3 CMDs and galaxy distances

During the last decade the tip of the red giant branch (TRGB) method has become an efficient tool for measuring galaxy distances. The TRGB distances agree with the distances derived from the Cepheid period-luminosity relation within a 5% error. As was shown by Lee et al. (1993), Salaris & Cassisi (1997), and Udalski et al. (2001), in the I band the TRGB position is relatively independent of age and metallicity within $\sim$0.1 mag for old stellar populations with [Fe/H] < - 0.7 dex. According to Da Costa & Armandroff (1990), for metal-poor systems the TRGB is located at M_I = -4.05 mag. Ferrarese et al. (2000) calibrated the zero point of the TRGB from galaxies with Cepheid distances and yielded $M_I = -4.06 \pm 0.07$. A new TRGB calibration $M_I = -4.04 \pm0.12$ mag was determined by Bellazzini et al. (2001) based on photometry and distance estimation from a detached eclipsing binary in the Galactic globular cluster $\omega$ Centauri. In the present paper we adopt M_I = -4.05 mag. The bottom left panels of Fig. 2 show I versus (V-I) color-magnitude diagrams (CMDs) for the eighteen observed galaxies.

We determined the TRGB location using a Gaussian-smoothed I-band luminosity function (LF) for red stars with colors (V-I) within $\pm0\hbox{$.\!\!^{\rm m}$ }5$of the mean <V-I> for expected red giant branch stars. Following Sakai et al. (1996), we applied a Sobel edge-detection filter. The position of the TRGB was identified with the peak in the filter response function. The resulting LFs and the Sobel-filtered LFs are shown in the bottom right corners of Fig. 2. The results are summarized in Table 1.

 

 

Table 1: New distances to galaxies in the Canes Venatici cloud.

