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Up: Photometric distances to six galaxies


3 Distance estimates for the galaxies

3.1 NGC 784 = UGC 1501 = PGC 7671 = IRAS 01582+2836

\includegraphics[angle=90,width=10cm]{dio_ikar1.eps}\end{figure} Figure 1: NGC 784 in the V band. As for all the other images, North is to the top, East is to the left. The upper right corner corresponds to X=0, Y=0of the frame coordinates given in the photometry tables, i.e. X increases to the East and Y to the South

This irregular galaxy seen nearly edge-on with angular dimensions of $6\hbox{$.\mkern-4mu^\prime$ }6\times1\hbox{$.\mkern-4mu^\prime$ }5$and an integral apparent magnitude of $B_{\rm T}= 12.23$ has a radial velocity of $V_{\rm h}= 198$ km s-1 (NASA Extragalactic Database = NED). In spite of its expected small distance, NGC 784 has not yet been resolved into stars. Broeils & van Woerden (1994) studied the structure and kinematics of NGC 784 in the H I line, showing that its neutral hydrogen content as well as rotational amplitude are typical of an irregular galaxy. The central part of NGC 784 is represented in Fig. 1 from our V CCD frame.

\includegraphics[angle=-90,width=10cm]{dio_ikar1.eps}\end{figure} Figure 2: Colour-Magnitude diagram for 84 stars in NGC 784

Among star-like objects found by DAOPHOT in this field we put into a Colour-Magnitude Diagram (=CMD) 84 stars, whose image parameters satisfy the conditions $\mid$SHARP$\mid < 2$, $\mid$CHI$\mid < 2$, and $\sigma(m) < 0\hbox{$.\!\!^{\rm m}$ }2$. The CMD for them, Fig. 2, shows the presence of some blue stars. Following Sandage & Tammann (1974) and de Vaucouleurs (1979) we estimated the distance modulus via the relation

\begin{displaymath}(m-M)_0= 1.51 \cdot \langle B(3B)\rangle - 0.51 \cdot B_{\rm T} - A_B + 4.14
\end{displaymath} (1)

between the mean apparent magnitude of three blue supergiant candidates, $\langle B(3B)\rangle$, and the integral magnitude of the galaxy, $B_{\rm T}$, where AB is the galactic extinction. The numerical coefficients were derived by Karachentsev & Tikhonov (1994) from galaxies with distances known via cepheids. In the cases where a galaxy was observed in the V and I bands only, the (B-V) color was derived from the relation $(B-V)= 0.83\cdot(V-I)$ defined by the standard Landolt's stars with V-I < 1. With $\langle B(3B)\rangle = 20.39$ and AB = 0.22 we derive for NGC 784 a distance modulus (m-M)0= 28.47 or D = 5.0 Mpc.

The basic parameters of NGC 784 and of the other considered galaxies are given in Table 2. Its first six lines indicate the standard major and minor diameters of the galaxy, its integral apparent magnitude, galactic extinction, morphological type, and heliocentric radial velocity from the NED. The next lines contain the radial velocity with respect to the centroid of the Local Group (Karachentsev & Makarov 1996), number of measured brightest stars, mean magnitude and colour of three brightest blue stars, the distance modulus and linear distance of the galaxy, and its absolute blue magnitude.

Note that recent H I observations by Huchtmeier et al. (2000) reveal in the NGC 784 neighbourhood two dwarf galaxies, kk16 and kk17, with radial velocities +399 and +348 km s-1, respectively, which are probable companions of NGC 784.

3.2 NGC 2683 = UGC 4641 = PGC 24930 = IRAS 08493+3336

\includegraphics[angle=90,width=10cm]{dio_ikar3.eps}\end{figure} Figure 3: Mosaic NGC 2683 image in the R band

A view of this Sb-type galaxy is shown in a mosaic (Fig. 3) constructed from three frames in the R filter with 30 s exposure each.

\includegraphics[angle=0,width=10cm]{dio_ikar4a.eps}\end{tabular}\end{figure} Figure 4: Northern (top) and southern (bottom) parts of NGC 2683 in V band

Figure 4 presents images of the northern (a) and southern (b) parts of the galaxy in the V band after subtracting the frames smoothed with a median filter with a $10\times$ FWHM window. As one can see from these images, the whole body of the galaxy is spotted with dusty lines, which impedes accurate photometry of stars, especially in the B band.
\includegraphics[angle=-90,width=10cm]{dio_ikar5.eps}\end{figure} Figure 5: CMD for 327 stars in NGC 2683. The filled circles are for stars measured in the northern region, the open squares indicate stars in the southern side

