DWARF GALAXIES OF THE LOCAL GROUP Mateo, Mario
Annual review of astronomy and astrophysics,
01/1998, Letnik:
36, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The Local Group dwarf galaxies offer a unique window to the detailed
properties of the most common type of galaxy in the Universe. In this review, I
update the census of Local Group dwarfs based on ...the most recent distance and
radial velocity determinations. I then discuss the detailed properties of this
sample, including (
a
) the integrated photometric parameters and optical
structures of these galaxies, (
b
) the content, nature, and distribution
of their interstellar medium (ISM), (
c
) their heavy-element abundances
derived from both stars and nebulae, (
d
) the complex and varied
star-formation histories of these dwarfs, (
e
) their internal kinematics,
stressing the relevance of these galaxies to the "dark matter
problem" and to alternative interpretations, and (
f
) evidence for
past, ongoing, and future interactions of these dwarfs with other galaxies in
the Local Group and beyond. To complement the discussion and to serve as a
foundation for future work, I present an extensive set of basic observational
data in tables that summarize much of what we know and do not know about these
nearby dwarfs. Our understanding of these galaxies has grown impressively in
the past decade, but fundamental puzzles remain that will keep the Local Group
at the forefront of galaxy evolution studies for some time.
Abstract
We use deep narrowband CaHK (F395N) imaging taken with the Hubble Space Telescope (HST) to construct the metallicity distribution function (MDF) of Local Group ultra-faint dwarf galaxy ...Eridanus
II
(Eri
II
). When combined with archival F475W and F814W data, we measure metallicities for 60 resolved red giant branch stars as faint as
m
F475W
∼ 24 mag, a factor of ∼4× more stars than current spectroscopic MDF determinations. We find that Eri
II
has a mean metallicity of Fe/H = −2.50
−
0.07
+
0.07
and a dispersion of
σ
Fe
/
H
=
0.42
−
0.06
+
0.06
, which are consistent with spectroscopic MDFs, though more precisely constrained owing to a larger sample. We identify a handful of extremely metal-poor star candidates (EMP; Fe/H < −3) that are marginally bright enough for spectroscopic follow-up. The MDF of Eri
II
appears well described by a leaky box chemical evolution model. We also compute an updated orbital history for Eri
II
using Gaia eDR3 proper motions, and find that it is likely on first infall into the Milky Way. Our findings suggest that Eri
II
underwent an evolutionary history similar to that of an isolated galaxy. Compared to MDFs for select cosmological simulations of similar mass galaxies, we find that Eri
II
has a lower fraction of stars with Fe/H < −3, though such comparisons should currently be treated with caution due to a paucity of simulations, selection effects, and known limitations of CaHK for EMPs. This study demonstrates the power of deep HST CaHK imaging for measuring the MDFs of UFDs.
We introduce a Bayesian method for fitting faint, resolved stellar spectra in order to obtain simultaneous estimates of redshift and stellar-atmospheric parameters. We apply the method to thousands ...of spectra – covering 5160–5280 Å at resolution
$\mathcal {R}\sim 20\,000$
– that we have acquired with the MMT/Hectochelle fibre spectrograph for red giant and horizontal branch candidates along the line of sight to the Milky Way's dwarf spheroidal satellite in Draco. The observed stars subtend an area of ∼4 deg2, extending ∼3 times beyond Draco's nominal ‘tidal’ radius. For each spectrum, we tabulate the first four moments – central value, variance, skewness and kurtosis – of posterior probability distribution functions representing estimates of the following physical parameters: line-of-sight velocity (v
los), effective temperature (T
eff), surface gravity (log g) and metallicity (Fe/H). After rejecting low-quality measurements, we retain a new sample consisting of 2813 independent observations of 1565 unique stars, including 1879 observations for 631 stars with (as many as 13) repeat observations. Parameter estimates have median random errors of
$\sigma _{v_{\mathrm{los}}}$
= 0.88 km s−1,
$\sigma _{T_{\mathrm{eff}}}$
= 162 K, σlog g
= 0.37 dex and σFe/H = 0.20 dex. Our estimates of physical parameters distinguish ∼470 likely Draco members from interlopers in the Galactic foreground.
Crater 2: An Extremely Cold Dark Matter Halo Caldwell, Nelson; Walker, Matthew G.; Mateo, Mario ...
