141 research outputs found
Fundamental Properties of the Highly Ionized Plasmas in the Milky Way
The cooling transition temperature gas in the interstellar medium (ISM),
traced by the high ions, Si IV, C IV, N V, and O VI, helps to constrain the
flow of energy from the hot ISM with T >10^6 K to the warm ISM with T< 2x10^4
K. We investigate the properties of this gas along the lines of sight to 38
stars in the Milky Way disk using 1.5-2.7 km/s resolution spectra of Si IV, C
IV, and N V absorption from the Space Telescope Imaging Spectrograph (STIS),
and 15 km/s resolution spectra of O VI absorption from the Far Ultraviolet
Spectroscopic Explorer (FUSE). The absorption by Si IV and C IV exhibits broad
and narrow components while only broad components are seen in N V and O VI. The
narrow components imply gas with T<7x10^4 K and trace two distinct types of
gas. The strong, saturated, and narrow Si IV and C IV components trace the gas
associated with the vicinities of O-type stars and their supershells. The
weaker narrow Si IV and C IV components trace gas in the general ISM that is
photoionized by the EUV radiation from cooling hot gas or has radiatively
cooled in a non-equilibrium manner from the transition temperature phase, but
rarely the warm ionized medium (WIM) probed by Al III. The broad Si IV, C IV, N
V, and O VI components trace collisionally ionized gas that is very likely
undergoing a cooling transition from the hot ISM to the warm ISM. The cooling
process possibly provides the regulation mechanism that produces N(C IV)/N(Si
IV) = 3.9 +/- 1.9. The cooling process also produces absorption lines where the
median and mean values of the line widths increase with the energy required to
create the ion.Comment: Accepted for publication in the ApJ. Only this PDF file contains all
the figures and tables in a single fil
CGM properties in VELA and NIHAO simulations; the OVI ionization mechanism: dependence on redshift, halo mass and radius
We study the components of cool and warm/hot gas in the circumgalactic medium
(CGM) of simulated galaxies and address the relative production of OVI by
photoionization versus collisional ionization, as a function of halo mass,
redshift, and distance from the galaxy halo center. This is done utilizing two
different suites of zoom-in hydro-cosmological simulations, VELA (6 halos;
) and NIHAO (18 halos; to ), which provide a broad theoretical basis
because they use different codes and physical recipes for star formation and
feedback. In all halos studied in this work, we find that collisional
ionization by thermal electrons dominates at high redshift, while
photoionization of cool or warm gas by the metagalactic radiation takes over
near . In halos of and above, collisions become
important again at , while photoionization remains significant down to
for less massive halos. In halos with , at most of the photoionized OVI is in a
warm, not cool, gas phase (~K). We also find that
collisions are dominant in the central regions of halos, while photoionization
is more significant at the outskirts, around , even in massive
halos. This too may be explained by the presence of warm gas or, in lower mass
halos, by cool gas inflows
Characterizing Transition Temperature Gas in the Galactic Corona
We present a study of the properties of the transition temperature (T~10^5 K)
gas in the Milky Way corona, based on measurements of OVI, NV, CIV, SiIV and
FeIII absorption lines seen in the far ultraviolet spectra of 58 sightlines to
extragalactic targets, obtained with Far-Ultraviolet Spectroscopic Explorer
(FUSE) and Space Telescope Imaging Spectrograph. In many sightlines the
Galactic absorption profiles show multiple components, which are analyzed
separately. We find that the highly-ionized atoms are distributed irregularly
in a layer with a scaleheight of about 3 kpc, which rotates along with the gas
in the disk, without an obvious gradient in the rotation velocity away from the
Galactic plane. Within this layer the gas has randomly oriented velocities with
a dispersion of 40-60 km/s. On average the integrated column densities are log
