Far Infrared Spitzer Observations of Elliptical Galaxies: Evidence for Extended Diffuse Dust

0.66 MB·English

by  Pasquale Temi

Draftversion February5,2008
PreprinttypesetusingLATEXstyleemulateapjv.10/09/06
FAR INFRARED SPITZER OBSERVATIONS OF ELLIPTICAL GALAXIES: EVIDENCE FOR EXTENDED
DIFFUSE DUST
Pasquale Temi1,2, Fabrizio Brighenti3,4, William G. Mathews3
Draft versionFebruary 5, 2008
ABSTRACT
Far-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 24µm correlates with optical fluxes, suggesting that this relatively
hot dust is largely circumstellar. But fluxes at 70µm and 160µmdo not correlate with optical fluxes.
7
Elliptical galaxies with similar L have luminosities at 70µm and 160µm (L and L ) that vary
0 B 70 160
0 overafactor∼100,implyinganadditionalsourceofdustunrelatedtothatproducedbyongoinglocal
2 stellar mass loss. Neither L70/LB nor L160/LB correlate with the stellar age or metallicity. Optical
n line fluxes from warmgas at T ∼104 K correlateweakly with L70 and L160, suggesting that the dust
mayberesponsibleforcoolingthisgas. Manynormalellipticalgalaxieshaveemissionat70µmthatis
a
extendedto5–10kpc. Extendedfar-infraredemissionwithsputteringlifetimesof∼108 yrsisdifficult
J
to maintain by mergers with gas-rich galaxies. Instead, we propose that this cold dust is buoyantly
5
transported from reservoirs of dust in the galactic cores which is supplied by mass loss from stars in
1
the core. Intermittent energy outbursts from AGNs can drive the buoyant outflow.

1 Subject headings: galaxies: elliptical and lenticular; galaxies: ISM; infrared: galaxies; infrared: ISM
v
1
3 1. INTRODUCTION Galaxymergers areanother obvious possible source of
4 The study of far-infrared (FIR) emission from ellipti- dust in elliptical galaxies, indeed it is widely thought
1 cal galaxies is particularly interesting because the inter- that ellipticals are formed by mergers. Elliptical-looking
0 galaxies can be produced after a few Gyrs following the
nal rates that dust is formed and destroyed can both be
7 merger of two disk galaxies (e.g. Barnes & Hernquist
estimated. Dust is produced by mass loss from evolv-
0 1992), but this simple recipe may not easily explain
ing red giant stars in the old stellar population typical
/ all structural and kinematic properties of these galax-
h of these galaxies. This mass loss is directly observed
ies (Naab, Jesseit & Burkert 2006). Many of the merg-
p in the cumulative mid-infrared 10–24µm emission from
- circumstellarregionsaroundgiantstars,causingthesur- ersthatproducedtheripples,shellsandcounter-rotating
o cores in elliptical galaxies probably occurreda very long
face brightness profiles at 10–24µmto resemble those at
r time ago. It is likely that massive boxy ellipticals, sim-
t optical wavelengths (Knapp et al. 1989; Athey et al.
s ilar to many of those with strong FIR emission, have
2002). The dust destruction rate by sputtering can be
a
experiencedmostly“dry”gasanddust-freemergersdur-
: estimated from the frequency of thermal impacts from
v ing the last few Gyrs (Bell et al. 2004; Faber et al.
ions in the hot interstellar gas which for many ellipti-
i 2006). Nevertheless, mergers do occasionally occur be-
X cals can be estimated from X-ray observations. During
tween low-redshift ellipticals and gas-rich galaxies and
their sputtering lifetimes, dust grains ejected from gi-
r these post-merger ellipticals often selectively appear in
a ant stars are heated by starlight and thermal electron
the observing lists of infrared observers. Our previous
impacts to temperatures that radiate in the FIR (e.g.
study of FIR emission from elliptical galaxies was based
Tsai & Mathews 1995, 1996; Temi et al. 2003). This
on a sample chosen from the ISO data archive (Temi et
radiation has been observed both by the Infrared Astro-
al. 2004) that included a significant number of galaxies
nomical Satellite, IRAS (Knapp et al. 1989) and by the
withabnormallylargemassesofdustandcoldgaswhich
Infrared Space Observatory, ISO (Temi et al. 2004). In
are uncharacteristic of elliptical galaxies in general. The
addition, small optically obscuring clouds and disks of
largerangeofFIRluminositiesthatwefoundintheISO
dusty gas have been detected in about half of the bright
sample was therefore not unexpected, and it was rea-
ellipticals studiedwiththe Hubble Space Telescope, HST
sonable to suppose that mergers with dusty galaxies ex-
(van Dokkum & Franx 1995; Ferrari et al. 1999; Lauer
plained the absence of correlations between optical and
et al. 2005). These clouds probably formed from dusty
FIR emission. The failure to find correlations between
gas ejected from stars within the central ∼kpc in these
theFIRandopticalluminositiesofellipticalgalaxieshas
elliptical galaxies (Mathews & Brighenti 2003).
been used in the past to argue that the cold dust is ac-
1AstrophysicsBranch,NASA/AmesResearchCenter,MS245- quired in a stochastic fashion by mergers (Forbes 1991).
6,MoffettField,CA94035. Tounderstandtherelativeroleofinternalandexternal
2SETIInstitute,MountainView,CA94043;andDepartmentof sourcesofdustinellipticalgalaxies,wedescribeherenew
Physics and Astronomy, University of Western Ontario, London, Spitzer observationsat 24, 70 and 160µm. Our full sam-
ONN6A3K7,Canada.
3University of California Observatories/Lick Observatory, ple consists of 46 elliptical galaxies, formed by combin-
Board of Studies in Astronomy and Astrophysics, University of ing our own proprietary Spitzer observations with those
California,SantaCruz,CA95064. of other observers. The improved sensitivity and angu-
4Dipartimento di Astronomia, Universita`di Bologna, viaRan-
lar resolution of Spitzer provides clear evidence that the
zani1,Bologna40127,Italy.
2 Temi, Brighenti & Mathews
70µm emission is spatially extended in many ellipticals. infrared (FIR) passbands of ∆λ= 5.3, 19 and 34µm for
In particular, we find extended FIR emission in ellipti- emission at 24, 70 and 160µm respectively. The PIs of
calgalaxiesthatdonotcontainuncharacteristicallylarge theoriginalSpitzerprogramfromwhichthegalaxieshave
massesofneutral,molecularoropticallyvisiblecoldgas. been selected are shown in column two of Table 1. All
The diffuse nature of the dust in these otherwise normal galaxies are classified as ellipticals in the NED catalog,
galaxies,combinedwiththerelativelyshort(ionsputter- and the morphological type T is taken from the LEDA
ing) lifetime for dust destruction in the hot gas, favors catalog. Nevertheless,weshallfindthatoursamplecon-
an internal source of dust rather than dust-rich merg- tainssomeS0galaxies. Mostofthesamplegalaxieshave
ers which are probably too infrequent. The luminosity knownX-rayluminositiesL ,whichareameasureofthe
x
ofextendedFIRdustemissioninmostellipticalsgreatly hot gas present in each galaxy.