The Astrophysical journal,
04/2017, Letnik:
839, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We present results from MMT/Hectochelle spectroscopy of 390 red giant candidate stars along the line of sight to the recently discovered Galactic satellite Crater 2. Modeling the joint distribution ...of stellar positions, velocities, and metallicities as a mixture of Crater 2 and Galactic foreground populations, we identify ∼62 members of Crater 2, for which we resolve a line-of-sight velocity dispersion of = km s−1 and a mean velocity of = km s−1 (solar rest frame). We also resolve a metallicity dispersion of = dex and a mean of = dex that is 0.28 0.14 dex poorer than estimated from photometry. Despite Crater 2's relatively large size (projected halflight radius Rh ∼ 1 kpc) and intermediate luminosity (MV ∼ −8), its velocity dispersion is the coldest that has been resolved for any dwarf galaxy. These properties make Crater 2 the most extreme low-density outlier in dynamical as well as structural scaling relations among the Milky Way's dwarf spheroidals. Even so, under assumptions of dynamical equilibrium and negligible contamination by unresolved binary stars, the observed velocity distribution implies a gravitationally dominant dark matter halo, with a dynamical mass of M and a mass-to-light ratio of enclosed within a radius of ∼1 kpc, where the equivalent circular velocity is km s−1.
We apply the Jeans equation to estimate masses for eight of the brightest dwarf spheroidal (dSph) galaxies. For Fornax, the dSph with the largest kinematic data set, we obtain a model-independent ...constraint on the maximum circular velocity, V max = 20+4 -3 km s-1. Although we obtain only lower limits of V max 10 km s-1 for the remaining dSphs, we find that in all cases the enclosed mass at the projected half-light radius is well constrained and robust to a wide range of halo models and velocity anisotropies. We derive a simple analytic formula that estimates M(r half) accurately with respect to results from the full Jeans analysis. Applying this formula to the entire population of Local Group dSphs with published kinematic data, we demonstrate a correlation such that M(r half) r 1.4+/-0.4 half, or in terms of the mean density interior to the half-light radius, Delta *r r -1.6+/-0.4 half. This relation is driven by the fact that the dSph data exhibit a correlation between global velocity dispersion and half-light radius. We argue that tidal forces are unlikely to have introduced this relation, but tides may have increased the scatter and/or altered the slope. While the data are well described by mass profiles ranging over a factor of 2 in normalization (V max ~ 10-20 km s-1), we consider the hypothesis that all dSphs are embedded within a 'universal' dark matter halo. We show that in addition to the power law M r 1.4, viable candidates include a cuspy 'Navarro-Frenk-White' halo with V max ~ 15 km s-1 and scale radius r 0 ~ 800 pc, as well as a cored halo with V max ~ 13 km s-1 and r 0 ~ 150 pc. Finally, assuming that their measured velocity dispersions accurately reflect their masses, the smallest dSphs now allow us to resolve dSph densities at radii as small as a few tens of pc. At these small scales, we find mean densities as large as Delta *r 5 M pc-3 (200 GeV cm-3).
ABSTRACT
The ultra-faint dwarf (UFD) galaxy Reticulum 2 (Ret 2) was recently discovered in images obtained by the Dark Energy Survey. We have observed the four brightest red giants in Ret 2 at high ...spectral resolution using the Michigan/
Magellan
Fiber System. We present detailed abundances for as many as 20 elements per star, including 12 elements heavier than the Fe group. We confirm previous detection of high levels of
r
-process material in Ret 2 (mean Eu/Fe = +1.69 ± 0.05) found in three of these stars (mean Fe/H = −2.88 ± 0.10). The abundances closely match the
r
-process pattern found in the well-studied metal-poor halo star CS 22892–052. Such
r
-process-enhanced stars have not been found in any other UFD galaxy, though their existence has been predicted by at least one model. The fourth star in Ret 2 (Fe/H = −3.42 ± 0.20) contains only trace amounts of Sr (Sr/Fe = −1.73 ± 0.43) and no detectable heavier elements. One
r
-process enhanced star is also enhanced in C (natal C/Fe ≈ +1.1). This is only the third such star known, which suggests that the nucleosynthesis sites leading to C and
r
-process enhancements are decoupled. The
r
-process-deficient star is enhanced in Mg (Mg/Fe = +0.81 ± 0.14), and the other three stars show normal levels of
α
-enhancement (mean Mg/Fe = +0.34 ± 0.03). The abundances of other
α
and Fe-group elements closely resemble those in UFD galaxies and metal-poor halo stars, suggesting that the nucleosynthesis that led to the large
r
-process enhancements either produced no light elements or produced light-element abundance signatures indistinguishable from normal supernovae.