Name	RA (1950) Dec	$a \times b$	$B_{\rm t}$	$A_{\rm b}$	$(V-I)_{\rm T}$	T	$V_{\rm LG}$	I(TRGB)	$(m-M)_{\rm o}$	D	(V-I)-3.5
	$^{\rm hh\ mm\ ss}\;\;\;\;\; \hbox{$^\circ$ }\hbox{$^\circ$ }\;\hbox{$^\prime$ }\hbox{$^\prime$ }\;\hbox{$^{\prime\prime}$ }\hbox{$^{\prime\prime}$ }$	$\hbox {$^\prime $ }$	mag	Ai	$\pm \sigma$		km s-1	mag	mag	Mpc	[Fe/H]
U6541	113045.2 493043	1.4 $\times$ 0.8	14.23	0.08	0.53	10	304	23.94	27.95	3.89	1.39
				0.04	$\pm$0.07			$\pm$0.28	$\pm$0.29	$\pm$0.47	-1.54
N3738	113304.4 544758	2.6 $\times$ 1.9	12.13	0.05	0.62	10	305	24.42	28.45	4.90	1.48
				0.02	$\pm$0.10			$\pm$0.24	$\pm$0.25	$\pm$0.54	-1.27
N3741	113325.2 453343	2.0 $\times$ 1.1	14.3	0.10	0.67	10	264	23.41	27.41	3.03	1.37
				0.05	$\pm$0.10			$\pm$0.21	$\pm$0.23	$\pm$0.33	-1.61
KK109	114433.5 435659	0.6 $\times$ 0.4	18.62	0.08	0.80	10	241	24.26	28.27	4.51	1.15
				0.04	$\pm$0.10			$\pm$0.15	$\pm$0.17	$\pm$0.34	-2.54
N4150	120801.2 304054	2.3 $\times$ 1.6	12.45	0.08	1.20	-1	198	-	-	20:	-
				0.04	$\pm$0.01
U7298	121400.6 523018	1.1 $\times$ 0.6	15.95	0.10	0.45	10	255	24.12	28.12	4.21	1.32
				0.05	$\pm$0.26			$\pm$0.15	$\pm$0.17	$\pm$0.32	-1.80
N4244	121459.8 380506	19.4 $\times$ 2.1	10.67	0.09	0.89	6	255	24.25	28.26	4.49	1.32
				0.04	$\pm$0.07			$\pm$0.22	$\pm$0.24	$\pm$0.47	-1.80
N4395	122320.8 334922	13.2 $\times$ 11.0	10.61	0.07	0.73	9	315	24.30	28.32	4.61	1.45
				0.03	$\pm$0.11			$\pm$0.28	$\pm$0.29	$\pm$0.57	-1.36
U7559	122437.1 372509	3.2 $\times$ 2.0	14.12	0.06	0.48	10	231	24.42	28.44	4.87	1.29
DDO126				0.03	$\pm$0.10			$\pm$0.24	$\pm$0.26	$\pm$0.55	-1.91
N4449	122545.1 442215	6.2 $\times$ 4.4	9.83	0.08	0.63	10	249	24.11	28.12	4.21	1.43
				0.04	$\pm$0.02			$\pm$0.26	$\pm$0.27	$\pm$0.50	-1.42
U7605	122611.0 355940	1.1 $\times$ 0.8	14.76	0.06	0.61	10	317	24.21	28.23	4.43	1.21
				0.03	$\pm$0.10			$\pm$0.27	$\pm$0.28	$\pm$0.53	-2.26
IC3687	123950.8 384633	3.4 $\times$ 3.0	13.75	0.09	0.57	10	385	24.29	28.30	4.57	1.32
DDO141				0.04	$\pm$0.26			$\pm$0.22	$\pm$0.24	$\pm$0.48	-1.80
KK166	124649.5 355305	1.7 $\times$ 1.0	17.62	0.06	1.20	-3	-	24.36	28.38	4.74	1.27
				0.03	$\pm$0.2			$\pm$0.34	$\pm$0.34	$\pm$0.69	-2.00
N4736	124832.3 412328	11.2 $\times$ 9.1	8.74	0.08	1.19	2	353	24.33	28.34	4.66	1.69
M94				0.04	$\pm$0.01			$\pm$0.28	$\pm$0.29	$\pm$0.59	-0.84
U8308	131110.0 463511	1.1 $\times$ 0.6	15.53	0.04	0.69	10	243	24.08	28.11	4.19	1.37
DDO167				0.02	$\pm$0.26			$\pm$0.25	$\pm$0.26	$\pm$0.47	-1.61
U8320	131216.6 461101	3.6 $\times$ 1.4	12.73	0.07	0.52	10	273	24.16	28.18	4.33	1.28
DDO168				0.03	$\pm$0.26			$\pm$0.25	$\pm$0.26	$\pm$0.49	-1.96
N5204	132743.8 584032	5.0 $\times$ 3.0	11.73	0.05	0.75	9	341	24.31	28.34	4.65	1.32
				0.02	$\pm$0.01			$\pm$0.26	$\pm$0.27	$\pm$0.53	-1.80
U8833	135236.0 360500	0.9 $\times$ 0.8	15.15	0.05	0.68	10	285	23.49	27.52	3.19	1.26
				0.02	$\pm$0.10			$\pm$0.12	$\pm$0.15	$\pm$0.21	-2.04

This table contains the following columns: (1) galaxy name; (2) equatorial coordinates corresponding to the galaxy center; (3,4) angular size in arcmin and apparent total magnitude from the NASA Extragalactic Database (NED), uncorrected for internal and external extinction; (5) Galactic extinction in the B, I-bands from Schlegel et al. (1998); (6) integrated or effective color of the galaxy, corrected for Galactic extinction; the data on colors are taken from Makarova et al. (1998), Makarova (1999), and Prugniel & Heraudeau (1998); for the galaxies NGC 3741, KK 109, and KK 166 we present V-I colors of their core from our measurements; (7) morphological type in de Vaucouleurs notation; (7) radial velocity with respect to the LG centroid; for some galaxies we used new accurate velocities measured recently by Huchtmeier et al. (2002); (9) position of the TRGB and its uncertainty derived with the Sobel filter; (10) true distance modulus with its uncertainty, which takes into account the uncertainty in the TRGB detection as well as uncertainties of the HST photometry zero point ($\sim$ $0\hbox{$.\!\!^{\rm m}$ }05$), the aperture corrections ($\sim$ $0\hbox{$.\!\!^{\rm m}$ }05$), and crowding effects ($\sim$ $0\hbox{$.\!\!^{\rm m}$ }06$) added in quadrature; uncertainties in extinction and reddening are assumed to be $10\%$ of their values from Schlegel et al. (1998); and (11) linear distance in Mpc and its uncertainty.

