1,721,068 research outputs found
The Ages of Elliptical Galaxies from Mid-Infrared Emission
No full textThe mid-infrared (10–20 m) luminosity of elliptical galaxies is dominated by the integrated emission from
circumstellar dust in red giant stars. As a single stellar population evolves, the rate of dusty mass loss from red giant
stars decreases with time, so the mid-infrared luminosity should also decline with stellar age. To seek such a
correlation, we have used archival Infrared Space Observatory (ISO) observations to determine surface brightness
profiles and central fluxes at 15 m in 17 early-type galaxies for which stellar ages have been determined from
optical spectral indices. The radial surface brightness distributions at 15 m generally follow the stellar de Vaucouleurs
profile, as expected. We find that the surface brightness ratio 15 m=I band is systematically higher in
elliptical galaxies with ages P5 Gyr and in galaxies that exhibit evidence of recent mergers. Within the accuracy
of our observations, 15 m=I band shows no age dependence for ages k5 Gyr. The corresponding flux ratios
F15 m=FI band within apertures scaled to the effective radius (Re=8) are proportional to the 15 m=I band ratios at
larger galactic radii, indicating that no 15 m emission is detected from central dust clouds visible in optical images
in some of our sample galaxies. Emission at 15 m is observed in noncentral massive clouds of dust and cold gas in
NGC 1316, an elliptical galaxy that is thought to have had a recent merger. Recent Spitzer Space Telescope data also
indicate the presence of polycyclic aromatic hydrocarbon (PAH ) emission at 8 m. Several ellipticals have extended
regions of 15 m emission that have no obvious counterparts at other frequencies
Spitzer Observations of Transient, Extended Dust in Two Elliptical Galaxies: New Evidence of Recent Feedback Energy Release in Galactic Cores
No full textSpitzer 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 m out to about 5 kpc. Both NGC 5044 and NGC 4636 have extended 70 m emission from cold dust
exceeding that expected from stellar mass loss. The sputtering lifetime of this extended dust in the 1 keVinterstellar gas,
107 yr, 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 NGC 4636 has been disrupted into many small filaments. It is remarkable that
the asymmetrically extended 8 m emission in NGC 5044 is spatially coincident with H+[N ii] 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
kiloparsecs in 107 yr, 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
107 years ago
The Mid-Infrared Spectral Energy Distribution, Surface Brightness, and Color Profiles in Elliptical Galaxies
No full textWe combine 2MASS data and Spitzer archival data to study the emission in mid-infrared passbands (1.2Y24 m)
from a sample of 18 elliptical galaxies. In general the surface brightness distributions resemble de Vaucouleurs pro-
files, indicating that most of the emission arises from the photospheres or circumstellar regions of red giant stars. The
spectral energy distribution peaks near the H band at 1.6 m. The half-light or effective radius has a pronounced minimum
near the K band (2.15 m) with a second, less consistent minimum in the 24 m passband. All sample-averaged
radial color profiles hki kji, where ki < kj (and j 61⁄4 24 m), have positive slopes within about twice the (K-band)
effective radius. Evidently this variation arises because of an increase in stellar metallicity toward the galactic cores.
Color profiles hK ji all have positive slopes, particularly when j 1⁄4 5:8 m, although no obvious absorption feature
is observed in spectra of elliptical galaxies near 5.8 m. This, and the minimum in Re, suggests that the K band may be
anomalously luminous in metal-rich stars in galaxy cores. Unusual radial color profiles involving the 24 m passband
may suggest that some 24 m emission comes from interstellar not circumstellar dust grains
Mid-Infrared Emission from Elliptical Galaxies: Sensitivity to Stellar Age
No full textMid-infrared observations (3.6–24 mm) of normal giant elliptical galaxies with the Spitzer Space Telescope
are consistent with pure populations of very old stars with no evidence of younger stars. Most of the stars in
giant elliptical galaxies are old, but the mean stellar age determined from Balmer absorption in optical spectra
can appear much younger due to a small admixture of younger stars. The mean stellar age can also be determined
from the spectral energy distribution in the mid-infrared, which decreases with time relative to the optical emission
and shifts to shorter wavelengths. The observed flux ratios and for elliptical galaxies with F8 mm 3.6 /F F mm 24 mm 3.6 /F mm
the oldest Balmer line ages are lower than predicted by recent models of single stellar populations. For elliptical
galaxies with the youngest Balmer line ages in our sample, 3–5 Gyr, the flux ratios are identical to F24 mm 3.6 /F mm
those of the oldest stars. When theoretical mid-IR spectra of old (12 Gyr) and young stellar populations are
combined, errors in the observations are formally inconsistent with a mass fraction of young stars F24 mm 3.6 /F mm
that exceeds ∼1%. This is less than the fraction of young stars expected in discussions of recent surveys of
elliptical galaxies at higher redshifts. However, this inconsistency between Balmer line ages and those inferred
from mid-IR observations must be regarded as provisional until more accurate observations and theoretical spectra
become available. Finally, there is no evidence to date that central disks or patches of dust commonly visible in
optical images of elliptical galaxies contribute sensibly to the mid-IR spectrum
Spitzer Observations of Passive and Star-Forming Early-Type Galaxies: An Infrared Color–Color Sequence
No full textWe describe the infrared properties of a large sample of early-type galaxies, comparing data from the Spitzer
archive with Ks-band emission from the Two Micron All Sky Survey. While most representations of this data
result in correlations with large scatter, we find a remarkably tight relation among colors formed by ratios of
luminosities in Spitzer-Multiband Imaging Photometer bands (24, 70, and 160 μm) and the Ks band. Remarkably,
this correlation among E and S0 galaxies follows that of nearby normal galaxies of all morphological types.
