29 research outputs found
Chemical compositions of six metal-poor stars in the ultra-faint dwarf spheroidal galaxy Bo\"otes I
Ultra-faint dwarf galaxies recently discovered around the Milky Way (MW)
contain extremely metal-poor stars, and might represent the building blocks of
low-metallicity components of the MW. Among them, the Bo\"otes I dwarf
spheroidal galaxy is of particular interest because of its exclusively old
stellar population. We determine chemical compositions of six red giant stars
in Bo\"otes I, based on the high-resolution spectra obtained with the High
Dispersion Spectrograph mounted on the Subaru Telescope. Abundances of 12
elements, including C, Na, alpha, Fe-peak, and neutron capture elements, were
determined for the sample stars. The abundance results were compared to those
in field MW halo stars previously obtained using an abundance analysis
technique similar to the present study. We confirm the low metallicity of
Boo-094 ([Fe/H]=-3.4). Except for this star, the abundance ratios ([X/Fe]) of
elements lighter than zinc are generally homogeneous with small scatter around
the mean values in the metallicities spanned by the other five stars
(-2.7-2.7 show
no significant enhancement of carbon. The [Mg/Fe] and [Ca/Fe] ratios are almost
constant with a modest decreasing trend with increasing [Fe/H] and are slightly
lower than the field halo stars. The [Sr/Fe] and [Sr/Ba] ratios also tend to be
lower in the Bo\"otes I stars than in the halo stars. Our results of small
scatter in the [X/Fe] ratios for elements lighter than zinc suggest that these
abundances were homogeneous among the ejecta of prior generation(s) of stars in
this galaxy.Comment: 16 pages, 12 figures. Accepted to A&A, language correcte
Structure of the Milky Way stellar halo out to its outer boundary with blue horizontal-branch stars
We present the structure of the Milky Way stellar halo beyond Galactocentric
distances of kpc traced by blue horizontal-branch (BHB) stars, which
are extracted from the survey data in the Hyper Suprime-Cam Subaru Strategic
Program (HSC-SSP). We select BHB candidates based on photometry,
where the -band is on the Paschen series and the colors that involve the
-band are sensitive to surface gravity. About 450 BHB candidates are
identified between kpc and 300 kpc, most of which are beyond the reach
of previous large surveys including the Sloan Digital Sky Survey. We find that
the global structure of the stellar halo in this range has substructures, which
are especially remarkable in the GAMA15H and XMM-LSS fields in the HSC-SSP. We
find that the stellar halo can be fitted to a single power-law density profile
with an index of () with (without) these fields and
its global axial ratio is (). Thus, the stellar halo may be
significantly disturbed and be made in a prolate form by halo substructures,
perhaps associated with the Sagittarius stream in its extension beyond kpc. For a broken power-law model allowing different power-law indices
inside/outside a break radius, we obtain a steep power-law slope of outside a break radius of kpc ( kpc) for the case
with (without) GAMA15H and XMM-LSS. This radius of kpc might be as close
as a halo boundary if there is any, although larger BHB sample is required from
further HSC-SSP survey to increase its statistical significance.Comment: 12 pages, 8 figures, revised version, accepted for publication in
PAS
The Progenitor of the Peculiar Galaxy NGC3077
We present a study of the structural properties and metallicity distribution
of the nearby peculiar galaxy NGC3077. Using data from our survey of the M81
Group with the Hyper Suprime-Cam on the Subaru Telescope, we construct deep
color-magnitude diagrams that are used to probe the old red giant branch
population of NGC3077. We map these stars out to and beyond the nominal tidal
radius, which allows us to derive the structural properties and stellar content
of the peripheral regions. We show that NGC3077 has an extended stellar halo
and pronounced ``S-shaped" tidal tails that diverge from the radial profile of
the inner region. The average metallicity of the old population in NGC3077 is
estimated from individual RGBs to be , which
decreases with the distance from the galaxy center as dex
. The metallicity of the S-shaped structure is similar to that
of the regions lying at ~kpc), indicating that
the stellar constituents of the tidal tails have come from the outer envelope
of NGC3077. These results suggest that this peculiar galaxy was probably a
rather normal dwarf elliptical galaxy before the tidal interaction with M81 and
M82. We also examine the evidence in our dataset for the six recently-reported
ultra-faint dwarf candidates around NGC3077. We recover a spatial overdensity
of sources coinciding with only one of these.Comment: 13 pages, 10 figures, Accepted for publication in Ap
