3.3 μm PAH observations of the central kiloparsecs of Centaurus A
AAstronomy & Astrophysics manuscript no. CenA˙astro-ph c (cid:13)
ESO 2018October 29, 2018 µ m PAH observations of the central kiloparsecs ofCentaurus A (cid:63) L.E. Tacconi-Garman and E. Sturm European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei M¨unchen, Germany, e-mail: [email protected] Max-Planck-Institut f¨ur extraterrestrische Physik, Giessenbachstraße, 85748 Garching bei M¨unchen, Germany, e-mail: [email protected]
Received date / Accepted date
ABSTRACT
Aims.
The aim of this work is to further investigate the nature of PAH excitation and emission especially in the context of tracing starformation in a variety of extragalactic environments. Here we turn our attention to the energetic environment of the closest AGN inour sample, Centaurus A.
Methods.
Using ISAAC on the ESO VLT UT1 (Antu) we have made high spatial resolution 3.3 µ m imaging observations of thecentral kiloparsec of CenA. These observations have been compared with star formation tracers in the near- and mid-infrared, as wellas with mid-infrared tracers of nuclear activity. Results.
The nucleus is not devoid of PAH emission, implying that the PAH particles are not destroyed in the nucleus as might beexpected for such a harsh environment. However, we see the feature to continuum ratio decrease towards the AGN. As well, the3.3 µ m PAH feature emission generally traces the sites of star formation in Cen A, but in detail there are spatial o ff sets, consistentwith an earlier study of the starburst galaxies NGC 253 and NGC 1808. However, the feature-to-continuum ratio does not drop at thepositions of star formation as was previously seen in that earlier study. The cause for this di ff erence remains uncertain. Finally, ourdata reveal possible evidence for a nearly face-on, circular or spiral, dust structure surrounding the nucleus. Key words.
Galaxies: active - Galaxies: individual: Centaurus A (NGC 5128) - Galaxies: ISM - Galaxies: Seyfert
1. Introduction
Over the last two decades it has become increasingly evidentthat emission from polycyclic aromatic hydrocarbons (PAHs)can be used as a powerful diagnostic of the physical condi-tions in a virtually unlimited number of di ff erent environments,from the environs of individual stars to globally-averaged entiregalaxies (e.g. Peeters et al. 2004, and references therein). Thereare prominent emission features at 3.3, 6.2, 7.7, 8.6, 11.3, and12.7 µ m. The strength of these features depend on the charac-teristics of the emitting particles (size, geometry, composition,charge state, etc.; e.g. Allamandola et al. 1989; Bauschlicheret al. 2008, 2009; Draine & Li 2001; Schutte et al. 1993). Thus,ratios of PAH emission at various of these wavelengths (espe-cially to emission at 3.3 µ m) can be used as powerful tracersof grain size, grain photoionization state, etc. (e.g. Bauschlicheret al. 2009). For the particular case of dusty galaxies, earlystudies with the Infrared Space Observatory (e.g. Genzel et al.1998; Rigopoulou et al. 1999; Tran et al. 2001) used the pres-ence and strengths of spectral features attributed to PAHs incomparison to ionic line and continuum emission to disentan-gle the relative contributions of starburst and active nuclei tothe overall luminosity in samples of ultraluminous
IRAS galaxies(ULIRGs). Later, with the low resolution spectroscopy a ff ordedby the Spitzer Space Telescope , it became not only possible touse the PAH emission features to estimate redshifts to IR brightgalaxies to at least z ∼ (cid:63) Based on observations collected at the European Organisation forAstronomical Research in the Southern Hemisphere, Chile (Program075.B-0618, PI: Tacconi-Garman). ies of those galaxies, thereby placing constraints on the contri-bution of star formation and AGN to the cosmic IR background(Schweitzer et al. 2006; Spoon et al. 2007; Teplitz et al. 2007;Valiante et al. 2007; Yan et al. 2007).A few key observational facts have emerged from these stud-ies and many others covering a wide range of source types(Peeters et al. 2002; van Diedenhoven et al. 2004; Galliano et al.2008, to cite but a few). First, PAH emission is seen in nor-mal, starbursting, and active galaxies. However, although thereis little di ff erence in the profiles of the PAH features from onegalaxy to another, there are observed variations in the ratios ofthe PAH feature emission at di ff erent wavelengths (e.g. Peeterset al. 2004). Such ratio variations have been used as probes ofthe details of the PAH characteristics (e.g. size and charge) in