exceeds that expected from normal mass loss from local Of the 46 galaxies in the sample three are dwarf ellip-
stars. However,small,opticallythicknucleardustclouds tical galaxies (NGC855, NGC1377, and NGC3265) with
observedinmanyellipticalgalaxiesmayprovideanalter- LB ≤5×109 LB,⊙. Although they arelisted in Table 1,
native, more plausible source for the extended FIR dust they are not included in our discussion.
emission. Consequently,wearguethatdustaccumulated
in the central regions of elliptical galaxies from stellar 3. OBSERVATIONSANDDATAREDUCTION
sources in the core is transported out to larger radii ThedatapresentedherewereacquiredwiththeMulti-
by buoyantly rising gas intermittently heated by low- band Imager Photometer (MIPS) (Rieke et al. 2004)
luminosity active galactic nuclei (AGN) in the galactic on the Spitzer Space Telescope (Werner et al. 2004) in
cores. Ifthishypothesiscanbeconfirmedinmoredetail, threewavebandscenteredat24,70and160µm. Sincethe
the presence of extended cold dust can be used to trace galaxysamplehasbeenconstructedbycollectingarchival
the radial circulation of hot gas in these galaxy/groups. datafromseveralSpitzer observingprograms,the image
Such circulation flows are known to be consistent with data were recorded using observing modes and integra-
X-ray observations in which very little hot gas cools to tions times tailored for their original scientific goals. As
low temperatures (Mathews, Brighenti & Buote 2004). a consequence, although the maps represent the deepest
FIRimagesacquiredtodate,somedataachievesensitiv-
2. THESAMPLE ities that are not always optimized for targets that are
For this study we construct a sample of 46 elliptical intrinsicallyfaintatfar-infraredwavelengths. For exam-
galaxies by combining far-infrared Spitzer observations ple, three ofthe brightestellipticals inthe VirgoCluster
from our own Spitzer data with additional galaxies in (NGC4472, NGC4636, and NGC4649) recorded under a
the public Spitzer archive. Our proprietary far-infrared guaranteed time program (PID 69, PI G. Fazio) reach
Spitzer observationsof elliptical galaxies(PID 20171,PI a relatively low sensitivity of 0.5MJy/sr and 1.1MJy/sr
P.Temi)consistsof11non-peculiar,bright(L ≥2×1010 (1σ) at 70 and 160µm, respectively, while other galax-
B
LB,⊙)ellipticalgalaxies,mosthavinghigh-qualityX-ray ies (i.e. PID 20171, PI P. Temi) have deeper maps at
observations. Galactic proximity and spatial scale were a sensitivity level of only 0.12MJy/sr and 0.3MJy/srfor
alsoimportantfactorsinchoosingthissamplesincehigh the same two wavebands. The SINGS data are recorded
angular resolution is needed to separate emission from in MIPS scan mode, covering a very large sky area
dusty cores and diffuse emission from truly interstellar (∼30′×10′),incorporatingtwoseparatedpassesateach
dust spread over the galactic volume. Furthermore, we sourcelocation. SINGSimagescorrespondtomapswith
avoidedsourceswithstrongradiosourcesandAGNthat intermediatesensitivity. Details onthe observingstrate-
have detectable signals in the far infrared. Ellipticals gies, field coverage and integration times for the SINGS
with strong infrared Milky Way cirrus or bright spiral programaredescribedbyKennicuttetal. (2003). Apart
galaxies in the vicinity were also avoided. Since one of from the SINGS observations, data have been acquired
our goals is to study the interaction between dust and in MIPS photometry mode, allowing appropriate cover-
hot gas, we included many galaxies for which the X-ray age of the sources and their extended emission. At all
emission is well-studied by either Chandra or XMM. wavelengths, sufficiently large field sizes were chosen to
In addition to our original sample we selected 35 ad- reliably measure the background emission. We used the
ditional ellipticals from data currently available in the BasicCalibratedData (BCD) products fromthe Spitzer
publicSpitzerarchivetargetedbyseveralPIsduringtheir Science pipeline (version 13.2) to construct mosaic im-
GeneralObservationinCycles1and2,GuaranteedTime ages for all objects. Pipeline reduction and post-BCD
Observations, and the Legacy Program SINGS (Kenni- processingusingtheMOPEXsoftwarepackage(Makovoz
cutt et al. 2003). All the galaxies added to our original et al. 2006) provide all necessary steps to process indi-
sample were chosen using selection criteria tailored to vidual frames: dark subtraction, flat-fielding, mux-bleed
our specific scientific goals. Thus, the 46 galaxies stud- correction, flux calibration, correction of focal plane ge-
ied here form a rather homogeneous sample and is large ometrical distortion, and cosmic ray rejection.
enough to be statistically meaningful. Foregroundstarsandbackgroundgalaxieswerepresent
Tables 1 and 2 summarize the relevant properties of in the final mosaiced images at all bands. These were
the sample galaxiesincluding the far-infraredphotomet- identifiedbyeyeandcross-checkedusingsurveysatother
ric measurements. Optical data are taken from the wavelengths(DigitalSkySurveyand2MASS).Theywere
LEDAcatalog(Patureletal. 1997)andtheRSAcatalog then masked out from each MIPS image before flux ex-
(Sandage & Tamman 1981), while X-ray data are from tractionwas performed. Also,a few asteroidswere iden-
Ellis and O’Sullivan et al. (2006). Luminosities and dis- tified and removed from the affected maps. Aperture
tances are calculated with H = 70 km s−1 Mpc−1 and photometry was performed by measuring fluxes in suit-
0
are derived from the flux densities in Table 1 using far- ablecircularaperturesaroundthecenters. Fluxdensities
FIR Emission from Elliptical Galaxies 3
wereextractedfromaperturesextendingouttotheentire FIR-emitting dust could either be diffusely distributed
optical disk of each galaxy (R ). Sky subtraction was inthe hotgasorconfinedto dense centralclouds similar
25
performed by averaging values from multiple apertures to those visible in optical absorption (e.g. Lauer et al.
placed around the target, avoiding any overlap with the 2005). We discuss below our preference for the diffuse
faint extended emission from the galaxy. Uncertainties component in producing most of the FIR-emission.
influx densities due to skysubtractionaregenerallyless It is remarkable that L and L range over more
70 160
than 1%, but can be of the order of tens of percent for than two orders of magnitude for galaxies with similar
faint sources. L – this is a significant new result that must be ex-
B
ThemeasuredfluxdensitiesarelistedinTable1. Most plained. InourprevioussurveyofarchivaldatafromIn-
of the galaxies in the sample have been detected by fraredSpaceObservatory(ISO)wefoundasimilarspread
MIPS,andamongthemalargefractionshowfar-infrared in FIR luminosities at 60-200µm, but the FIR luminosi-
emission well beyond the sensitivity capabilities of pre- ties in the ISO data spanned only one order of magni-
viousinfraredspaceobservatories. Althoughmostofthe tude for fixed L (Temi et al. 2004). At the time we
B
galaxies presented in this paper are unpublished either attributed much of this scatter to the unusual nature of
becausetheyareourownproprietarydataorbecausethe the ISO sample which contained many (post-merger) el-
analysis is still in progress by the original PIs, results liptical galaxies with massive disks of cold, dusty gas.
for a few galaxies are already present in the literature Now with this Spitzer sample of (mostly) normal ellipti-
(i.e. the SINGS galaxies). We find that our flux den- calgalaxieswe find thatthe rangeofFIR luminosities is
sities are in good agreement with previously published evenlarger,althoughwe show in Appendix A that some
data. The only exception is the measured flux density of the FIR-brightestsample galaxiescannotbe regarded
at 160µm reported by Dale et al. (2005) for the SINGS as normal. In Figure 4 we show that there is no obvious
galaxy NGC584. Very likely this discrepancy is due to a trend of dust color F /F (i.e. mean dust tempera-
70 160
misidentification of the target in their very large 160µm ture)withL exceptperhapsatendencyforcoolerdust
70
scan map. Corrections for extended emission were ap- in more FIR-luminous galaxies. A similar conclusion re-
plied to the fluxes as described in the Spitzer Observer’s sults if we replace F on the horizontal axis with F ,
70 160
manual. PhotometricerrorslistedinTable1referonlyto L or L .
