A&A 401, 863-872 (2003)
DOI: 10.1051/0004-6361:20021819

Stellar content of NGC 404 - The nearest S0 Galaxy

N. A. Tikhonov^1,2 - O. A. Galazutdinova^1,2 - 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 $3.42 \pm 0.25$ Mpc for this galaxy. Global photometric characteristic of this galaxy were then measured out to a radius of 3 $\hbox {$^\prime $ }$, giving $V = 10\hbox{$.\!\!^{\rm m}$ }04$ and $I = 8\hbox{$.\!\!^{\rm m}$ }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 $\hbox {$^\prime $ }$ 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 $\rm [Fe/H] = -1.11$.

Key words: galaxies: individual: NGC 404 - galaxies: stellar content - galaxies: photometry - galaxies: distance and redshifts

1 Introduction

NGC 404 is a prominent example of an early-type galaxy that gives strong evidence of being in a more active state than the "classical" early type. A detailed study of the stellar content of the dwarf S0 galaxy NGC 404 is crucial because of the following reasons. Firstly, its relatively small radial velocity (Stromberg 1925) indicates that it is likely to be a member of the Local group (Baars & Wendker 1976). Secondly, the galaxy is rather isolated (i.e. not affected by interaction and environmental effects), making it an interesting source for exploring the evolutionary scenarios of galaxies. Thirdly, the galaxy is classified as a LINER, due to the presence of very narrow emission lines [OII] 3727, [OIII] 5007, H$_{\alpha}$, [NII] 6583, [SII] 6716, 6731, together with high order Balmer absorption (Humason et al. 1956; Burbidge & Burbidge 1965; Ho et al. 1993; Pogge et al. 2000) in its nuclear spectrum. Fourthly, NGC 404 contains both atomic gas and a prominent dust component (Barbon et al. 1982; Gallagher 1986; Ravindranath et al. 2001).

The gas-dust system observed close to the center of the galaxy is likely to turn into a region of active star-formation. Wiklind & Henkel (1990) supposed that CO emission was associated with a prominent dust lane in the central region of NGC 404, which might be part of an annular structure around the nucleus. The kinematical data indicate rotational motion of the gas around the center. Baars & Wendker (1976) noted peculiar radio properties of NGC 404. Larkin et al. (1998) obtained NIR spectra and reported the detection of strong [Fe II] emission suggesting X-ray heating to be at work. The detection of the UV core with HST was presented by Maoz et al. (1998), who explained the data by the presence of a central star cluster. The X-ray emission of NGC 404 is consistent with originating completely from discrete stellar sources, given the galaxy's blue luminosity (Komossa et al. 1999).

The physical parameters of the galaxy (diameter, mass, luminosity) depend on which distance is accepted. Measurements of the distance to this galaxy give estimates from 1 Mpc (Baars & Wendker 1976) via 3.9 Mpc (De Vaucouleurs & Olson 1984) to as high as 10 Mpc (Plana et al. 1998). So far NGC 404 has not been resolved into stars, making it impossible to measure the distance directly. We filled this gap and determined distance, size, stellar content, metallicity, colour, luminosity of the NGC 404 on the basis of stellar and surface photometry.

2 Ground based observations and data reduction

Observations of NGC 404 were carried out on the 2.5 Nordic telescope with the High Resolution Adaptive Camera and a $2048 \times 2048$ Loral CCD binned to $2 \times 2$ with a scale of 0.220 $\hbox{$^{\prime\prime}$ }$/pix after binning. Images were obtained under photometric weather conditions and with a seeing of $0 \hbox{$.\!\!^{\prime\prime}$ }6$ (Table 1). Data reduction included bias and flatfielding using twilight flats. Then the DAOPHOT and ALLSTAR packages (Stetson 1994) were used to obtain the instrumental magnitudes of the stars. Photometric standards from the list of Landolt (1992) were observed in all observing runs to bring the instrumental magnitudes on the Johnson-Cousins standard photometric system. The transformation equations are:

$$V - v = 25.205 - 0.106\times(V - I) - A_v\times X_v, \; \; \sigma = 0.005$$ (1)

$$I - i = 24.498 + 0.011\times(V - I) - A_i\times X_i,\;\; \sigma = 0.004,$$ (2)

where the coefficients were derived from the standard stars (Landolt 1992) observed on the same night.