N(OVI)=14.3, log N(NV)=13.5, log N(CIV)=14.2, log N(SiIV)=13.6 and log
N(FeIII)=14.2, with a dispersion of just 0.2 dex in each case. In sightlines
around the Galactic Center and Galactic North Pole all column densities are
enhanced by a factor ~2, while at intermediate latitudes in the southern sky
there is a deficit in N(OVI) of about a factor 2, but no deficit for the other
ions. We compare the column densities and ionic ratios to a series of
theoretical predictions: collisional ionization equilibrium, shock ionization,
conductive interfaces, turbulent mixing, thick disk supernovae, static
non-equilibrium ionization (NIE) radiative cooling and an NIE radiative cooling
model in which the gas flows through the cooling zone. None of these models can
fully reproduce the data, but it is clear that non-equilibrium ionization
radiative cooling is important in generating the transition temperature gas.Comment: 99 pages, 11 figures, with appendix on Cooling Flow model; only a
sample of 5 subfigures of figure 2 included - full set of 69 available
through Ap
An Exact Integration Scheme for Radiative Cooling in Hydrodynamical Simulations
A new scheme for incorporating radiative cooling in hydrodynamical codes is
presented, centered around exact integration of the governing semi-discrete
cooling equation. Using benchmark calculations based on the cooling downstream
of a radiative shock, I demonstrate that the new scheme outperforms traditional
explicit and implicit approaches in terms of accuracy, while remaining
competitive in terms of execution speed.Comment: 7 pages, accepted by ApJS. Revision 2, with error in eqn. 13 fixe
Exploring the Origin and Fate of the Magellanic Stream with Ultraviolet and Optical Absorption
(Abridged) We present an analysis of ionization and metal enrichment in the
Magellanic Stream (MS), the nearest gaseous tidal stream, using HST/STIS and
FUSE ultraviolet spectroscopy of two background AGN, NGC 7469 and Mrk 335. For
NGC 7469, we include optical spectroscopy from VLT/UVES. In both sightlines the
MS is detected in low-ion and high-ion absorption. Toward NGC 7469, we measure
a MS oxygen abundance [O/H]_MS=[OI/HI]=-1.00+/-0.05(stat)+/-0.08(syst),
supporting the view that the Stream originates in the SMC rather than the LMC.
We use CLOUDY to model the low-ion phase of the Stream as a photoionized plasma
using the observed Si III/Si II and C III/C II ratios. Toward Mrk 335 this
yields an ionization parameter log U between -3.45 and -3.15 and a gas density
log (n_H/cm^-3) between -2.51 and -2.21. Toward NGC 7469 we derive sub-solar
abundance ratios for [Si/O], [Fe/O], and [Al/O], indicating the presence of
dust in the MS. The high-ion column densities are too large to be explained by
photoionization, but also cannot be explained by a single-temperature
collisional-ionization model (equilibrium or non-equilibrium). This suggests
the high-ion plasma is multi-phase. Summing over the low-ion and high-ion
phases, we derive conservative lower limits on the ratio N(total H II)/N(H I)
of >19 toward NGC 7469 and >330 toward Mrk 335, showing that along these two
directions the vast majority of the Stream has been ionized. The presence of
warm-hot plasma together with the small-scale structure observed at 21 cm
provides evidence for an evaporative interaction with the hot Galactic corona.
This scenario, predicted by hydrodynamical simulations, suggests that the fate
of the MS will be to replenish the Galactic corona with new plasma, rather than
to bring neutral fuel to the disk.Comment: Accepted for publication in ApJ. 18 pages, 7 figures, all in colo
Cos observations of metal line and broad lyman alpha absorption in the multi-phase o vi and ne viii system toward he 02226-4110
Observations of the QSO HE 0226-4110 (zem = 0.495) with the Cosmic Origins
Spectrograph (COS) from 1134 to 1796 {\AA} with a resolution of ~17 km s-1 and
signal-to- noise (S/N) per resolution element of 20 to 40 are used to study the
multi-phase absorption system at z = 0.20701 containing O VI and Ne VIII. The
system was previously studied with lower S/N observations with FUSE and STIS.