70 160
the statistical uncertainties. Total errors should include Figures 5 and 6 show no discernable correlation of ei-
systematic uncertainties due to flux calibration and to ther L /L or L /L with stellar Balmer line ages
70 B 160 B
sky subtraction errors. The uncertainties on the final or metallicity. However, L /L may show a slight ten-
24 B
absolute calibration are estimated at 10% for the 24µm dency to decrease with age
data, and 20% at 70 and 160µm.
logL /L =(30.35±0.05)−(0.021±0.005)age (3)
Figure 1 shows gray-scale images of a representative 24 B
sub-sample ofgalaxies. The columnof panels onthe left or increase with metallicity
isthe 24µmdata,while70and160µmdataareshownin
logL /L =(29.95±0.06)+(0.7±0.2)log[Z/H]. (4)
the center and right columns, respectively. Galaxies are 24 B
ordered by ascending IR flux in a common field of view A negative correlationof L /L with age would be ex-
24 B
of 3′×3′. Some very faint galaxies are shown, including pected since the rate of mass loss from evolving stars
anondetectionat160µmandfewsourceswithextended decreases with stellar age (Piovan et al. 2003), although
far-infrared emission. Detailed images of the diffuse ex- thisexpectedtrendisnotsupportedbyourstudyofellip-
tended emission in NGC4636 and NGC5044 will be pre- tical galaxies at mid-IR wavelengths (Temi et al. 2005).
sented in a companion paper currently in preparation. It must be noted however that Balmer line ages for the
same galaxy often show considerable variation when de-
4. COMPARISONOFFIR,OPTICALANDX-RAYEMISSION termined by different authors, so the anticorrelation of
In Figure 2 we compare fluxes at 24, 70 and 160µm L /L with age needs further confirmation. A posi-
24 B
withopticalB-bandfluxesandarelatedplotofFIRand tive correlation of L /L with metallicity in Figure 6
24 B
B-bandluminositiesisshowninFigure3. Boththe24µm is expected if more interstellar grains are produced by
andB-bandfluxesandluminositiesareclearlycorrelated, mass-losing stars of higher metallicity. However, this is
inconsistentwiththenegativecorrelationat70µminFig-
logF24 =(8.37±0.42)+(0.716±0.043)logFB,⊙ (1) ure 6,
and
logL /L =(30.7±0.1)−(1.9±0.4)log[Z/H]. (5)
70 B
logL24 =(32.37±0.69)+(0.799±0.066)logLB,⊙. (2) No correlations with X-ray luminosity with any of the
The approximate consistency with unit slope F ∝ F FIR luminosities are visible in Figure 7. Decreasing FIR
24 B
indicates that the emission at 24µm, like that at mid-IR luminositieswithincreasinghotgasdensitymightbeex-
wavelengths (Knapp et al. 1989; Athey et al. 2002), is pected since interstellar grains are destroyedby sputter-
dominated by circumstellar emission that closely traces ing which is proportional to the density of the hot gas.
optical starlight. However, the relationship between L and gas density
x
However, the huge scatter seen in the 70µm–Bband is unclear. For example, there is no correlation between
panels in Figures 2 and 3 suggest that this colder dust L and the hot gas density at ∼ 10 kpc in the ellipti-
x
is unassociated with stars and is truly interstellar. The cal galaxies observed with Chandra by Humphrey et al.
absence of any clear correlation between FIR and opti- (2006). ThisdisconnectbetweengasdensityandL may
x
calemissionfromellipticalshasbeendiscussedbyForbes arise because the hot gas density on kpc-galactic scales
(1991),Goudfrooij&deJong(1995),Trinchieri&Goud- is influencedby intermittent energysupplied by the cen-
frooij (2002) and Temi et al. (2004). In principle cold, tral black holes, while L is largely determined by more
x
4 Temi, Brighenti & Mathews
extensive gas that fills the dark halos that surround the FW0.1M/2R is plotted against F /F , there is no
e 70 160
elliptical galaxies (Mathews et al. 2006). Consequently, clearevidencethatgalaxieswithmoreextendedF have
70
the absence of correlations in Figure 7 is not surprising. colder dust.
Finally, we note that all qualitative features in Figure 7
are preserved if luminosities are replaced with fluxes. 5. ASIMPLEFAR-INFRAREDSPECTRALENERGY
DISTRIBUTION
In Figure 8 we compare FIR fluxes with optical emis-
sion line fluxes determined by three different observers: We estimate the FIR spectral energy distribution
Goudfrooijetal. (1994a,b),Macchetto et al. (1996)and (SED) from diffusely distributed interstellar dust in el-
Sarzi et al. (2006). The optical observations require dif- liptical galaxies using a simple model similar to that de-
ficult background subtraction and the procedures used scribed in Temi et al. (2003). For completeness and
vary among the observers. For this reason, we compare clarity,webrieflydescribethis modelagainhere. We as-
FIR fluxes in Figure 8 with each set of optical emission sume that dust grains ejected from evolving stars come
line fluxes independently. into direct contact with the hot interstellar gas after a
We believe that both F and F correlate weakly short time, <∼ 105 − 106 yr, when the stellar gas has
60 170
with observed optical line fluxes, but that F does not. heated to the (virial) temperature of the hot interstellar
24
The recent SAURON data for F (Sarzi et al. 2006) gas. This time scale is established by comparing typical
Hβ
show the tightest relationship. The Goudfrooij et al. Balmer emission line observations with the luminosity
data (left column of panels in Figure 8) are marginally expected if all the gas expelled from stars (at the ex-
correlated,butthesignificanceincreasesifitiscombined pected rate for an old stellar population) is ionized and
withtheMacchettoetal. data(showninthecentralcol- in hydrostatic equilibrium with the hot gas (Mathews &
umn). Brighenti1999). After ∼106 yrsthis warmgasat104 K
In Figure 9 we compare the fluxes in the optical emis- must merge with the hot gas since otherwise its Balmer
sion lines with B-band fluxes, using only data for non- line luminosity would exceed typically observed values.
