MID OBSERVATIONS OF EARLY-TYPE

S. C. Madden1 , L. Vigroux1, M. Sauvage1 1 CEA Saclay, Service d'Astrophysique, 91191 Gif-sur- Yv ette CEDEX, France.

Abstract We present results from an ISOCAM survey of early-type galaxies imaged in broad­ band filters centered on 4.5, 6. 7 and 15µm. We find that the 4.5µm emission traces mainly the continuum from the evolved stellar population, while the 15µm, and often the 6.7µm emission show a different morphology and trace the dust. About half of the galaxies in our sample have significant MIR excess which originates from stochastically heated small grains and PAHs. The 6.7 µm/15µm ratio can be used as a tool to differentiate dusty early-type galaxies from dust-free ones.

1 Introduction

Early-type galaxies, once believed to be simple systems of coeval stars with little or no dust or gas, are now known to contain a stellar distribution which often includes main-sequence stars more massive than spectral type G in addition to the evolved red giant population. The ISM of early-type galaxies can be very diverse and is often dominated by extensive, hot (T � 106 K) X-ray emitting gas [6] [18]. Neutral atomic gas [19] [21] and molecular gas [22] [21] [17] are also often present in early-type galaxies giving ISM characteristics similar to spiral galaxies. From IRAS lOOµm observations, we know that early-type galaxies can harbor small amounts of cold (T � 20K) dust [9] which is more prominantly observed in spiral galaxies. Considerable mass loss from red giant stars, thought to reside in early-type galaxies, results in the expulsion of dust through stellar winds into circumstellar envelopes and subsequently into the ISM. The 12µm dust emission seen in spiral galaxies by IRAS is normally attributed to an additional population of small interstellar grains heated stochastically to high temperatures [16] [2]. How­ ever, compared to spiral galaxies, the early-type galaxies generally exhibit excess 12µ IRAS

229 emission relative to the lOOµm which has led [10] to conclude that most of the 12µm emission is photospheric and circumstellar emission. On the other hand, analyses by [15] conclude that the 12µm emission as interstellar in origin. The precise origin of the mid-infrared (MIR) emission in early-type galaxies, however, has been difficult to decipher with IRAS observations alone. To further unveil the nature and distribution of the MIR dust in elliptical and lenticular galaxies, we are conducting a survey using the CAM instrument [4] onboard the Infrared Space Observatory (ISO) [7] and present here initial results for 11 of the galaxies from our sample.

2 The Data

Observations were made in the following 3 bands: LW l, from 4.0 to 5.0µm (center: 4.5), LW2, from 5.0 to 8.5µm (center: 6.7µm) and LW3, from 12.0 to 18.0µm (center: 15.0µm). These bands were chosen as they can sample various components of the MIR emission. The 15µm emission can be attributed mostly to the presence of the small hot grains which are heated stochastically, as opposed to the larger grains seen in the FIR by IRAS which are in ther­ mal equilibrium. The 6.7µm band is dominated by emission from the unidentified infrared bands (UIBs) at 6.6, 7.7 and 8.6µm, which are often modeled as polycyclic aromatic hydrocar­ bons (PAHs) [5] - large 2-dimensional molecules, also stochastically heated (see review by M. Sauvage, this volume). Additionally, an associated underlying continuum, the nature of which is uncertain, is contained in this band. Photospheric emission from evolved stars dominates the 4.5µm band and can also be present to some degree in the other bands. As an example, we present the results for the E4 , NGC 5266, superposed on the optical image fromthe Digital Sky Survey in Figure 1. From comparison of the distribution of the various band emission, we can qualitatively conclude the following: 1) Since the 4.5µm emission distribution coincides with the bulk of the optical emission, it is tracing the stellar emission (as there is not expected to be much contribution at this wavelength from dust). 2) The 15µm emission depicts a more confined, prominent dust lane [20], oriented along the minor axis and decoupled from the stellar distribution. 3) In the 6. 7 µm band, we see a component of the emission with a morphology corresponding to that of the dust lane. An additional component extends beyond the dust lane that can originate from stars and from PAHs in the ISM or circumstellar envelopes. Detailed modeling is necessary to disentangle the complex emission from 6.7µm band. ISOCA'.\1 observations toward the SO galaxy NGC 3998 show a similar distribution in these 3 bands [11].

230 08 LW l 4.0 lo 5.0µm 05 LW2 5.0 lo 8.5µm

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33.1 Spectral Energy Distributions For most of the sample, the relative variations of the 3 MIR bands can be described by one of the two characteristic signatures shown in Figure 2. For example, the SO galaxy, NGC 5102 and the E2 NGC 4649 (Figure 2: left) demonstrate the typical spectral energy distribution in the MIR of sources emitting predominantly stellar continuum and lacking significant amounts of dust. In contrast, Figure 2 (right) shows examples of the E3p galaxy NGC 5363 and the E4 galaxy, l\GC 5266, in which the PAH emission is present in the enhanced 6.7µm band. Additionally, the emission from the very small hot grains may be present to some extent in the 15µm band.