 

Table 1: Observational log.

Region	Date	Band	Exposure	PEP ID
S0	06.09.1995	F814w	160	5999
S1	13.01.2001	F606w	600	8601
		F814w	600	8601
S2	25.12.1994	F606w	1200	5369
		F814w	2100	5369
S3	25.12.1994	F606w	1200	5369
		F814w	$2\times2100$	5369
Nordic	27.07.1997	I	$3\times400$
		V	$3\times300$
		I	30

3 HST WFPC2 observations

To study the resolved stellar population of the galaxy and to measure its distance we obtained images of the four regions (S0, S1, S2, S3) of NGC 404 (Fig. 1) from the HST archive. We used data from programs 8601, 5369, 5999 (Table 1). All observations were preprocessed through the standard STScI pipeline, as described by Holtzmann et al. (1995a). After removing cosmic ray hits we applied point-spread-function (PSF) fitting packages DAOPHOTII and ALLSTAR in MIDAS (Stetson 1994). These programs use automatic star-finding algorithms and then measure stellar magnitudes by fitting a PSF that is constructed from other uncrowded parts of images. A total of about 12 000 (S1 region), 860 (S2 region), 1250 (S3 region) stars were measured in the F606W and F814W filters with aperture radius of 1.5 pixels. Then we determined the aperture correction from the 1.5 pixel radius aperture to the standard $0\hbox{$.\!\!^{\prime\prime}$ }5$ radius aperture size for the WFPC2 photometric system using bright uncroweded stars. The F606W and F814W instrumental magnitudes has been transformed to standard magnitudes in the Jonson-Cousins system using the prescriptions of Holtzmann et al. (1995b). Background galaxies, unresolved blends and stars contaminated by CCD blemishes which have "quality" parameters as defined in ALLSTAR in MIDAS of $\vert{\it SHARP}\vert>0.15, \vert{\it CHI}\vert>1.2, \vert{\it ST}\vert>0\hbox{$.\!\!^{\rm m}$ }4$ were excluded from list.

	Figure 1: DSS-2 20 $\hbox {$^\prime $ }\times 20\hbox {$^\prime $ }$ image of NGC 404 with Nordic and the HST/WFPC2 footprints superposed, indicating the 5 regions (S0, S1, S2, S3 and Nordic field) observed. The bright star $\beta$ And is situated 7 $\hbox {$^\prime $ }$ south-east of the center of the galaxy.
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	Figure 2: The isophotes of NGC 404 (Nordic, $30^{\rm s}$ in I band). The size of the image is $3\hbox {$.\mkern -4mu^\prime $ }5$.
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	Figure 3: The change of ellipticity with radius. A significant difference in value is due to the influence of gas-dust clouds.
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4 Morphology and integral photometry of NGC 404

NGC 404 is a S0 galaxy with well-resolved gaseous clouds near the center. The galaxy isophotes (Fig. 2) show that the galaxy is viewed face-on (Barbon et al. 1982), and this allows the study of the stellar population of the galaxy out to large radii. The presence of the second-magnitude star $\beta$ And 7 $\hbox {$^\prime $ }$ south-east of the nucleus of the galaxy (see Fig. 1) distorts isophotal fitting and increases the photometric uncertainty. The surface photometry of the obtained images was done with the SURFPHOT routine of the MIDAS package developed by ESO. Before the surface photometry of the NGC 404 we removed background stars by fitting a second-degree surface in a circular pixel-area. The sky background was then approximated by a tilted plane, created from a 2-dimensional polynomial, using the least-squares method (FIT/FLAT-SKY). The accuracy of the sky background determination is about 0.3% of the original sky level. The ellipticity profile of NGC 404 is shown in Fig. 3. It is seen from this figure that the ellipticity ( ${\rm ELL}=1-b/a$) near the center 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 $R=60\hbox{$^{\prime\prime}$ }$. The symmetry of the galaxy and its smooth profile permitted us to measure global photometric parameters using only one quadrant of NGC 404, viz. the one opposite to $\beta$ And. This quadrant was cut from images of NGC 404; the vertex of the quadrant coincides with the center of the galaxy.

