A&A 401, 863–872 (2003) Astronomy DOI: 10.1051/0004-6361:20021819 & c ESO 2003 Astrophysics Stellar content of NGC 404 – The nearest S0 Galaxy N. A. Tikhonov1,2, O. A. Galazutdinova1,2, and A. Aparicio3 1 Special Astrophysical Observatory, Russian Academy of Sciences, N. Arkhyz, KChR 369167, Russia 2 Isaac Newton Institute of Chile, SAO Branch, Russia 3 Instituto de Astrofisica de Canarias, 38200 La Laguna, Tenerife, Spain Recieved 11 December 2000 / Accepted 6 December 2002 Abstract. We report V−band and I−band CCD stellar and surface photometry of the galaxy NGC 404 , taken with HST WFPC2 and the 2.5 m Nordic telescope. The colour-magnitude diagram for the stars in this galaxy is typical of that of spheroidal systems (i.e. it lacks luminous, young stars but contains a large number of Asymptotic Giant Branch (AGB) and Red Giant Branch (RGB) stars). The disk of the galaxy is mostly dominated by red giant stars while its bulge consists of both the AGB and RGB population. Using the distance indicator technique, based on the tip of the red giant branch (TRGB), we find a distance of 3.42 ± 0.25 Mpc for this galaxy. Global photometric characteristic of this galaxy were then measured out to a radius of 3, giving V = 10m. 04 and I = 8m. 95. We find (V − I) = 0.3 for the nucleus of NGC 404. The integral colour of the galaxy changes slightly along the radius, and its mean value is (V − I) = 1.1. On the HST images situated at 9 from the galaxy center there are many red giants. This means that the size of the disk of NGC 404 exceeds 20 kpc. The value of the mean metallicity of the red giants in the disk is [Fe/H] = −1.11. Key words. galaxies: individual: NGC 404 – galaxies: stellar content – galaxies: photometry – galaxies: distance and redshifts 1. Introduction NIR spectra and reported the detection of strong [Fe II] emis- sion suggesting X-ray heating to be at work. The detection of NGC 404 is a prominent example of an early-type galaxy that the UV core with HST was presented by Maoz et al. (1998), gives strong evidence of being in a more active state than the who explained the data by the presence of a central star cluster. “classical” early type. A detailed study of the stellar content The X-ray emission of NGC 404 is consistent with originat- of the dwarf S0 galaxy NGC 404 is crucial because of the ing completely from discrete stellar sources, given the galaxy’s following reasons. Firstly, its relatively small radial velocity blue luminosity (Komossa et al. 1999). (Stromberg 1925) indicates that it is likely to be a member of The physical parameters of the galaxy (diameter, mass, lu- the Local group (Baars & Wendker 1976). Secondly, the galaxy minosity) depend on which distance is accepted. Measurements is rather isolated (i.e. not affected by interaction and environ- of the distance to this galaxy give estimates from 1 Mpc (Baars mental effects), making it an interesting source for exploring & Wendker 1976) via 3.9 Mpc (De Vaucouleurs & Olson 1984) the evolutionary scenarios of galaxies. Thirdly, the galaxy is to as high as 10 Mpc (Plana et al. 1998). So far NGC 404 has classified as a LINER, due to the presence of very narrow emis- not been resolved into stars, making it impossible to measure sion lines [OII] 3727, [OIII] 5007, H α, [NII] 6583, [SII] 6716, the distance directly. We filled this gap and determined dis- 6731, together with high order Balmer absorption (Humason tance, size, stellar content, metallicity, colour, luminosity of the et al. 1956; Burbidge & Burbidge 1965; Ho et al. 1993; Pogge NGC 404 on the basis of stellar and surface photometry. et al. 2000) in its nuclear spectrum. Fourthly, NGC 404 con- tains both atomic gas and a prominent dust component (Barbon et al. 1982; Gallagher 1986; Ravindranath et al. 2001). 2. Ground based observations and data reduction The gas-dust system observed close to the center of the galaxy is likely to turn into a region of active star-formation. Observations of NGC 404 were carried out on the 2.5 Nordic Wiklind & Henkel (1990) supposed that CO emission was as- telescope with the High Resolution Adaptive Camera and a × × sociated with a prominent dust lane in the central region of 2048 2048 Loral CCD binned to 2 2 with a scale of / NGC 404, which might be part of an annular structure around 0.220 pix after binning. Images were obtained under photo- . the nucleus. The kinematical data indicate rotational motion of metric weather conditions and with a seeing of 0 6 (Table 1). the gas around the center. Baars & Wendker (1976) noted