theemission regions and / or of the local conditions (e.g. Bolatto et al.2007; Bauschlicher et al. 2009; Haynes et al. 2010; Diamond-Stanic & Rieke 2010; Galliano et al. 2008; Haan et al. 2011).Secondly, there appears to be a threshold at a metallicity of12 + log( O / H ) ∼ . µ m PAH feature emissionin a sample of nearby, IR bright, well-studied low-metallicitydwarf galaxies, starburst galaxies, and Seyferts spanning a rangeof physical environments. High spatial resolution 3 µ m imag-ing has been obtained for the entire sample, and the results fortwo starburst galaxies, NGC 253 and NGC 1808, have been pub-lished in Tacconi-Garman et al. (2005). a r X i v : . [ a s t r o - ph . C O ] F e b acconi-Garman and Sturm: PAH in CenA The results of that study show that although generally thePAH emission does peak in the inner star forming regions, thelack of detailed correlation (either positive or negative) betweenPAH emission and sites of recent star formation indicates theconnection between the instantaneous star formation rate andPAH emission may not be so direct as advocated in the stud-ies of nearby galaxies from large aperture work (e.g. Tran et al.2001, and references therein). This may signal that the PAHemission better traces the general B star population than sites ofrecent massive star formation (Spoon 2003; Boselli et al. 2004;Calzetti 2010). Moreover, in those galaxies we found that thePAH feature-to-continuum emission actually decreases at thestar formation sites (see Fig. 6 and 7 in Tacconi-Garman et al.2005), perhaps owing to mechanical energy deposited into theISM, photoionization of the PAH, or photodissociation of thePAH. In addition, in NGC 253 we find the first evidence for PAHmolecules in a starburst-driven galactic superwind.In the present work we turn our attention to the more en-ergetic environment presented by the nearest Seyfert galaxy,Centaurus A. This galaxy is by far one of the most well-studiedof all galaxies, and a comprehensive review of this work has beenpresented in Israel (1998). The proximity of Cen A allows us aunique opportunity to separate the active galactic nucleus andjet from surrounding structures. This, in turn, provides us withthe ability to cleanly separate the impact that the nuclear activ-ity has on the PAH feature emission. Our observations and thetechniques employed for data reduction are described in § §
3. In § §
2. Observations and Data Reduction
Data on Cen A have been obtained in Service Mode with the in-frared camera and spectrograph ISAAC on ESO’s Very LargeTelescope UT1 (ANTU) on 11 /
12 July 2005. ISAAC was oper-ated in the LWI3 mode, with a pixel scale of 0.1478 arcsec / pixel,and a corresponding field of view of 151 (cid:48)(cid:48) × (cid:48)(cid:48) . We used thetwo narrow band filters NB 3.28 and NB 3.21, centered on the3.28 µ m PAH feature and the underlying continuum at 3.21 µ m,respectively. The spectrum near the 3.3 µ m PAH feature showscontinuum on the blue side and a weaker PAH feature at 3.4 µ m(Sturm et al. 2000; van Diedenhoven et al. 2004) making the useof a single blue side filter for continuum more accurate than ifthe feature were bracketed with two filters.In both filters we took a series of exposures, each consist-ing of a stack of 160 0.5 second sub-exposures, with the nucleusplaced in turn near the middle of each of the four detector quad-rants with jitter applied. This strategy was adopted under theassumption that the region over which PAH and / or continuumemission would be detectable would be small enough not to fallin any of the other three detector quadrants which for the expo-sure in question did not contain the nucleus, making data in thosequadrants viable for sky measurement purposes. After frame se-lection the total exposure time was 53 min and 57 min for theNB 3.21 and NB 3.28 filters, respectively. The mean seeing at3.3 µ m was about 0.6 arcseconds under photometric skies. The data were reduced with IRAF using standard techniquesas outlined in the ISAAC Data Reduction Guide 1.5 . However,during data reduction it was realized that the assumption of smallsource size adopted when planning the observations was unsub-stantiated. This meant that a procedure had to be adopted whichwent beyond the usual pair-wise subtraction of images to re-move the sky emission. The procedure adopted consisted of twopasses. The first pass involved simple pair-wise subtraction ofimages to remove sky, albeit imperfectly. The resulting imageswere then co-aligned such that the very bright and well-definednuclear peaks lined up. A mask was then constructed from a