ISOCAM Observations of the Seyfert 2 NGC 3147

Laurent 1, I. F. Mirabel 1, V. Charmandaris 1, M. Sauvage 1, P. Gallais 1 L. Vigroux 1, C.J. Cesarsky 1 0.1 Service d'Astrophysique CEA/DSM/DAPNIA/ Centre d'Etudes de Saclay Gif-sur- Yvette, France. F-91191

Abstract We have obtained mid-infrared maps of the face-on Seyfert 2 galaxy NGC 3147. Its average properties are typical of normal . However, the mid-infrared properties of the Seyfert nucleus and the immediate environment are peculiar. The nucleus presents low silicate absorption and low emission from Polycyclic Aromatic Hydrocarbon (PAH) which suggest that we observe a hot inner dusty torus with a radius smaller than 0.6 kpc. Between 0.6 kpc and 1.2 kpc from the center, there is an extended PAH emission which may be attributed to the narrow line region (NLR) heated directly by the non-thermal emission of the (AGN). A forming ring at 2-2.5 kpc from the center is observed in the mid-infrared. Beyond this ring, the thermal emission near the strong star forming regions (SFRs) increases faster than the PAH emission. The ratio of 7 µm to 15 µm fluxis less than 1 in star forming regions and becomes close to outside these regions. Even though the star formation rate is very high in the ring, we still detect PAH emissions which suggest that PAHs are not completely destroyed in starburst regions. 1

1 Introduction

Although NGC 3147 was first classified as a normal galaxy [5], it wasproposed that this galaxy has a low luminosity AGN, because of broad optical emission lines [8] and an excess of X-ray = 2 1043ergs-1) [2). luminosity ( Lx 104 - ASCA X-ray observations (0.4-10 keV) have shown that the spectrum is similar to a typical Seyfert 1 galaxy but the X-ray variability is not detected as we would expect for a low luminosity AGN. According to the unified model in which a molecular torus surrounds the broad line region 2 (BLR), this behavior could be explained by a Seyfert model assuming that a molecular torus

321 seen edge-on hides the non-thermal AGN emission [13]. The observed X-ray spectrum may originate from an obscured AGN seen only in scattered light. The optical nuclear spectrum has a strongly inverted [NII]/Ha ratio (typical of the AGN region) as well as a broad [NII] line (FWHM 400 km s-1 ). The [NII] line could originate from photons of an AGN by scattering on matter located above and below the dusty torus. The detection in addition of the narrow Ha line seems� to prove that the central region is visually obscured. 0 NGC 3147 contains a large total amount of molecular gas (M(H2) = 1.6 x 101 M0 ) with a total infrared luminosity Lrn = .5.75 x 1010£0 [21]. For NGC 3147, the Lrn/M(H2) equal to 3.6, which is about the same value as found in normal non-interacting, non-starburst gas-rich spiral galaxies like the Milky Way or NGC 891 [15]. If Lrn/M(H2 ) measures the star formation efficiency then this efficiency in NGC 3147 is similar to that in normal gas-rich spirals. We assume the distance of the galaxy to be D 43.2 Mpc (Ho = 75 kms-1 Mpc-1 ). In this context, one pixel of 3" in our infraredmaps corresponds to 0.6 kpc. = 2 Observations and data reduction

NGC 3147 was observed by ISOCAM1 with the filters LW 2, LW3, LW4 and LW7 which cor­ respond to 6.75, 15, 6, and 9.62 µm respectively [3]. For each filter, maps have been obtained using a raster mode (2x2 positions). The spatial resolution range from 5" to 811 depends on the point spread function (PSF) with 3" pixel field of view. We obtained final rasters covering a 114" x114" field of view. For each filter, we have approximately 50 exposures with 2.1 seconds of integration time at each position. The ISOCAM data presented in this paper were analyzed using the CAM Interactive Anal­ ysis software (CIA) 2. The dark current subtraction is done using a library of calibration data. Cosmic ray hits were subtracted applying a multi-resolution method using the wavelet transform [19]. The detector memory effects were corrected using an inversion method. The photometric uncertainty which is estimated to 30 % is principally due to this memory effect. The back­ ground is estimated using an area which does not contain a source. To improve the spatial resolution, Lucy's algorithm associated to wavelet transforms has been applied to get the de­ convolved images for each filter. The flux is conserved in the total map as well as in bright isolated regions. We have also tried to reconstruct images by convolving each raster with its own PSF. The finalmaps obtained are almost identical to the initial non-deconvolved images.

