318 research outputs found
Chandra Detection of Massive Black Holes in Galactic Cooling Flows
Anticipating forthcoming observations with the Chandra X-ray telescope, we
describe the continuation of interstellar cooling flows deep into the cores of
elliptical galaxies. Interstellar gas within about r = 50 parsecs from the
massive black hole is heated to T > 1 keV and should be visible unless thermal
heating is diluted by non-thermal pressure. Since our flows are subsonic near
the massive black holes, distributed cooling continues within 300 pc from the
center. Dark, low mass stars formed in this region may be responsible for some
of the mass attributed to central black holes.Comment: 6 pages with 3 figures; accepted by Astrophysical Journal Letter
Spitzer Observations of Transient, Extended Dust in Two Elliptical Galaxies: New Evidence of Recent Feedback Energy Release in Galactic Cores
Spitzer observations of extended dust in two optically normal elliptical
galaxies provide a new confirmation of buoyant feedback outflow in the hot gas
atmospheres around these galaxies. AGN feedback energy is required to prevent
wholesale cooling and star formation in these group-centered galaxies. In NGC
5044 we observe interstellar (presumably PAH) emission at 8 microns out to
about 5 kpc. Both NGC 5044 and 4636 have extended 70 microns emission from cold
dust exceeding that expected from stellar mass loss. The sputtering lifetime of
this extended dust in the ~1keV interstellar gas, ~10^7 yrs, establishes the
time when the dust first entered the hot gas. Evidently the extended dust
originated in dusty disks or clouds, commonly observed in elliptical galaxy
cores, that were disrupted, heated and buoyantly transported outward. The
surviving central dust in NGC 5044 and 4636 has been disrupted into many small
filaments. It is remarkable that the asymmetrically extended 8 micron emission
in NGC 5044 is spatially coincident with Halpha+[NII] emission from warm gas. A
calculation shows that dust-assisted cooling in buoyant hot gas moving out from
the galactic core can cool within a few kpc in about ~10^7 yrs, explaining the
optical line emission observed. The X-ray images of both galaxies are
disturbed. All timescales for transient activity - restoration of equilibrium
and buoyant transport in the hot gas, dynamics of surviving dust fragments, and
dust sputtering - are consistent with a central release of feedback energy in
both galaxies about 10^7 yrs ago.Comment: 13 pages. Accepted by ApJ; minor typos correcte
Evolution of Hot Gas and Dark Halos in Group-Dominant Elliptical Galaxies: Influence of Cosmic Inflow
We study the complete dynamical evolution of hot interstellar gas in massive
elliptical galaxies born into a simple flat universe beginning with an
overdense perturbation. Within the turn-around radius dark matter flows in a
self-similar fashion into a stationary Navarro-Frenk-White halo and the
baryonic gas shocks. After a few gigayears, when enough gas accumulates within
the accretion shock, the de Vaucouleurs stellar system is constructed and the
energy from Type II supernovae is released. The stars and dark halo are matched
to NGC 4472. Gas continues to enter the galaxy by secondary infall and by
stellar mass loss based on a Salpeter IMF. After about 13 Gyrs the temperature
and density distribution in the hot gas agree quite well with the hot
interstellar gas observed in NGC 4472. As a result of supernova-driven outflow,
the present day baryonic fraction has a deep minimum in the outer galactic
halo. When relatively gas-rich, X-ray luminous models are spatially truncated
at early times, simulating tidal events that may have occurred during galaxy
group dynamics, the current locus of truncated models lies just along the
, X-ray size correlation among well-observed ellipticals, providing
another striking confirmation of our simple model of elliptical evolution.Comment: 16 pages in AASTEX LaTeX with 14 figures; accepted by Astrophysical
Journa
Recent X-ray Observations and the Evolution of Hot Gas in Elliptical Galaxies: Evidence for Circumgalactic Gas
X-ray emitting gaseous halos, such as that in elliptical galaxies like NGC
4472, cannot have been produced solely from gas expelled from galactic stars.