Given the distance moduli of the galaxies, we can estimate their mean metallicity, [Fe/H], from the mean color of the TRGB measured at an absolute magnitude M_I = -3.5, as recommended by Da Costa & Armandroff (1990). Based on a Gaussian fit to the color distribution of the giant stars in a corresponding I- magnitude interval $(-3.5\pm 0.3)$, we derived their mean colors, (V - I)_-3.5, which lie in a range of [1.15-1.68] after correction for Galactic reddening. Following the relation of Lee et al. (1993), this provides us with mean metallicities, [Fe/H] = [ -0.8, -2.5] dex, listed in the last column of Table 1. With a typical statistical scatter of the mean color ($\sim$ $0\hbox{$.\!\!^{\rm m}$ }05$), and the uncertainty of the HST photometry zero point we expect an uncertainty in metallicity to be about 0.3 dex. Therefore within the measurement accuracy the metallicity of the galaxies satisfy the required limitation, [Fe/H] < -0.7 dex.

Below, some individual properties of the galaxies are briefly discussed.

UGC 6541 = Mkn 178. A blue compact galaxy from Markarian's lists is located on the NW edge of the CVn I cloud. It was resolved into stars for the first time by Georgiev et al. (1997), who derived a distance of $3.5\pm 0.7$ Mpc via the brightest blue stars. The distance to UGC 6541 from the luminosity of TRGB is $D = 3.89\pm0.47$ Mpc, which is in reasonable agreement with the previous estimate.

NGC 3738 = UGC 6565 = Arp 234. This dwarf irregular galaxy appears to be semi- resolved into brightest stars on the reproduction given in the Atlas of Peculiar Galaxies (Arp 1966). Georgiev et al. (1997) estimated its distance as $3.5\pm 0.7$ Mpc from the magnitudes of the brightest blue stars. The images obtained with WFPC2 reveal about 17 700 stars seen in both filters. The CM diagram for NGC 3738 shows a large number of blue stars, as well as AGB stars. From the TRGB position we derive a distance of $4.90\pm 0.54$ Mpc.

NGC 3741 = UGC 6572. Like two previous objects, NGC 3741 lies at the NW periphery of the CVn I cloud. The galaxy has an asymmetric "cometary'' shape. Its size in H  I exceeds its optical diameter significantly (Haynes & Giovanelli 1991). Georgiev et al. (1997) derived its distance to be $3.5\pm 0.7$ Mpc from the brightest blue stars, while the TRGB distance from our measurements is $3.03\pm 0.33$ Mpc.

KK 109. This dwarf irregular galaxy of low surface brightness was found by Karachentseva & Karachentsev (1998). Huchtmeier et al. (2000) detected it in the H  I line and determined its radial velocity, $V_{\rm LG} = 241$ km s^-1, which is typical for CVn I members. The CM diagram of KK 109 shows the TRGB magnitude to be $24.26\pm 0.15$ mag, which yields a distance of $4.51\pm 0.34$ Mpc. Based on the WFPC2 images, we carried out surface photometry of KK 109, obtaining total magnitudes $V_{\rm t} = 18.14 \pm 0.2$ mag, $(V - I)_{\rm t} = 0.8 \pm 0.1$ mag, and a central surface brightness of $23.6 \pm 0.2$ mag arcsec^-2 in the V band. With the derived apparent magnitude and distance, KK 109 has an absolute magnitude of M_V = -10.19 mag, placing it among the faintest known dIrr galaxies such as LGS 3 and Antlia.