In particular, the tight infrared color–color correlation for S0 galaxies alone follows that of the entire Hubble
sequence of normal galaxies, roughly in order of galaxy type from ellipticals to spirals to irregulars. The specific
star formation rate (SFR) of S0 galaxies estimated from the 24 μm luminosity increases with decreasing K-band
luminosity (or stellar mass) from essentially zero, as with most massive ellipticals, to rates typical of irregular
galaxies. Moreover, the luminosities of the many infrared-luminous S0 galaxies can significantly exceed those
of the most luminous (presumably post-merger) E galaxies. SFRs in the most infrared-luminous S0 galaxies
approach 1–10 solar masses per year. Consistently, with this picture we find that while most early-type galaxies
populate an infrared red sequence, about 24% of the objects (mostly S0s) are in an infrared blue cloud together
with late-type galaxies. For those early-type galaxies also observed at radio frequencies, we find that the farinfrared
luminosities correlate with the mass of neutral and molecular hydrogen, but the scatter is large. This
scatter suggests that the star formation may be intermittent or that similar S0 galaxies with cold gaseous disks
of nearly equal mass can have varying radial column density distributions that alter the local and global SFRs
Evidence of Star Formation in Local S0 Galaxies: Spitzer Observations of the Sauron Sample
No full textWe discuss infrared Spitzer observations of early-type galaxies in the SAURON sample at 24, 60, and 170 μm. When compared with 2MASS Ks band luminosities, lenticular (S0) galaxies exhibit a much wider range of mid-to-far-infrared luminosities than elliptical (E) galaxies. Mid- and far-infrared emission from E galaxies is a combination of circumstellar or interstellar emission from local mass-losing red giant stars, dust buoyantly transported from the galactic cores into distant hot interstellar gas and dust accreted from the environment. The source of mid- and far-IR emission in S0 galaxies is quite different and is consistent with low levels of star formation, 0.02-0.2 M sun yr-1, in cold, dusty gaseous disks. The infrared 24 μm-70 μm color is systematically lower for (mostly S0) galaxies with known molecular disks. Our observations support the conjecture that cold dusty gas in some S0 galaxies is created by stellar mass loss at approximately the same rate that it is consumed by star formation, so the mass depletion of these disks by star formation will be slow. Unlike E galaxies, the infrared luminosities of S0 galaxies correlate with both the mass of molecular gas and the stellar Hβ spectral index, and all are related to the recent star formation rate (SFR). However, star formation rates estimated from the Hβ-emission-line luminosities L Hβ in SAURON S0 galaxies are generally much smaller. Since L Hβ does not correlate with 24 μm emission from dust heated by young stars, optical emission lines appear to be a poor indicator of SFRs in SAURON S0 galaxies. The absence of Hβ emission may be due to a relative absence of OB stars in the initial mass function or to dust absorption of Hβ emission lines
Far-Infrared Spitzer Observations of Elliptical Galaxies: Evidence for Extended Diffuse Dust
No full textFar-infrared Spitzer observations of elliptical galaxies are inconsistent with simple steady state models of dust
creation in red giant stars and destruction by grain sputtering in the hot interstellar gas at T 107 K. The flux at 24 m
correlates with optical fluxes, suggesting that this relatively hot dust is largely circumstellar. But fluxes at 70 and 160 m
do not correlate with optical fluxes. Elliptical galaxies with similar LB have luminosities at 70 and 160 m (L70 and L160)
that vary over a factor of 100, implying an additional source of dust unrelated to that produced by ongoing local stellar
mass loss. Neither L70/LB nor L160/LB correlate with the stellar age or metallicity. Optical line fluxes from warm gas at
T 104 K correlate weakly with L70 and L160, suggesting that the dust may be responsible for cooling this gas. Many
normal elliptical galaxies have emission at 70 m that is extended to 5Y10 kpc. Extended far-infrared emission with
sputtering lifetimes of 108 yr is difficult to maintain by mergers with gas-rich galaxies. Instead, we propose that this
cold dust is buoyantly transported from reservoirs of dust in the galactic cores, which are supplied by mass loss from
stars in the core. Intermittent energy outbursts from AGNs can drive the buoyant outflow
Mechanical AGN feedback: controlling the thermodynamical evolution of elliptical galaxies
