9 research outputs found
The Origins of the Circumgalactic Medium in the FIRE Simulations
We use a particle tracking analysis to study the origins of the
circumgalactic medium (CGM), separating it into (1) accretion from the
intergalactic medium (IGM), (2) wind from the central galaxy, and (3) gas
ejected from other galaxies. Our sample consists of 21 FIRE-2 simulations,
spanning the halo mass range log(Mh/Msun) ~ 10-12 , and we focus on z=0.25 and
z=2. Owing to strong stellar feedback, only ~L* halos retain a baryon mass
>~50% of their cosmic budget. Metals are more efficiently retained by halos,
with a retention fraction >~50%. Across all masses and redshifts analyzed >~60%
of the CGM mass originates as IGM accretion (some of which is associated with
infalling halos). Overall, the second most important contribution is wind from
the central galaxy, though gas ejected or stripped from satellites can
contribute a comparable mass in ~L* halos. Gas can persist in the CGM for
billions of years, resulting in well-mixed halo gas. Sight lines through the
CGM are therefore likely to intersect gas of multiple origins. For low-redshift
~L* halos, cool gas (T<10^4.7 K) is distributed on average preferentially along
the galaxy plane, however with strong halo-to-halo variability. The metallicity
of IGM accretion is systematically lower than the metallicity of winds
(typically by >~1 dex), although CGM and IGM metallicities depend significantly
on the treatment of subgrid metal diffusion. Our results highlight the multiple
physical mechanisms that contribute to the CGM and will inform observational
efforts to develop a cohesive picture.Comment: 23 pages, 22 figures. Minor revisions from previous version. Online
interactive visualizations available at zhafen.github.io/CGM-origins and
zhafen.github.io/CGM-origins-pathline
Coronal Properties of the Seyfert 1.9 Galaxy MCG -05-23-016 Determined from Hard X-ray Spectroscopy with NuSTAR
Measurements of the high-energy cut-off in the coronal continuum of active
galactic nuclei have long been elusive for all but a small number of the
brightest examples. We present a direct measurement of the cut-off energy in
the nuclear continuum of the nearby Seyfert 1.9 galaxy MCG -05-23-016 with
unprecedented precision. The high sensitivity of NuSTAR up to 79 keV allows us
to clearly disentangle the spectral curvature of the primary continuum from
that of its reflection component. Using a simple phenomenological model for the
hard X-ray spectrum, we constrain the cut-off energy to keV
with 90% confidence. Testing for more complex models and nuisance parameters
that could potentially influence the measurement, we find that the cut-off is
detected robustly. We further use simple Comptonized plasma models to provide
independent constraints for both the kinetic temperature of the electrons in
the corona and its optical depth. At the 90% confidence level, we find
keV and assuming a slab (disk-like)
geometry, and keV and assuming a spherical
geometry. Both geometries are found to fit the data equally well and their two
principal physical parameters are correlated in both cases. With the optical
depth in the optically thick regime, the data are pushing the currently
available theoretical models of the Comptonized plasma to the limits of their
validity. Since the spectral features and variability arising from the inner
accretion disk have been observed previously in MCG -05-23-016, the inferred
high optical depth implies that a spherical or disk-like corona cannot be
homogeneous.Comment: 7 pages, 2 figures. Accepted for publication in Ap
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The fates of the circumgalactic medium in the FIRE simulations
We analyse the different fates of the circumgalactic medium (CGM) in FIRE-2 cosmological simulations, focusing on the redshifts z = 0.25 and 2 representative of recent surveys. Our analysis includes 21 zoom-in simulations covering the halo mass range Mh(z = 0) ∼ 1010-1012 M⊙. We analyse both where the gas ends up after first leaving the CGM (its 'proximate' fate) and its location at z = 0 (its 'ultimate' fate). Of the CGM at z = 2, about half is found in the ISM or stars of the central galaxy by z = 0 in Mh(z = 2) ∼ 5 × 1011 M⊙ haloes, but most of the CGM in lower mass haloes ends up in the intergalactic medium (IGM). This is so even though most of the CGM in Mh(z = 2) ∼ 5 × 1010 M⊙ haloes first accretes on to the central galaxy before being ejected into the IGM. On the other hand, most of the CGM mass at z = 0.25 remains in the CGM by z = 0 at all halo masses analysed. Of the CGM gas that subsequently accretes on to the central galaxy in the progenitors of Mh(z = 0) ∼ 1012 M⊙ haloes, most of it is cool (T ∼ 104 K) at z = 2 but hot (∼Tvir) at z ∼ 0.25, consistent with the expected transition from cold mode to hot mode accretion. Despite the transition in accretion mode, at both z =0.25 and 2 ≥ 80 per cent of the cool gas inMh ≥ 1011 M⊙ haloes will accrete on to a galaxy. We find that the metallicity of CGM gas is typically a poor predictor of both its proximate and ultimate fates. This is because there is in general little correlation between the origin of CGM gas and its fate owing to substantial mixing while in the CGM