2 BVRI photometry

Data in the broad-band BVRI Johnson-Cousins system were obtained under photometric conditions at the 1.23 m telescope of the Calar Alto Observatory (Spain) in January 1999. The images covered a field of view of $\sim$8$^\prime$.5$\times$8$^\prime$.5 with a spatial scale of 0.5 arcsec pixel^-1. Observational details are summarized in Table 1.

The images were reduced with IRAF following the standard steps of bias subtraction, flat fielding and cosmic rays removal. An additional defringing procedure was applied to the I band image. A Landolt (1992) standard field was observed three times during the night for photometric calibration purposes. The calibration constants (see Table 2) were determined by means of the IRAF package PHOTCAL through a uniformly weighted fit of the following transformation equations:

 

B = b + B₀ - k_B X_B + C_B (B-V) (1)

 

V = v + V₀ - k_V X_V + C_V (B-V) (2)

 

R = r + R₀ - k_R X_R + C_R (V-R) (3)

I = i + I₀ - k_I X _I + C_I (R-I) (4)

where B, V, R, and I are calibrated magnitudes, b, v, r, and i are sky-subtracted instrumental magnitudes normalized to 1 s exposure and X_B, X_V, X_R, and X_I are the airmasses at the time of the observations. We have identified seven galaxies in the field, two of which form an interactive pair known as IRAS 02443+4437 (Fig. 1). We have studied the morphology of each galaxy and calculated the total magnitudes in the four bands by means of GIM2D (Simard 1998; Simard et al. 2001), an IRAF package written to perform an automated bulge-disk decomposition of the surface brightness profile. GIM2D is well suited for low signal-to-noise images of distant galaxies since it has the considerable advantage of including a PSF deconvolution in the bi-dimensional fit. The effects of the distance in dimming the outer parts of galaxies' disks can be considered comparable to the effects produced by heavy Galactic foreground extinction in nearby galaxies. Therefore the suitability of the program to the bulge-disk decomposition of distant galaxies makes it also particularly appropriate in the case of low-Galactic-latitude galaxies - like those studied in this work - which might be heavily affected by foreground Galactic extinction.

 

 

Table 2: Photometric calibration constants $^{{\rm a}}$.

Band	M0	$k_{\rm M}$	$C_{\rm M}$
B	22.193 $\pm$ 0.054	0.227 $\pm$ 0.041	0.110 $\pm$ 0.009
V	22.624 $\pm$ 0.041	0.138 $\pm$ 0.030	$-0.028 \pm 0.007$
R	23.102 $\pm$ 0.036	0.086 $\pm$ 0.026	0.064 $\pm$ 0.009
I	22.526 $\pm$ 0.162	0.071 $\pm$ 0.117	0.274 $\pm$ 0.061

$^{{\rm a}}$

With M₀, $k_{\rm M}$, and $C_{\rm M}$ we indicate the zero point, the extinction coefficient and color term, respectively, in each of the four photometric bands.

Figure 1: R-band exposure of the galaxy system around the position $\alpha$ = $02^{\rm h}47^{\rm m}36^{\rm s}.3$, $\delta$ = 44$^\circ$51$^\prime$39 $^{\prime \prime }$(J2000). The galaxies identified in the field of view are marked with ellipses and labeled with small-case letters. The minimum circle containing all geometrical centers of the galaxies is drawn, as well. Objects "e'' and "f'' are the components of the interactive pair IRAS 02443+4437.

The program gives as output, the values of the best-fit parameters and asymmetry indices, besides an image of the galaxy model and a residual image obtained by subtracting the model from the original one. This fitting method has already been successfully applied by Tran et al. (2001) to a sample of galaxies belonging to poor galaxy groups and spanning a wide range in morphological type. They found that the ratio between the bulge and the total luminosity (B/T) can be used on average as a robust morphology indicator to discriminate between early-type bulge-dominated (B/T > 0.4) and late-type disk-dominated (B/T < 0.4) galaxies.