pecu- Data reduction included bias and flatfielding using twilight liar radio properties of NGC 404. Larkin et al. (1998) obtained flats. Then the DAOPHOT and ALLSTAR packages (Stetson 1994) were used to obtain the instrumental magnitudes of the Send offprint requests to: N. A. Tikhonov, stars. Photometric standards from the list of Landolt (1992) e-mail: [email protected] were observed in all observing runs to bring the instrumental Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20021819 864 N. A. Tikhonov et al.: Stellar content of NGC 404 – The nearest S0 Galaxy Table 1. Observational log. the nucleus of the galaxy (see Fig. 1) distorts isophotal fitting and increases the photometric uncertainty. The surface photom- Region Date Band Exposure PEP ID etry of the obtained images was done with the SURFPHOT rou- S0 06.09.1995 F814w 160 5999 tine of the MIDAS package developed by ESO. Before the sur- S1 13.01.2001 F606w 600 8601 face photometry of the NGC 404 we removed background stars F814w 600 8601 by fitting a second-degree surface in a circular pixel-area. The S2 25.12.1994 F606w 1200 5369 sky background was then approximated by a tilted plane, cre- F814w 2100 5369 ated from a 2-dimensional polynomial, using the least-squares S3 25.12.1994 F606w 1200 5369 / F814w 2 × 2100 5369 method (FIT FLAT-SKY). The accuracy of the sky background Nordic 27.07.1997 I 3 × 400 determination is about 0.3% of the original sky level. The el- V 3 × 300 lipticity profile of NGC 404 is shown in Fig. 3. It is seen from I 30 this figure that the ellipticity (ELL = 1 − b/a) near the cen- ter of the galaxy varies from 0.02 to 0.12 with an average value of 0.07, and decreases to 0.045 beyond the radius R = 60 . The magnitudes on the Johnson-Cousins standard photometric sys- symmetry of the galaxy and its smooth profile permitted us to tem. The transformation equations are: measure global photometric parameters using only one quad- rant of NGC 404, viz. the one opposite to β And. This quadrant V − v = 25.205 − 0.106 × (V − I) − Av × Xv,σ= 0.005 (1) was cut from images of NGC 404; the vertex of the quadrant I − i = 24.498 + 0.011 × (V − I) − Ai × Xi,σ= 0.004, (2) coincides with the center of the galaxy. The reason for this unusual approach was associated with where the coefficients were derived from the standard stars the insufficient size of the CCD and the presence of nearby (Landolt 1992) observed on the same night. bright star. Our photometry was done by calculating “growth curves” where the light is integrated in a concentric quadrant of 3. HST WFPC2 observations increasing radius, corresponding to a simulated aperture pho- tometry. We used the results of the photometry of the one quad- To study the resolved stellar population of the galaxy and to rant to obtain global parameters of the whole galaxy, assuming measure its distance we obtained images of the four regions that the other three quadrants resemble the first one closely. (S0, S1, S2, S3) of NGC 404 (Fig. 1) from the HST archive. The correctness of this assumption is confirmed by the consis- We used data from programs 8601, 5369, 5999 (Table 1). All tency of our results with those obtained by other authors. The observations were preprocessed through the standard STScI derived results are shown in Fig. 4. In Fig. 4a we present radial pipeline, as described by Holtzmann et al. (1995a). After re- surface brightness profiles in the V and I bands. These pro- moving cosmic ray hits we applied point-spread-function(PSF) files are typical for disk galaxies. The regions of the bulge and fitting packages DAOPHOTII and ALLSTAR in MIDAS exponential disk are clearly visible. In Fig. 4b we show the to- (Stetson 1994). These programs use automatic star-finding al- tal V and I magnitudes as a function of radius. It is seen that gorithms and then measure stellar magnitudes by fitting a PSF the curve of luminosity variation with increasing aperture has that is constructed from other uncrowded parts of images. A not reached its asymptotic value, i.e. the radius of the galaxy is total of about 12 000 (S1 region), 860 (S2 region), 1250 (S3 more that 3.0. The extent of the disk beyond 3.0 can be seen region) stars were measured in the F606W and F814W fil- in studying the distribution of red giants; however, it is diffi- ters with aperture radius of 1.5 pixels. Then we determined cult to extend integral photometry to a radius of over 3.0 be- the aperture correction from the 1.5 pixel radius aperture to cause of the insufficient size of the CCD field.