The reason for this unusual approach was associated with the insufficient size of the CCD and the presence of nearby bright star. Our photometry was done by calculating "growth curves" where the light is integrated in a concentric quadrant of increasing radius, corresponding to a simulated aperture photometry. We used the results of the photometry of the one quadrant to obtain global parameters of the whole galaxy, assuming that the other three quadrants resemble the first one closely. The correctness of this assumption is confirmed by the consistency of our results with those obtained by other authors. The derived results are shown in Fig. 4. In Fig. 4a we present radial surface brightness profiles in the V and I bands. These profiles are typical for disk galaxies. The regions of the bulge and exponential disk are clearly visible. In Fig. 4b we show the total V and I magnitudes as a function of radius. It is seen that the curve of luminosity variation with increasing aperture has not reached its asymptotic value, i.e. the radius of the galaxy is more that 3 $\hbox{$.\mkern-4mu^\prime$ }$0. The extent of the disk beyond 3 $\hbox{$.\mkern-4mu^\prime$ }$0 can be seen in studying the distribution of red giants; however, it is difficult to extend integral photometry to a radius of over 3 $\hbox{$.\mkern-4mu^\prime$ }$0 because of the insufficient size of the CCD field. In Figs. 4c, d are shown the radial and total (V - I) colour profiles as a function of radius. The integral colour of the galaxy (Fig. 4d) changes slightly with increasing aperture since the contribution of the outer regions of the galaxy is small because of their low brightness. The gradual variation of the galaxy colour along the radius (Fig. 4c) can be explained by the variation of the stellar population. The sharp change in colour near the limiting aperture value ( $R = 150\hbox{$^{\prime\prime}$ }$) is probably due to our errors of measurement at low surface brightness.

	Figure 4: Global photometric parameters of the NGC 404: a) Radial surface brightness profile in the V and I bands. b) The total V and I magnitudes with respect to the radius from the galaxy center. c) Radial (V - I) colour profile. d) The total (V - I) colour as a function radius.
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	Figure 5: The colour map (V - I) of the central part of the NGC 404. The black areas correspond to red regions and the white areas to bluish ones. The size of the image is 28 $\hbox {$^{\prime \prime }$ }\times 28\hbox {$^{\prime \prime }$ }$.
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The errors in the photometry have multiple sources: possible asymmetry of the galaxy, calibration errors, flatfielding and sky subtraction, readout noise, foreground stars and background galaxies. In our measurements the largest contributions to the errors in the global photometric parameters are dominated by the non-flatness of the sky background because of the nearby bright star. The calibration error is negligible (< $0\hbox{$.\!\!^{\rm m}$ }006$). The errors in the total magnitude determination are less than  $0\hbox{$.\!\!^{\rm m}$ }03$.

The photometric profile of the galaxy in the B band up to $R = 90\hbox{$^{\prime\prime}$ }$ is presented in the paper by Barbon et al. (1982). A comparison of these results with our measurements (Fig. 4a) shows good agreement provided that the mean colour of the galaxy is assumed to be (B-V) = 0.94. In an earlier work on the measurement of the brightness profile (Johnson 1961) only relative measurements of brightness were made, and we did not compare them with ours. Quite recently, based on photographic observations Baggett et al. (1998) have represented the surface brightness of NGC 404 by the sum of two components (bulge and disk) up to a radius of 5 $\hbox {$^\prime $ }$, without discussing the effect of the close bright star. Our surface brightness measurements differ from the result of these researchers by about 0 $.\!\!^{\rm m}$4.

