The Magellanic Clouds are surrounded by an extended network of gaseous structures. Chief among these is the Magellanic Stream, an interwoven tail of filaments trailing the Clouds in their orbit ...around the Milky Way. When considered in tandem with its Leading Arm, the Stream stretches over 200° on the sky. The Stream is thought to represent the result of tidal interactions between the Clouds and ram-pressure forces exerted by the Galactic corona, and its kinematic properties reflect the dynamical history of the pair of dwarf galaxies closest to the Milky Way. The Stream is a benchmark for hydrodynamical simulations of accreting gas and cloud corona interactions. If the Stream survives these interactions and arrives safely in the Galactic disk, its cargo of over a billion solar masses of gas has the potential to maintain or elevate the Galactic star-formation rate. In this article, we review the current state of knowledge of the Stream, including its chemical composition, physical conditions, origin, and fate. We also review the dynamics of the Magellanic System, including the proper motions and orbital history of the Large and Small Magellanic Clouds, the first-passage and second-passage scenarios, and the evidence for a Magellanic Group of galaxies.
Abstract
We present new simulations of the formation of the Magellanic Stream based on an updated first-passage interaction history for the Magellanic Clouds, including both the Galactic and ...Magellanic Coronae and a live dark matter halo for the Milky Way. This new interaction history is needed because previously successful orbits need updating to account for the Magellanic Corona and the loosely bound nature of the Magellanic Group. These orbits involve two tidal interactions over the last 3.5 Gyr and reproduce the Stream’s position and appearance on the sky, mass distribution, and velocity profile. Most importantly, our simulated Stream is only ∼20 kpc away from the Sun at its closest point, whereas previous first-infall models predicted a distance of 100–200 kpc. This dramatic paradigm shift in the Stream’s 3D position would have several important implications. First, estimates of the observed neutral and ionized masses would be reduced by a factor of ∼5. Second, the stellar component of the Stream is also predicted to be <20 kpc away. Third, the enhanced interactions with the MW’s hot corona at this small distance would substantially shorten the Stream’s lifetime. Finally, the MW’s UV radiation field would be much stronger, potentially explaining the H
α
emission observed along most of the Stream. Our prediction of a 20 kpc Stream could be tested by searching for UV absorption lines toward distant MW halo stars projected onto the Stream.
Abstract
We analyze new far-ultraviolet spectra of 13 quasars from the
COS-Halos survey that cover the H
i
Lyman limit of 14 circumgalactic medium (CGM) systems. These data yield precise estimates or ...more constraining limits than previous COS-Halos measurements on the H
i
column densities
. We then apply a Monte-Carlo Markov chain approach on 32 systems from COS-Halos to estimate the metallicity of the cool (
K) CGM gas that gives rise to low-ionization state metal lines, under the assumption of photoionization equilibrium with the extragalactic UV background. The principle results are: (1) the CGM of field
L
* galaxies exhibits a declining H
i
surface density with impact parameter
(at
confidence), (2) the transmission of ionizing radiation through CGM gas alone is 70 ± 7%; (3) the metallicity distribution function of the cool CGM is unimodal with a median of
and a 95% interval
to
; the incidence of metal-poor (
) gas is low, implying any such gas discovered along quasar sightlines is typically unrelated to
L
* galaxies; (4) we find an unexpected increase in gas metallicity with declining
(at
confidence) and, therefore, also with increasing
; the high metallicity at large radii implies early enrichment; and (5) a non-parametric estimate of the cool CGM gas mass is
, which together with new mass estimates for the hot CGM may resolve the galactic missing baryons problem. Future analyses of halo gas should focus on the underlying astrophysics governing the CGM, rather than processes that simply expel the medium from the halo.
ABSTRACT The total contribution of diffuse halo gas to the galaxy baryon budget strongly depends on its dominant ionization state. In this paper, we address the physical conditions in the highly ...ionized circumgalactic medium (CGM) traced by absorption lines observed in COS-Halos spectra. We analyze the observed ionic column densities, absorption-line widths and relative velocities, along with the ratios of / for 39 fitted Voigt profile components of O vi. We compare these quantities with the predictions given by a wide range of ionization models. Photoionization models that include only extragalactic UV background radiation are ruled out; conservatively, the upper limits to / and measurements of imply unphysically large path lengths 100 kpc. Furthermore, very broad absorption (b > 40 km s−1) is a defining characteristic of the CGM of star-forming L* galaxies. We highlight two possible origins for the bulk of the observed : (1) highly structured gas clouds photoionized primarily by local high-energy sources or (2) gas radiatively cooling on large scales behind a supersonic wind. Approximately 20% of circumgalactic O vi does not align with any low-ionization state gas within 50 km s−1 and is found only in halos with < 1012 . We suggest that this type of unmatched O vi absorption traces the hot corona itself at a characteristic temperature of K. We discuss the implications of these very distinct physical origins for the dynamical state, gas cooling rates, and total baryonic content of L* gaseous halos.
