Stellar population and kinematics of NGC 404

¹ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, Republic of South Africa e-mail: bouchard@ast.uct.ac.za
² Université de Lyon, 69000 Lyon; Université Lyon 1, 69622 Villeurbanne; Centre de Recherche Astrophysique de Lyon; Observatoire de Lyon, 69561 St. Genis Laval; CNRS, UMR 5574, France
³ Instituto de Astrofísica de Canarias, La Laguna, 38200 Tenerife, Spain
⁴ Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
⁵ Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhnij Arkhyz, Karachaevo-Cherkessia, Russia
⁶ Isaac Institute Chile, SAO Branch, Casilla 8-9, Correo 9, Santiago, Chile

Received: 18 August 2009
Accepted: 8 January 2010

Abstract

Context. NGC 404 is a nearly face-on, nearby low-luminosity lenticular galaxy. Probing its characteristics provides a wealth of information on the details of the possible evolution processes of dS0 galaxies, which may not be possible in other, more distant objects.

Aims. We study the internal kinematics and the spatial distribution of the star formation history in NGC 404.

Methods. We obtained long-slit spectroscopy at the OHP 1m93 telescope along the major and minor axes of NGC 404. The spectra had a resolution R = 3600 covering a wavelength range from 4600 to 5500 Å. The data were fitted against the Pegase. HR stellar population models to derive the internal stellar kinematics, ages, and metallicities simultaneously. All this was done while taking any instrumental contamination to the line-of-sight velocity distribution into account. First, the global properties of the galaxy were analysed by fitting a single model to the data and looking at the kinematic variations and SSP equivalent age and metallicities as a function of radius. Afterwards, the stellar populations were decomposed into 4 individually analysed components.

Results. NGC 404 clearly shows two radial velocity inversions along its major axis. The kinematically decoupled core rotates in the same direction as the neutral hydrogen shell that surrounds the galaxy. We resolved the star formation history in the core of the galaxy into 4 events: a very young (< 150 Myr, and [Fe/H] = 0.4) component with constant ongoing star formation, a second young (430 Myr) component with [Fe/H] = 0.1, an intermediate population (1.7 Gyr) that has [Fe/H] = -0.05, and finally an old (12 Gyr) component with [Fe/H] = -1.26. The two young components fade very quickly with radius, leaving only the intermediate and old population at a radius of 25´´ (370 pc) from the centre.

Conclusions. We conclude that NGC 404 had a spiral morphology about 1 Gyr ago and that one or many merger events has triggered a morphological transition. The interstellar medium in the galaxy has two components, the cold molecular gas is most probably a remnant from its past spiral incarnation and the outer neutral hydrogen layer that has probably been acquired in one of the latest mergers.