(ProQuest: ... denotes formulae and/or non-USASCII text omitted) We present results from spectroscopic observations with the Michigan/Magellan Fiber System (M2FS) of 147 stellar targets along the ...line of sight to the newly discovered "ultrafaint" stellar systems Tucana 2 (Tuc 2) and Grus 1 (Gru 1). Based on simultaneous estimates of line of sight velocity and stellar-atmospheric parameters, we identify 8 and 7 stars as probable members of Tuc 2 and and Gru 1, respectively. Our sample for Tuc 2 is sufficient to resolve an internal velocity dispersion of ... km s super(-1) about a mean of ... km s super(-1)(solar rest frame), and to estimate a mean metallicity of Fe/H = ... These results place Tuc 2 on chemodynamical scaling relations followed by dwarf galaxies, suggesting a dominant dark matter component with dynamical mass ... M sub(middot in circle) enclosed within the central ~160 pc, and dynamical mass-to-light ratio ... M sub(middot in circle)/Lv .middot in circle. For Gru 1 we estimate a mean velocity of ... km s super(-1) and a mean metallicity of Fe/H = ... but our sample does not resolve Gru 1's velocity dispersion. The radial coordinates of Tuc 2 and Gru 1 in Galactic phase space suggest that their orbits are among the most energetic within a distance of <, ~ 300 kpc. Moreover, their proximity to each other in this space arises naturally if both objects are trailing the Large Magellanic Cloud.
We present spectroscopic data for individual stars observed from 2004 March through 2008 August as part of our Michigan/MIKE Fiber System (MMFS) survey of four dwarf spheroidal (dSph) galaxies: ...Carina, Fornax, Sculptor, and Sextans. Using MMFS at the Magellan/Clay Telescope at Las Campanas Observatory, we have acquired 8855 spectra from 7103 red-giant candidates in these Galactic satellites. We list measurements of each star's line-of-sight velocity (median error ±2.1 km s -1) and spectral line indices for iron and magnesium absorption features. We use globular cluster spectra to calibrate the indices onto standard Fe/H metallicity scales, but comparison of the resulting metallicities with published values suggests that the MMFS indices are best used as indicators of relative, not absolute, metallicity. The empirical distributions of velocity and spectral indices also allow us to quantify the amount of contamination by foreground stars. In a companion paper, we develop an algorithm that evaluates the membership probability for each star, showing that the present MMFS sample contains more than 5000 dSph members, including 774 Carina members, 2483 Fornax members, 1365 Sculptor members, and 441 Sextans members.
Abstract
We present uniformly measured stellar metallicities of 463 stars in 13 Milky Way (MW) ultra-faint dwarf galaxies (UFDs;
M
V
= −7.1 to −0.8) using narrowband CaHK (F395N) imaging taken with ...the Hubble Space Telescope. This represents the largest homogeneous set of stellar metallicities in UFDs, increasing the number of metallicities in these 13 galaxies by a factor of 5 and doubling the number of metallicities in all known MW UFDs. We provide the first well-populated MDFs for all galaxies in this sample, with 〈Fe/H〉 ranging from −3.0 to −2.0 dex, and
σ
Fe/H
ranging from 0.3–0.7 dex. We find a nearly constant Fe/H∼ −2.6 over 3 decades in luminosity (∼10
2
–10
5
L
⊙
), suggesting that the mass–metallicity relationship does not hold for such faint systems. We find a larger fraction (24%) of extremely metal-poor (Fe/H< −3) stars across our sample compared to the literature (14%), but note that uncertainties in our most metal-poor measurements make this an upper limit. We find 19% of stars in our UFD sample to be metal-rich (Fe/H > −2), consistent with the sum of literature spectroscopic studies. MW UFDs are known to be predominantly >13 Gyr old, meaning that all stars in our sample are truly ancient, unlike metal-poor stars in the MW, which have a range of possible ages. Our UFD metallicities are not well matched to known streams in the MW, providing further evidence that known MW substructures are not related to UFDs. We include a catalog of our stars to encourage community follow-up studies, including priority targets for ELT-era observations.
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