Our measurements for $D = 200 \hbox{$^{\prime\prime}$ }$ have yielded $V=10\hbox{$.\!\!^{\rm m}$ }27$ and $I=9\hbox{$.\!\!^{\rm m}$ }18$ with an accuracy of $\pm0\hbox{$.\!\!^{\rm m}$ }01$, and for the maximum diameter $D = 360\hbox{$^{\prime\prime}$ }$ we have obtained $V = 10\hbox{$.\!\!^{\rm m}$ }04$ and $I = 8\hbox{$.\!\!^{\rm m}$ }95$ with an accuracy of $0\hbox{$.\!\!^{\rm m}$ }02$. For diameter $D = 200 \hbox{$^{\prime\prime}$ }$ Longo et al. (1983) presented colour and luminosity values: (B-V)=0.95 and $V=10\hbox{$.\!\!^{\rm m}$ }29$. One can see a good agreement of this result with ours in the V band in spite of the great difference in the methods used.

We obtained the distribution of (V-I)throughout the central parts of the galaxy by dividing I and V images. Figure 5 shows the colour of the central part of the NGC 404. The dark parts on the picture correspond to the reddened regions of the galaxy due to the light absorption in gas-dust clouds. The huge dark cloud visible in the galaxy image (Fig. 5) has been investigated by Wiklind & Henkel (1990). Sage (1990) believes that the gas of this cloud formed while the stars evolved, and there is no evidence of an external origin. However Bertola et al. (1992), based on measurement of the rotation curves of the gas and the galaxy, consider the gas to be of external origin. The white areas in Fig. 5 correspond to regions that are bluer than the main body of the galaxy. The existence of this particular region near the nucleus and a second region that is similar but fainter and further extended from the nucleus, is likely to indicate the possibility of weak star formation. This is confirmed by the revealing of an extended region with emission in H$_{\alpha}$ and OIII (Plana et al. 1998). At the edge of the central blue diffuse region (Fig. 5) there is a star-like object with colour (V-I) = 0.3. The construction of isophotes has shown that this blue bright object is the center of the galaxy. Its size is less than  $0\hbox{$.\!\!^{\prime\prime}$ }1$ (Maoz et al. 1995). We have done median smoothing of the V image and subtracted the smoothed image from the original one. After this procedure we revealed the extended system of gas-dust clouds (Fig. 6) which cannot be seen on the initial images. We used the B image of the 6 m telescope (BTA) (Fig. 6, bottom) and HST image (S0 region) (Fig. 6, middle) to confirm the reality of this structure. In the HST image the structure is faint because of the short exposure. In the V band of the Nordic image the faintest gas-dust filaments have a contrast of 2%. In the I band this structure is not visible. The white areas in Fig. 6 correspond to gas-dust clouds which form a radial and slightly spiral structure. It can be seen from this figure that the single, very massive cloud near the center is not the only one. The colour (V-I) of this massive cloud and neighbouring regions differ by only $0\hbox{$.\!\!^{\rm m}$ }03$. The other gas-dust clouds have the same or smaller colour contrast. After comparing the distribution of colour (Fig. 5) and brightness (Fig. 6), we concluded that in these spiral-like regions there is no violent star formation. NGC 404 is not a unique S0 galaxy with a gas-dust spiral-like structure. The galaxy NGC 4550 has a similar asymmetrical structure (Wiklind & Henkel 2001).

