Abstract
ABSTRACT The ratio of calcium ii H plus Hϵ to calcium ii K inverts as a galaxy stellar population moves from being dominated by older stars to possessing more A and B class stars. This ratio – the H:K ratio – can serve as an indicator of stellar populations younger than 200 Myr. In this work, we provide a new method to determine H:K, and apply it to spectra taken of cluster galaxies in Abell 3888. Although H:K is on average systematically lower for the cluster than for a wider field sample, we show that H:K does not have a simple relationship with other indices such as the equivalent widths of Hδ and [O ii] beyond having a high value for strong [O ii] emission. Moreover, strongly inverted galaxies with H:K > 1.1 have no preferred location within the cluster and are only slightly lower in their velocity dispersions around the cluster compared to strongly emitting [O ii] galaxies. Our results indicate that selecting galaxies on H:K inversion results in a heterogeneous sample formed via a mixture of pathways that likely includes, but may not be limited to, merging spiral galaxies, and quiescent galaxies accreting lower mass, gas-rich companions. In concert with other selection criteria, H:K can provide a means to select a more ‘pure’ passive sample or to aid in the selection of highly star-forming galaxies, especially where other spectral line indicators such as H α may not have been observed.
Highlights
The hierarchical assembly of galaxies in the cold dark matter paradigm unambiguously demonstrates that they build-up their stellar mass through gravitational collapse and through repeated merger events with smaller to similar mass objects over their life histories (e.g. White & Rees 1978; Kauffmann, White & Guiderdoni 1993; Cole et al 1994; Fakhouri & Ma 2008; Neistein & Dekel 2008; Rodriguez-Gomez et al 2015)
We introduce a new methodology for measuring the dips of each of these lines
We have investigated the Ca H plus H to Ca K ratio for cluster galaxies belonging to Abell 3888
Summary
The hierarchical assembly of galaxies in the cold dark matter paradigm unambiguously demonstrates that they build-up their stellar mass through gravitational collapse and through repeated merger events with smaller to similar mass objects over their life histories (e.g. White & Rees 1978; Kauffmann, White & Guiderdoni 1993; Cole et al 1994; Fakhouri & Ma 2008; Neistein & Dekel 2008; Rodriguez-Gomez et al 2015). White & Rees 1978; Kauffmann, White & Guiderdoni 1993; Cole et al 1994; Fakhouri & Ma 2008; Neistein & Dekel 2008; Rodriguez-Gomez et al 2015) Such mergers may be ‘wet’ in the sense that a given galaxy accretes gas and dust – the fuel for star formation – during these mergers, or they may be ‘dry’ and add pre-existing stellar populations on to themselves (Bell et al 2006; Ciotti, Lanzoni & Volonteri 2007). Observations show that some galaxies (large ellipticals) have probably formed their stars very early in the history of the Universe and passively evolved ever
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