An Hα Study of the Kinematics of NGC 3627
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A&A 405, 89–97 (2003) Astronomy DOI: 10.1051/0004-6361:20030590 & c ESO 2003 Astrophysics An Hα study of the kinematics of NGC 3627 L. Chemin1, V. Cayatte1, C. Balkowski1, M. Marcelin2, P. Amram2, W. van Driel1, and H. Flores1 1 Observatoire de Paris, GEPI, UMR 8111, associ´e au CNRS eta ` l’Universit´e Paris VII, 5 place J. Janssen, 92195 Meudon, France 2 Observatoire Astronomique Marseille-Provence, 2 place Le Verrier, 13248 Marseille, France Received 24 September 2002 / Accepted 15 April 2003 Abstract. We report here on the first observations in the Hα line to investigate the kinematics of the spiral galaxy NGC 3627, the brightest member of the Leo Triplet. The velocity field observed at high angular resolution shows signatures of non-circular motion. They can be attributed to dynamical perturbations due to the bar and to external interactions. In particular, we confirm the presence of an inner ring recently detected in CO observations (Regan et al. 2002). We find a corotation radius significantly smaller than that derived in previous studies and we show that the inner ring location is consistent with the ultra-harmonic resonance induced by the bar pattern speed. We also show that a warped tilted-ring model can be fitted to the outer parts of the southern, receding half of the disc, which is inclined by some 15◦ relative to the inner parts of the galaxy. Key words. galaxies: individual: NGC 3627 – galaxies: kinematics and dynamics – galaxies: structure 1. Introduction field: the interarm smoothly distributed field and the S-shaped component related to the bar and spiral arms. These compo- NGC 3627 (Messier 66) is a spiral galaxy classified as SABb in nents sometimes meet each other at a very different pitch angle the RC3 catalogue (de Vaucouleurs et al. 1991), showing signa- (defined as the angle between the local magnetic field orien- tures of a Liner/Seyfert 2-type nuclear activity in its spectrum tation and the azimuthal direction). While in the western arm (Ho et al. 1997). The optical broad-band image of NGC 3627 the S-shaped component follows well the density wave flows reveals a pronounced spiral pattern with heavy dust lanes, in- and the dust lane, in the south-eastern disc it crosses the opti- dicating a strong density wave action. The pattern is asym- cal arm at a high angle, despite strong gas compression effects metric with respect to the major axis. Moreover, this image as indicated by a heavy dust lane. High spatial resolution ob- shows a perturbed morphology of the eastern arm which ap- servations of the kinematics of the ionized gas are needed to pears to have been displaced from the plane of the galaxy. understand the origin of this peculiar magnetic field behaviour With NGC 3623 and NGC 3628, it forms the Leo Triplet. in the galaxy. Low resolution H mapping of the Triplet (Rots 1978; Haynes Although it was this problem that initially motivated the et al. 1979) has revealed signs of past interactions between the present study, we want to focus here only on the kinematical two largest spirals, NGC 3627 and NGC 3628: a tail between properties of the ionized gas in NGC 3627, which was observed these galaxies, a long plume extending about 50 to the east as part of the GHASP1 project (Garrido et al. 2002, 2003), a of NGC 3628 and a non-corotating component in the eastern survey of the ionized hydrogen kinematics of about 200 nearby outskirts of the H disc of NGC 3627. Using a restricted three- galaxies by Fabry-Perot interferometry. We want to examine body code, Toomre (1977) and Rots (1978) have proposed a possible gas kinematic perturbations in the galactic disc and to model of a retrograde encounter of NGC 3627 moving along present the supporting observational data needed for future 3D a parabolic orbit around NGC 3628. H and CO-line studies of dynamo simulations of the radio polarization and of the mag- NGC 3627 (Zhang et al. 1993; Reuter et al. 1996) show clear netic field (see e.g. Otmianowska-Mazur et al. 2002). We will asymmetries in the velocity field of the atomic and molecular address the relationship between the gas flows and the peculiar gas. magnetic field in a forthcoming paper. A high resolution total power and polarization VLA map- Moreover, in a recent paper, Regan et al. (2002) noted the ping of this galaxy in the radio continuum (Soida et al. 2001) presence of an unseen inner ring in NGC 3627 from BIMA has revealed the existence of two components of the magnetic CO-line observations at a resolution of ∼6 (Regan et al. 1999). Send offprint requests to: L. Chemin, Rings are usually observed in barred galaxies and are easily e-mail: [email protected] revealed on optical photographic plates because intense star Based on observations collected at the Observatoire de Haute- Provence (France). 