spa-tially smoothed version of the co-addition of these images, suchthat the regions that contained source emission were zero andeverywhere else was unity. The final master mask was the su-perset of the two individual masks for the two filters. That is, ifthere was emission seen in either individual mask it was set tozero in the master mask. Knowing by how much each individ-ual exposure had to be shifted to align the nuclear peak meantalso knowing by how much the master mask had to be shiftedin the opposite sense to mask the object in any given exposure.In the second data reduction pass the sky for any given expo-sure was estimated from the median of the nearest exposuresin which the nucleus was placed in the other quadrants, wherein each case masking was applied as described. The resultingsky-subtracted images were then co-aligned as in the first pass,by ensuring that the bright nuclear peaks were on top of eachother. For purposes of absolute coordinates we have fixed thecoordinates of the nuclear peak to be that of the VLBI peak,13 h m . s − ◦ (cid:48) (cid:48)(cid:48) (Ma et al. 1998). The resultingimages from the NB 3.21 filter (continuum) and NB 3.28 filter(PAH + continuum) are shown in Figs. 1 and 2. For direct com-parison with optical observations of the same region we presentin Figs. 3 and 4 a reproduction of Figure 4 of Marconi et al.(2000). As preparation for continuum subtraction of the NB 3.28 data,we modeled the continuum slope as one consistent with that ofthe range of nearby galaxies represented by the SINGS sample(Kennicutt et al. 2003) or with an AGN-dominated spectrum.In analysing the 1–850 µ m SEDs of the SINGS galaxies Daleet al. (2005) find that they can be well fit by a combination ofa dust-only component and a stellar component represented bya 900 Myr continuous star formation with solar metallicity andSalpeter IMF (see their Figs. 3–10). Over the wavelength rangeof our observations only the latter component contributes. Wehave thus used Starburst99 to produce such a continuum spec-trum (Fig. 5) and have determined the relative flux that wouldbe observed with the two ISAAC filters if the continuum hadthat shape. We find that in such a case scaling the flux in theNB 3.21 filter image by a factor of 1.099 before subtracting fromthe NB 3.28 image would remove the continuum from that im-age.To model an AGN-dominated continuum spectrum we reliedon the result of Netzer et al. (2007) who have analysed the IRSEDs of QSOs in the QUEST sample (Schweitzer et al. 2006). In IRAF is distributed by the National Optical AstronomyObservatories, which are operated by the Association of Universitiesfor Research in Astronomy, Inc., under cooperative agreement with theNational Science Foundation. http: // / sci / facilities / paranal / instruments / isaac / doc / drg / html / drg.html2acconi-Garman and Sturm: PAH in CenA Fig. 1.
Logarithmically-scaled 3.21 µ m continuum emissionfrom Cen A. The data have been scaled by a factor of 1.042 (seetext) and spatially smoothed by an 8 × Fig. 2.
Logarithmically-scaled 3.28 µ m PAH feature + continuumemission from Cen A. The data have been spatially smoothed byan 8 × Fig. 3.
Reproduction of the WFPC2 mosaic of Cen A (Figure 4of Marconi et al. 2000) with the ISAAC field of view indicatedwith a red box. For alignment with the ISAAC data it was nec-essary to apply a small (1 ◦ ) rotation to the HST image.
Fig. 4.
The same as Fig. 3 but with contours of the 3.28 µ mPAH feature + continuum emission superposed. The contours arebased on the image shown in Fig. 2 and have values of 0, 0.25,. . . , 2.0 in the arbitrary units of the map.it from the flux in the NB 3.28 filter would result in the latterbeing fully continuum-subtracted .Scaling our observed continuum by the star formation factorof 1.099 resulted in a continuum level near the nucleus whichwas higher than the PAH feature + continuum flux measured inthe same region by the NB 3.28 observations. Thus the centralregions are better represented by an AGN-dominated continuumslope. Since the observed continuum levels in the regions whichlie further from the nucleus are inherently much lower than thoseof the PAH + continuum image there is little di ff erence in adopt-ing one continuum scaling versus the other. Hence, for the en- Note that despite the fact that the model continua are both blueover this wavelength range, the derived continuum-correction factorsare both larger than unity owing to the fact that the throughput of theNB 3.28 filter is larger than that of the NB 3.21 filter. 3acconi-Garman and Sturm: PAH in CenA
Fig. 5.