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
Searches for New Milky Way Satellites from the First Two Years of Data of the Subaru/Hyper Suprime-Cam Survey: Discovery of Cetus~III
We present the results from a search for new Milky Way (MW) satellites from
the first two years of data from the Hyper Suprime-Cam (HSC) Subaru Strategic
Program (SSP) ~deg and report the discovery of a highly
compelling ultra-faint dwarf galaxy candidate in Cetus. This is the second
ultra-faint dwarf we have discovered after Virgo~I reported in our previous
paper. This satellite, Cetus~III, has been identified as a statistically
significant (10.7) spatial overdensity of star-like objects, which are
selected from a relevant isochrone filter designed for a metal-poor and old
stellar population. This stellar system is located at a heliocentric distance
of 251~kpc with a most likely absolute magnitude of ~mag estimated from a Monte Carlo analysis. Cetus~III is extended with
a half-light radius of ~pc, suggesting that this is a
faint dwarf satellite in the MW located beyond the detection limit of the Sloan
Digital Sky Survey. Further spectroscopic studies are needed to assess the
nature of this stellar system. We also revisit and update the parameters for
Virgo~I finding ~mag and ~pc. Using simulations of -dominated cold dark matter
models, we predict that we should find one or two new MW satellites from ~deg HSC-SSP data, in rough agreement with the discovery rate so far.
The further survey and completion of HSC-SSP over ~deg will
provide robust insights into the missing satellites problem.Comment: 12 pages, 12 figures, accepted for publication in PASJ special issu
A New Milky Way Satellite Discovered In The Subaru/Hyper Suprime-Cam Survey
We report the discovery of a new ultra-faint dwarf satellite companion of the
Milky Way based on the early survey data from the Hyper Suprime-Cam Subaru
Strategic Program. This new satellite, Virgo I, which is located in the
constellation of Virgo, has been identified as a statistically significant (5.5
sigma) spatial overdensity of star-like objects with a well-defined main
sequence and red giant branch in their color-magnitude diagram. The
significance of this overdensity increases to 10.8 sigma when the relevant
isochrone filter is adopted for the search. Based on the distribution of the
stars around the likely main sequence turn-off at r ~ 24 mag, the distance to
Virgo I is estimated as 87 kpc, and its most likely absolute magnitude
calculated from a Monte Carlo analysis is M_V = -0.8 +/- 0.9 mag. This stellar
system has an extended spatial distribution with a half-light radius of 38
+12/-11 pc, which clearly distinguishes it from a globular cluster with
comparable luminosity. Thus, Virgo I is one of the faintest dwarf satellites
known and is located beyond the reach of the Sloan Digital Sky Survey. This
demonstrates the power of this survey program to identify very faint dwarf
satellites. This discovery of VirgoI is based only on about 100 square degrees
of data, thus a large number of faint dwarf satellites are likely to exist in
the outer halo of the Milky Way.Comment: typos are corrected, 6 pages, 4 figures, accepted for publication in
Ap
Stellar population and structural properties of dwarf galaxies and young stellar systems in the M81 group
We use Hyper Suprime-Cam on the Subaru Telescope to investigate the
structural and photometric properties of early-type dwarf galaxies and young
stellar systems at the center of the M81 Group. We have mapped resolved stars
to magnitudes below the tip of the red giant branch over almost 6.5
square degrees, corresponding to a projected area of at
the distance of M81. The resulting stellar catalogue enables a homogeneous
analysis of the member galaxies with unprecedented sensitivity to low surface
brightness emission. The radial profiles of the dwarf galaxies are
well-described by Sersic and King profiles, and show no obvious signatures of
tidal disruption. The measured radii for most of these systems are larger than
the existing literature values and we find the total luminosity of IKN
() to be almost 3 magnitudes brighter than
previously-thought. We identify new dwarf satellite candidates, d1006+69 and
d1009+68, which we estimate to lie at a distance of Mpc and
Mpc. With and
, d1006+69 is one of the faintest and most metal-poor
dwarf satellites currently-known in the M81 Group. The luminosity functions of
young stellar systems in the outlying tidal HI debris imply continuous star
formation in the recent past and the existence of populations as young as 30
Myr old. We find no evidence for old RGB stars coincident with the young
MS/cHeB stars which define these objects, supporting the idea that they are
genuinely new stellar systems resulting from triggered star formation in
gaseous tidal debris.Comment: 24 pages, 22 figures, Accepted for publication in Ap