75 research outputs found
Mid-Infrared Emission from Elliptical Galaxies: Sensitivity to Stellar Age
Mid-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
The Mid-Infrared Spectral Energy Distribution, Surface Brightness and Color Profiles in Elliptical Galaxies
We describe photometry at mid-infrared passbands (1.2 - 24 microns) for a
sample of 18 elliptical galaxies. All surface brightness distributions resemble
de Vaucouleurs profiles, indicating that most of the emission arises from the
photospheres or circumstellar regions of red giant stars. The spectral energy
distribution peaks near 1.6 microns, but the half-light or effective radius has
a pronounced minimum near the K band (2.15 microns). Apart from the 24 micron
passband, all sample-averaged radial color profiles have measurable slopes
within about twice the (K band) effective radius. Evidently this variation
arises because of an increase in stellar metallicity toward the galactic cores.
For example, the sampled-averaged color profile (K - 5.8 microns) has a
positive slope although no obvious absorption feature is observed in spectra of
elliptical galaxies near 5.8 microns. This, and the minimum in the effective
radius, suggests that the K band may be anomalously luminous in metal-rich
stars in galaxy cores. Unusual radial color profiles involving the 24 micron
passband may suggest that some 24 micron emission comes from interstellar not
circumstellar dust grains.Comment: 18 pages. Accepted by Ap
Hot gaseous atmospheres in galaxy groups and clusters are both heated and cooled by X-ray cavities
Expanding X-ray cavities observed in hot gas atmospheres of many galaxy
groups and clusters generate shock waves and turbulence that are primary
heating mechanisms required to avoid uninhibited radiatively cooling flows
which are not observed. However, we show here that the evolution of buoyant
cavities also stimulates radiative cooling of observable masses of
low-temperature gas. During their early evolution, radiative cooling occurs in
the wakes of buoyant cavities in two locations: in thin radial filaments
parallel to the buoyant velocity and more broadly in gas compressed beneath
rising cavities. Radiation from these sustained compressions removes entropy
from the hot gas. Gas experiencing the largest entropy loss cools first,
followed by gas with progressively less entropy loss. Most cooling occurs at
late times, yrs, long after the X-ray cavities have disrupted
and are impossible to detect. During these late times, slightly denser low
entropy gas sinks slowly toward the centers of the hot atmospheres where it
cools intermittently, forming clouds near the cluster center. Single cavities
of energy ergs in the atmosphere of the NGC 5044 group create
of cooled gas, exceeding the mass of extended
molecular gas currently observed in that group. The cooled gas clouds we
compute share many attributes with molecular clouds recently observed in NGC
5044 with ALMA: self-gravitationally unbound, dust-free, quasi-randomly
distributed within a few kpc around the group center.Comment: 12 pages, 11 figure; accepted for publication by Ap
Spectral Energy Distribution Mapping of Two Elliptical Galaxies on sub-kpc scales
We use high-resolution Herschel-PACS data of 2 nearby elliptical galaxies,
IC1459 & NGC2768 to characterize their dust and stellar content. IC1459 &
NGC2768 have an unusually large amount of dust for elliptical galaxies (1-3 x
10^5 Msun), this dust is also not distributed along the stellar content. Using
data from GALEX (ultraviolet) to PACS (far-infrared), we analyze the spectral
energy distribution (SED) of these galaxies with CIGALEMC as a function of the
projected position, binning images in 7.2" pixels. From this analysis, we
derive maps of SED parameters, such as the metallicity, the stellar mass, the
fraction of young star and the dust mass. The larger amount of dust in FIR maps
seems related in our model to a larger fraction of young stars which can reach
up to 4% in the dustier area. The young stellar population is fitted as a
recent (~ 0.5 Gyr) short burst of star formation for both galaxies. The
metallicities, which are fairly large at the center of both galaxies, decrease
with the radial distance with fairly steep gradient for elliptical galaxies.Comment: 14 pages, 26 figures, to be published in Ap
Mid-IR Enhanced Galaxies in the Coma & Virgo Cluster: lenticulars with a high star formation rate
We explore the properties of early-type galaxies (ETGs), including
ellipticals (E) and lenticulars (S0), in rich environments such as clusters of
galaxies (Virgo and Coma). The L_24/L_K distribution of ETGs in both Virgo and
Coma clusters shows that some S0s have a much larger L_24/L_K ratio (0.5 to ~2