Illustration of the di ff erences of continuum slope as afunction of the dominant contributor to the continuum. The solidcurve represents the AGN-dominated case (Netzer et al. 2007),while the dotted line shows our SB99 model results consistentwith star formation (Dale et al. 2005). The dashed line indicatesthe continuum slope that would have to be present to result in nodetected PAH emission at the position of the nucleus (see text).The continua have been normalized to unity at λ = . µ m. Forreference the two ISAAC filter curves are shown.tire field of view we correct the flux in the NB 3.21 image bya factor of 1.042 before subtracting it from the NB 3.28 imageto produce a continuum-free PAH feature image. The resultingcontinuum-free 3.3 µ m PAH feature image is presented in Fig. 6.The emission at the position of the nucleus itself is consistentwith a point source in the NB 3.21 and NB 3.28 observations,as well as in the resulting continuum-free PAH image.We have used the continuum-subtracted PAH feature map(Fig. 6) together with the pure continuum map (Fig. 1) to derivea feature to continuum ratio map. During the division of thesetwo maps only those pixels which were at least 3 σ from theirrespective mean background levels were considered. The result-ing 3.3 µ m PAH feature-to-continuum map is shown in Fig. 7.
3. Results
The continuum emission presented in Fig. 1 is strongly peakedtowards the position of the nucleus, with faint traces of emis-sion to the northwest and southeast. As well there are clearindications of extinction due to the well-known dust lanes inCen A. The PAH feature + continuum emission map (Fig. 2),while also clearly peaked towards the nucleus, shows much moreprominent emission towards the northwest and southeast witha parallelogram-shaped morphology. The regions in the contin-uum image that display evidence for the extinction are com-pletely filled in in this image. These clear di ff erences in mor-phology between the observations made with the NB 3.21 andNB 3.28 filters are strongly indicated in both the continuum-freePAH feature emission map (Fig. 6) as well as the PAH feature tocontinuum ratio map (Fig. 7).Figure 6 reveals that it is the PAH feature emission that bettertraces out a symmetric parallelogram centered on the nucleusthan does the 3.3 µ m continuum emission. This morphology was Fig. 6.
Linearly-scaled 3.3 µ m continuum-free PAH featureemission from Cen A, derived from the subtraction of the scaledNB 3.21 data from the NB 3.28 data (see text) and spatiallysmoothed by an 8 × α , δ = h m . s , − ◦ (cid:48) (cid:48)(cid:48) ) described in the text.Logarithmically-spaced contours are drawn at the levels 0.60,0.90, 1.34, and 2.00 in the arbitrary units of the map.already seen in a number of di ff erent observations, covering awide range of wavelengths (Table1).Finally, we note that the ISAAC data are the first to clearlyshow that the parallelogram feature is traced by continuum-subtracted PAH feature emission . This PAH feature emissionis seen to consist of a number of knots throughout the parallel- Quillen et al. (2008) present
Spitzer Space Telescope maps of the7.7 µ m, 8.6 µ m, and 11.3 µ m PAH features (their Figs. 13 and 3) inthe central region of Cen A, though those maps are not continuum-subtracted.4acconi-Garman and Sturm: PAH in CenA Fig. 7. µ m PAH feature to continuum ratio emission fromCen A, derived from the images shown in Figs. 1 and 6. The barin the lower right corner of each panel illustrates 200 pc at thedistance to Cen A. The wedges to the right indicate the valuesof the ratio. The bottom panel shows a blowup of the centralregion to better illustrate details. The arrows in that panel are atthe positions of the bright spots as defined in the PAH featuremap (Fig.6) to facilitate comparison with that figure. We plot acontour at a feature to continuum value of 0.1 to make the lowestlevels more visible against the black background. On top of theimage we show logarithmically-spaced contours of the mid-IR[O iv ] emission (Quillen et al. 2008) indicating the Narrow LineRegion (see text) and two red lines indicative of the positionangle of the radio / X-ray jet (Burns et al. 1983; Kraft et al. 2000).ogram structure, but most notably in the inner regions (see theblowup in Fig. 6) and in the northwestern and southeastern ends.
In the central regions of Cen A (blowups in Figs. 6 and 7) onesees a wealth of structure. First, there are 4 bright knots as in-dicated with arrows in those figures. These spots are the sameas have been seen at 8 and 24 µ m by Quillen et al. (2006a),and are amongst a number of features which define the knotty“sides” of the parallelogram structure. The southeastern “arm” Table 1.
