56 research outputs found
Characterizing the Circumgalactic Medium of the Lowest-mass Galaxies: A Case Study of IC 1613
Using 10 sight lines observed with the Hubble Space Telescope/Cosmic Origins Spectrograph, we study the circumgalactic medium (CGM) and outflows of IC 1613, which is a low-mass (M_* ~ 10⁸ M_⊙), dwarf irregular galaxy on the outskirts of the Local Group. Among the sight lines, four are pointed toward UV-bright stars in IC 1613, and the other six sight lines are background QSOs at impact parameters from 6 kpc (<0.1R_(200)) to 61 kpc (0.6R_(200)). We detect a number of Si ii, Si iii, Si iv, C ii, and C iv absorbers, most of which have velocities less than the escape velocity of IC 1613 and thus are gravitationally bound. The line strengths of these ion absorbers are consistent with the CGM absorbers detected in dwarf galaxies at low redshifts. Assuming that Si ii, Si iii, and Si iv comprise nearly 100% of the total silicon, we find 3% (~8 × 10³ M_⊙), 2% (~7 × 10³ M_⊙), and 32%–42% [~(1.0–1.3) × 10⁵ M_⊙] of the silicon mass in the stars, interstellar medium, and within 0.6R_(200) of the CGM of IC 1613. We also estimate the metal outflow rate to be Ṁ_(out,Z) ⩾ 1.1 x 10⁻⁵ M_⊙ yr⁻¹ and the instantaneous metal mass loading factor to be η_Z ≥ 0.004, which are in broad agreement with available observation and simulation values. This work is the first time a dwarf galaxy of such low mass is probed by a number of both QSO and stellar sight lines, and it shows that the CGM of low-mass, gas-rich galaxies can be a large reservoir enriched with metals from past and ongoing outflows
The emptiness inside: Finding gaps, valleys, and lacunae with geometric data analysis
Discoveries of gaps in data have been important in astrophysics. For example,
there are kinematic gaps opened by resonances in dynamical systems, or
exoplanets of a certain radius that are empirically rare. A gap in a data set
is a kind of anomaly, but in an unusual sense: Instead of being a single
outlier data point, situated far from other data points, it is a region of the
space, or a set of points, that is anomalous compared to its surroundings. Gaps
are both interesting and hard to find and characterize, especially when they
have non-trivial shapes. We present in this paper a statistic that can be used
to estimate the (local) "gappiness" of a point in the data space. It uses the
gradient and Hessian of the density estimate (and thus requires a
twice-differentiable density estimator). This statistic can be computed at
(almost) any point in the space and does not rely on optimization; it allows to
highlight under-dense regions of any dimensionality and shape in a general and
efficient way. We illustrate our method on the velocity distribution of nearby
stars in the Milky Way disk plane, which exhibits gaps that could originate
from different processes. Identifying and characterizing those gaps could help
determine their origins. We provide in an Appendix implementation notes and
additional considerations for finding under-densities in data, using critical
points and the properties of the Hessian of the density.Comment: 17 pages, 10 figures. Submitted to AJ. Comments welcomed. Revision:
added 3D gridding + restructured outline: implementation notes (Quadratic
Kernel) and methods for approx critical points and 1d-valley now in Anne
The many streams of the Magellanic Stream
We present results from neutral hydrogen (HI) observations of the tip of the
Magellanic Stream (MS), obtained with the Arecibo telescope as a part of the
on-going survey by the Consortium for Galactic studies with the Arecibo L-band
Feed Array. We find four large-scale, coherent HI streams, extending
continously over a length of 20 degrees, each stream possessing different
morphology and velocity gradients. The newly discovered streams provide strong
support for the tidal model of the MS formation by Connors et al. (2006), which
suggested a spatial and kinematic bifurcation of the MS. The observed
morphology and kinematics suggest that three of these streams could be
interpreted as a 3-way splitting of the main MS filament, while the fourth
stream appears much younger and may have originated from the Magellanic Bridge.
We find an extensive population of HI clouds at the tip of the MS. Two thirds
of clouds have an angular size in the range 3.5'--10'. We interpret this as
being due to thermal instability, which would affect a warm tail of gas
trailing through the Galactic halo over a characteristic timescale of a few
Myrs to a few hundred Myrs. We show that thermal fragments can survive in the
hot halo for a long time, especially if surrounded by a <10^6 K halo gas. If
the observed clumpy structure is mainly due to thermal instability, then the
tip of the MS is at a distance of ~70 kpc. A significant fraction of HI clouds
at the tip of the MS show multi-phase velocity profiles, indicating the
co-existence of cooler and warmer gas.Comment: Accepted by Ap
Compact HI clouds from the GALFA-HI survey
The Galactic Arecibo L-band Feed Array HI (GALFA-HI) survey is mapping the
entire Arecibo sky at 21-cm, over a velocity range of -700 to +700 km/s (LSR),
at a velocity resolution of 0.18 km/s and a spatial resolution of 3.5 arcmin.
The unprecedented resolution and sensitivity of the GALFA-HI survey have
resulted in the detection of numerous isolated, very compact HI clouds at low
Galactic velocities, which are distinctly separated from the HI disk emission.
In the limited area of ~4600 deg surveyed so far, we have detected 96 of
such compact clouds. The detected clouds are cold with a median T
(the kinetic temperature in the case in which there is no non-thermal
broadening) of 300 K. Moreover, these clouds are quite compact and faint, with
median values of 5 arcmin in angular size, 0.75 K in peak brightness
temperature, and cm in HI column density. Most of the
clouds deviate from Galactic rotation at the 20-30 km/s level, and a
significant fraction show evidence for a multiphase medium and velocity
gradients. No counterparts for these clouds were found in other wavebands. From
the modeling of spatial and velocity distributions of the whole compact cloud
population, we find that the bulk of the compact clouds are related to the
Galactic disk, and their distances are likely to be in the range of 0.1 to a
few kpc. We discuss various possible scenarios for the formation and
maintenance of this cloud population and its significance for Galactic ISM
studies.Comment: Accepted for publication in the Astrophysical Journa
A peculiar HI cloud near the distant globular cluster Pal 4
We present 21-cm observations of four Galactic globular clusters, as part of
the on-going GALFA-HI Survey at Arecibo. We discovered a peculiar HI cloud in
the vicinity of the distant (109 kpc) cluster Pal 4, and discuss its properties
and likelihood of association with the cluster. We conclude that an association
of the HI cloud and Pal 4 is possible, but that a chance coincidence between
Pal 4 and a nearby compact high-velocity cloud cannot be ruled out altogether.
New, more stringent upper limits were derived for the other three clusters: M
3, NGC 5466, and Pal 13. We briefly discuss the fate of globular cluster gas
and the interaction of compact clouds with the Galactic Halo gas.Comment: Accepted for publication in MNRA
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