NGC 4150. The core of this lenticular galaxy is crossed by a curved dusty furrow (see insert in Fig. 2). In spite of its low radial velocity, $V_{\rm LG} = 198$ km s^-1, NGC 4150 appears unresolved into stars on the WFPC2 images. Its TRGB magnitude appears to exceed $I_{\rm lim} = 25$ mag (beyond our detection limit), yielding a lower limit of 6.3 Mpc for its distance. We suggest that NGC 4150 belongs to the Virgo cluster outskirts, and not to the CVn I cloud. Most of the objects seen in the galaxy body seem to be slightly extended and diffuse with integrated colors of V-I = 0.8-1.6, which raises the possibility that they are globular clusters. If they are indeed globular clusters, we can use the turnover magnitude of the globular cluster luminosity function (GCLF), $V\sim 24$ mag, as a distance indicator (Ferrarese et al. 2000). With this assumption we derive a rough distance estimate of $\sim$20 Mpc consistent with the Virgo cluster distance.

UGC 7298. This is a dIrr galaxy of low surface brightness. UGC 7298 has been resolved into stars by Tikhonov & Karachentsev (1998), who estimated its distance to be $8.6\pm 1.5$ Mpc via the brightest stars. The CM diagram (Fig. 2) shows populations of blue stars and AGB stars. We determine the TRGB magnitude to be I(TRGB) $= 24.12 \pm 0.15$ mag, which gives a distance of $4.21\pm 0.32$ Mpc. The TRGB distance is approximately two times smaller than the distance from the brightest stars. A possible cause of this difference is a lack of very luminous blue stars in this galaxy, i.e., no very recent massive star formation.

NGC 4244. A large edge-on Sc galaxy extends far beyond the WFPC2 field. Its periphery was resolved into stars by Karachentsev & Drozdovsky (1998), who estimated the galaxy distance to be $4.5 \pm 0.9$ Mpc from the photometry of the brightest stars. The CM diagram (Fig. 2) shows $\sim$15 000 stars, in particular pronounced populations of blue stars and AGB stars. The TRGB position, $24.25\pm 0.22$ mag, corresponds to a distance of $4.49\pm 0.47$ Mpc in close agreement with the distance estimate via the brightest stars.

NGC 4395. This face-on Sd galaxy with a Seyfert 1 type nucleus also extends beyond the WFPC2 field. According to Karachentsev & Drozdovsky (1998), its distance via the brightest blue stars is $4.2\pm 0.8$ Mpc. The CM diagram in Fig. 2 reveals about 21 800 stars seen both in the V and I bands. The majority of the detected stars are likely RGB stars. From the TRGB position we derive a distance of $4.61\pm 0.57$ Mpc, which agrees well with the previous distance estimate.

UGC 7559 = DDO 126. This irregular dwarf galaxy has been resolved into stars by Hopp & Schulte-Ladbeck (1995), Georgiev et al. (1997), and Makarova et al. (1998), who derived distance estimates of 4.8 Mpc, 3.9 Mpc, and 5.1 Mpc, respectively. Our distance for UGC 7559 based on the TRGB ( $4.87\pm 0.55$ Mpc) is in the middle of the range previously obtained by other authors.

NGC 4449. This boxy-shaped Magellanic irregular galaxy of high surface brightness is a second ranked member of the CVn I cloud according to its luminosity. NGC 4449 is enveloped in a huge H  I "fur coat'', whose angular size (75') exceeds the Moon's diameter (Bajaja et al. 1994). Based on photometry of the brightest stars, Karachentsev & Drozdovsky (1998) estimated its distance to be $2.9\pm 0.6$ Mpc. The WFPC2 photometry reveals about 27 000 stars seen in both images. The CM diagram in Fig. 2 shows stellar populations of different kinds including RGB stars. From the TRGB magnitude, $24.11\pm 0.26$ mag, we derive a distance of $4.21\pm 0.50$ Mpc.