peculiar E or E/S0 galaxies, excluding peculiar and However, the dust in this gas can resist sputtering de-
known S0 galaxies. The top and bottom panels from structioninthehotinterstellargas(T ∼106−107 K)for
Macchetto et al. (1996) and Sarzi et al. (2006) show much longer times, ∼ 108 yrs. Consequently, we model
that the line flux is weakly correlated with the stellar “recently”produceddust thatwascreatedby localstars
continuum(seealsoPhillipsetal. 1986;Macchettoetal. during the previous ∼108 yrs. Once in contact with the
1996;Goudfrooij1999). A correlationbetween extended hotgas, the dust grainsare destroyedby sputtering ata
line emission and the optical continuum suggests that rate
the warm gas with T ∼ 104 K has a stellar origin. But da
=−3.2×10−14n [1+(2×106/T)2.5]−1 µms−1, (6)
the quality ofthese difficult opticalobservationsis poor. dt p
Thereareveryfew objectsincommonamongthe Goud-
where a is the grainradius (in µm) and n is the proton
frooij, Macchetto and Sarzi data sets, but the flux-flux p
density in the hot gas (Draine & Salpeter 1979; Tsai &
plots of these common galaxies show large scatter with
Mathews 1995).
almost no clear monotonic relationship. Five of the six
If N(r,a,t)da be the number of grains cm−3 with ra-
galaxies in common between Goudfrooij and Macchetto
dius between a and a+da at galactic radius r at time t,
correlate negatively in a flux-flux plot.
the grain population evolves according to
ThecorrelationinFigure9,ifreal,maysimplyindicate
that the warm gas and the dust associated with it both ∂ da
N =S(r,a), (7)
have a stellar origin – more stars of a similar age and ∂a(cid:20) (cid:18)dt(cid:19)(cid:21)
metallicity eject more gas. However, the optical emis-
sion line flux depends sensitively on the density of the where S(r,a)da is the rate at which grains with radii a
warm gas and therefore on the pressure in the local hot areexpelledfromstarsper cm3 (Tsai& Mathews1995).
interstellar gas. At present there are too few accurate We assumeS(a)=Soa−s for 0<a<amax, withs=3.5
observations to interpret Figure 9 with confidence. (Mathis et al. 1977), so the coefficient
Allellipticalsinoursampleshowevidence ofextended 3δα∗ρ∗ (4−s)
eimmaisgseison–adtet2a4ilµsmofsitmhiislawritllobtehadtisocfustsheedoipntiacaslepstaerlalater So(r)= 4πρg10−12 a4m−asx (8)
publication. However, spatial extension is also evident dependsontheratethattheoldstellarpopulationejects
at70µmasshowninTable1,buttheemissionat160µm mass α∗ = M∗−1|dM∗/dt| ≈ 4.7×10−20 s−1 (Mathews,
is not spatially resolved due to decreasing resolution at 1989),the stellar density ρ∗, the initial dust to gas mass
larger wavelengths. Figure 10 shows the surface bright- ratioscaledto the stellarmetalabundance insolarunits
ness at 70µm for eight galaxies together with the point δ =(1/150)z(r), and the density of (amorphous)silicate
response function (PRF) for the MIPS detector (includ- grains, ρ =3.3 gm cm−3. The solution of eq. (7) is
g
ing the first Airy ring). All contours in Figure 10 are
−1
arbitrarily normalized to 50 at the center. The (loga- N(r,a)= da So(r) a1−s a≤a . (9)
rithmic) extension in arseconds beyond the PRF is best (cid:12)dt(cid:12) (s−1) max
(cid:12) (cid:12)
measuredat0.08ofthecentralfluxes,shownwitharrows (cid:12) (cid:12)
in Figure 10. The full width at 4/50 = 0.08 maximum Thegraintempe(cid:12)rat(cid:12)ureTdisdeterminedbythebalance
ofheating(bybothabsorptionofstellarradiationandby
(subsequently referred to as FW0.1M; last two columns
electron-graincollisions) and radiative cooling:
in Table 1) of the PRF is 37′′. Twelve galaxies in our
full sample have FW0.1M greater than 40′′, indicating ∞
thatthesegalaxiesarespatiallyresolvedat70µm. When Z 4πJ∗(r,λ)Qabs(a,λ)πa2dλ+4πa241nehveEeiτ(a)
0
FIR Emission from Elliptical Galaxies 5
=4πa2σ T (r,a)4hQ i(T ,a). (10) the observed fluxes for the two remaining FIR-extended
SB d abs d
galaxies – NGC 5044 and NGC 4636 – are very much
In view of the high electron-graincollision frequency, we
larger than predicted by our steady state model for the
neglect stochastic heating due to single impacts.
FIR spectral energy distribution. We believe that the
We assume that all grains have properties similar
dust in these galaxies has an entirely different origin.
to astronomical silicates (Laor & Draine 1993) using
The large and extended FIR emission from NGC 5044
the following approximations for the absorption coeffi-
and NGC 4636, which is related to the enormous range
cient Q ≈ aψ(λ), and for the Planck-averaged values
abs of FIR luminosities in Figure 3, will be discussed in a
hQ i ≈ 1.35×10−5T2a (see Draine & Lee 1984). The
abs d separate forthcoming publication.
mean intensity of starlight is found by integrating over
a de Vaucouleurs stellar profile using the appropriateef- 6. DISCUSSION
fective radius R and stellar luminosity for each galaxy. 6.1. Large Range of L
e FIR
The collisional heating term in the equation above in-
The first important conclusion to be drawn from Fig-
cludesacorrectionτ(a)forsmallgrainsthatdonotcom-
ures 2 and 3 is that warm circumstellar dust and colder
pletely stop the incident thermal electrons (Dwek 1986)
interstellar dust behave very differently. The relatively
and hveEei = (32/πme)1/2(kT)3/2 is an average over a smallscatterinF24 inFigure2isslightlylargerthanour
Maxwellian distribution. The electron density and tem- estimated observational errors. Nevertheless, we have
peratureofthehotgasarecalculatedusingdeconvoluted been unable to find correlations between stellar age or
profiles from X-ray observations. The FIR emissivity at metallicity and the residuals in F from the mean cor-
24
each wavelength is calculated from relation. We conclude that the warm dust emitting at
1 amax 24µmislargelycircumstellarandthisisconfirmedbythe
j(r,λ)= N(r,a)4πa210−8Q (a,λ) similarityofopticaland24µmsurfacebrightnessprofiles
abs
4π Z
0 which will be discussed in detail elsewhere.
The second and most remarkable result in Figures 2
×πB(T (r,a),λ)da. (11)
d
and 3 is that the emission from colder dust at 70 and
In Figure 11 we compare the expected FIR spectral 160µm varies by a huge factor relative to that at 24µm
energy distribution for three galaxies in our sample that and optical wavelengths. In the previous section and
have well-known hot gas density and temperature pro- in Figure 11 we show that 70 and 160µm fluxes from
filesfromX-rayobservations: NGC1399,1404and4472. three galaxies having well-observed interstellar gas den-
None of these galaxies are spatially resolved at 70µm. sity profiles can be fit reasonably well with our simple
TheX-raydataforthesegalaxieshasbeentakenfromthe dust heating model without varying any of the dust pa-
followingsources: NGC1404&1399: (Paolilloetal2002; rameters from those we assumed at the outset.
