50 research outputs found
Two distinct red giant branch populations in the globular cluster NGC 2419 as tracers of a merger event in the Milky Way
Recent spectroscopic observations of the outer halo globular cluster (GC) NGC
2419 show that it is unique among GCs, in terms of chemical abundance patterns,
and some suggest that it was originated in the nucleus of a dwarf galaxy. Here
we show, from the Subaru narrow-band photometry employing a calcium filter,
that the red giant-branch (RGB) of this GC is split into two distinct
subpopulations. Comparison with spectroscopy has confirmed that the redder RGB
stars in the [=(Ca] index are enhanced in [Ca/H] by 0.2
dex compared to the bluer RGB stars. Our population model further indicates
that the calcium-rich second generation stars are also enhanced in helium
abundance by a large amount (Y = 0.19). Our photometry, together with
the results for other massive GCs (e.g., Cen, M22, and NGC 1851),
suggests that the discrete distribution of RGB stars in the index might be
a universal characteristic of this growing group of peculiar GCs. The planned
narrow-band calcium photometry for the Local Group dwarf galaxies would help to
establish an empirical connection between these GCs and the primordial building
blocks in the hierarchical merging paradigm of galaxy formation.Comment: 4 pages, 4 figures, 1 table, accepted for the publication in ApJ
Nonlinear Color-Metallicity Relations of Globular Clusters. III. On the Discrepancy in Metallicity between Globular Cluster Systems and their Parent Elliptical Galaxies
One of the conundrums in extragalactic astronomy is the discrepancy in
observed metallicity distribution functions (MDFs) between the two prime
stellar components of early-type galaxies-globular clusters (GCs) and halo
field stars. This is generally taken as evidence of highly decoupled
evolutionary histories between GC systems and their parent galaxies. Here we
show, however, that new developments in linking the observed GC colors to their
intrinsic metallicities suggest nonlinear color-to-metallicity conversions,
which translate observed color distributions into strongly-peaked, unimodal
MDFs with broad metal-poor tails. Remarkably, the inferred GC MDFs are similar
to the MDFs of resolved field stars in nearby elliptical galaxies and those
produced by chemical evolution models of galaxies. The GC MDF shape,
characterized by a sharp peak with a metal-poor tail, indicates a virtually
continuous chemical enrichment with a relatively short timescale. The
characteristic shape emerges across three orders of magnitude in the host
galaxy mass, suggesting a universal process of chemical enrichment among
various GC systems. Given that GCs are bluer than field stars within the same
galaxy, it is plausible that the chemical enrichment processes of GCs ceased
somewhat earlier than that of field stellar population, and if so, GCs
preferentially trace the major, vigorous mode of star formation events in
galactic formation. We further suggest a possible systematic age difference
among GC systems, in that the GC systems in more luminous galaxies are older.
This is consistent with the downsizing paradigm of galaxies and supports
additionally the similar nature shared by GCs and field stars. Our findings
suggest that GC systems and their parent galaxies have shared a more common
origin than previously thought, and hence greatly simplify theories of galaxy
formation.Comment: 55 pages, 7 figures, 5 tables; Accepted for publication in Ap
The Presence of Two Distinct Red Giant Branches in the Globular Cluster NGC 1851
There is a growing body of evidence for the presence of multiple stellar
populations in some globular clusters, including NGC 1851. For most of these
peculiar globular clusters, however, the evidence for the multiple red
giant-branches (RGBs) having different heavy elemental abundances as observed
in Omega Centauri is hitherto lacking, although spreads in some lighter
elements are reported. It is therefore not clear whether they also share the
suggested dwarf galaxy origin of Omega Cen or not. Here we show from the CTIO
4m UVI photometry of the globular cluster NGC 1851 that its RGB is clearly
split into two in the U - I color. The two distinct RGB populations are also
clearly separated in the abundance of heavy elements as traced by Calcium,
suggesting that the type II supernovae enrichment is also responsible, in
addition to the pollutions of lighter elements by intermediate mass asymptotic
giant branch stars or fast-rotating massive stars. The RGB split, however, is
not shown in the V - I color, as indicated by previous observations. Our
stellar population models show that this and the presence of bimodal
horizontal-branch distribution in NGC 1851 can be naturally reproduced if the
metal-rich second generation stars are also enhanced in helium.Comment: 13 pages, 4 figures, accepted for publication in the Astrophysical
Journal Letter