 2×10 5 , 100 Myr cluster, and also did not find any evidence for age spreads. It is important to note that the YMCs studied so far have current masses (and escape velocities) well in excess of the intermediate age clusters that show eMSTOs.…”

A young cluster with an extended main-sequence turnoff: confirmation of a prediction of the stellar rotation scenario

“…In order to subtract the background field stars, we followed the same procedure as used in Niederhofer et al (2015aNiederhofer et al ( , 2016. We adopt a cluster field, centred on the cluster, with a radius of 2 * rc (where rc = 11.1" -McLaughlin & van der Marel 2005), and a reference field located near the edge of the images (i.e.…”

“…The first is that massive clusters should show onc 2015 RAS going star-formation for the first few hundred Myr of their lives, whereas no clusters with ages beyond 10 Myr have been found with active star-formation (Bastian et al 20132016a;Niederhofer et al 2015a). Moreover, In order to form a second generation of stars, clusters must be able to retain and/or accrete material from their surroundings (e.g., Conroy & Spergel 2011).…”