Jones et al. 1997); NGC 4472: (Mathews & Brighenti However, notice that all three galaxies in Figure 11
2003; Brighenti & Mathews 1997). The profiles in Fig- that agree with our model SED – NGC 1399, 1404 and
ure 11 show the expected FIR spectral energy distribu- 4472 – have values of L and L in Table 2 that lie
70 160
tions computedwiththe equationsabove. The solidand near the bottom of the observed distributions in Figure
dashed lines correspond respectively to emission from 3. We think it is impossible to explain the huge disper-
grains with original maximum sizes a = 0.3µm and sion of L and L in Figure 3 by simply varying the
max 70 160
1.0µm. TheobservedfluxesforNGC1399andNGC4472 dust parameters in the SED models used for the three
at 70µmand 160µmare in excellent agreementwith our galaxiesinFigure11,i.e.,the stellarmasslossrate,dust
simple model. The fit to NGC 1404 is also satisfactory, composition, dust size distribution, etc. are unlikely to
given the many assumptions in our model. While the be in error by an order of magnitude or more. Our SED
flux at 24µm is also shownin Figure 11, this circumstel- model clearly fails to fit most of the FIR observations.
laremissionisnotincludedinourmodel,butitsupports Some of the large spread of L and L in Figure
70 160
ourassumptionthatmuchofthe diffuse interstellardust 3 can be understood because our sample includes very
in these three galaxies has a stellar origin. FIR-luminous elliptical galaxies that contain abnormal
It is of particular interest to determine FIR spectral masses of interstellar dust and cold gas, probably ac-
energy distributions for galaxies that have accurate in- quired by a recent merger. In Appendix A we review
terstellar gas density and temperature profiles and are relevant observations of 12 sample galaxies having the
also spatially resolved at 70µm. Seven galaxies in Ta- highest 70µm luminosities in order of descending L .
70
ble 1 have FW0.1M greater than 50′′ but only six of We conclude from this limited information that most el-
these are in the Ellis and O’Sullivan (2006) list of X-ray lipticalswiththelargestL andL inFigure3cannot
70 160
galaxies detected with ROSAT: NGC 2768, 2974, 4125, be regarded as typical or normal. Of the 12 galaxies in
4636, 4696, and 5044. Of these six galaxies, only three AppendixA,weregardthefollowing8aseitherlikelyS0s
– NGC 4636, 4696 and 5044 – have X-ray luminosities (whichoftenhaveextended,rotationallysupported,star-
(logLx >∼ 41.5) large enough for an accurate determina- forming disks of cold dusty gas)or having massive HI or
tion of the gas density and temperature profiles. How- molecular lanes or disks probably acquired by mergers:
ever, the optically peculiar elliptical NGC 4696 is the IC 3370, NGC 5018, 2974, 4125, 5322, 2768, 3962, and
central galaxy in the Centaurus Cluster with powerful 5077. These atypical or mis-classified elliptical galaxies
FRI-type radio emission, a large mass of dusty neutral arethebrightestgalaxiesinoursample. However,were-
gas and highly disturbed hot cluster gas (Fabian et al. gard the following galaxies in Appendix A with slightly
2006). In view of these complexities, NGC 4696 is an lower L as normal or quasi-normal ellipticals: NGC
70
inappropriate galaxy to learn about the nature of FIR 3557, 5044, 4589, and (possibly) 4697. If we disregard
emission from normal elliptical galaxies. We find that the eight galaxies with atypical dust content, the range
6 Temi, Brighenti & Mathews
inL andL occupiedbynormalellipticalsisreduced gin,resultingfromarecentmergerwithasmalldust-rich
70 160
from ∼100 (shown in Figure 3) to about ∼30, but this galaxythatisotherwisedifficulttoobserve. Weadopted
is still larger than can be explained by varying dust pa- this explanation previously in our discussion of the ex-
rameters in our SED model. These normal ellipticals cess dust in NGC 4636 (Temi, et al. 2003), which we
with excess FIR are the most interesting galaxies in our interpreted as a dusty merger that occurred within the
sample. last ∼ 3×108 yrs, the estimated sputtering time in the
hot gas in this galaxy. However, it has since been de-
6.2. Location of FIR-Emitting Dust
termined that the mean stellar age in NGC 4636 is very
WeassumethatgalaxieswithL significantlyabove old (10.3 Gyrs, Sanchez-Blazquez et al. 2006), and this
FIR
that predicted by our model SEDs in Figure 11 have strongly argues against a merger ∼3×108 yrs ago with
dust contents that exceed those that can be understood a dusty, star-forming galaxy. Moreover, we observe that
byasteadystatebalancebetweendustcreationbymass- the additional dust in NGC 4636 is extended (FW0.1M
losingstarsanddustdestructionbysputtering. Galaxies =54.4′′ inTable1)andappearstobe distributedrather
in Figure 3 with logL70 >∼ 40.5 must contain additional evenly about the center unlike the irregular distribution
colddustfromanothersource. Thisadditionalcolddust that might be expected to result from a recent merger
iseither(1)spreaddiffuselythroughoutalargeregionof undergoing dynamical friction with accompanying ram-
the galactic hot interstellar gas or (2) located in dense stripping and tidal disruption. So our previous interpre-
optically thick clouds similar to those seenin absorption tation of NGC 4636 needs to be reevaluated.
by optical observers. In Appendix B we describe a sim- Thedustsputteringtimeforgrainsofinitialradiusain
pleargumentthatsupportsthefirstoptionabove,diffuse thehotgasofanellipticalgalaxyist ≈a/|da/dt| ≈
sp sput
FIR emission. We suppose inAppendix B that the FIR- 1.2×108a (n /10−2)−1 yrs(wherea isinmicrons)pro-
µ e µ
emitting dust is in a small spherical cloud of radius rc videdtheambientgastemperatureisT >∼106K.(Draine
located at the center of an elliptical galaxy. The dust is &Salpeter1979;Tsai&Mathews1995). Evenifthedust
heated both by internal stars and by starlight incident is mixed with warm gas at T ∼104 K, where sputtering
from the entire galaxy. The dust cloud is assumed to be is negligible, its overall sputtering destruction time t
sp
opticallythicktomoststellarradiation. Asthecloudra- cannot be significantly reduced since, as discussed be-
dius rc is increased, the estimated 70µm luminosity also low, we generally expect that the warm gas is thermally
increases because the cloud encloses more stars and in- heated and joins the (spatially extended) hot gas phase
tercepts more light from external stars. However, this onatimescale∼106 yrsthatisverymuchlessthant .
sp
simple dusty cloud model cannot account for the lumi- ThereforeitisreasonabletoassumethatthediffuseFIR-
nosity range in Figure 3 unless rc is so large (>∼ 200 pc) emitting dust in our sample galaxies is in direct contact
that it would be easily apparent in moderate resolution with the hot gas during most of its sputtering lifetime.
optical images. The UV-ionized surfaces of these clouds The dynamical (freefall) time of a merging dusty
mayalsobevisible. WhileAGNheatingwithinoptically galaxyacrosstheopticalimageofalargeellipticalissev-
thick dusty clouds could easily provide the needed FIR eral ∼ 108 years, comparable to t . Therefore, on the
sp
energy, this seems unlikely since AGNs would need to merger hypothesis, diffusely distributed cold dust must
be bolometrically brighter than the stars within rc and be resupplied by mergers in a regular fashion at least
would therefore be seen in those ellipticals in which the every∼108 yrs. Therefore,anentourageof dusty merg-
dusty clouds are in disks or not present at all. Such lu- ing galaxies should be commonly observed in the outer
minous AGN would almost certainly be visible at other regions of the majority of elliptical galaxies having FIR
frequencies. excess emission – but they are not. In fact, mergers be-
ComparisonsofthemostFIR-luminous(normal)ellip- tween elliptical and (small) dust rich galaxies are rarely
ticals in our sample with recent HST images (e.g. van observedatzeroredshift. While webelieve that mergers
Dokkum&Franx1995;Laueretal. 2005),showacorre- dooccurandthatsomeofthemostFIRluminousgalax-
spondence between the absence of nuclear dust and low ies in our sample can only be explained by mergers, the
LFIR. For example, of the 19 galaxies in common be- similarity of the sputtering and dynamical time scales
tween our sample and that of Lauer et al. (2005), all makes it difficult to accept the proposition that merg-
11 of the ellipticals with L70 > 1040.5 erg s−1 have de- ers are the dominant or only source of (extended) dust
tectable nuclear dust, while all but one of the 9 galaxies in most luminous elliptical galaxies. This point of view
with L70 <1040.5 have no nuclear dust detected. Never- is supported by Figure 5, showing that the incidence of
theless,evenforthese galaxieswithlowerL70 webelieve youngstarsinelliptical galaxiesis uncorrelatedwith the
that most of the LFIR comes from a diffuse component. FIR excess luminosities seen in Figure 3.