The results of our photometry of 327 stars and star-like objects are presented in the diagram $V\propto V-I$ (Fig. 5). The stars from the northern galaxy side are indicated by dark circles, and the southern part stars are marked by open boxes. A considerable part of the brightest blue objects have a diffuse or elongated shape. After removing them we selected three stars as blue supergiant candidates. Their mean apparent magnitude, 20.64, yields a distance modulus of 29.81 with AB = 0.07. Note that the the brightest red (B-V > 1.6) stars in the galaxy have V = 22.0, which corresponds to a distance modulus of about 30.0 that agrees with the "blue'' modulus. According to Tully (1988) NGC 2683 is situated in the scattered cloud "Leo spur'', which probably moves with a peculiar velocity of about -200 km s-1 with respect to the Local "pancake''. This can explain the low radial velocity of the galaxy, V0= 364 km s-1, at its rather large distance of 9.2 Mpc. From Karachentseva & Karachentsev (1998) and Huchtmeier et al. (2000) there are two dwarf companions in the vicinity of NGC 2683: kk69 with $V_{\rm h} = 420$ km s-1 and the dwarf spheroidal system kk70. The rotation curve for NGC 2683 was studied in the H I line by Broeils & van Woerden (1994).

3.3 NGC 2903 = UGC 5079 = KIG 347 = PGC 27077 = IRAS 09292+2143

\includegraphics[angle=-90,width=10cm]{dio_ikar6.eps}\end{figure} Figure 6: Digital Sky Survey image of NGC 2903. The squares indicate the position of our frames

This large ( $12\hbox{$.\mkern-4mu^\prime$ }6\times6\hbox{$.\mkern-4mu^\prime$ }0$) bright ( $B_{\rm T}= 9.68$) spiral galaxy similar to M 81 still remained unresolved into stars. In its wide vicinity there is no other bright galaxy. This is why Karachentseva (1973) included NGC 2903 in her Catalog of isolated galaxies (KIG). Within $3\hbox{$^\circ$ }$ around NGC 2903 there are two dwarf galaxies: UGC 5086 and F565-v1 imaged by Makarova & Karachentsev (1998). An image of NGC 2903 from the Digital Sky Survey is shown in Fig. 6. Our two CCD frames indicated by the boxes cover the basic part of its spiral pattern except the faint outer arms.

\includegraphics[angle=180,width=10cm]{dio_ikar7b.eps}\end{tabular}\end{figure} Figure 7: Northern (top) and southern (bottom) parts of NGC 2903 in the V band

Reproductions of the northern and southern CCD images are presented in Fig. 7 after subtracting frames smoothed with a median filter of $10\times$FWHM window.
\includegraphics[angle=-90,width=10cm]{dio_ikar8.eps}\end{figure} Figure 8: CMD for 273 brightest stars in NGC 2903. The filled circles correspond to stars measured in the northern region and open squares indicate stars in the southern side

The results of the photometry of 273 brightest stars are shown in $V\propto V-I$ diagram (Fig. 8). As in the case of NGC 2683, slightly diffuse objects prevail amongst the brightest blue ones. They are probably compact H II regions and multiple stars. As blue supergiant candidates, we pick three stars. Their mean apparent magnitude, 20.27, yields with galactic extinction of $0\hbox{$.\!\!^{\rm m}$ }07$ a distance modulus of 29.74 or D = 8.9 Mpc. Being located in the same Leo spur cloud as NGC 2683, the spiral galaxy NGC 2903 has also a low radial-velocity to distance ratio: H= V0/D= 50 km s-1/Mpc (line 14 in Table 2).

3.4 NGC 5204 = UGC 8490 = PGC 47368 = IRAS 13274+5840

\includegraphics[angle=90,width=10cm]{dio_ikar9.eps}\end{figure} Figure 9: Composite B+V image of the central part of NGC 5204 after subtraction of the median smoothed frame

A complete image of this irregular galaxy in the B+V bands is shown in Fig. 9 after subtraction of the median smoothed frame. It shows many more resolved stars than the reproduction in the Atlas of Sandage & Bedke (1988).

\includegraphics[angle=-90,width=10cm]{dio_ikar10.eps}\par\end{figure} Figure 10: V vs. (B-V) CM diagram of the 131 stars in the area of NGC 5204

Despite the short exposures (100 and 200 s), photometry of our frames reveals 131 stars whose CMD is presented in Fig. 10. For three brightest blue stars we derived $\langle B(3B)\rangle = 19.81$ which is in good agreement with the previous estimate by Karachentsev et al. (1994) 19.92. According to this paper, NGC 5204 is located in the outskirts of the M 101 group. However, its distance, 4.1 Mpc, seems to be considerably lower than the average distance of other members of the group, 6.9 Mpc.