Full text linkA fundamental gap in the current understanding of galaxies concerns the thermodynamical evolution of ordinary, baryonic matter. On the one hand, radiative emission drastically decreases the thermal energy content of the interstellar plasma (ISM), inducing a slow cooling flow towards the centre. On the other hand, the active galactic nucleus (AGN) struggles to prevent the runaway cooling catastrophe, injecting huge amount of energy into the ISM. The present study intends to investigate thoroughly the role of mechanical AGN feedback in (isolated or massive) elliptical galaxies, extending and completing the mass range of tested cosmic environments. Our previously successful feedback models in galaxy clusters and groups demonstrated that AGN outflows, self-regulated by cold gas accretion, are able to quench the cooling flow properly without destroying the cool core. Via three-dimensional hydrodynamic simulations (FLASH 3.3), also including stellar evolution, we show that massive mechanical AGN outflows can indeed solve the cooling-flow problem for the entire life of the galaxy, at the same time reproducing typical observational features and constraints such as buoyant under dense bubbles, elliptical shock cocoons, sonic ripples, dredge-up of metals, subsonic turbulence and extended filamentary or nuclear cold gas. In order to avoid overheating and totally emptying the isolated galaxy, the frequent mechanical AGN feedback should be less powerful and efficient (ϵ ∼ 10−4) compared with the heating required for more massive and bound ellipticals surrounded by the intragroup medium (ϵ ∼ 10−3)
Raining on black holes and massive galaxies: The top-down multiphase condensation model
Full text linkThe plasma haloes filling massive galaxies, groups and clusters are shaped by active galactic nucleus (AGN) heating and subsonic turbulence (σv ~ 150 km s-1), as probed by Hitomi. Novel 3D high-resolution simulations show the soft X-ray, keV hot plasma cools rapidly via radiative emission at the high-density interface of the turbulent eddies, stimulating a top-down condensation cascade of warm 104 K filaments. The kpc-scale ionized (optical/ultraviolet) filaments form a skin enveloping the neutral filaments (optical/infrared/21 cm). The peaks of the warm filaments further condense into cold molecular clouds (< 50 K; radio) with total mass of several 107M⊙ and inheriting the turbulent kinematics. In the core, the clouds collide inelastically, mixing angular momentum and leading to Chaotic Cold Accretion (CCA). The black hole accretion rate (BHAR) can be modelled via quasi-spherical viscous accretion, M ̇ • ∝ νc, with clump collisional viscosity νc ≡ λc σc and λc ~ 100 pc. Beyond the core, pressure torques shape the angular momentum transport. In CCA, the BHAR is recurrently boosted up to 2 dex compared with the disc evolution, which arises as turbulence becomes subdominant.With negligible rotation too, compressional heating inhibits the molecular phase. The CCA BHAR distribution is lognormal with pink noise, f-1 power spectrum characteristic of fractal phenomena. Such chaotic fluctuations can explain the rapid luminosity variability of AGN and high-mass X-ray binaries. An improved criterium to trace non-linear condensation is proposed: σν/νcool ≲ 1. The three-phase CCA reproduces key observations of cospatial multiphase gas in massive galaxies, including Chandra X-ray images, SOAR Hα filaments and kinematics, Herschel [C+] emission and ALMA molecular associations. CCA plays important role in AGN feedback and unification, the evolution of BHs, galaxies and clusters
Cold Dust in Early-Type Galaxies. I. Observations
No full textWe describe far-infrared observations of early-type galaxies selected from the Infrared Space Observatory
(ISO) archive. This rather inhomogeneous sample includes 39 giant elliptical galaxies and 14 S0 (or later)
galaxies. These galaxies were observed with the array photometer PHOT on-board the ISO satellite using a
variety of different observing modes—sparse maps, mini-maps, oversampled maps, and single pointings—each
of which requires different and often rather elaborate photometric reduction procedures. The ISO background
data agree well with the COBE-DIRBE results to which we have renormalized our calibrations. As a further
check, the ISO fluxes from galaxies at 60 and 100 m agree very well with those previously observed with IRAS
at these wavelengths. The spatial resolution of ISO is several times greater than that of IRAS, and the ISO
observations extend out to 200 m, which views a significantly greater mass of colder dust not assessable to
IRAS. Most of the galaxies are essentially point sources at ISO resolution, but a few are clearly extended at FIR
wavelengths with image sizes that increase with FIR wavelength. The integrated far-infrared luminosities do not
correlate with optical luminosities, suggesting that the dust may have an external, merger-related origin. In
general, the far-infrared spectral energy distributions can be modeled with dust at two temperatures, 43 and
20 K, which probably represent limits of a continuous range of temperatures. The colder dust component
dominates the total mass of dust, 106–107 M, which is typically more than 10 times larger than the dust masses
previously estimated for the same galaxies using IRAS observations. For S0 galaxies we find that the optically
normalized far-infrared luminosity LFIR=LB correlates strongly with the mid-infrared luminosity L15 m=LB, but
that correlation is weaker for elliptical galaxies
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