	Figure 6: After median smoothing one can see the radial structure of the gas-dust clouds. The black areas correspond to higher brightness and white areas to lower brightness. The size of the Nordic and HST (S0 region) images is $28\hbox {$^{\prime \prime }$ }\times 28\hbox {$^{\prime \prime }$ }$. The BTA image has a size of $54\hbox {$^{\prime \prime }$ }\times 54\hbox {$^{\prime \prime }$ }$.
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5 Colour-magnitude diagrams

The closeness of the bright star $\beta$ And has a strong effect on the accuracy of the integral parameters of the surface photometry, but it has absolutely no influence on the stellar photometry with DAOPHOT II (Stetson 1987, 1994). We have conducted stellar photometry of the HST (S1, S2, S3) and Nordic images (Fig. 1). Figures 7a-c show the final I vs. (V-I) colour-magnitude diagrams (CMD) of NGC 404. The deepest images are S2 and S3 (see Table 1). As has been expected, bright blue and red supergiants are lacking completely in the diagrams, and the main population of the galaxy consists of red giants (RGB) and asymptotic giant branch stars (AGB). We show the CMD of the S2 and S3 regions in Fig. 7a. This picture shows that there is a narrow RGB extending up to I=23.67. Also a small number of bright red stars is seen extending up to $I\simeq21.5$ above the tip of the RGB. These are AGB stars. We show the CMD of the S1 region in Fig. 7b. There is a wide band of RGB stars with a number of AGB stars. The groundbased CMD (Fig. 7c) mainly consists of AGB stars. The RGB stars are poorly seen due to their faintness. A number of objects in this CMD are unresolved background galaxies. Figure 8 shows the error distributions for the stellar photometry. Completeness tests were performed using the usual procedure of artificial star trials (Stetson 1987). A total of 6000 artificial stars was added to the F606W and F814W frames of NGC 404 (we used only the WF4 chip frames in S1 and S3) in several steps of 600 stars each with magnitudes and colours in the range $21.5\leq$ F814W$\leq26$. The completeness curves (input magnitudes - fraction recovered) are shown in Figs. 9a, b.

	Figure 7: [ (V - I), I] Colour-Magnitude diagrams of different fields of NGC 404. a) CMD of stars in far regions of NGC 404 (S2 and S3) obtained with WFPC2 HST. b) CMD of stars in the bulge of NGC 404 (S1). c) CMD of images of the Nordic telescope. The dashed line shows the position of the TRGB.
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	Figure 8: Residuals of the fitting of the PSF models versus magnitude for stars included in the final photometric list of NGC 404: a) in the far region (S3). b) in the bulge (S1). c) in the Nordic images.
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	Figure 9: Completeness levels of the WFPC2 photometry (S1(a) and S3(b) regions, WF4-chip) based on artificial star tests.
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	Figure 10: Smoothed luminosity function LF (solid lines) and edge-detection filter output (dotted lines). The position of the TRGB is indentified with the peak in the filter output function: a) LF for S2 and S3 regions. $I_{\rm{TRGB}} {\rm (S2+S3)} = 23\hbox{$.\!\!^{\rm m}$ }67$, b) LF for S1 region. $I_{\rm {TRGB}}$ ${\rm (S1)} = 23\hbox{$.\!\!^{\rm m}$ }76$.
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6 The distance and metal abundance