1 Gassendi H Alpha survey of SPirals. Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20030590 90 L. Chemin et al.: An Hα study of the kinematics of NGC 3627 formation which take place in them leads to a high intensity Table 1. Journal of the Fabry-Perot observations. contrast with respect to their surroundings. However, surpris- ingly, NGC 3627 has never been classified as a ringed galaxy. Observations Regan et al. (2002) pointed out that the combination of a high Telescope OHP 1.93 m inclination with the presence of intense star formation through- Equipment GHASP @ Cassegrain focus out the bar region makes the detection of the inner ring difficult Date March 2000 from optical images. Seeing ∼3 According to the classical dynamical scenario in a barred Interference Filter potential, an inner ring forms preferentially at ultra-harmonic Central Wavelength 6576 Å resonant orbits (see Buta & Combes 1996, and references FWHM 11 Å therein) as confirmed by observations (see e.g. Buta et al. Transmission 0.74 2001). If the presence of an inner ring is confirmed in Calibration NGC 3627 from the Hα observations, the knowledge of its lo- Neon Comparison light λ6598.95 Å cation and of the disc rotation curve will constrain the pattern Fabry–Perot speed of the bar. Interference Order 793 @ 6562.78 Å Section 2 describes the observations and the data reduction Free Spectral Range at Hα ∼380 km s−1 techniques, Sect. 3 presents the distribution of the ionized gas. Finesse at Hα 10 In Sect. 4 we analyze the observed kinematics of the ionized Spectral resolution at Hα 18750 at the sample step gas. The discussion of the results is presented in Sect. 5, the Sampling main conclusions are summarized in Sect. 6. Number of Scanning Steps 24 Sampling Step 0.35 Å(16 km s−1) Total Field 4.1 × 4.1 (256 × 256 px2) 2. Observations and data reduction Pixel Size 0.96 2.1. Observations Exposures times Total exposure 2 hours α We observed NGC 3627 in the H line in March 2000 using the Elementary scanning 2 GHASP instrument , a focal reducer with a scanning Fabry- exposure time 15 s per channel Perot interferometer and a bi-dimensional photon counting de- tector, mounted on the 1.93 m telescope of the Observatoire Table 2. Basic parameters of NGC 3627. de Haute-Provence (OHP) in France. Using the old configuration of GHASP, the field of view Right ascension (J2000) 11h20m15s was 4.1 × 4.1 on the sky, covered by 256 × 256 pixels of 0 .96 × Declination (J2000) +12◦5930 0.96. The interference filter was centered at λ = 6576.00 Å Classificationa SAB(s)b and has a FWHM of 11.00 Å. The p = 793 interference or- Nucleusb LINER/Sy2 der provided a Free Spectral Range (FSR = c/p) of 378 km s −1 Distancec (Mpc) 11.1 (or 8.3 Å at the rest wavelength of the Hα line, 6562.78 Å). The Linear scale (pc arcsec−1)54 FSR was scanned through 24 channels, resulting in a spectral e ◦ ± −1 Kinematical inclination ( )655 sampling of 15.8 km s . The total exposure time for the galaxy ◦ Kinematical position anglee ( ) 170 ± 5 was about 2 hours and the seeing was about 3 . As the angular Heliocentric systemic velocitye (km s−1) 727± 5 size of the galaxy exceeds the field of view of the GHASP in- Optical major axis diameter D d () 8.7 strument, the southernmost part of the western spiral arm and 25 Mean Hα bar position anglee (◦) 155 ± 5 the northernmost tip of the eastern arm could not be observed. Bar radius f ()49 Table 1 gives the journal of the observations and Table 2 summarizes the basic properties of NGC 3627, as well as some a de Vaucouleurs et al. (1991). parameters deduced from this study. b Ho et al. (1997). c Saha et al. (1999). d Paturel et al. (1991). 2.2. Data reduction e This paper. f Das et al. (2003). The data reduction procedure for the raw 256 × 256 × 24 data cube has been described in several papers (e.g. Marcelin et al. 1987; Amram et al. 1992). First, using a calibration data cube to increase the signal-to-noise ratio in the faintest parts of the λ = . obtained with a Neon lamp ( 6598 95 Å) before and after galaxy. This was done by applying a Gaussian smoothing to the observation of the galaxy, a wavelength calibrated map is the Hα profiles, as well as a spatial smoothing in X and Y with made, in which each pixel has its own wavelength reference a FWHM of 3 pixels.