Other observations revealing the parallelogram struc-ture in Cen A.
Wavelength / Line Telescope / Instrument References NotesH α AAT / TAURUS 11.25 µ m, 1.65 µ m, 2.2 µ m NTT / SOFI 2 a µ m, 4.5 µ m, 5.8 µ m, 8.0 µ m Spitzer / IRAC 3, 47 µ m, 15 µ m ISO / ISOCAM 5450 µ m, 850 µ m JCMT / SCUBA 5, 61.3 mm CO(2-1) SMA 7 b CO(2-1) ALMA 821 cm H i ATCA 9
Notes. ( a ) after extinction correction and ellipse subtraction; ( b ) centralpart of the structure only References. (1) Bland et al. (1987); (2) Kainulainen et al.(2009, private communication); (3) Quillen et al. (2006b); (4)Brookes et al. (2008); (5) Mirabel et al. (1999); (6) Leeuw et al.(2002); (7) Espada et al. (2009); (8) ESO Photo Release eso1222,http: // / public / news / eso1222 / ; (9) Struve et al. (2010). shows more bright knots than the other parts of that structure.Given the general radial fallo ff of the continuum emission, theseknotty structures represent regions of relatively high feature-to-continuum ratio (see Section 4.3.1 for discussion of the brightestfour knots). In addition, PAH feature emission is seen betweenthese straight lines and the bright nucleus itself in a roughlynorth-south bar-like structure, which appears to end at a nearlycomplete, nearly circular clumpy ring of PAH feature emissionabout 6.5 (cid:48)(cid:48) (120 pc) in diameter surrounding the nucleus (seealso the blow-up of this ring in Fig. 11). Unlike at the positionsof the knots in the linear features, the locations of the clumps inthe ring show only slight rises in the PAH feature-to-continuumratio while the ring as a whole is visible in Fig. 7.Interior to this ring, spatially unresolved nuclear PAH fea-ture emission is clearly detected, despite the potentially hostileenvironment of the active nucleus. Indeed it is impossible toscale the continuum emission prior to subtraction from the fea-ture + continuum emission in such a way that the nuclear PAHfeature disappears without strongly over-subtracting the contin-uum at all other locations with the field of view. Moreover, ifone considers only the nucleus itself, its SED would have tohave a slope as indicated by the dashed line in Fig. 5 for thePAH emission to vanish there. The slope of that SED is phys-ically unrealistic. Thus, the nucleus does show a very low, butnon-zero, feature-to-continuum emission ratio, perhaps explain-ing why the PAH feature emission has gone undetected to date(e.g. Laurent et al. 2000).This is not to suggest that the nucleus has no influence atall on the PAH emission. In Fig. 7 one clearly sees the featureto continuum decreasing roughly radially towards the nucleus.Indeed, the contours of [O iv ] could serve almost as well as con-tours of the PAH feature to continuum ratio. Thus, assuming thatthe [O iv ] does trace the Narrow Line Region rather than localshocks , the nucleus is seen to have a marked e ff ect on the e ffi -ciency of the PAH emission in its vicinity. There is no obviousimpact on the PAH emission stemming from the radio / X-ray jet(see Fig. 7 where lines indicate the position of the jet).Finally, there is a faint, arc-like feature ∼ (cid:48)(cid:48) to the north ofthe nucleus seen in the PAH feature image and marginally in the Krajnovi´c et al. (2007) cite evidence for the radial increase in theimportance of shocks though their analysis is restricted to the centralfew arcseconds. 5acconi-Garman and Sturm: PAH in CenA feature-to-continuum image. There is no detected counterpart ofthis feature to the south of the nucleus.