We estimate the distance of NGC 404 using the tip of the red giants branch (TRGB method) as described by Lee et al. (1993) and by Da Costa & Armandroff (1990). The best uncrowded fields for measuring the distance are the S2 and S3 regions. These fields are situated at large radii from the galaxy center and they have lower contamination by AGB stars. To obtain the magnitude of the TRGB we used the luminosity function (LF) of the stars in the colour interval $1\hbox{$.\!\!^{\rm m}$ }2\leq(V-I)\leq2\hbox{$.\!\!^{\rm m}$ }2$. We computed the LF by counting the stars lying inside a bin of $\pm 0\hbox{$.\!\!^{\rm m}$ }1$. The central value was varied in steps of $0\hbox{$.\!\!^{\rm m}$ }03$ to obtain the value of the LF while reducing the dependence of the results upon the particular choice of bin center. Measurements of the RGB tip positions can be made using a Sobel filter (Sakai et al. 1996) to determine objectively the positions of the TRGB. The LF and the Sobel filtered LF are shown in Fig. 10a. The position of the TRGB is identified with the highest peak in the filter output function. For the S2 and S3 regions we obtained $I_{\rm {TRGB}} = 23\hbox{$.\!\!^{\rm m}$ }67\pm0\hbox{$.\!\!^{\rm m}$ }05$. The position of the S1 region is in the bulge of NGC 404 and its CMD is contaminated by AGB stars, which tend to smear out the "edge'' defining the TRGB in the luminosity function (Sakai & Madore 1999). To estimate the distance of NGC 404 we used the WF3 and WF4 chip only, because of the stellar crowding in other chips. For S1 we obtained $I_{\rm {TRGB}} = 23\hbox{$.\!\!^{\rm m}$ }76\pm0\hbox{$.\!\!^{\rm m}$ }05$ (Fig. 10b). One can see that $I_{\rm {TRGB}}$(S1) $\sim I_{\rm {TRGB}}$(S2,S3). This means that RGB stars of the S2 and S3 region belong to NGC 404 and that its disk has a very extended size of up to 20 $\hbox {$^\prime $ }$. The second argument that regions S2 and S3 belong to NGC 404 consists of star distribution. The star density in the S2 and S3 regions increases towards the center of NGC 404 and remains constant in the direction perpendicular to the radius (see the discussion in Sect. 8). We estimated the mean metallicity of the RGB stars of the S2 and S3 regions using the (V-I) colours at $M_I=-3\hbox{$.\!\!^{\rm m}$ }5$ ($\sim$ $0\hbox{$.\!\!^{\rm m}$ }5$fainter than the TRGB) (Da Costa & Armandroff 1990; Lee et al. 1993). Based on a gaussian fit to the colour distribution of the giant stars in the range $23\hbox{$.\!\!^{\rm m}$ }95 \le I \le 24\hbox{$.\!\!^{\rm m}$ }35$ we derive a mean colour of the RGB stars of (V-I)_-3.5 = 1.60. Galactic extinction values by Schlegel et al. (1998) are A_I =0.114 and A_V = 0.194. The dereddened colour of the RGB stars is (V-I)_0,-3.5 = 1.52. The values for mean metallicity and intrinsic magnitude of the TRGB are $\rm [Fe/H] = -1.11$ and M_I = -4.11. Finally, the distance modulus of NGC 404 is obtained: $(m-M)_0 = 27\hbox{$.\!\!^{\rm m}$ }67 \pm 0\hbox{$.\!\!^{\rm m}$ }15$(corresponding to a distance of 3.42 Mpc). The CMD of the S1 field has a wide RG branch with numerous AGB stars. The considerable width of the RGB can be the result of different factors such as crowding, uncertainty of internal extinction, intrinsic spread of the RGB stars metallicity and insufficient photometric limit of these images. The mean metallicity of S1 is about the same as for the S2 and S3 fields, but it has a higher uncertainty.

7 The age of AGB and RGB stars

The comparison between theoretical isochrones (Bertelli et al. 1994) and the CMD of the stars in NGC 404, corrected with $(m - M) = 27\hbox{$.\!\!^{\rm m}$ }67$ and $E(V - I) = 0\hbox{$.\!\!^{\rm m}$ }08$, is shown in Figs. 11a, b. We obtain Z=0.001 which corresponds to the value of the mean metallicity derived by us (see Sect. 6). The brightest AGB stars are seen near the center of the galaxy, i.e. on the Nordic CMD (Fig. 11a). This figure supports the result that the youngest stars in the bulge of NGC 404 were formed 0.3 Gyr ago. The age of the remaining stars is essentially greater, up to 6 Gyr. There may also be even older stars among the bulge population, but our results are insufficient to select these. The age of the red giants of the disk (S2 + S3 fields) is in the range of 6.0-15 Gyr (Fig. 11b). In this figure one can see a few stars which have M_I< -4.0. They may be foreground stars. In this case, the age of the disk stars is at least 15 Gyr. Our results imply that star formation in this S0 galaxy took place 300 Myr ago in the central regions only and that the disk has an ancient stellar population. At first sight our results contradict the hypothesis that the older population should be in the bulge and the younger in the disk of S0 and Sa galaxies. (Larson 1976; Caldwell 1983; Bothun & Gregg 1990). A new model for the origin of bulges and disks (Noguchi 1999) can explain our results. Our recent investigations of NGC 5206 (unpublished), an S0 galaxy in the Centaurus group, indicate the same space distribution of AGB and RGB stars. The younger stars are concentrated towards the center of the galaxy and the older stars constitute an extended disk.

