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Hubbl E Space T El Escope Observations and Photoionization Modeling of the Liner Galaxy Ngc 10521 J

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Hubbl E Space T El Escope Observations and Photoionization Modeling of the Liner Galaxy Ngc 10521 J THE ASTROPHYSICAL JOURNAL, 532:883È894, 2000 April 1 ( 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A. HUBBL E SPACE T EL ESCOPE OBSERVATIONS AND PHOTOIONIZATION MODELING OF THE LINER GALAXY NGC 10521 J. R. GABEL,F.C.BRUHWEILER,D.M.CRENSHAW,S.B.KRAEMER, AND C. L. MISKEY Institute for Astrophysics and Computational Sciences, Department of Physics, The Catholic University of America, Washington, DC 20064; gabel=iacs.gsfc.nasa.gov Received 1999 May 20; accepted 1999 December 18 ABSTRACT We present a study of available Hubble Space Telescope (HST ) spectroscopic and imaging obser- vations of the low-ionization nuclear emission-line region (LINER) galaxy NGC 1052. The WFPC2 imagery clearly di†erentiates extended nebular Ha emission from that of the compact core. Faint Object Spectrograph (FOS) observations provide a full set of optical and UV data (1200È6800Ó). These spec- tral data sample the innermost region(0A.86] 0A.86D 82 ] 82 pc) and exclude the extended Ha emission seen in the WFPC2 image. The derived emission-line Ñuxes allow a detailed analysis of the physical \ conditions within the nucleus. The measured Ñux ratio for Ha/Hb,FHa/FHb 4.53, indicates substantial intrinsic reddening, E(B[V ) \ 0.42, for the nuclear nebular emission. This is the Ðrst Ðnding of a large extinction of the nuclear emission-line Ñuxes in NGC 1052. If the central ionizing continuum is assumed to be attenuated by a comparable amount, then the emission-line Ñuxes can be reproduced well by a simple photoionization model using a central power-law continuum source with a spectral index of a \[1.2 as deduced from the observed Ñux distribution. A multidensity, dusty gas gives the best Ðt to the observed emission-line spectrum. Our calculations show that the small contribution from a highly ionized gas observed in NGC 1052 can also be reproduced solely by photoionization modeling. The high gas covering factor determined from our model is consistent with the assumption that our line of sight to the central engine is obscured. Subject headings: galaxies: active È galaxies: individual (NGC 1052) È galaxies: ISM È galaxies: nuclei 1. INTRODUCTION luminous AGNs (Ferland & Netzer 1983; Ho, Filippenko, & Sargent 1993) and shock-heating (Heckman 1980; Low-ionization nuclear emission-line regions, (LINERs) Dopita & Sutherland 1995; Dopita 1996). LINERs might have been found to occur in nearly one-third of all bright, represent a heterogeneous class of objects since the pro- nearby galaxies (Ho et al. 1995). They share many of the posed ionization mechanisms can all explain the low appar- emission characteristics of active galactic nuclei (AGNs), ent luminosity of LINERs. Perhaps more than one of these but are distinguished by their low luminosities and rela- various mechanisms is at work in the same object. I tively strong, low-ionization emission lines (e.g., [O ] NGC 1052 is a nearby (z \ 0.0045; de Vaucouleurs et al. II II II jj6300, 6363, [O ] j3727, [N ] jj6548, 6584, [S ] 1991) elliptical galaxy (E4) with a strong nuclear emission- jj6716, 6731). A recent study by Ho (1999) has demon- line spectrum dominated by low ionization features. For strated that the UVÈX-ray spectral energy distribution example, the [O I] j6300 and [O II] j3727 Ñuxes exceed (SED) of LINERs is harder than in higher luminosity that of [O III] j5007, plus [N II] j6584/Ha[1 (Fosbury et AGNs. This is consistent with the general trend exhibited in al. 1978; Ho et al. 1993, 1997). Fosbury et al. (1978, hereafter AGNs of decreasingaox with decreasing luminosity (Avni & F78) showed that the emission-line gas is made up of a Tananbaum 1982). The physical relation between LINERs luminous central component (d ¹ 2@@) and a di†use extended and AGNs is unclear. They may be simply the low- region (d D 20@@). Each emission region contributes approx- luminosity extension of the AGN phenomenon, powered by imately equally in total Balmer line Ñux. Furthermore, the accretion onto a compact object or, alternatively, they may extended component emits many of the other prominent be the result of entirely di†erent mechanisms. optical lines observed in the spectrum of the central region, Although central-source photoionization appears to be such as [O II] j3727 and [O III] j5007 (F78). the dominant mechanism powering the nebular emission The observed optical-continuum emission is dominated seen in the high-luminosity Seyfert galaxies, it has not by the host galaxy, with no clear detection of any under- been thoroughly tested for galaxies containing LINERs. lying nonthermal emission (F78; Fosbury et al. 1981). Competing ionization mechanisms that have been used Radio studies, on the other hand, clearly