In addition to the argument in Appendix B, our pref-
erence for extended excess dust is supported by noting 6.4. Internal Origin for Excess Dust
thatallsixgalaxieswithextendedFIRemissioninTable
1 (e.g. FW0.1M > 50′′) have excess FIR emission, i.e. 6.4.1. Evolution of Dust Ejected from Stars
L >1040.5 erg s−1. Before exploring the origin of the additional dust re-
70
quired to produce FIR luminosities in excess of L ∼
6.3. External Origin for the Excess Dust 1040.5 ergs−1, it is useful to review the evolutionof7c0old
IftheFIRemissioninexcessofthatexpectedfromour dustygasthatisintroducedintothehot,X-rayemitting
simplemodelSEDinFigure11isfromcolddustdiffusely interstellar gas by mass-losing red giant stars. Our SED
distributed throughout the hot interstellar gas, where modelisbasedonaninternalstellarsourceofdustwhich
has it come from? It is often supposed that optically- is confirmed by mid-IR observations of (apparently out-
observed dust in elliptical galaxies has an external ori- flowing)circumstellardust(Knappet al. 1989;Athey et
FIR Emission from Elliptical Galaxies 7
al. 2002; Bressan et al. 2006;Bregman et al. 2006). We luminosity expected in a large group-centered elliptical
think that the fate of this dusty gas varies with galactic galaxy by dusty mass loss just within the central kpc
radius. where rapid dust-assistedcooling is expected. Accretion
Throughout most of the volume occupied by galactic isexpectedtoquicklyfollowcooling. TheAGNluminos-
starsbeyond∼1kpc,weassumethatboththedustand ityisLagn =ǫM˙ c2 =6×1045[M˙1/(M⊙ yr−1)](ǫ/0.1)erg
gas ejected from red giant stars ultimately merge with yr−1whereM˙ isthetotalrateofstellarmasslosswithin
1
the hot gas which is the dominant interstellar phase. the central kpc. Since a 1010 year old stellar population
We expect that gas recently ejected from stars can be ofmassM∗ ejects1.5(M∗/1012M⊙)M⊙ yr−1,thestellar
maintained in pressure equilibrium at warm tempera-
tures (T ≈ 104 K) by photoionization by UV emitted mass within 1 kpc, ∼5×1010 M⊙, generates M˙1 ≈0.07
by post-AGB stars. Emission from warm gas at r ≫ 1 M⊙ yr−1. When accreted by the central massive black
hole, this translates into a luminosity L ≈ 4×1044
kpc is faint and has not been observed. Eventually this agn
ergs s−1 that exceeds the entire X-ray luminosity of the
warm gas thermally mixes with nearby hot gas. When
most luminous galaxy groups, L ∼ 3×1043 ergs s−1.
the dust in this gas confronts the hot gas it will be de- x
stroyed by sputtering in ∼108 yrs. Consequently, dusty stellar mass loss within the central
kpc of bright ellipticals can generate enough accretion
However, within ∼1 kpc of the centers of elliptical
energy to balance the entire bolometric X-ray luminos-
galaxies the evolution of dusty stellar ejecta is different.
ity of a large galaxy group. The heating of the hot gas
Extendedwarmgasinthisregioniscommonlyobserved,
as a consequence of this accretion also provides enough
but its velocities differ from those of nearby stars (e.g.
energy to drive buoyant mass circulation in the hot gas
Caon, Macchetto & Pastoriza 2000). Evidently the hot
out to 5-10 kpc.
andwarmgasexchangedmomentumwhenthewarmgas
The evolution of dusty disks in galactic cores that we
had a much lower density. After dusty stellar envelopes
just described cannot continue very long. If small core
and winds are ejected from orbiting red giant stars, the
gas expands until its density approaches (but still ex- disksaccretedustygasatM˙1 ≈0.07M⊙yr−1,for∼1010
ceeds)the localhotgasdensity. Atthis time the ejected yrs, and allowing for a larger mass-loss rate in the past,
gas,whichisstillmovingwiththestellarorbitalvelocity, currently observed disks should have a typical mass of
is disrupted by hydrodynamic instabilities and shocked ∼1010 M⊙ and a dust mass of ∼108, both far in excess
to a few million degrees where its density is comparable of those observed. (Small stellar disks with masses ∼
to that of the hot interstellar gas. During this period 109−1010 M⊙ are not commonly observed in the cores
ofinteractionbetween the stellar ejecta and the hotgas, ofmostellipticals,sonormalstar-formationisruledout.)
momentum is exchanged and the ejected gas acquires Alargeminorityofbrightellipticals showno evidenceof
some of the velocity of the ambient hot gas. Even if the central dusty disks/clouds. Where has this dusty gas
gas ejected from stars is (transiently) heated completely gone?
tothetemperatureofthelocalhotgas(∼107K),itsdust
contentwillcoolthegasrapidlybackto∼104whereitis 6.4.2. Explosive Ejection of Dust from the Galactic Core
stabilzed by UV from post-AGB stars. This rapid dust- The excess diffuse dust observed in FIR-emission may
assistedcooling occurs because ofveryfrequentinelastic have been exploded out from the centers of elliptical
collisions between thermal electrons in the hot gas and galaxies by the release of AGN energy. We estimate
the dust grains. Mathews & Brighenti (2003) showed from our model SED that the total mass of excess dust
that efficient dust cooling of the hot gas is also faster required to explain the FIR luminosities in Figure 3 is
thanthesputteringlifetimeofthedust,sothegasretains Md ∼ 3 × 105 M⊙. This is comparable to the esti-
somedustafteritcoolsto∼104K.Sincethecoolingtime mated dust masses in the central dusty disks or lanes
to ∼104 K within 1 kpc is less than the dynamical time (e.g. Goudfrooij & de Jong 1995). If the (very uncer-
in the local (stellar) potential, these dense warm clouds tain) dust to gas mass ratio is δ ≈ 0.01, then the mass
freefalltowardthe galacticcenter andaccumulate there. ofgasthatinitially containedthis dustisM ∼107−108
As dusty warm gas collects into a disk near the center, M⊙. If cold dusty gas is ballistically ejected from the
similartothoseobservedbyvanDokkum&Franx(1995) core out to a few kpc, velocities of ∼100 km s−1 are re-
and Lauer et al. (2005), it eventually becomes optically quired. The kinetic energy needed to explosively trans-
thick to the stellar UV radiation. Becauseof its residual portdusty gasfrom the galactic coreto ∼1 kpc is E ≈
k
dust, we expect molecular formation and eventual cool- 0.5Mv2 =1055(0.01/δ)(Md/106 M⊙)(v/100km/s)2 ergs.