In traditional models for the evolution of hot interstellar gas (cooling flows)
in ellipticals, the galaxies are assumed to have been cleared of gas by
SNII-driven winds at some early time then gas is subsequently replenished by
mass loss from an evolving population of old stars. To test this, we accurately
determine the stellar and dark halo mass of NGC 4472 using hydrostatic
equilibrium, then solve the standard time-dependent cooling flow equations to
recover the observed hot gas temperature and density distributions when evolved
to the present time. This procedure fails: the computed gas density gradient is
too steep, the total gas mass is too low, and the gas temperatures are much too
low. All variants on this basic procedure also fail: increasing the SNIa rate,
using the mass dropout assumption, arbitrarily adjusting uncertain
coefficients, etc. However, agreement is achieved if the galaxy is supplied
with additional, spatially-extended hot gas early in its evolution. This old
``circumgalactic'' gas can be retained to the present time and may be related
to cosmological ``secondary infall''.Comment: 15 pages in two-column AASTEX LaTeX including 1 table and 8 figures;
abstract corrected in replacement; accepted by Astrophysical Journa
Cooling Flow Star Formation and the Apparent Stellar Ages of Elliptical Galaxies
Observational constraints and theoretical arguments indicate that cooled
interstellar gas in bright elliptical galaxies forms into a young stellar
population throughout the region within the half-light radius. The young
population has a bottom-heavy, but optically luminous IMF extending to 1 - 2
M_sun. When the colors and spectral features of this young population are
combined with those of the underlying old stellar population, the apparent ages
are significantly reduced, similar to the relatively young apparent ages
observed in many ellipticals. Galactic mergers are not required to resupply
young stars. The sensitivity of continuous star formation to L_B and L_x/L_B is
likely to account for the observed spread in apparent ages among elliptical
galaxies. Local star formation is accompanied by enhanced stellar H_beta
equivalent widths, stronger optical emission lines, enhanced thermal X-ray
emission and lower apparent temperatures in the hot gas. The young stars should
cause M/L to vary with galactic radius, perturbing the fundamental plane
occupied by the old stars.Comment: 6 pages with 2 figures; accepted by Astrophysical Journal Letter
Where Do Cooling Flows Cool?
Although only about 5 percent of the total baryonic mass in luminous
elliptical galaxies is in the form of cooled interstellar gas, it is
concentrated within the optical effective radius r_e where it influences the
local dynamical mass. The mass of cooled gas must be spatially distributed
since it greatly exceeds the masses of central black holes. We explore here the
proposition that a population of low mass, optically dark stars is created from
the cooled gas. We consider a wide variety of radial distributions for the
interstellar cooling, but only a few are consistent with observed X-ray surface
brightness profiles. In a region of concentrated interstellar cooling, the
X-ray emission can exceed that observed, suggesting the presence of additional
support by magnetic stresses or non-thermal pressure. In general we find that
the mass of cooled gas contributes significantly to stellar dynamical mass to
light ratios which vary with galactic radius. If the stars formed from cooled
interstellar gas are optically luminous, their influence on the the mass to
light ratio would be reduced. The mass of cooled gas inside r_e is sensitive to
the rate that old stars lose mass, which is nearly independent of the initial
mass function of the old stellar population.Comment: 18 pages with 6 figures; accepted by Astrophysical Journa
Self-Generated Magnetic Fields in Galactic Cooling Flows
Interstellar magnetic fields in elliptical galaxies are assumed to have their
origin in stellar fields that accompany normal mass loss from an evolving
population of old stars. The seed fields are amplified by interstellar
turbulence driven by stellar mass loss and supernova events. These disordered
fields are further amplified by time-dependent compression in the inward moving
galactic cooling flow and are expected to dominate near the galactic core.
Under favorable circumstances, fields similar in strength to those observed G can be generated solely from these natural
galactic processes. In general the interstellar field throughout elliptical
galaxies is determined by the outermost regions in the interstellar gas where
the turbulent dynamo process can occur. Because of the long hydrodynamic flow
times in galactic cooling flows, currently observed magnetic fields may result
from periods of intense turbulent field amplification that occurred in the
outer galaxy in the distant past. Particularly strong fields in ellipticals may
result from ancient galactic mergers or shear turbulence introduced at the
boundary between the interstellar gas and ambient cluster gas.Comment: 21 pages in AASTEX LaTeX with 2 figures; accepted by Astrophysical
Journa
Why Are Rotating Elliptical Galaxies Less Elliptical at X-ray Frequencies?