show it to have to successfully explain at least some of the emission both compact and extended continuum emission com- characteristics of LINERs include photoionization by hot ponents. At D1.5 GHz, its radio emission is characterized stars (Terlevich & Melnick 1985; Filippenko & Terlevich by a Ñat, unresolved (¹1A) core and two weaker lobes 1985), photoionization by nonthermal emission typical of extending 10AÈ15A on either side of the compact source (Wrobel 1984). NGC 1052 has been shown to have an IR excess in the 5È20 km band within the inner 2A of the 1 Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science nucleus (Becklin, Tokunaga, & Wynn-Williams 1982). A Institute. STScI is operated by the Association of Universities for Research recent analysis of ROSAT /HRI imagery shows some in Astronomy, Inc., under NASA contract NAS 5-26555. extended soft X-ray emission, but the low spatial resolution 883 884 GABEL ET AL. Vol. 532 at these energies prevents isolating any compact emission at form a comprehensive picture of the activity of NGC 1052. scales less than 10A (Guainazzi & Antonelli 1999). X-ray Finally, we summarize our results and present our conclu- observations with ASCA indicate a very Ñat spectrum for sions in ° 5. the hard X-ray emission (2È10 keV) of NGC 1052 OBSERVATIONS AND EMISSION LINE MEASUREMENTS (Guainazzi & Antonelli 1999). 2. - As with LINERs in general, there is much disagreement 2.1. FOS Observations on the nature of the excitation mechanism responsible for The spectral data used in this study were obtained with the nuclear activity in NGC 1052. Initial Ðndings have sug- the Faint Object Spectrograph aboard the Hubble Space gested that shock-heating could produce the observed line Telescope (HST /FOS) on 1997 January 13. They were emission (Koski & Osterbrock 1976; F78). This claim is retrieved from the Space Telescope Science Institute (STScI) based largely upon the high electron temperature (T D III data archives. All observations were of the active nucleus of 33,000 K) implied by the strong [O ] j4363 emission. NGC 1052 through the0A.86 pair square aperture (post- Furthermore, simple shock-excitation models have been COSTAR), covering 1200È6800Ó. Thus, the aperture shown to match the prominent optical emission features sampled the central 82 ] 82 pc of the nuclear region of (Koski & Osterbrock 1976; F78). Alternate interpretations \ \ ~1 NGC 1052, assuming D 19.6 Mpc forH0 75 km s by Ferland & Netzer (1983), and more recently by Ho et al. Mpc~1. The optical and NUV data were obtained with (1993), have shown that many of the strong emission lines high-resolution gratings giving j/*j D 1300, while the UV observed in NGC 1052 (and in LINERs in general) could be spectrum has a lower resolution given by j/*j D 250. The produced by photoionization from a nonthermal contin- individual spectra are described in Table 1 along with their uum typical of Seyfert galaxies, namely a power law with [ exposure times, grating settings, and spectral resolutions. spectral index, aB 1.5. The major di†erence in the Because of a guide-star lock failure, the data may have models between the typically more luminous Seyfert gal- slightly degraded spectral resolution and sample a slightly axies and the LINERs is that the ionizing Ñux is inferred to larger spatial region than the aperture size would indicate. be signiÐcantly weaker in LINERs. Subsequent detailed If the extended regions emit di†erently than the nucleus, photoionization modeling of NGC 1052 by Pe quignot this may have an e†ect on the observed emission-line ratios (1984, hereafter P84) has shown that many of the optical III and absolute Ñuxes. emission lines, including [O ] j4363, could be Ðtted well Using the maps of Burstein & Heiles (1982), we Ðnd a by photoionization of a multidensity component gas. His foreground Galactic reddening value of E(B[V ) \ 0.02 in study assumed a central photoionizing source consisting of the direction of NGC 1052. Figure 1 shows the spectrum in a blackbody continuum with an X-ray tail extending to the rest frame of NGC 1052, corrected for Galactic extinc- higher energies. Recently, a polarimetric study has shown tion using the extinction law of Savage & Mathis (1979). NGC 1052 to have a broad Ha emission-line component with FWHM D 5000 km s~1 (Barth et al. 1998). This pro- 2.1.1. UV Emission vides compelling evidence for the presence of a compact Examination of the UV spectrum below 2000Ó in Figure object and lends strong support to the nonstellar photoion- 1 shows a weak UV continuum compared to the optical ization hypothesis. continuum, although, there appears to be a slight rise This paper presents a detailed analysis of the nuclear toward lower wavelengths. This UV continuum shows no activity in the prototypical LINER galaxy NGC 1052. Spe- evidence for recent starburst activity in the nucleus of NGC ciÐcally, we use Hubble Space Telescope (HST ) observations 1052.
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