ing far below 104 K. Dense, cold nuclear dust disks and Althoughveryuncertain,theenergyinvolvediscertainly
cloudssimilartothoseobservedcanformnaturallyfrom larger than that of a single supernova. Since multiple
stellar mass loss in elliptical galaxy cores (Mathews & starburstactivity in elliptical galaxiesis very rare,AGN
Brighenti2003). Dusty galaxymergers arenot required. energy is required for dust ejection from the core. If the
We speculate below that this central dust reservoir may AGN dust ejection process is intermittent, this would
be the source of extended diffuse FIR emission in many explain those elliptical galaxies that are not observed to
ellipticals. have optically obscuring dust in their cores or extended
Central dusty clouds and disks in ellipticals are not FIR excess emission. The FIR variance expected from
merely an amusing curiosity, but are likely to be key to intermittent AGN outbursts obviates the argument of
understanding the energetics of AGN feedback in reg- Forbes (1991) and Simoes Lopes et al. (2006) that the
ulating cooling flows in the hot gas within and around failureofanL -L correlationnecessarilyrequiresan
FIR B
elliptical galaxies. Indeed, Simoes Lopes et al. (2006) externalsourceofdustbymergers. Someexplosivechar-
findacorrelationbetweennucleardustandAGNactivity acter to the AGN-dust cloud interaction is implied by
in elliptical galaxies. Consider the black hole accretion the chaotic nature of the central dust observed in many
8 Temi, Brighenti & Mathews
HST images (Lauer et al. 2005). This chaotic, denser nectionbetweenthewarmgasand(excess)dust. Similar
dusty gasishighly transientandmusthavebeen ejected morphologiesofgasanddustimageshavebeendiscussed
from the core. These chaotic clouds cannot be under- by optical observers (e.g. Goudfrooij et al. 1994). Al-
stood as infalling stellar ejecta (as suggested by Lauer though optical line fluxes appear to correlate with the
et al.), formed by the coalescence of many dusty stellar FIR and B-band fluxes, Figure 2 shows that the FIR
envelopesproducedbyindividualstars,sincethefreefall and B-band fluxes do not mutually correlate. This can
time(∼106yrs)tothegalaxycenteristooshortforsuch be understood if most of the FIR-emitting dust comes
accumulations to occur. not from recent stellar mass loss, but is transported out
However,explosiveejectionsofcentraldustto∼5−10 from a reservoir in the galactic core, which contains a
kpc with energies Ek >∼ 1055 ergs would produce kpc- large mass of dust unrelated to the the global stellar B-
sizedcentralcavities,strongshocksandheatedcoresthat band flux or luminosity.
are rarely observed in X-ray images while extended ex- 7. CONCLUSIONS
cess FIR emission is common. This may be the best
argument against explosive dust transport. Our main conclusions are:
(1)The opticalluminosities L ofellipticalgalaxiescor-
6.4.3. Buoyant Ejection of Dust form the Galactic Core B
relate with luminosities at 24µm (L ) but not with L
24 70
Density irregularities and multi-temperature X-ray or L160. This indicates that the transition between cir-
spectra of the hot interstellar gas in elliptical galaxies cumstellar and interstellar dust occurs for dust grains
reveal buoyant activity, presumably related to a less ex- with emission that peaks somewhere between 24µ and
plosive heating of the hot gas with AGN energy (Buote, 70µm.
Lewis, Brighenti & Mathews 2003a,b). In view of these
(2)EllipticalgalaxieswithsimilarL haveL andL
X-ray observations, it is plausible that central dust can B 70 160
that vary over a factor ∼100. This variation is reduced
bebuoyantlytransportedoutto∼5−10kpc,producing
to ∼ 30 if abnormally dust-rich ellipticals are excluded
the extended FIR emission that we observe.
from the sample.
The buoyant rise time t of heated gas is expected
buoy
to be comparable to or somewhat less than the sputter- (3)NeitherL /L norL /L correlateswiththestel-
70 B 160 B
ing destruction time in the buoyant gas. For example lar age or metallicity of the galaxies.
inthe galaxy/groupNGC 5044the freefalltime fromra-
diusr tothegalacticcenterist =4.5×106r0.87 yrs, (4) Weak correlations appear between optical line emis-
kpc ff kpc
based on a mass model that combines a de Vaucouleurs sion fluxes from warm (∼ 104 K) gas and F70 and F160.
stellar profile (M∗ = 3.37 × 1011, M⊙; re = 10 kpc) This suggests a relationship between dust and gas that
with an Navarro-Frenk-White dark halo (mass M = may have cooled from the hot phase.
halo
4×1013 M⊙; concentration c = 8.67). The buoyancy (5) Seven galaxies in our sample are clearly extended at
rise time tbuoy =βtff is greater than tff by some factor 70µ with FW0.1M exceeding 50′′ corresponding to 5–10
β >∼ 1. ¿From Draine & Salpeter (1979) the approxi- kpc.
mate grainsputteringtime is t ≈2.4×106a n (r)−1
sput µ e
where a is the grain radius in microns. Adopting (6) The luminosities and spectral energy distributions
µ
ne ≈ 0.085rk−p0c.87 cm−3 for the observed hot gas density of elliptical galaxies with the lowest L70 and L160 are
consistentwithasimplesteadystatedustmodelinwhich
in NGC 5044 within 30 kpc, the local sputtering time is
dust expelled from stars is eroded by sputtering in the
nearly proportional to t , t ≈ 1.8a t . However,
ff sput µ ff
hot interstellar gas.
the buoyant gas will have a density that is δ < 1 times
lower than the observed hot gas density. Therefore, we
(7)WeshowinAppendixBthattheobservedemissionat
seethattheratioofthelocalsputteringtimeinthebuoy-
70 and 160µm cannot come from small optically visible
antgastothe buoyancyrisetime is∼1.8a /(δβ),which
µ clouds in the galactic cores similar to those seen in HST
may well exceed unity considering the many uncertain-
images.
ties involved.
TheexactnatureoftheAGNenergyreleasethatiniti- (8) In view of the common occurrance of spatially ex-
atesbuoyantactivityisunclearatpresent. Inabouthalf tended emission at 70µm in many ellipticals, it is un-
of the ellipticals with optically visible dust, the images likely that this dust can be supplied by successive merg-
of the obscuring dust are morphologically “chaotic” and erswithgas-richgalaxies,eachoccurringduringtherela-
distributedinanasymmetricfashionawayfromthecen- tively shortsputtering lifetime of the diffuse dust, ∼108
terofthegalaxy(Laueretal. 2005). EvidentlytheAGN yrs. Addiiontal supporting evidence for such mergers is
energy is sufficient to disrupt the dense central dusty lacking.
disks and lanes which must reform again later by local
(9) The appearance of extended FIR-emitting dust in
stellar mass loss. It is interesting that the radii of the
otherwise normal elliptical galaxies can be understood
dustydisksobservedbyLaueretal. (2005)donotcorre-
by the disruption and buoyanttransport of centrally ac-
late with the stellar rotation, also suggesting short disk
cumulateddustbyintermittentAGNactivity. Thebuoy-
lifetimes and intermittent disruption of the dusty disks.