3 Results and discussion

3.1 Mid-infrared morphology of the disk

In figure 1, we present deconvolved images of NGC 3147 in each filter. Several SFRs are detected in the spiral arms. In NGC 3147, we can distinguish two types of mid-infrared emission. Inside SF Rs, the young heat the dust (very small grains) producing strong thermal emission at 15 µm (LW3). The emission bands at 6.2, 7.7, 8.6 and 11.3 µm (LW4,LW2) are due to stretching

1Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with participation of !SAS and NASA. 2CJA is a joint development by the ESA astrophysics division and the ISOCAM consortium led by the ISOCAM PI, C. Cesarsky, Direction des Sciences de la matiere, C.E.A. France

322 and bending modes of the C-C and C-H bonds in PAHs which are excited by ionizing photons. The silicate band absorption centered on 10 µm corresponds to LW7 filter. A star formation ring lying between 2 and 3 kpc in radius around the nucleus appears al­ ready slightly with the non-deconvolved images. The fact that this ring has identical dimensions for each wavelength indicates that it is really detected in all the wavelength ranges. Moreover, this ring is also observed with the Ha image.

lw4 lw7 - 10.7 +60 (5.5 - 6.5 ,um] (8.5 ,um]

+30

lw2 (5 - 8.5 ,um J lw3 - 18 ,um] -30 (12

-60

-60 ARC SECONDS0 +30 +60 CENTER: R.A. 10 16 52.95 DEC +73 24 0.0 EQUINOX· J2000

Figure 1: Deconvolved images using theoret­ Figure 2: An image from J. Young with ical PSFs. superimposed 7µm contours. Ho In figure 2, the Ho image kindly provided by J. Young and LW2 image show a good corre­ lation. The well-defined SFRs are also detected in LW3, LW4 and LW7.

3.2 Mid-infrared properties of the nuclear region

The PAH emissions (LW2 and LW4) from central regions which are larger than their corre­ sponding PSFs indicate that PAH emission is extended. The telescope jitter may produce some photometric artifacts in spreading out the intensity. However, the jitter which is estimated to 111 should not change so much images and PSFs because we have 311 pixel field of view. In figure 3, the Ha image shows an asymmetric shape near the nucleus. This is not unusual for Seyfert galaxies and can be explained as the emission from the NLR heated by the AGN core along the molecular torus axis. \Ve have detected such an extended emission elongated in the same direction with LW2 and LW4 filters. On the other hand, the silicate band absorption (LW7) and the thermal emission (LW3) of the nuclear region appear point-like, as a result they must originate from the central pixel (0.6 kpc).

In figure 4, we present the SED of the nuclear region. VVe do not observe strong PAH emission (LW4 is close to LW2) nor significant silicate absorption features. The lack of silicate absorp­ tion led to the idea that we see in the mid-infrared the inner part of the dusty torus. Pier et al. [11] using a model of dusty torus heated by the AGN core, have suggested that the silicate absorption depends on the inclination of the line of sight. The inner torus of hot dust which is heated directly by the BLR has to emit in the silicate

323 band according to this model. This way, we could observe a silicate emission in a type 1 Seyfert. The outer part of the torus may produce absorption features as we can see in normal SFRs. As a result, the silicate absorption must decrease when the viewing angle decreases (toward the torus axis). Thus, we would see the inner molecular torus of NGC 3147, if the line of sight is passing just above it. The lack of PAH emissions leads to the same conclusion. The temperature must be higher than in SFRs to destroy the PAHs.

Nucleus - Peak F'ux

•10 ;o 15 0 WavelengthW (microns 20 s E c 0 H : l�--L-\lnteg��-=ated Flux-� : 9x9),,,l arcsecs D Nucleus s 50 >: 40 -10 0 -� 13x 20 '----��---'LW3 LW4 ·o WavelengthLW2 LW7 microns 15 20 -10 •10 ARC 5ECOMDS d: 1 a CEITTER: R.R. 10 16 53.95 DEC •73 24 4,5 EQUUIOX: J2000 Figure 4: The spectral energy distribution O � ) Figure Ha image from J. Young [25] show­ of the nuclear region after deconvolution. (a) ing the star formation ring and the asymmet­ The point-like nuclear region (311 x 3") . (b) ric shape the nucleus. We have superim­ Integrated (911 911) around the nuclear 3: of flux x posed 15 µm contours. region.

