Heterogeneity of Inverted Calcium II H:K Ratio Cluster Galaxies. (arXiv:1909.04956v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Pimbblet_K/0/1/0/all/0/1">K. A. Pimbblet</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Crossett_J/0/1/0/all/0/1">J. P. Crossett</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fraser_McKelvie_A/0/1/0/all/0/1">A. Fraser-McKelvie</a>
The ratio of calcium II H plus H$epsilon$ 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 H$delta$ and
[OII]. 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 [OII] 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$alpha$ may not have been observed.
The ratio of calcium II H plus H$epsilon$ 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 H$delta$ and
[OII]. 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 [OII] 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$alpha$ may not have been observed.
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