ant rise time out to 5-10 kpc, ∼107 yrs, is less than the
Regardlessofthe mode of outwarddust transport,ex-
sputtering lifetime of the dust, ∼108 yrs.
plosiveorbuoyant,whenthedustrisesto5-10kpcitmay
causethehotgastocoolbacktowarmgastemperatures
∼ 104 K. The correlations in Figure 8 between F and This work is based on observations made with the
70
F and the optical line fluxes, if real, indicate a con- Spitzer Space Telescope, which is operated by the Jet
160
FIR Emission from Elliptical Galaxies 9
Propulsion Laboratory, California Institute of Technol- NASAgrantsNAG5-8409&ATP02-0122-0079andNSF
ogy, under NASA contract 1407. Support for this work grant AST-0098351 for which we are very grateful. We
was provided by NASA through Spitzer Guest Observer thank Justin Harker for his help with the photometric
grant RSA 1276023. Studies of the evolution of hot gas properties of old stellar populations.
in elliptical galaxies at UC Santa Cruz are supported by
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10 Temi, Brighenti & Mathews
TABLE 1
Name Samplea Tb FluxDensity Dc Re 70µmFWHM 70µmFW0.1M
24µm 70µm 160µm
(PI) (mJy) (mJy) (mJy) (Mpc) (′′) (′′) (Kpc) (′′) (Kpc)
NGC507 Temi -3.3 ··· <2.8 ··· 71.99 77.1
NGC533 Temi -4.8 7.1±3.1 23.7±4.9 26.1±5.6 68.23 47.5
NGC584 Kennicutt -4.6 47.6±7.6 49.8±6.9 <17.2 23.76 27.4
NGC596 Surace -4.3 15.9±4.6 21.4±5.1 16.7±7.3 23.87 30.0
NGC636 Surace -4.8 10.3±3.6 <16.4 <24.5 22.28 18.9
NGC720 Temi -4.8 25.6±5.1 18.2±4.9 <21.0 22.29 39.5
NGC855 Kennicutt -4.6 72.3±8.5 1335±36 2086±46 9.33 18.9 22.5 1.0 58.2 7.6
NGC1377 Kennicutt -2.1 1603±40 4988±71 2375±48 22.20
NGC1395 Kaneda -4.8 45.4±6.5 129±11 194±14 21.98 45.4
NGC1399 Temi -4.5 59.9±7.3 15.9±7.6 24.1±8.6 19.40 42.4
NGC1404 Kennicutt -4.7 56.6±6.3 32.6±6.3 <32.2 19.40 26.7
NGC1407 Kaneda -4.5 42.5±6.4 ··· <35.4 22.08 71.8
NGC1700 Surace -4.7 18.7±4.9 29.3±5.3 40.3±7.2 38.04 13.7
NGC2325 Temi -4.7 27.0±6.4 36.5±6.8 21.7±6.4 31.92 86.5
NGC2768 Temi -4.3 46.6±8.2 694±26 377±29 22.38 53.5 21.4 2.3 56.4 6.1
NGC2974 Kaneda -4.7 62.5±12.4 682±18 1979±54 30.33 36.9 22.8 3.4 55.4 8.1
NGC2986 Temi -4.7 13.7±4.8 <2.8 <14.5 32.51 41.4
NGC3265 Kennicutt -4.8 287±17 1952±44 2312±49 21.33
NGC3379 Fazio -4.8 65.5±8.8 60.5±8.0 59.1±8.2 10.71 35.2
NGC3557 Temi -4.8 30.5±4.5 263±16 247±18 34.51 37.8 20.0 3.3 38.8 6.5
NGC3610 Surace -4.2 18.1±4.5 19.1±7.3 32.8±16.5 29.24 12.8
NGC3923 Temi -4.6 42.9±6.7 22.7±5.4 44.0±7.8 19.14 53.3
NGC3962 Kaneda -4.7 21.5±5.6 374±18 495±24 23.23 34.4 22.6 2.5 44.0 5.0
NGC4125 Kennicutt -4.8 53.7±6.9 773±28 1395±37 27.79 59.9 21.6 2.9 56.8 7.7
NGC4365 Cote’ -4.8 21.7±4.7 63.8±8.0 52.9±7.3 17.06 57.2
NGC4472 Fazio -4.8 73.3±8.6 58.1±7.6 60.4±7.8 17.06 104.0
NGC4552 Kennicutt -4.6 57.3±7.8 92.1±10.2 171±16 17.06 30.0
NGC4589 Kaneda -4.8 14.9±4.5 262±16 360±21 26.30 41.4 22.6 2.9 45.6 5.8
NGC4621 Cote’ -4.8 34.2±6.3 31.7±5.7 43.4±6.6 17.06 46.5
NGC4636 Fazio -4.8 31.2±5.6 188±12 169±14 17.06 101.7 22.3 1.8 54.4 4.5
NGC4649 Fazio -4.6 106±10 46.3±6.8 <27.5 17.06 73.6
NGC4660 Cote’ -4.7 15.2±4.3 36.4±6.2 53.7±8.2 17.06 12.8 20.0 1.7 39.0 3.2
NGC4696 Sparks -3.7 22.9±4.7 127±13 269±21 39.65 212.4 22.6 4.4 52.2 10.1
NGC4697 Surace -4.8 43.8±6.7 589±25 755±28 16.22 75.3
NGC4915 Surace -4.5 11.5±5.5 29.2±4.8 54.2±8.9 46.98 7.9
NGC5018 Surace -4.6 60.5±9.1 1119±34 1687±41 32.36 24.9 20.4 3.2 39.6 6.21
NGC5044 Temi -4.7 23.0±5.3 230±14 242±18 32.36 78.6 23.1 3.6 53.6 8.4
NGC5077 Fazio -4.8 32.4±5.7 127±11 141±13 32.36 25.0 20.2 3.2 49.5 7.8
NGC5322 Fazio -4.8 41.4±6.4 455±23 625±27 29.79 35.2
NGC5557 Surace -4.8 13.2±5.0 25.1±10.5 <42 49.02 28.6
NGC5813 Fazio -4.8 14.9±4.9 58.4±7.6 34.8±5.9 30.78 48.6 19.6 2.9 49.8 7.4
NGC5846 Fazio -4.7 38.5±6.2 102±10 117±12 24.55 82.6 20.4 2.4 43.1 5.1
NGC5982 Surace -4.8 13.5±5.1 37.2±7.2 59.9±11.7 40.18 29.4
NGC6703 Fazio -2.8 19.9±4.5 41.4±6.4 17.9±5.1 32.06 23.8
NGC7619 Temi -4.7 9.9±4.7 <3.8 <14.5 42.85 32.1
IC3370 Kaneda -4.7 43.8±6.7 649±25 873±30 45.90 18.5 20.5 4.5 39.7 8.8
a
Galaxies are selected from the following Spitzer observing programs: SINGS Legacy Program, R. Kennicutt (PI); GTO program number 69,
G. Fazio (PI); GO program number 3403, J. Surace (PI); GO program number 3506, Sparks (PI); GO program number 3619, H. Kaneda (PI);
b
GO program number 3649, P. Cote’ (PI); GO program number 20171, P. Temi (PI). The morphological type T is taken from the HyperLeda
database.c DistancesarecalculatedwithH0=70kms−1 Mpc−1.