3.5 NGC 5474 = UGC 9013 = VV 344 = Arp 26 = PGC 50216 = IRAS 14031+5354

\includegraphics[angle=0,width=10cm]{dio_ikar11.eps}\par\end{figure} Figure 11: R band image of NGC 5474 from the Isaac Newton Telescope Archive. The two squares correspond to 6 m BTA CCD frames

The structure of this nearest companion to M 101 is strongly disturbed by tidal interaction. A view of the galaxy imaged in the R band is shown in Fig. 11. It has been taken from the archive of the Isaac Newton telescope (La Palma). Positions of two CCD frames obtained with the 6-m telescope are indicated by boxes.

\includegraphics[angle=0,width=10cm]{dio_ikar12.eps}\par\end{figure} Figure 12: I frame of the southern part of NGC 5474

The reproduction of the southern image in the I band is presented in Fig. 12.
\includegraphics[angle=-90,width=10cm]{dio_ikar13.eps}\par\end{figure} Figure 13: Colour-magnitude diagram for 298 stars in NGC 5474

\includegraphics[angle=-90,width=10cm]{dio_ikar14.eps}\par\end{figure} Figure 14: I band inlay of NGC 5585 from the 1 m Kapteyn telescope image with the position of the 6 m BTA CCD frame

The results of photometry of 298 brightest stars in both fields are given in the diagram $V\propto V-I$ (Fig. 13). For three brightest blue supergiant candidates we derived the mean apparent magnitude $\langle B(3B)\rangle= 20.37$, which coincides with the previous estimate, 20.36, made by Karachentsev et al. (1994). The distance modulus of NGC 5474, (m-M)0 = 29.15, agrees well with the modulus of M 101 (29.26) measured via cepheids (Alves & Cook 1995).

3.6 NGC 5585 = UGC 9179 = KIG 624 = PGC 51210 = IRAS 14181+5657

As the two previous galaxies, NGC 5585 is a probable member of the M 101 group, but it looks rather isolated with respect to the neighbouring galaxies. Fig. 14 reproduces a view of NGC 5585 in the Iband taken from the archive of the John Kapteyn telescope (La Palma).

Figure 15 shows the main body of the galaxy from the CCD image obtained with the 6-m telescope in the I band after subtraction of the median smoothed frame.

The results of stellar photometry averaged over two observing runs are presented as a CMD, Fig. 16. The mean apparent magnitude of the three brightest blue stars is $\langle B(3B)\rangle= 20.71$, which agrees better with the old estimate of Sandage & Tammann (1974) 20.9, than with the value of 20.02 derived by Karachentsev et al. (1994). However, all three brightest blue stars used by Karachentsev et al. (1994) are situated on the NE side of NGC 5585 beyond the boundaries of our present frame. Despite the somewhat uncertain distance of NGC 5585, 5.7 Mpc (former estimate), and 8.7 Mpc (the present paper), this galaxy seems to be peripheric member of the M 101 group.


Table 2: Basic parameters of the resolved galaxies
Parameter N 784 N 2683 N 2903 N 5204 N 5474 N 5585
$a_25,\; (\hbox{$^\prime$ }$) 6.6 9.3 12.6 5.0 4.8 5.8
$b_25,\; (\hbox{$^\prime$ }$) 1.5 2.2 6.0 3.0 4.3 3.7
$B_{\rm T},\;$ (mag) 12.23 10.64 9.68 11.73 11.28 11.20
$A_B,\;$ (mag) 0.22 0.07 0.07 0.01 0.00 0.00
Type SBdm SA(rs)b SAB(rs)bc SA(s)m SA(s)cd SAB(s)d
$V_{\rm h}, \;$ (km s-1) 198 411 556 201 273 305
$V_0,\; $ (km s-1) 389 364 443 341 412 459
N** 84 327 273 131 298 102
$\langle B(3B)\rangle,\;$ (mag) 20.39 20.64 20.27 19.81 20.37 20.71
$\langle B-V \rangle_3$ -0.17 0.10 0.14 0.14 0.15 0.08
(m-M)0 28.47 29.81 29.74 28.06 29.15 29.70
D, (Mpc) 5.0 9.2 8.9 4.1 6.8 8.7
MB -16.46 -19.24 -20.13 -16.34 -18.22 -18.50
H=V0/D 78 40 50 83 61 53
SGB, $\; (\hbox{$^\circ$ })$ -6.3 -33.4 -36.4 17.8 22.9 24.7

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