We analyze the physical conditions of the cool, photoionized (T similar to 10 super(4) K) circumgalactic medium (CGM) using the COS-Halos suite of gas column density measurements for 44 gaseous halos ...within 160 kpc of L ~ L super(*) galaxies at z similar to 0.2. These data are well described by simple photoionization models, with the gas highly ionized (n sub(HII)/n sub(H) gap 99%) by the extragalactic ultraviolet background. Scaling by estimates for the virial radius, R sub(vir), we show that the ionization state (tracked by the dimensionless ionization parameter, U) increases with distance from the host galaxy. The ionization parameters imply a decreasing volume density profile n sub(H) = (10 super(-4+ or -0.25))(R/R sub(vir)) super(-0+ or -0.3). Our derived gas volume densities are several orders of magnitude lower than predictions from standard two-phase models with a cool medium in pressure equilibrium with a hot, coronal medium expected in virialized halos at this mass scale. Applying the ionization corrections to the HI column densities, we estimate a lower limit to the cool gas mass M super(cool) sub(CGM) > 6.5 x 10 super(10) Mmiddot in circle for the volume within R < R sub(vir). Allowing for an additional warm-hot, O VI-traced phase, the CGM accounts for at least half of the baryons purported to be missing from dark matter halos at the 10 super(12) Mmiddot in circle scale.
Properties of the Magellanic Corona Lucchini, Scott; D’Onghia, Elena; Fox, Andrew J.
The Astrophysical journal,
05/2024, Volume:
967, Issue:
1
Journal Article
Peer reviewed
Open access
Abstract We characterize the Magellanic Corona, the warm gaseous halo around the Large Magellanic Cloud (LMC). The Corona is a key ingredient in the formation of the Magellanic Stream and has ...recently been observed in high-ion absorption around the LMC. In this work, we present a suite of high-resolution hydrodynamical simulations to constrain its total mass and temperature prior to the infall of the Magellanic Clouds to our Galaxy. We find that the LMC is able to host a stable Corona before and during its approach to the MW through to the present day. With a Magellanic Corona of >2 × 10 9 M ⊙ at 3 × 10 5 K, our simulations can reproduce the observed total mass of the neutral and ionized components of the Trailing Stream, the size of the LMC disk, the ionization fractions along the Stream, the morphology of the neutral gas, and the on-sky extent of the ionized gas. The Corona plays an integral role in the survival, morphology, and composition of the Magellanic Clouds and the Trailing Stream.
ABSTRACT
We present spatially resolved measurements of cool gas flowing into and out of the Milky Way (MW), using archival ultraviolet spectra of background quasars from the Hubble Space ...Telescope/Cosmic Origins Spectrograph. We co-add spectra of different background sources at close projected angular separation on the sky. This novel stacking technique dramatically increases the signal-to-noise ratio of the spectra, allowing the detection of low-column-density gas (down to EW > 2 mÅ). We identify absorption as inflowing or outflowing, by using blue/redshifted high-velocity cloud absorption components in the Galactocentric rest frame, respectively. The mass surface densities of both inflowing and outflowing gases vary by more than an order of magnitude across the sky, with mean values of 〈Σin〉 ≳ 104.6 ± 0.1$\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}$ for inflowing gas and 〈Σout〉 ≳ 103.5 ± 0.1$\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}$ for outflowing gas. The mass flow rate surface densities (mass flow rates per unit area) also show large variation across the sky, with $\langle \dot{\Sigma }(d)_{\rm in}\rangle \gtrsim (10^{-3.6\pm 0.1})(d/12\, \mathrm{kpc})^{-1}$ $\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}\, \mathrm{yr}^{-1}$ for inflowing gas and $\langle \dot{\Sigma }(d)_{\rm out}\rangle \gtrsim (10^{-4.8\pm 0.1})(d/12\, \mathrm{kpc})^{-1}$ $\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}\, \mathrm{yr}^{-1}$ for outflowing gas. The regions with highest surface mass density of inflowing gas are clustered at smaller angular scales (θ < 40°). This indicates that most of the mass in inflowing gas is confined to small, well-defined structures, whereas the distribution of outflowing gas is spread more uniformly throughout the sky. Our study confirms that the MW is predominantly accreting gas, but it is also losing a non-negligible mass of gas via outflow.