4. Discussion
The fact that the 3.3 µ m PAH feature traces out the large, sym-metric parallelogram may not be surprising in light of the factthat it is better traced by the longer wavelength (5.8 µ m and8.0 µ m) IRAC imaging than at the shorter IRAC bands (3.6 µ mand 4.5 µ m; Quillen et al. 2006b). The longer wavelength bandsare more dominated by a dust component and therefore lessinfluenced by (di ff use) starlight than is the case at the shorterwavelength bands. However, what is less anticipated is the factthat the emission from the parallelogram is so faint relativeto the nucleus in the 3.3 µ m continuum. The feature is seenat 3.6 µ m and at J (1.247 µ m) and K s (2.162 µ m), once theveil of ∼ H (1.653 µ m),Kainulainen, private communication). In the J image the largenumbers of fairly bright sources seen in the extended structureare about a factor of 300 fainter than the nuclear peak. Thebrightness ratio in the case of the 3.6 µ m imaging is not easyto infer from Fig. 1 of Quillen et al. (2006b) but is likely notsubstantially lower than this. This would be consistent with re-sults of Dale et al. (2005) and of Netzer et al. (2007) that togetherwould imply similar ratios of the nuclear brightness to that in thelarger scale structure. The dynamic range in our 3.3 µ m contin-uum image (Fig. 1) is about a factor of 125. Thus, it may be thecase that our observations are not deep enough to have detectedthe continuum at this wavelength. Nevertheless, the fact that wehave made a clear detection of the PAH feature emission, whileat the same time not having detected the continuum emissionmeans that at this wavelength the emission from the parallelo-gram is clearly dominated by dust emission. This is in contrastto the case at 3.6 µ m. That the 3.3 µ m PAH feature emission is related to star formationis demonstrated in Fig. 8. In this figure we show as a backgroundimage a colorized version of Fig. 10 (right) of Marconi et al.(2000), illustrating HST Pa α data with VLA α , but with some small o ff sets and other discrepancies clearlyvisible. For example, while the PAH emission is seen to alignquite well with the features in the brighter SE part of the paral-lelogram, the PAH emission at the positions of the three brightknots (as indicated by arrows in Fig. 6, the fourth knot lying out-side the field of view of the Pa α data) is seen to be displacedfrom the position of the Pa α peaks. Another clear mis-alignment Fig. 8.
A comparison of the 3.3 µ m PAH feature emission (yel-low / black contours), Pa α emission (underlying grayscale im-age), and 6 cm radio continuum emission (light black contours,the latter two taken from Fig. 10 of Marconi et al. 2000). Fig. 9.
A blowup of the northeast portion of the Pa α and PAHfeature emission shown in Fig. 8, illustrating a spatial anti-corelation between the two emission features.example is the case of the Pa α patch to the northwest of thenortheastern bright PAH / Pa α knot (Fig.9). In that case the PAHemission appears to lie on either side of the Pa α but not at itsposition. (See also Fig. 11 for similar o ff sets between Pa α andPAH in the very center of Cen A). Any reasonable extinctionlaw would indicate that the extinction for Pa α is higher than at3.3 µ m, which implies that one cannot account for such spatialdistribution di ff erences with di ff erential extinction.As an alternate tracer of star formation we present in Fig. 10the 12.8 µ m [Ne ii ] emission from Quillen et al. (2008) as con-tours on top of the ISAAC 3.3 µ m PAH feature emission fromthe present work. The [Ne ii ] observations also show a relativelygood agreement with the PAH peaks, though given the resolutiondisparity of a factor of ∼ Fig. 10. µ m PAH feature emission (image) with contours of[Ne ii ] emission from Quillen et al. (2008). The white circlesin the lower right represent the spatial resolution of the ISAACobservations (left) and the Spitzer observations (right).conclusively rule out that there are spatial o ff sets between thetwo emission features. Quillen et al. (2006a) report on the discovery of a shell of emis-sion some 30 (cid:48)(cid:48) in radius from the nucleus of Cen A in
Spitzer observations at 4.5, 5.8, 8, and 24 µ m (Fig. 1 of Quillen et al.2006a). The nature of the shell is unknown at present, at leastin part owing to a current lack of velocity information. Quillenet al. present plausibility arguments that suggest that a moder-ate starburst a few thousand solar masses interior to the shellcould have led to its formation. Further, based on size argumentsand the perceived requirement for a gap in the dust distribution(Quillen et al. 2006b), they note that there could exist multiplecircum-nuclear shells in Cen A.The fact that the shell detected in the Spitzer observationsis undetected in the current study is puzzling in light of ourNGC 253 3.3 µ m imaging results (Tacconi-Garman et al. 2005).In that work we found the first evidence for PAH molecules ina starburst-driven