The COS observations provide more reliable measures of the H I and metal lines
present in the system and reveal the clear presence of broad Lyman {\alpha}
(BLA) absorption with b = 72(+13, -6) km s-1 and logN(H I) = 13.87\pm0.08.
Detecting BLAs associated with warm gas absorbers is crucial for determining
the temperature, metallicity and total baryonic content of the absorbers. The
BLA is probably recording the trace amount of thermally broadened H I in the
collisionally ionized plasma with log T ~5.7 that also produces the O VI and Ne
VIII absorption. The total hydrogen column in the collisionally ionized gas,
logN(H) ~ 20.1, exceeds that in the cooler photoionized gas in the system by a
factor of ~22. The oxygen abundance in the collisionally ionized gas is [O/H] =
-0.89\pm0.08\pm0.07. The absorber probably occurs in the circumgalactic
environment (halo) of a foreground L = 0.25L* disk galaxy with an impact
parameter of 109h70-1 kpc identified by Mulchaey & Chen (2009).Comment: 20 pages and 5 figures. Accepted by the Astrophysical Journa
Turbulence and the formation of filaments, loops and shock fronts in NGC 1275 in the Perseus Galaxy Cluster
NGC1275, the central galaxy in the Perseus cluster, is the host of gigantic
hot bipolar bubbles inflated by AGN jets observed in the radio as Perseus A. It
presents a spectacular -emitting nebulosity surrounding NGC1275,
with loops and filaments of gas extending to over 50 kpc. The origin of the
filaments is still unknown, but probably correlates with the mechanism
responsible for the giant buoyant bubbles. We present 2.5 and 3-dimensional MHD
simulations of the central region of the cluster in which turbulent energy,
possibly triggered by star formation and supernovae (SNe) explosions is
introduced. The simulations reveal that the turbulence injected by massive
stars could be responsible for the nearly isotropic distribution of filaments
and loops that drag magnetic fields upward as indicated by recent observations.
Weak shell-like shock fronts propagating into the ICM with velocities of
100-500 km/s are found, also resembling the observations. The isotropic outflow
momentum of the turbulence slows the infall of the intracluster medium, thus
limiting further starburst activity in NGC1275. As the turbulence is subsonic
over most of the simulated volume, the turbulent kinetic energy is not
efficiently converted into heat and additional heating is required to suppress
the cooling flow at the core of the cluster. Simulations combining the MHD
turbulence with the AGN outflow can reproduce the temperature radial profile
observed around NGC1275. While the AGN mechanism is the main heating source,
the supernovae are crucial to isotropize the energy distribution.Comment: accepted by ApJ Letter
MHD numerical simulations of colliding winds in massive binary systems - I. Thermal vs non-thermal radio emission
In the past few decades detailed observations of radio and X-rays emission
from massive binary systems revealed a whole new physics present in such
systems. Both thermal and non-thermal components of this emission indicate that
most of the radiation at these bands originates in shocks. OB and WR stars
present supersonic and massive winds that, when colliding, emit largely due to
the free-free radiation. The non-thermal radio and X-ray emissions are due to
synchrotron and inverse compton processes, respectively. In this case, magnetic
fields are expected to play an important role on the emission distribution. In
the past few years the modeling of the free-free and synchrotron emissions from
massive binary systems have been based on purely hydrodynamical simulations,
and ad hoc assumptions regarding the distribution of magnetic energy and the
field geometry. In this work we provide the first full MHD numerical
simulations of wind-wind collision in massive binary systems. We study the
free-free emission characterizing its dependence on the stellar and orbital
parameters. We also study self-consistently the evolution of the magnetic field
at the shock region, obtaining also the synchrotron energy distribution
integrated along different lines of sight. We show that the magnetic field in
the shocks is larger than that obtained when the proportionality between
and the plasma density is assumed. Also, we show that the role of the
synchrotron emission relative to the total radio emission has been
underestimated.Comment: MNRAS accepte
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