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Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

The Continuum and Line Spectra of the Seyfert 2 Galaxy NGC 5252: Radiation and Shock Contributions

M. Contini
School of Physics and Astronomy, Tel-Aviv University, Tel-Aviv, 69978, Israel

M. A. Prieto
Max-Planck-Institut für Extraterrestrisch Physick, D-85748 Garching-bei-München, Germany

S. M. Viegas
Instituto Astronômico e Geofísico, USP, Av. M. Stefano 4200, 04301 São Paulo, SP, Brasil

Introduction

NGC 5252 is an S0 galaxy at a redshift of z 0.0230. Its optical size is 0.9 arcmin, the major axis being at a position angle PA 15, and the apparent blue magnitude is m = 13.6 (NED). NGC 5252 harbors a Seyfert 2 nucleus which illuminates a defined biconical structure of ionized gas extending up to 17 kpc ( = 100 ) from the center of the galaxy (Tadhunter & Tsvetanov 1989, Prieto & Freudling 1995).

The present modelling makes use of all observational data available so far for this galaxy from the radio to the hard X-ray regimes. Data on the continuum energy distribution of NGC 5252 have mostly been taken from Kotilanien & Prieto (1995) and references therein (Figure 1). The hard (1--10 keV) X-ray spectrum has been taken from ASCA by Cappi et al. (1995). The optical nebular emission is given by Osterbrock & Martel (1993) complemented by Acosta-Pulido et al. (1995).

Interpreting the Continuum and Line Spectra

The complete modelling of NGC 5252 by SUMA is discussed in Contini, Prieto & Viegas (1995). The SUMA code (Viegas & Contini 1994, and references therein) calculates the emission spectra (lines and continuum) from a shocked cloud which may also be affected by photoionizing radiation, in a plane-parallel geometry. The comparison of calculation results with the corrected observational nuclear data is given in Table 1. The power-law radiation dominated model (R1) alone cannot fit all the lines, particularly those from high ionization levels ([Ne V] and [Fe X]) and [O III] 4363 which depends on the temperature of the gas. The weighted average of R1 with shock dominated models (S1 and S2) characterized by higher shock velocities better fits the observations and explains the soft X-ray excess. The relative weights, R1:S1:S2 :: 100:5:0.02, are deduced from the fit of the continua (Figure 1).

The calculated physical conditions allow the estimation of the column density of hydrogen ions, of the free path length for ionizing photons in the narrow line region (NLR), and of the distance of the NLR clouds from the active center.

The interpretation of the spectra in the extended NLR (Acosta-Pulido et al. 1995) confirms the ejection of matter throughout the cones.

The agreement of both the continuum and emission line spectra with the observations provides a validation of the model and a self consistent picture of NGC 5252.

 
Figure: Continuum spectrum of NGC 5252. is in , is in Hz. Long dashed lines refer to model R1, dashed lines to models S1 and S2, and dotted line to the nuclear emission. The observational data are indicated by the crosses (and upper limits by arrows), the stellar-subtracted by encircled crosses. The solid line corresponds to the total theoretical continuum, assuming different weights for models R1, S1 and S2.

 
Table 1: The nuclear emission line spectrum relative to H =1

References:

Acosta-Pulido, J., Vila-Vilaro, B., Perez, I., Wilson, A., & Tsvetanov, Z. 1995, preprint

Cappi, M., Mihara, T., Matsuoka, M., Brinkmann, W., Prieto, M. A., & Palumbo, G. G. C. 1995, ApJ, in press

Contini, M., Prieto, M. A., & Viegas, S. M. 1995, MNRAS, submitted

Kotilanien, J. K. & Prieto, M. A. 1995, A&A, 295, 646

Osterbrock, D. E. & Martel, A. 1993, ApJ, 418, 668

Prieto, M. A. & Freudling, W. 1995, MNRAS, in press

Tadhunter, C. & Tsvetanov, Z. 1989, Nature, 341, 422

Viegas, S. M. & Contini, M. 1994, ApJ, 428, 113



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