The Large-scale Ionization Cones in the Galaxy Bland-Hawthorn, Joss; Maloney, Philip R.; Sutherland, Ralph ...
The Astrophysical journal,
11/2019, Volume:
886, Issue:
1
Journal Article
Peer reviewed
Open access
There is compelling evidence for a highly energetic Seyfert explosion (1056-57 erg) that occurred in the Galactic center a few million years ago. The clearest indications are the X-ray/γ-ray "10 kpc ...bubbles" identified by the ROSAT and Fermi satellites. In an earlier paper, we suggested another manifestation of this nuclear activity, i.e., elevated H emission along a section of the Magellanic Stream due to a burst (or flare) of ionizing radiation from Sgr A*. We now provide further evidence for a powerful flare event: UV absorption line ratios (in particular / , Si iv/Si ii) observed by the Hubble Space Telescope reveal that some Magellanic Stream clouds toward both galactic poles are highly ionized by a source capable of producing ionization energies up to at least 50 eV. We show how these are clouds caught in a beam of bipolar, radiative "ionization cones" from a Seyfert nucleus associated with Sgr A*. In our model, the biconic axis is tilted by about 15° from the south Galactic pole with an opening angle of roughly 60°. For the Magellanic Stream at such large Galactic distances (D 75 kpc), nuclear activity is a plausible explanation for all of the observed signatures: elevated H emission and H ionization fraction (xe 0.5), enhanced / and Si iv/Si ii ratios, and high and Si iv column densities. Wind-driven "shock cones" are ruled out because the Fermi bubbles lose their momentum and energy to the Galactic corona long before reaching the Magellanic Stream. Our time-dependent Galactic ionization model (stellar populations, hot coronal gas, cloud-halo interaction) is too weak to explain the Magellanic Stream's ionization. Instead, the nuclear flare event must have had a radiative UV luminosity close to the Eddington limit (fE 0.1-1). Our time-dependent Seyfert flare models adequately explain the observations and indicate that the Seyfert flare event took place To = 3.5 1 Myr ago. The timing estimates are consistent with the mechanical timescales needed to explain the X-ray/γ-ray bubbles in leptonic jet/wind models ( 2-8 Myr).
Background IL-25 and IL-33 belong to distinct cytokine families, but experimental mouse studies suggest their immunologic functions in type 2 immunity are almost entirely overlapping. However, only ...polymorphisms in the IL-33 pathway ( IL1RL1 and IL33 ) have been significantly associated with asthma in large-cohort genome-wide association studies. Objective We sought to identify distinct pathways for IL-25 and IL-33 in the lung that might provide insight into their roles in asthma pathogenesis and potential for therapeutic intervention. Methods IL-25 receptor–deficient (Il17rb −/− ) , IL-33 receptor–deficient (ST2, Il1rl1−/− ), and double-deficient (Il17rb −/− Il1rl1 −/− ) mice were analyzed in models of allergic asthma. Microarrays, an ex vivo lung slice airway contraction model, and Il13+/eGFP mice were then used to identify specific effects of IL-25 and IL-33 administration. Results Comparison of IL-25 and IL-33 pathway–deficient mice demonstrates that IL-33 signaling plays a more important in vivo role in airways hyperreactivity than IL-25. Furthermore, methacholine-induced airway contraction ex vivo increases after treatment with IL-33 but not IL-25. This is dependent on expression of the IL-33 receptor and type 2 cytokines. Confocal studies with Il13+/eGFP mice show that IL-33 more potently induces expansion of IL-13–producing type 2 innate lymphoid cells, correlating with airway contraction. This predominance of IL-33 activity is enforced in vivo because IL-33 is more rapidly expressed and released in comparison with IL-25. Conclusion Our data demonstrate that IL-33 plays a critical role in the rapid induction of airway contraction by stimulating the prompt expansion of IL-13–producing type 2 innate lymphoid cells, whereas IL-25–induced responses are slower and less potent.
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