dex) than the bulk of the ETG population. This could be interpreted as an
enhanced star formation rate in these lenticulars. We compare the optical
colors of galaxies in these two clusters and investigate the nature of these
sources with a large L24/L_K ratio by looking at their spatial distribution
within the cluster, by analyzing their optical spectra and by looking at their
optical colors compared to late-types. We obtain 10 Coma and 3 Virgo early-type
sources with larger L24/L_K ratios than the bulk of their population. We call
these sources Mid-Infrared Enhanced Galaxies (MIEGs). In Coma, they are mostly
located in the South-West part of the cluster where a substructure is falling
onto the main cluster. MIEGs present lower g-r color than the rest of the ETG
sample, because of a blue continuum. We interpret the excess L24/L_K ratio as
evidence for an enhanced star-formation induced as a consequence of their
infall into the main cluster.Comment: Accepted for publication in Ap
Spitzer Observations of Passive and Star Forming Early-type Galaxies: an Infrared Color-Color Sequence
We describe the infrared properties of a large sample of early type galaxies,
comparing data from the Spitzer archive with Ks-band emission from 2MASS. 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-MIPS (24, 70 and 160 um) bands 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 of S0 galaxies estimated from the
24um luminosity increases with decreasing Ks-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. Star formation rates 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 far-infrared 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 SF rates.Comment: 14 Pages, 13 figures, Accepted by Ap
AGN Feedback in Galaxy Groups: the two interesting cases of AWM 4 and NGC 5044
We present AGN feedback in the interesting cases of two groups: AWM 4 and NGC
5044. AWM 4 is characterized by a combination of properties which seems to defy
the paradigm for AGN heating in cluster cores: a flat inner temperature profile
indicative of a past, major heating episode which completely erased the cool
core, as testified by the high central cooling time (> 3 Gyrs) and by the high
central entropy level (~ 50 keV cm^2), and yet an active central radio galaxy
with extended radio lobes out to 100 kpc, revealing recent feeding of the
central massive black hole. A recent Chandra observation has revealed the
presence of a compact cool corona associated with the BCG, solving the puzzle
of the apparent lack of low entropy gas surrounding a bright radio source, but
opening the question of its origin. NGC 5044 shows in the inner 10 kpc a pair
of cavities together with a set of bright filaments. The cavities are
consistent with a recent AGN outburst as also indicated by the extent of dust
and H_alpha emission even though the absence of extended 1.4 GHz emission
remains to be explained. The soft X-ray filaments coincident with H_alpha and
dust emission are cooler than those which do not correlate with optical and
infrared emission, suggesting that dust-aided cooling can contribute to the
overall cooling. For the first time sloshing cold fronts at the scale of a
galaxy group have been observed in this object.Comment: 4 pages, 1 figure, to appear in proceedings of the conference "The
Monster's Fiery Breath: Feedback in Galaxies, Groups, and Clusters", June
2009, Madison Wisconsi
ALMA observations of molecular clouds in three group centered elliptical galaxies: NGC 5846, NGC 4636, and NGC 5044
We present new ALMA CO(2--1) observations of two well studied group-centered
elliptical galaxies: NGC~4636 and NGC~5846. In addition, we include a revised
analysis of Cycle 0 ALMA observations of the central galaxy in the NGC~5044
group that has been previously published. We find evidence that molecular gas,
in the form of off-center orbiting clouds, is a common presence in bright
group-centered galaxies (BGG). CO line widths are times broader
than Galactic molecular clouds, and using the reference Milky Way , the
total molecular mass ranges from as low as in NGC~4636
to in NGC~5044. With these parameters the virial
parameters of the molecular structures is . Complementary observations
of NGC~5846 and NGC~4636 using the ALMA Compact Array (ACA) do not exhibit any
detection of a CO diffuse component at the sensitivity level achieved by
current exposures. The origin of the detected molecular features is still
uncertain, but these ALMA observations suggest that they are the end product of
the hot gas cooling process and not the result of merger events. Some of the
molecular clouds are associated with dust features as revealed by HST dust
extinction maps suggesting that these clouds formed from dust-enhanced cooling.
