372 research outputs found
Are Some Milky Way Globular Clusters Hosted by Undiscovered Galaxies?
The confirmation of a globular cluster (GC) in the recently discovered
ultrafaint galaxy Eridanus II (Eri II) motivated us to examine the question
posed in the title. After estimating the halo mass of Eri II using a published
stellar mass - halo mass relation, the one GC in this galaxy supports extending
the relationship between the number of GCs hosted by a galaxy and the galaxy's
total mass about two orders of magnitude in stellar mass below the previous
limit. For this empirically determined specific frequency of between 0.06 and
0.39 globular clusters per 10 of total mass, the surviving Milky
Way (MW) subhalos with masses smaller than could host as many
as 5 to 31 GCs, broadly consistent with the actual population of outer halo MW
GCs, although matching the radial distribution in detail remains a challenge.
Using a subhalo mass function from published high resolution numerical
simulations and a Poissonian model for populating those halos with the
aforementioned empirically constrained frequency, we find that about 90 of
these GCs lie in lower-mass subhalos than that of Eri II. From what we know
about the stellar mass-halo mass function, the subhalo mass function, and the
mass-normalized GC specific frequency, we conclude that some of the MW's outer
halo GCs are likely to be hosted by undetected subhalos with extremely modest
stellar populations.Comment: 5 pages, 4 figures; ApJL in pres
Numerically Modeling the First Peak of the Type IIb SN 2016gkg
Many Type IIb supernovae (SNe) show a prominent additional early peak in
their light curves, which is generally thought to be due to the shock cooling
of extended hydrogen-rich material surrounding the helium core of the exploding
star. The recent SN 2016gkg was a nearby Type IIb SN discovered shortly after
explosion, which makes it an excellent candidate for studying this first peak.
We numerically explode a large grid of extended envelope models and compare
these to SN 2016gkg to investigate what constraints can be derived from its
light curve. This includes exploring density profiles for both a convective
envelope and an optically thick steady-state wind, the latter of which has not
typically been considered for Type IIb SNe models. We find that roughly
of extended material with a radius of
reproduces the photometric light curve data,
consistent with pre-explosion imaging. These values are independent of the
assumed density profile of this material, although a convective profile
provides a somewhat better fit. We infer from our modeling that the explosion
must have occurred within of the first observed data
point, demonstrating that this event was caught very close to the moment of
explosion. Nevertheless, our best-fitting one-dimensional models overpredict
the earliest velocity measurements, which suggests that the hydrogen-rich
material is not distributed in a spherically symmetric manner. We compare this
to the asymmetries seen in the SN IIb remnant Cas A, and we discuss the
implications of this for Type IIb SN progenitors and explosion models.Comment: 8 pages, 8 figures, updated version accepted for publication in The
Astrophysical Journa
Dynamical evidence for a strong tidal interaction between the Milky Way and its satellite, Leo V
We present a chemodynamical analysis of the Leo~V dwarf galaxy, based on Keck
II DEIMOS spectra of 8 member stars. We find a systemic velocity for the system
of kms, and barely resolve a
velocity dispersion for the system, with kms, consistent with previous studies of Leo~V. The
poorly resolved dispersion means we are unable to adequately constrain the dark
matter content of Leo~V. We find an average metallicity for the dwarf of
[Fe/H], and measure a significant spread in the iron abundance
of its member stars, with [Fe/H] dex, which cleanly
identifies Leo~V as a dwarf galaxy that has been able to self-enrich its
stellar population through extended star formation. Owing to the tentative
photometric evidence for tidal substructure around Leo~V, we also investigate
whether there is any evidence for tidal stripping or shocking of the system
within its dynamics. We measure a significant velocity gradient across the
system, of kms per
arcmin (or kms~kpc), which points almost directly
toward the Galactic centre. We argue that Leo~V is likely a dwarf on the brink
of dissolution, having just barely survived a past encounter with the centre of
the Milky Way.Comment: 14 pages, 12 figures, accepted for publication in MNRAS. Updated to
include minor revisions from referee proces
The Density Profiles of Massive, Relaxed Galaxy Clusters. I. The Total Density Over Three Decades in Radius
Clusters of galaxies are excellent locations to probe the distribution of
baryons and dark matter (DM) over a wide range of scales. We study a sample of
seven massive, relaxed galaxy clusters with centrally-located brightest cluster
galaxies (BCGs) at z=0.2-0.3. Using the observational tools of strong and weak
gravitational lensing, combined with resolved stellar kinematics within the
BCG, we measure the total radial density profile, comprising both dark and
baryonic matter, over scales of ~3-3000 kpc. Lensing-derived mass profiles
typically agree with independent X-ray estimates within ~15%, suggesting that
departures from hydrostatic equilibrium are small and that the clusters in our
sample (except A383) are not strongly elongated along the line of sight. The
inner logarithmic slope gamma_tot of the total density profile measured over
r/r200=0.003-0.03, where rho_tot ~ r^(-gamma_tot), is found to be nearly
universal, with a mean = 1.16 +- 0.05 (random) +0.05-0.07
(systematic) and an intrinsic scatter of < 0.13 (68% confidence). This is
further supported by the very homogeneous shape of the observed velocity
dispersion profiles, obtained via Keck spectroscopy, which are mutually
consistent after a simple scaling. Remarkably, this slope agrees closely with
numerical simulations that contain only dark matter, despite the significant
contribution of stellar mass on the scales we probe. The Navarro-Frenk-White
profile characteristic of collisionless cold dark matter is a better
description of the total mass density at radii >~ 5-10 kpc than that of dark
matter alone. Hydrodynamical simulations that include baryons, cooling, and
feedback currently provide a poorer match. We discuss the significance of our
findings for understanding the assembly of BCGs and cluster cores, particularly
the influence of baryons on the inner DM halo. [abridged]Comment: Updated to matched the published version in Ap
Separating baryons and dark matter in cluster cores: a full 2-D lensing and dynamic analysis of Abell 383 and MS2137-23
(abridged) We utilize existing imaging and spectroscopic data for the galaxy
clusters MS2137-23 and Abell 383 to present improved measures of the
distribution of dark and baryonic material in the clusters' central regions.
Our method, based on the combination of gravitational lensing and dynamical
data, is uniquely capable of separating the distribution of dark and baryonic
components at scales below 100 kpc. We find a variety of strong lensing models
fit the available data, including some with dark matter profiles as steep as
expected from recent simulations. However, when combined with stellar velocity
dispersion data for the brightest member, shallower inner slopes than predicted
by numerical simulations are preferred. For Abell 383, the preferred shallow
inner slopes are statistically a good fit only when the multiple image position
uncertainties associated with our lens model are assumed to be 0\farcs5, to
account for unknown substructure. No statistically satisfactory fit was
obtained matching both the multiple image lensing data and the velocity
dispersion profile of the brightest cluster galaxy in MS2137-23. This suggests
that the mass model we are using, which comprises a pseudo-elliptical
generalized NFW profile and a brightest cluster galaxy component may
inadequately represent the inner cluster regions. This may plausibly arise due
to halo triaxiality or by the gravitational interaction of baryons and dark
matter in cluster cores. However, the progress made via this detailed study
highlights the key role that complementary observations of lensed features and
stellar dynamics offer in understanding the interaction between dark and
baryonic matter on non-linear scales in the central regions of clusters.Comment: 18 pages, 9 figures; accepted for publication in the Astrophysical
Journa
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