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October 9th, 2000

The double barred galaxy NGC 3504

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The double barred galaxy NGC 3504

Credit: Image courtesy of E. Emsellem
CRAL-Observatoire de Lyon, Lyon, France

(Collaborators: Didier Greusard, Francoise Combes, Daniel Friedli)

Bars are ubiquitous in spiral galaxies (they are found in more than 70% of them). Bars are density waves generated by gravitational instabilites or galaxy collisions. They are made of stars and gas and play an essential role both in the dynamics and evolution of galaxies. In particular, they may help fuel the central regions of galaxies with gas and dust by allowing these components to lose part of their angular momentum. Bars may thus be responsible for the intense activity seen in the so-called Active Galactic Nuclei or AGN.

However there seems to be no link between the presence of an active nucleus and the existence of a bar. One of the reason invoked to explain this is that a large-scale bar can usually drag the gas within the central kpc or so, but then other processes should take the relay. Recent studies have revealed bars within bars, which may play such a role. These smaller scale nuclear bars have now been observed in many spirals, but up to now, mostly studied through imaging. Spectroscopic observations are required to determine the stellar and gas dynamics in the central kpc of double-barred galaxies, and understand the link with the central activity or starburst.

We present here observations obtained with OASIS at the CFHT of NGC 3504, a nearby double-barred galaxy. OASIS in an integral field spectrograph working in the optical domain allowing to get simultaneously 1200 spectra over a field of view of typically 10 arcseconds. The images in the top show direct images of the central regions of NGC 3504 obtained respectively with OASIS (I band), the HST (I band) and the adaptive optics of the CFHT (PUEO). The central elongated structure is clearly seen as well as a strong asymmetry appearing as a one-arm spiral arm in the visible and as an offset nuclear structure in the infrared. The bright secondary source at about 6 arcsecond south is a supernova discovered during the OASIS observations (1998cf). The bottom left panels show some OASIS spectra tracing the stellar component of the Galaxy. The Ca triplet seen in absorption in the central regions is used to derive the stellar velocity field. Broad Ca lines are also seen in emission at the location of the supernova. The right panels show reconstructed images in different emission lines which trace the ionised gas distribution. The varying morphology of these maps reflect the different physical states of the core and nuclear arm regions. The study of this data set will help determine the detailed internal dynamics in the nuclear regions, and understand the role of the nuclear bar and the asymmetry in the context of the central nuclear fueling mechanism.

Technical description:

OASIS is an integral field spectrograph working in the optical domain and providing simultaneously 1200 spectra. A micro-lens array splits the input image into small micro-pupils. Each pupil is then dispersed in the spectrograph stage and imaged on the detector. The observations presented here were obtained in March 1998 and November 1999. The sampling used was 0.4 arcsec giving a total field of view of 12"x15". Two spectral configurations were used: one in the red (around 8500 Angs) to study the stellar kinematics and one in the blue (6300 to 7000 Angs) to study the gaseous component The seeing was around 1 arsec in the red, 0.8 arcsec in the blue.

next week: Inside the hourglass: a young massive star reveals its hatchlings

editors: François Ménard, Jean-Charles Cuillandre & Catherine Dougados
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CFHT is funded by the Governments of Canada and France, and by the University of Hawaii.