The PAH emission is more extended in LW2 and LW4 filters. An extended ionizing region in the emission cone of the AGN may produce this emission of PAHs. The hypothesis that young stars could excite PAH molecules is ruled out. Actually, OB stars produced by star formation close to the AGN would give rise to much more thermal emission compared to PAH emission. However, we do not observe an extended thermal emission. Thus, our integrated spectrum (9" by 9") could be due to an ionized region (the NLR) producing the PAH emission, plus the thermal emission arising from the molecular dusty torus. Contrary to NGC 3147, silicate absorption is detected in a hyper-luminous infrared galaxy IRAS P09104+4109 [20] considered as a dust enshrouded type 2 quasar and in the Seyfert 2 galaxy Circinus [9] . In these cases, we do not see the inner section of the dusty torus. It is seen edge-on and hides completely the central part of the AGN. The Seyfert galaxies should have two different temperatures of thermal emission. The colder temperature centered on 30 K corresponds to classical dust emission of SFRs. The warmer dust region which would be heated by the AGN has a temperature centered on 170 K [14]. The hot component may destroy PAHs near the central region and as a result an observer would see only the hot thermal emission without the PAH emission.

3.3 Mid-infrared properties of star forming regions

In figure 5, the spectrum of the star formingring and the brightest SFR located on northwest have been calculated by integrating the infrared total flux. The inner radius of the ring measures

324 approximately 2 kpc and the thickness of the ring is 1 kpc. The strongest isolated SFR and the ring present a similar SED. We conclude that the different intensities of star formation do not affect significantly the shape of the mid-infrared spectrum.

-� S1or forming region - Integrated f x (3•3 p xels) .. Hll re o s (lw3) - lnteoroted Flux / (63 pixels) 0 . 25 i gi g 0.20 LW3 � 01. 5 0.10  - 0.05 l lu l n � o.oo L______� �= ,., E � 10 15 20 10 15 20 Wovelengtti (microns) F:I Wovelengtr (microns) I Ring - integrated Flux (39 pixels) H:I regio s (lw2) - lntegrcted Flux / (159 pixels) � 0.15 LW2 LW3 � 0.10 LW3 I a.as x n O.OD �------10 15 20 � �10 15 20 Wavelength {microns) Wavelength (microns) � Figure 5: The spectral energy distribution forl Figure 6: (a) Typical broad band mid­ the brightest region located on northwest at infrared spectrum for star forming regions the end of the arm (top) and the ring obtained (upper to 8u for LW3). (b) Extended regions after the deconvolution (bottom). close to star forming regions ( 4u up to 8u for LW2).

In the top graph of figure 6, we present the integrated SED of the regions of the galaxy with the highest thermal emission, as traced by LW3. At bottom, we present a similar plot for the remaining regions of the galaxy with strong diffuse emission (high LW2 flux). The results are given in mJy/arcsec2• We notice that the LW2/LW4 ratio is approximately equal to 1.5 for SFRs while it decreases towards 1.3 for the highest LW3 regions. Given that LW2 and LW4 correspond to PAH band emissions, we conclude that the PAH emission still exists even though the interstellar radiation field heating the dust is high. These SEDs also show that the PAH emission is higher in absolute intensity when the dust emission becomes important. It is evident that the PAHs are not significantly destroyed by ionizing photons in SFRs when the thermal emission increases. In the inter-arm regions, the PAH contribution becomes higher than the dust emission even though the intensity decreases. As the radiation field decreases, the dust temperature drops exponentially as in a black-body emission. However as PAH emissions result from heating by ionizing photons, they vary linearly with the temperature [16].

4 Conclusions

(I) The mid-infraredspectrum of the nucleus of NGC 3147 is distinctly different from all other regions of the galaxy. The low silicate absorption and low PAH emission indicate that the dusty torus in the nuclear region is not seen completely edge-on. It is plausible that we see a section of the hot inner torus heated by the BLR where the temperature is high enough to destroy the PAHs and produce silicate emission. However, the optical spectroscopy suggests that the nucleus of NGC 3147 is a Seyfert 2, and therefore, the BLR is hidden from the line of sight.

325 (II) At 5-8.5 µm, PAH emission around the central pixel is due to the N1R heated by ion­ izing photons from the B1R. The N1R is not sufficiently close to the hot B1R for heating the dust. As a result, the contribution from the PAH emission dominates the thermal emission from small grains.

(III) The mid-infrared spectra for different SFRs always have 1W2/1W3 ratio lower than 1. However, the 1W2/1W3 ratio is close to 1 in the diffuse emission around SFRs as in M51 [16]. Ionizing photons from OB stars excite PAHs which emit in 1W2 filter range and the dust emitting in 1W3 filter range is also heated by the photons from these young stars. The thermal emission from the dust increases exponentially with the temperature whereas the PAH emission varies linearly with the temperature. As a result, the dust emission relative to the PAH emission becomes higher when the radiation field from young stars increases.

Acknowledgements. We are grateful to Judith Young forproviding the Ha image. We would like to thank D.B. Sanders for helpful suggestions to the manuscript.

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