If mass and angular momentum were conserved in cooling flows associated with
luminous, slowly rotating elliptical galaxies, the inflowing hot gas would spin
up, resulting in disks of cold gas and X-ray images that are highly flattened
along the equatorial plane out to several effective radii. Such X-ray
flattening is not observed at the spatial resolution currently available to
X-ray observations. Evidently mass and angular momentum are not in fact
conserved. If cooling flows are depleted by localized radiative cooling at
numerous sites distributed throughout the flows, then disks of cooled gas do
not form and the X-ray images appear nearly circular. However, the distribution
of young stars formed from the cooled gas is still somewhat flattened relative
to the stellar light. X-ray images of galactic cooling flows can also be
circularized by the turbulent diffusion of angular momentum away from the axis
of rotation, but the effective viscosity of known processes -- stellar mass
loss, supernovae, cooling site evolution, etc. -- is insufficient to
appreciably circularize the X-ray images. Radial gradients in the interstellar
iron abundance are unaffected by the expected level of interstellar turbulence
since these gradients are continuously re-established by Type Ia supernovae.Comment: 17 pages with 6 figures; accepted by Astrophysical Journa
Influence of Cooled Interstellar Gas on the Fundamental Plane for Elliptical Galaxies
We explore the possibly important influence of cooled interstellar gas on the
fundamental plane of elliptical galaxies. Interstellar cooling is described by
a parameterized sink term in the equation of continuity. Parameters that give
the best fits to the X-ray observations of NGC 4472 are used as a template for
the radial distribution of interstellar cooling in structurally homologous
elliptical galaxies of lower mass. Gas that cools within an effective radius
can contribute an additional 10 - 30 percent to the mass of the old stellar
population. If the cooled gas forms into stars of very low mass, , as is commonly assumed, the cooled mass is optically dark. As a
result, the mass to light ratios determined from stellar velocities
systematically overestimate that of the old stellar population. Moreover, the
total mass and spatial distribution of the optically dark young stellar
population does not scale homologously with galactic luminosity or radius and
the total stellar mass to light ratio varies with galactic radius. We
investigate the non-homologous perturbations of cooled gas on the mass to light
ratio for several idealized homologous elliptical galaxies and show that they
appear to be incompatible with the observed thinness of the fundamental plane.
If optically luminous young stars formed from the cooled gas, the disturbance
of the fundamental plane would be lessened.Comment: 10 pages with 2 figures; accepted by Astrophysical Journa
Probing the Dark Matter and Gas Fraction in Relaxed Galaxy Groups with X-ray observations from Chandra and XMM
We present radial mass profiles within 0.3 r_vir for 16 relaxed galaxy
groups-poor clusters (kT range 1-3 keV) selected for optimal mass constraints
from the Chandra and XMM data archives. After accounting for the mass of hot
gas, the resulting mass profiles are described well by a two-component model
consisting of dark matter (DM), represented by an NFW model, and stars from the
central galaxy. The stellar component is required only for 8 systems, for which
reasonable stellar mass-to-light ratios (M/L_K) are obtained, assuming a Kroupa
IMF. Modifying the NFW dark matter halo by adiabatic contraction does not
improve the fit and yields systematically lower M/L_K. In contrast to previous
results for massive clusters, we find that the NFW concentration parameter
(c_vir) for groups decreases with increasing M_vir and is inconsistent with no
variation at the 3 sigma level. The normalization and slope of the c_vir-M_vir
relation are consistent with the standard LambdaCDM cosmological model with
sigma_8 = 0.9. The small intrinsic scatter measured about the c_vir-M_vir
relation implies the groups represent preferentially relaxed, early forming
systems. The mean gas fraction (f =0.05 +/- 0.01) of the groups measured within
an overdensity Delta=2500 is lower than for hot, massive clusters, but the
fractional scatter (sigma_f/f=0.2) for groups is larger, implying a greater
impact of feedback processes on groups, as expected.Comment: Accepted for Publication in ApJ; 30 pages, 9 figures. No changes from
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