galactic superwind. The Cen A shell is sim-ilar to though smaller in scale than the one seen in NGC 253.Ratios of PAH emission at various wavelengths (especially toemission at 3.3 µ m) can be used as indicators of grain size, grainphotoionization state, etc. (e.g. Bauschlicher et al. 2009). Ournon-detection at 3 microns could indicate that the Cen A shellcontains preferentially large grains (Bregman et al. 1994), per-haps as a result of smaller grains having been destroyed (Smithet al. 2007), though this seems implausible, especially in lightof our detection of 3.3 µ m PAH emission in the nucleus itself.An alternative explanation could be that the PAH grains in theshell are preferentially ionized. This would tend to strengthenC-C stretching mode transitions (6.2, 7.7 , and 8.6 µ m) relativeto the C-H stretching mode at 3.3 µ m (van Diedenhoven et al.2004, and references therein). Further, deeper observations at3 microns would be required to better understand the composi- The 7.7 µ m transition is a blend of C-C stretching and C-H in-planebending (Bauschlicher et al. 2008, 2009). tion and physical state of the shell, perhaps shedding light on itsorigin.While there is no current detection of the 3.3 µ m PAH featureemission counterpart to the shell at 30 (cid:48)(cid:48) radius, the detection ofthe faint, arc-like feature ∼ (cid:48)(cid:48) to the north perhaps suggests thatthere have indeed been episodic events that have resulted in suchshells. In the Pa α data of Marconi et al. (2000) there is also a sug-gestion of an arc roughly 20 (cid:48)(cid:48) north of the nucleus (see Fig. 8),lending further support for this. Although the Spitzer imagingshows evidence for a southern counterpart for the northern loopof the shell, neither the Pa α arc at 20 (cid:48)(cid:48) nor the 3.3 µ m are at 15 (cid:48)(cid:48) has a counterpart to the south of the nucleus. The four knots that are clearly evident in 3.3 µ m PAH fea-ture emission (Figs. 6 and 7) are also present in numerousother line observations. They are seen in Spitzer observations of[S iii ], [Si ii ], [Fe iii ], [Ne ii ], H S(0), and to a lesser extent [Fe ii ](Quillen et al. 2008). In addition, three of the four spots are alsoevident in the Pa α imaging of Marconi et al. (2000), with thelast one falling outside their field of view. The fact that thesespots are evident in as many line and continuum observationsas they are suggests that they cannot be due to a chance super-position of the warped disk and the shell, as posited by Quillenet al. (2006a). Rather, it appears more likely that these locationsrepresent locations at which star formation is enhanced, as all ofthese lines can be found at or near sites of active star formation.Indeed, it may be the case that an interaction between an expand-ing shell and the warped disk at those locations has resulted inan enhancement of molecular cloud compression and hence anincrease in the local start formation rate. Certainly knowledge ofthe kinematics of the shell would not only help in defining it asa true coherent structure as noted by Quillen et al. (2006a), butwould also yield clues as to what influence it could have whenconfronting gas in the warped disk.Finally, we note that if these knots do represent sites of en-hanced star formation then there must be di ff erences in the phys-ical conditions and / or grain population relative to those at similarsites in the starburst galaxies NGC 253 and NGC 1808. In boththose cases Tacconi-Garman et al. (2005) found a decrease in thePAH feature-to-continuum ratio at star formation sites, while inthe present case the same ratio is seen to peak at the positionsof the knots. This implies that neither mechanical energy inputinto the local ISM nor photoionisation of the PAH nor photodis-sociation of the PAH, all of which could explain a drop in the ef-ficiency of 3.3 µ m PAH emission (Tacconi-Garman et al. 2005)can play dominant roles at the location of these knots. It couldthough be the case that the stellar contribution to the continuumis lower at these sites than it is in NGC 253 and NGC 1808.Spatially resolved PAH observations at longer wavelengths aswell as consistent star formation age-dating observations of allof these systems would be invaluable to better understand thepresence of feature-to-continuum variations at these sites. To explain the trapezoidal structure seen in the mid-infrared,Quillen et al. (2006b) modeled the system using a warped thindisk. In order to avoid the appearance in the model of an unob-served bright central bar-like feature, they posited a truncationof the dust distribution over the range 6 (cid:48)(cid:48) < ∼ r < ∼ (cid:48)(cid:48) . They ex-plored the possibility of mimicking a gap through decreasing the Fig. 11. µ m PAH feature emission with contours of Paschen α emission (Schreier et al. 1998, their Fig. 4). The bar below thenucleus represents 100 pc at the distance to Cen A.inclination of the innermost disk, but dismissed