The global nonlinear condensation may be triggered via the chaotic turbulent
field or buoyant uplift. The large virial parameter of the molecular structures
and correlation with the warm ()/hot () phase velocity
dispersion provide evidence that they are unbound giant molecular associations
drifting in the turbulent field, consistently with numerical predictions of the
chaotic cold accretion process. Alternatively, the observed large CO line
widths may be generated by molecular gas flowing out from cloud surfaces due to
heating by the local hot gas atmosphere.Comment: Revised version to be published in ApJ, 16 pages, 10 figures, 4
table
CIGALEMC: Galaxy Parameter Estimation using a Markov Chain Monte Carlo Approach with Cigale
We introduce a fast Markov Chain Monte Carlo (MCMC) exploration of the
astrophysical parameter space using a modified version of the publicly
available code CIGALE (Code Investigating GALaxy emission). The original CIGALE
builds a grid of theoretical Spectral Energy Distribution (SED) models and fits
to photometric fluxes from Ultraviolet (UV) to Infrared (IR) to put contraints
on parameters related to both formation and evolution of galaxies. Such a
grid-based method can lead to a long and challenging parameter extraction since
the computation time increases exponentially with the number of parameters
considered and results can be dependent on the density of sampling points,
which must be chosen in advance for each parameter. Markov Chain Monte Carlo
methods, on the other hand, scale approximately linearly with the number of
parameters, allowing a faster and more accurate exploration of the parameter
space by using a smaller number of efficiently chosen samples. We test our MCMC
version of the code CIGALE (called CIGALEMC) with simulated data. After
checking the ability of the code to retrieve the input parameters used to build
the mock sample, we fit theoretical SEDs to real data from the well known and
studied SINGS sample. We discuss constraints on the parameters and show the
advantages of our MCMC sampling method in terms of accuracy of the results and
optimization of CPU time.Comment: 12 pages, 8 figures, 4 tables, updated to match the version accepted
for publication in ApJ; code available at http://www.oamp.fr/cigale
The Ages of Elliptical Galaxies from Infrared Spectral Energy Distributions
The mean ages of early-type galaxies obtained from the analysis of optical
spectra, give a mean age of 8 Gyr at z = 0, with 40% being younger than 6 Gyr.
Independent age determinations are possible by using infrared spectra (5-21
microns), which we have obtained with the Infrared Spectrograph on the Spitzer
Observatory. This age indicator is based on the collective mass loss rate of
stars, where mass loss from AGB stars produces a silicate emission feature at
9-12 microns. This feature decreases more rapidly than the shorter wavelength
continuum as a stellar population ages, providing an age indicator. From
observations of 30 nearby early-type galaxies, 29 show a spectral energy
distribution dominated by stars and one has significant emission from the ISM
and is excluded. The infrared age indicators for the 29 galaxies show them all
to be old, with a mean age of about 10 Gyr and a standard deviation of only a
few Gyr. This is consistent with the ages inferred from the values of M/L_B,
but is inconsistent with the ages derived from the optical line indices, which
can be much younger. All of these age indicators are luminosity-weighted and
should be correlated, even if multiple-age components are considered. The
inconsistency indicates that there is a significant problem with either the
infrared and the M/L_B ages, which agree, or with the ages inferred from the
optical absorption lines.Comment: Accepted for publication in Ap
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