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Astronomy Astrophysics A&A 433, 79–86 (2005) Astronomy DOI: 10.1051/0004-6361:20034537 & c ESO 2005 Astrophysics Asymmetrical structure of ionization and kinematics in the Seyfert galaxy NGC 5033 E. Mediavilla1, A. Guijarro2, A. Castillo-Morales3 , J. Jiménez-Vicente3, E. Florido3, S. Arribas4,, B. García-Lorenzo1, and E. Battaner3 1 Instituto de Astrofísica de Canarias, Tenerife, Spain e-mail: [email protected] 2 Centro Astronómico Hispano Alemán, Almería, Spain e-mail: [email protected] 3 Dpto. Física Teórica y del Cosmos, Universidad de Granada, Spain 4 Space Telescope Science Institute, Baltimore, USA Received 18 October 2003 / Accepted 29 November 2004 Abstract. We present integral field spectroscopy of NGC 5033, a low luminosity Seyfert galaxy. The observations were made with INTEGRAL, a fiber based system operating at the WHT. The intensity map of the Hβ emission line represents a spi- ral or ring-like pattern of HII regions. On the contrary, the [OIII] intensity map morphology is markedly anisotropic. The strong morphological differences imply that the [OIII] emitters represent highly ionized gas illuminated by the central source. The [OIII] map morphology is compatible with a biconical structure of ionization induced by strong extinction in the galaxy disc that also obscures half of the spheroidal stellar bulge. We identify the spectrum corresponding to the Seyfert 1 nucleus from the presence of Hβ broad emission lines. This spectrum is located in a region where strong extinction is expected but exhibits the bluest spectral energy distribution. The Seyfert 1 nucleus seems to be offcenter with respect to the stellar rotation center. This result has been also found in other Seyfert galaxies and interpreted in terms of a past merger. The offcentering could indicate the presence of nonsymmetric departures in the gravitational potential which could be fueling the active nucleus. The kinematics of the [OIII] emitters show important deviations at a kpc scale with respect to the stellar velocity field and show features related to the asymmetrical morphology of the high ionization region. Key words. galaxies: active – galaxies: individual: NGC 5033 – galaxies: kinematics and dynamics – galaxies: spiral – galaxies: Seyfert – galaxies: nuclei 1. Introduction gaseous systems of different spatial location and kinematics are ionized. In this case a kinematic analysis (as in NGC 1068, According to observations, the narrow emission line regions in Arribas et al. 1996) is needed to reveal the bipolar structure of Seyferts and other active galaxies extend anisotropically over a ionization. The study of the ENLR in the circumnuclear region significant fraction of the host galaxy. These Extended Narrow of typical high luminosity Seyfert galaxies is also compromised Line Regions (ENLR) trace the illumination of the gas by the since most of them are very distant. ionizing continuum coming from the active nucleus that, in the framework of the unified models, should be collimated in two Nearby galaxies that host Low Luminosity Active Galactic opposite cones along the axis of symmetry of a central obscur- Nuclei (LLAGNs) are potentially very interesting scenarios to ing torus. Sometimes, this biconical structure of ionization can study the ionization structure and, in general, to probe the uni- be directly seen (e.g. NGC 5728, Arribas & Mediavilla 1993; fied scheme for AGNs. However the detection and mapping of Wilson et al. 1993), but in most cases the scenario is complex the line emission in the circumnuclear region of LLAGNs is and the shape of the ionization structure appears not clearly de- difficult. In these objects the emission lines are weak with re- fined. The shape and size of the ENLR depend not only on the spect to the continuum and filter imaging can give very doubt- relative inclination and widening of the radiation cones but also ful results. A better observational option to study the ENLR on the spatial distribution of the gas in the galaxy. Thus, the fi- is to obtain integral field spectroscopy from the circumnuclear nal appearance of the ENLR can be very amorphous if, as it region to directly measure the narrow line emission after prop- seems to be the rule for high luminosity Seyfert nuclei, several erly fitting the continuum (see, for instance, the case of the LINER galaxy NGC 7331, Mediavilla et al. 1997; Battaner Affiliated with the RSS Department of the European Space et al. 2003). With this aim, we present and discuss new obser- Agency. On leave from the IAC - CSIC. vations of the low-luminosity Seyfert galaxy NGC 5033, taken Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20034537 80 E. Mediavilla et al.: Asymmetrical structure of ionization in NGC 5033 with