that as a viablealternative since the resulting model (their Fig. 10b) does notresemble their IRAC data as well as does a model with a gap.Nevertheless, the nearly circular (or perhaps spiral) clumpy ringof PAH feature emission apparent in our observations (Fig. 6and blown up in Fig. 11) may be indicative of a more face-onstructure in the very center of Cen A. Note that this PAH featurebears further confirmation but that support for its existence isgiven by the rough spatial coincidence with observed Pa α emis-sion (Marconi et al. 2000).More recently Espada et al. (2009) have presented interfero-metric CO J = → (cid:48)(cid:48) × (cid:48)(cid:48) ofCen A. While their data is broadly consistent with a morphologylike that adopted by Quillen et al. (2006b), they do see depar-tures from what such a model would predict in terms of bothspatial distribution of gas and, importantly, the gas kinematics.In addressing these di ff erences they find that a combination ofa warped disk model together with a weak central bar potentialcould qualitatively explain their CO observations. In particular,such a model naturally reproduces the S-shape morphology seenin the interferometric CO data. This morphology is also seen inthe H S(0) J = µ m, but is not evident in theH S(3) J = µ m or the H S(5) J = µ m, as was already noted by Quillen et al. (2008). Thefact that the H S(0) line emission shows a similar morphologywith that of the CO leads Espada et al. to suggest that the warmmolecular gas traces the locations of weak-bar-induced spiralarms, where the gas is heated by the resulting shocks and starformation in the arms. As further support for this idea they claimthat the data of Quillen et al. (2008) show that the star formationtracers of [Si ii ], [S iii ] and [Fe ii ] are seen to be brighter on theNW and SE sides of the inner parallelogram. While some of thestructure seen in those data may be related to the S-shaped mor-phology seen in the CO and H S(0) there are departures fromthis which weaken this interpretation. For example, in both the[S iii ] and [Si ii ] maps (Fig. 4 of Quillen et al. 2008) the SW sideis brighter than the NW arm. This is also seen to be the case inthe Pa α observations of Marconi et al. (2000) as well as in our own measurements (see Fig. 6). The origin of this discrepancyremains unclear.
5. Conclusions
Based on our 3.3 µ m PAH imaging of Cen A we find that the nu-cleus is not devoid of PAH emission, but rather is very bright, de-spite the fact that the feature-to-continuum emission, which de-creases towards the AGN, is lowest at that position. This meansthat the PAH particles are not systematically destroyed in thenucleus as might be expected for such a harsh environment.Perhaps this is due to a large quantity of dust having been de-posited in and near the nucleus as a result of the same merg-ing process believed to have caused the larger scale dust struc-tures seen in this galaxy. However, we note that this may not bethe whole explanation, as similar results have been obtained in3.3 µ m imaging of NGC 1068 (Tacconi-Garman, in preparation).In addition, the 3.3 µ m PAH feature emission generallytraces the sites of star formation in Cen A, but in detail thereare spatial o ff sets. This is as was seen in the earlier study of thestarburst galaxies NGC 253 and NGC 1808 (Tacconi-Garmanet al. 2005). However, unlike in those other cases the feature-to-continuum ratio is not seen to drop at the sites of star forma-tion. Instead the ratio peaks at those positions, most notably atthe locations of the four bright knots discussed in Section 4.3.1.This implies that neither mechanical energy input into the localISM nor photoionisation of the PAH nor photodissociation of thePAH, all of which could explain a drop in the e ffi ciency of 3.3 µ mPAH emission (Tacconi-Garman et al. 2005) can play dominantroles at the location of these knots. In addition, what remainsunclear at present is the age of the star formation in the knots,especially when compared to those in NGC 253 and NGC 1808.Finally, our data reveal possible evidence for a nearly face-on, circular or spiral, dust structure surrounding the nucleus.Although this feature bears confirmation by additional obser-vations, support for its existence is given by the rough spatialcoincidence with observed Pa α emission (Marconi et al. 2000). Acknowledgements.
The authors would like to extend their thanks to the anony-mous referee for comments which resulted in a clearer and stronger paper. Aswell, the authors wish to express their gratitude to the dedicated sta ff on Paranalfor their support of these Service Mode observations. Finally, the authors wish towarmly thank Alice Quillen, Joel Green, and Jouni Kainulainen for making the Spitzer and SOFI data available to us, and to Ethan Schreier, Alessandro Capetti,and Alessandro Marconi for their help in locating the original of a previouslypublished image.
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