It is interesting to note that in irregular galaxies the intermediate and old populations have similar space distributions (Drozdovsky et al. 2001; Aparicio & Tikhonov 2000; Lynds et al. 1998).

	Figure 11: Isochrones (Bertelli et al. 1994) for metallicity z = 0.001 are shown superposed on CMD of NGC 404: a) Nordic data. Isochrones are for populations of ages 0.3, 1.0, 6.3 Gyr. b) HST data for S2 and S3 regions. Isochrones are for populations of ages 6.3, 15.0 Gyr.
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8 Radial distribution of AGB and RGB stars

 

Using the TRGB as the border we can separate all visible stars into two groups: AGB (M_I<-4.2) and RGB+AGB (M_I>-4.2) stars. The first group consists of AGB stars of different ages (Fig. 11) from 300 Myr ( $M_I\simeq-6.7$) to 6 Gyr ( $M_I\simeq-5.0$). In the second group the number of AGB stars is negligible and their presence does not affect a statistical investigation of the RGB stars. The mean age of RGB stars exceeds the mean age of the AGB stars (Fig. 11). We have derived the distribution of surface density of the AGB and RGB stars along the radius of the galaxy in the regions S1, S2 and S3 (Fig. 12). The surface density of red giants decreases slowly from the center to the edge of the galaxy, which corresponds to an extended disk with an exponential falloff. The decrease of the surface density of red giants in the bulge and their complete absence near the galaxy center is due only to the crowding of the stars. To prove this we added artificial stars ( $I_{\rm {average}} = 24\hbox{$.\!\!^{\rm m}$ }8$) uniformly over the whole field and conducted the usual photometry procedure. We show the results of this test in Fig. 13. One can see the absence of stars near the centre of the galaxy $(X_{\rm {center}} =340, Y_{\rm {center}} =-90)$ and the increase in their number with increasing galactocentric radius. We find the following dependence of the surface density of RGB stars in the disk on galactocentric radius:

$${\rm log}N_{\rm S} = -0.0041 \times RAD + 0.167 , \; \; \sigma = 0.081$$ (3)

where $N_{\rm S}$ is the surface density of RGB stars per square arcsecond and RAD is the galactocentric radius.

When studying the spatial distribution of stars we found a segregation of stars by luminosity. This is due to the fact that younger AGB stars are located only in the bulge, whereas the disk consists of old RGB stars and a small number of faint AGB stars. The study of the surface distribution of bright stars shows that all stars brighter than $I = 21\hbox{$.\!\!^{\rm m}$ }0$ are background stars. Stars from $21\hbox{$.\!\!^{\rm m}$ }0 < I < 23\hbox{$.\!\!^{\rm m}$ }7$ are bright AGB stars and most of them are located in the bulge of the galaxy.

	Figure 12: The distribution of AGB and RGB stars along galactocentric radius of NGC 404. The data at $RAD<170\hbox {$^{\prime \prime }$ }$ belong to S1 field and $RAD>420\hbox {$^{\prime \prime }$ }$ to S2 + S3 fields. The absence of data for $170\hbox {$^{\prime \prime }$ }<RAD< 420\hbox {$^{\prime \prime }$ }$is due to insufficient images in these regions.
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	Figure 13: The map of the resulting distribution of artificial stars from centre to outskirts of the galaxy after using FIND and ALLSTAR routines. The absence of the stars near the central part of the galaxy is due to crowding.
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It is seen from Fig. 12 that the surface density of bright AGB stars quickly drops with increasing distance from the center and becomes negligible at $RAD>180\hbox{$^{\prime\prime}$ }$. The following relation obtained:

$${\rm log}N_{\rm S} = -0.015 \times RAD + 0.136 , \; \; \sigma = 0.069.$$ (4)