The PSF images in the four photometric bands to be used during the galaxy fitting procedure were obtained with the DAOPHOT (Stetson & Harris 1988) package inside IRAF. The isophotal area of the galaxies was defined by means of the galaxy photometry package SExtractor V2.2.2 (Bertin & Arnouts 1996). The fit uses an exponential law for the disk, while a classical de Vaucouleurs r^1/4-law (de Vaucouleurs 1948) or a Sérsic profile (Sérsic 1968) can be chosen for the bulge. In the last case, the n index of the Sérsic law is one of the fitting parameters. We attempted fits of the seven galaxies with a Sérsic law plus exponential disk. When the n index was equal or very close to 4, we applied a de Vaucouleurs law for the bulge. In Table 3 we list the best-fit parameters from GIM2D: bulge to total light ratio (B/T), bulge effective radius ($R_{\rm e}$), ellipticity (e), disk scale length ($R_{\rm d}$), inclination (i), Sérsic law index (n), half-light radius ( $R_{\rm half}$), asymmetry index ($R_{\rm A}$), together with the reduced $\chi^2$. We show in Fig. 2 the BVRI thumbnails of each galaxy and in Fig. 3 the corresponding residuals after subtraction of the best-fit galaxy models.

 

 

Table 3: GIM2D best-fit parameters.

Band	B/T	$R_{\rm e}~(^{\prime\prime})$	e	$R_{\rm d}~(^{\prime\prime})$	$i~(^{\circ})$	n	$\chi^2$	$R_{\rm half}~(^{\prime\prime})$	$R_{\rm A}$
CG J0247+449a
B	0.31	0.62	0.21	2.76	54	0.91	1.01	2.86	0.02
V	0.36	0.67	0.20	3.19	56	1.15	1.02	2.81	0.01
R	0.38	0.57	0.21	3.19	57	1.31	1.07	2.65	0.01
I	0.45	0.63	0.22	3.87	58	2.33	1.00	2.70	0.00
CG J0247+449b
B	0.59	1.08	0.20	4.06	56	4	1.05	2.74	0.02
V	0.57	0.97	0.28	3.82	52	4	1.04	2.61	0.01
R	0.59	0.95	0.22	4.19	53	4	1.11	2.51	0.02
I	0.55	0.80	0.26	4.05	48	4	1.03	2.57	0.01
CG J0247+449c
B	0.33	1.22	0.39	4.86	45	4	1.00	5.57	0.00
V	0.42	1.65	0.33	5.12	46	4	1.05	5.33	0.01
R	0.40	1.40	0.32	5.67	46	4	1.09	5.69	0.01
I	0.39	1.05	0.32	4.77	48	4	1.00	4.78	0.01
CG J0247+449d
B	0.22	3.99	0.69	9.29	76	1.64	1.07	12.44	0.10
V	0.84	14.91	0.70	34.54	85	2.48	1.22	18.63	0.07
R	0.60	8.88	0.70	15.29	73	2.01	1.13	14.09	0.07
I	0.94	13.25	0.41	2.33	82	1.64	1.07	12.31	0.08
CG J0247+449e
B	0.32	2.87	0.64	6.06	55	0.38	1.60	6.24	0.17
V	0.42	3.38	0.65	4.29	55	0.48	2.90	4.86	0.17
R	0.35	2.93	0.69	4.67	55	0.41	3.00	5.06	0.16
I	0.39	2.86	0.67	6.30	60	0.73	1.40	5.95	0.09
CG J0247+449f
B	0.77	3.17	0.38	4.80	60	4	1.17	4.27	0.02
V	0.65	2.86	0.39	5.68	43	4	1.16	4.98	0.04
R	0.73	3.04	0.42	5.92	54	4	1.44	4.61	0.04
I	0.67	2.53	0.46	4.97	45	4	1.17	4.21	0.02
CG J0247+449g
B	0.66	2.60	0.34	3.00	69	4	1.00	3.57	0.01
V	0.72	3.27	0.41	5.64	59	4	0.99	4.86	0.01
R	0.64	2.12	0.40	3.23	25	4	1.00	3.34	0.01
I	0.53	1.51	0.51	3.42	38	4	1.08	3.34	0.02

Figure 2: Thumbnails showing the BVRI-band images of the seven galaxies identified in Fig. 1. The short bars on the bottom-right corner of the I images of each galaxy are 5 arcsec in size.