231 NGC 51 02 Spectral Engergy Distribution ( 30") 36_3 Spectro Engergy Distr b tion ( 36") ��o �5__ �_�___l__�r-_=�-_�i _u___�

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10 10 15 20 Wavelength (microns) :tc.�--���--���-'1Wavelength (microns) Figure 2:0 Examples of the MIR 15spectral 20 ener gy distributions for the 4-5, 6. 7 and 15µm bands. The left distributions appear to resemble mainly emission from the evolved stars, while those on the right are examples of galaxies displaying enhanced 6. 7µ m emission from the PA H features. 2 The 3 ISOCAM data points fill in a critical missing wavelength window in the spectral energy diagram from the near UV to the FIR for these galaxies. Simple fits to the SED assuming a single temperature black body yield a first order approximation of a dominant underlying population of 4300K to 4500K stars of type K5 for the central 30" to 36" regions of NGC 5102 and NGC 4373 (Figure 3). The MIR emission from NGC 5102 appears to originate strictly from the stellar continuum, not from dust. NGC 5102 is morphologically a true SO (having a nucleus, bulge and exponential disk) but contains a blue nuclear region due to recent . Hence, the excess seen in the near UV is from an extended stellar population as has been observed in some elliptical galaxies [3]. NGC 5363, on the other hand, clearly exhibits a MIR excess in the 6.7 to l5µm region of the energy spectrum. Half of the energy density emitted in the LW2 6. 7 µm band and at least 80% of that in the LW3 15µm band originate from dust. The 12µm IRAS detection, also shown on the SED for NGC 5363, covers a lower range in wavelength than the LW3, thus observing additional stellar emission. Also, the IRAS emission samples the entire galaxy, while we have shown here emission from a smaller aperature. Overall, the CAM data is consistent with the IRAS 12µm emission.

232 NGC 5102 NGC 5363 0 1 1 0 •o er> 14 er> 14 • E E, Li;� 1 3 ';- 13 ;:, 11 i ;:, 5 152l1 0 12 10�'������������ 1110 I Figure 3: Single16 temperature15 14 black13 body fits (T=4300 to 16450 0K)15 to the 14SEDs of13 NGC 125363 12 logv and NGC 5102. The ISOCAMlogv MIR points are filled circles. The open points in the FIR are the IR AS points. The 12µm IRAS emission is greater than that observed in the CA M data since here we only present the inner 30" and 36" of the galaxies, while IRAS beams sampled the fu ll galaxies. Th e data from the UV to NIR are from [13} [BJ [14} {12}

3.2 Comparison of the mid-infrared and IRAS far-infrared data

To see the overall variations of the properties of the galaxies in our sample, we compare our galaxy sample with a sample of mostly spiral galaxies of the and Coma clusters [1] plotted in IRAS 60µm/l00µm and CAM 6.7µm/l5µm ratios(Figure 4). A correlation has been found for the spirals [1] such that galaxies with higher 60µm/lOOµm ratios have lower 6.7µm/15µm ratios. As the temperature of the larger dust grains increases in higher radiation fields, as measured by the 60µm/lOOµm ratio, the 15µm emission, due to small hot grains increases.

Hence the negative slope in Figure 4. In the region where 60µm/l00µm � 0.3, some of the early-type galaxies deviate strongly from the correlation rising to much larger 6. 7µm/l5µm ratios (6.7µm/15µm i, 2.0). When comparing the MIR SEDs, with the distribution of the early-type galaxies on this figure, we find that the galaxies that are relatively dust-free, do not lie on the color-color correlation, but on the vertical sequence, while those with enhanced 6.7µm emission relative to the 4.5µm band, fall on this correlation. The early-type galaxies, which lie on the correlation which exists for the spiral galaxies, indeed exhibit features seen in spiral galaxies, such as young stars and molecular gas disks. Thus, we can use the 6. 7µm/l5µm and 60µm/100µm color ratios as a tool to distinguish the dusty early-type galaxies from the dust-free ones.

4 Conclusion

From an ISOCAM survey of early-type galaxies in the wavelength bands .of 4.5, 6.7 and 18µm, we find emission from hot grains and PAHs dominating the spectral energy distribution in about half of the sample. We can determine the infrared excess originating from the non­ thermal equilibrium heating of these small grains and PAHs. In some galaxies, no significant amount of dust in the form of small grains or PAHs is present. In these galaxies the MIR emission arises soley from a population of cool evolved stars. The ISOCAM 6.7µm/15µm ratios can be a tool to differentiate dusty early-type galaxies fromdust -free ones.

233 1.0

� D ::t 0.5 • 0 . E " • Jt:i� D u)" • D D •D D D 'iS'D� lill D j E 0.0 §b ::t D D lilJD � D I u:i D jI o; @;iClJ 1 .2 El j 8 -0.5 _J__ ,--'---'--� -1.0 -0.8 -0.6 -0.4 -0.2 0.0 log(f60µm/f1 OOµm)

Figure 4: Comparison of Early- Type Sample (filled diamonds) with Coma and Virgo galaxies [1} (open squares) in the 60µm/100µm vs. 6. 7µ m/15µm ratios. Some of the early type galaxies from our sample fall on the correlation while the dust-free galaxies deviate from the correlation.

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