Thus the distribution of two types of stars in NGC 404 suggests that the majority of AGB stars are located within the bulge, while RGB stars constitute the disk, which has a size of more than 20 $\hbox {$^\prime $ }$, corresponding to 20 kpc. We defined the size of the disk of NGC 404 using the stellar density. What surface brightness corresponds to the edge of the disk? With mean magnitude of the RGB stars $I=24\hbox{$.\!\!^{\rm m}$ }8$ and V-I=1.6 we measured the surface brightness in the S2 and S3 regions. Its value is $\mu_v = 31.5 ^{\rm m}/\sq\hbox{$^{\prime\prime}$ }$. The surface photometry does not make it possible to reach that photometric limit. It should be noted that the AGB spatial distribution is not sensitive to the choice of the border of the AGB stars. Dividing AGB stars (with I-magnitudes $(23\hbox{$.\!\!^{\rm m}$ }1<I<23\hbox{$.\!\!^{\rm m}$ }7)$ into two groups (bright and faint) we found that they have similar spatial distributions. Hence AGB stars are concentrated to the center of the galaxy and the fraction of younger stars is greater in the bulge than in the disk.

9 Summary

We have measured integral characteristics of NGC 404: ELL = (1 - b/a) = 0.05 for $R > 65\hbox{$^{\prime\prime}$ }$, $(V - I) = 1.08,\; V = 10\hbox{$.\!\!^{\rm m}$ }04$and $I = 8\hbox{$.\!\!^{\rm m}$ }95$ for $R = 180\hbox{$^{\prime\prime}$ }$. The galaxy colour (V-I) has been found to decrease along the galactocentric radius from 1.12 to 0.9. A system of gas-dust clouds which form a radially-spiral structure has been revealed in the bulge. The morphology of the central parts of the galaxy, which has a blue star-like nucleus and complex system of gas clouds has been studied.

For the first time this nearest S0 galaxy has been resolved into stars and the CMD for its stars has been constructed. Using the TRGB as the border we separated all stars of the galaxy into two groups: AGB + RGB and AGB stars. Examination of the stellar population has shown that the bulge of the galaxy is composed mainly of AGB stars with ages of more that 300 Myr, whereas the galaxy disk consists of RGB stars with an age from 6.0 up to 15 Gyr. The mean metallicity estimated using RGB colour, is $\rm [Fe/H] = -1.11$ in the outer disk. Using the brightness of the tip of the RGB we derive a mean distance modulus to NGC 404 of $(m-M)_0 = 27\hbox{$.\!\!^{\rm m}$ }67 \pm 0\hbox{$.\!\!^{\rm m}$ }15$ corresponding to a distance of 3.42 Mpc. The space distribution of two groups of stars in NGC 404 suggests that the majority of AGB stars are located within the bulge, while RGB stars constitute the disk, which has a size of more than 20 $\hbox {$^\prime $ }$, corresponding to 20 kpc. Up to now the stellar population of the S0 galaxies has been insufficiently investigated and we were unable to compare the spatial distribution of AGB and RGB stars along the radius in NGC 404 with results for other galaxies. There are many theoretical models for the formation of S0 galaxies (Farouki & Shapiro 1980; Larson et al. 1980; Icke 1985; Byrd & Valtonen 1990; Evrard 1991; Bekki 1998), but almost all of them are related to S0 galaxies that are members of clusters. Applying these models to our data may be invalid as NGC 404 is a rather isolated galaxy. Nevertheless, it should be remarked that these models predict that the disk of the galaxy is younger than the bulge. This is in contrast with results of the spatial distribution of the stars in NGC 404. Hopefully our data will be useful for future modelling of S0 galaxies.

Acknowledgements

This work has been financially supported by grant RFBR 00-02-16584. Data from the NASA/IPAC Extragalactic Database have been used.

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