INTEGRAL, a fiber-based system to obtain integral field For several spectral features of interest we have deter- spectroscopy at the William Herschel Telescope. This galaxy mined a grid of points at the locations of the fibers at the fo- has been alternatively classified as a Seyfert of type 1.5 (Ho cal plane and have interpolated a map of the selected feature et al. 1997c) or 1.9 (Veron-Cetty & Veron 2001). in the region of the central rectangle (see, e.g., Arribas et al. NGC 5033 presents a very pronounced spiral structure and 1999). In Fig. 1 we present six maps obtained from the ob- 10 a nuclear bulge with a mass estimated to be 2 × 10 M,toa servations of NGC 5033. The stellar velocity field was ob- distance of r = 1700 pc from the nucleus (Ford et al. 1971). tained cross-correlating the spectra in the ∼5250 Å–6000 Å The dynamics of nuclear activity of stars and gas in NGC 5033 wavelength range. The average error of the cross-correlation have been studied. Bosma (1981) remarked that NGC 5033 has is 50 ± 17 km s−1. The continuum was integrated in the a giant neighbor within a short distance: NGC 5005, a Seyfert ∼4700 Å–5840 Å wavelength interval. To obtain the inten- SAB(rs) galaxy. Using the Very Large Array, Thean et al. sity and the velocity maps for the ionized gas we performed (1997) found neutral gas within a few kiloparsecs (1 = 58 pc, Gaussian fits to the [OIII]λ5007 and the Hβ emission lines us- −1 −1 D = 11.8Mpc,H0 = 75 km s Mpc ) of the galactic cen- ing DIPSO commands. To avoid the influence of strong stellar ter which appears to be unaffected by the active nucleus. From Hβ absorption we considered in the fits an extra component measurements of the optical emission lines (mainly Hα)and representing this absorption feature. In the case of the [OIII] Westerbork HI-observations, van der Kruit & Bosma (1978) we obtain an average estimate for the velocity centroid uncer- did not detect any strong non-circular motion attributable to a tainties of 23 ± 10 km s−1. The average uncertainty in the inten- bar. However, more recently Curran et al. (2001) with the 20 m sity is ∼10%. In the case of Hβ we obtain similar uncertainties Onsala and 15 m SEST telescopes detected a bar in HCN in in the spectra where appreciable emission was detected but we NGC 5033. Ho & Ulvestad (2001), using VLA for radio con- could not find emission in several fibers. Four of them close to tinuum observations at 6 cm and 20 cm emission, found that the the center of the field correspond to a region without Hβ emis- morphology of the radio emission is predominantly that of a sion in Fig. 1. compact core slightly resolved surrounded by a fluffy envelope with a major-axis diameter of ≈0.9 kpc, roughly along the east- west direction, accompanied by elongated, jet-like features. 3. Results Koratkar et al. (1995) and Terashima et al. (1999) found that the 3.1. Continuum and line emission maps X-ray emission is mostly or entirely nuclear (≤500 pc) with a luminosity of 2.3×1041 erg s−1 in the 2–10 keV band. They also The continuum intensity map looks very asymmetric with the detected the Fe Kα emission line at 6.4 keV. Recently, Kohno emission peak (optical nucleus) apparently off-centered to- et al. (2003) have detected a perturbed distribution of CO in wards the west. To quantify this displacement we have fit- the central region of this galaxy with two bright peaks near the ted to the outermost closed isophote an ellipse (of semimajor nucleus. axis a = 13. 1) obtaining that the center of the ellipse is lo- In this paper we present integral field spectroscopy of the cated ∼2. 1 towards the east of the optical emission peak. The circumnuclear region of NGC 5033 to (i) identify its structure orientation of the ellipse major axis (apparent photometric ma- ◦ ◦ of ionization from the [OIII]λλ4959, 5007 maps; and (ii) study jor axis) is PA 171.1 ± 0.3. The continuum intensity map is the kinematics of the ionized emitters from the comparison be- morphologically identical to a map computed from the CaFe tween the kinematics of stars and gas. absorption feature (not shown here). This implies that it repre- sents the distribution of stars and that the contribution from the Seyfert continuum is considerably smaller than the bulge emis- 2. Observations and reduction sion. Clear evidence of extinction in the west side of the galaxy NGC 5033 was observed with the fiber system INTEGRAL (closer to us) is found in an optical image from the HST archive (Arribas et al. 1998) in combination with the fiber spectro- (Fig. 2). Thus, we are likely observing half of the spheroidal graph WYFFOS (Bingham et al. 1994) at the William Herschel stellar bulge of the galaxy, the other half being strongly ob- Telescope on 10 April 2001.
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