Figure 3: Thumbnails showing the BVRI-band residuals of the seven galaxies identified in Fig. 1, after subtraction of the GIM2D bidimensional best-fit models. Bars are the same as in Fig. 2.

 

 

Table 4: Magnitudes and colors $^{{\rm a}}$.

Obj. Id. $^{{\rm b}}$	B	V	R	I	B-V	V-R	V-I
a	17.96	16.75	16.07	15.38	1.21	0.68	1.38
	16.25	15.47	14.99	14.59	0.78	0.47	0.89
b	17.10	15.95	15.24	14.55	1.15	0.71	1.40
	15.91	15.05	14.48	13.98	0.86	0.58	1.07
c	16.64	15.41	14.62	14.00	1.23	0.79	1.41
	15.14	14.28	13.67	13.29	0.86	0.61	0.99
d	18.12	16.90	16.51	15.70	1.23	0.39	1.20
	15.11	14.58	14.54	14.22	0.54	0.04	0.36
e	16.13	15.69	14.98	14.30	0.44	0.72	1.39
	14.22	14.23	13.74	13.36	-0.02	0.50	0.87
f	16.85	15.47	14.84	14.20	1.38	0.63	1.27
	15.66	14.57	14.08	13.63	1.09	0.49	0.94
g	19.14	17.18	16.58	15.91	1.96	0.60	1.27
	17.95	16.28	15.82	15.34	1.67	0.46	0.94

$^{{\rm a}}$

Values in bold are corrected for inclination and total Galactic foreground extinction.

$^{{\rm b}}$

Objects are labeled according to Fig. 1.

The two parameters B/T and n were used for the morphological classification of the galaxies (see Sect. 4). Our B/T values in the B-band were compared with those plotted as a function of the morphological type T by Simien & de Vaucouleurs (1986). Additionally we took into account the growing evidence that late-type spirals host non-classical bulges, better reproduced by an exponential law (n = 1) (Andredakis & Sanders 1994; Andredakis 1998; Carollo et al. 2001).

The inclination correction terms A_Bi, E(B-V)_i and the selective extinction R_V have been calculated as a function of the morphological type T and of the B-band disk inclination i (instead of the ratio R₂₅ between the major and minor diameters at the $\mu_{B} = 25$ mag arcsec^-2 isophote) following the instructions given in the introduction to the Third Reference Catalogue of Bright Galaxies (de Vaucouleurs et al. 1991, RC3). The inclination corrections in the other photometric bands were derived by application of the Cardelli et al. (1989, CCM) extinction law. The inclination corrected total and effective colors B-V, V-R, and V-I of three galaxies, selected for being sufficiently separated one another and not too much inclined, were used to estimate the total foreground Galactic extinction. For this purpose the three colors available for each galaxy were compared with the mean values for the relevant morphological type published by Buta & Williams (1995), and the corresponding color excesses E(B-V) were calculated and averaged. Finally the weighted mean of the values obtained for these three objects, $E(B-V) = 0.29 \pm 0.06$ ( A_B = 1.19 mag, assuming a selective extinction R_V = 3.1) was used as a measure of the Galactic extinction in the direction of the galaxy group. We stress that this value is significantly different from the ones available from NED, A_B = 0.610 and A_B = 0.642 mag, based on the maps of Schlegel et al. (1998) and Burstein & Heiles (1982), respectively. This discrepancy could be due to small-scale fluctuations in the foreground extinction as a consequence of the patchy dust distribution at low Galactic latitudes. Such fluctuations cannot be detected by interpolating data from large scale maps like those cited above. In Table 4 we list observed magnitudes and colors of the seven galaxies in the field and the corresponding values (in bold) corrected for inclination and Galactic extinction.