UGC 7605. This is a blue irregular galaxy shaped like a horseshoe. The brightest blue stars in UGC 7605 are concentrated towards the core, and the outlying galaxy parts are redder and smooth. From the luminosity of the brightest stars Makarova et al. (1998) derived a galaxy distance of $4.4\pm 0.9$ Mpc. The CM diagram of UGC 7605 (Fig. 2) shows the RGB population giving a distance of $4.43\pm 0.53$ Mpc, which is in close agreement with the previous estimate.

IC 3687 = DDO 141 = UGC 7866. IC 3687 is an irregular dwarf galaxy with several regions of current star formation activity. Its CM diagram shows a mixed stellar population with a pronounced RGB with I(TRGB $) = 24.29\pm 0.22$ mag, which corresponds to a distance of $4.57\pm 0.48$ Mpc. Our distance for IC 3687 differs from the previous distance, $3.0\pm 0.6$ Mpc, obtained by Makarova et al. (1998) via the brightest stars.

KK 166. This galaxy is unique in terms of being the only dwarf spheroidal (dSph) galaxy of very low surface brightness identified so far in the CVn I region. (Another possible dSph in CVn I is DDO 113 = KDG 90.) The galaxy has been observed but not detected in the H  I line by Huchtmeier et al. (2000). The CM diagram shows a dominant RGB population with I(TRGB $) = 24.36\pm 0.34$ mag, yielding a distance of $4.74\pm 0.69$ Mpc, which confirms KK 166 as a likely member of the CVn I cloud. Apart from stellar photometry, we also carried out surface photometry in circular apertures. From our measurements KK 166 has a total magnitude $V_{\rm t} = 16.8 \pm 0.2$ mag, $(V-I)_{\rm t} = 1.2 \pm 0.1$ mag, and a central surface brightness of $25.0\pm 0.2 ^{\rm m}/\sq\hbox{$^{\prime\prime}$ }$ in the V band.

NGC 4736 = M 94. NGC 4736 is the brightest galaxy of type Sa in CVn I. We resolve it into stars for the first time. The WFPC2 was pointed at the galaxy periphery to avoid stellar crowding. In the galaxy halo the CM diagram (Fig. 2) shows numerous RGB stars with I(TRGB $) = 24.33\pm 0.28$ mag, which yields a distance of $4.66\pm 0.59$ Mpc. Karachentseva & Karachentsev (1998) carried out a proper search for dwarf companions to NGC 4736 based on the POSS-II plates. Surprisingly, they found no companions with a central surface brightness brighter than 25 $^{\rm m}/\sq\hbox{$^{\prime\prime}$ }$ in the B band within a radius of $\sim$3 degrees or 230 kpc around this giant galaxy. Such a pronounced degree of isolation of an Sa galaxy situated in the middle of the CVn I cloud seems rather unusual.

UGC 8308 = DDO 167. This is an asymmetric irregular galaxy of low surface brightness, becoming redder from its core to the periphery. It has been resolved into stars by Tikhonov & Karachentsev (1998), who derived a distance of $3.7\pm 0.7$ Mpc from the brightest star photometry. The CMD (Fig. 2) shows blue and red stellar populations with the TRGB position yielding a distance of $4.19\pm 0.47$, which is in close agreement with the previous estimate.

UGC 8320 = DDO 168. This irregular galaxy is located $14 \hbox{$^\prime$ }$away from UGC 8308, forming a probable pair of dwarf galaxies. Bresolin et al. (1993), Hopp & Schulte-Ladbeck (1995), and Tikhonov & Karachentsev (1998) have resolved it into stars and estimated its distance to be 3.3 Mpc, 3.9 Mpc, and 4.0 Mpc, respectively. The WFPC2 photometry gives a TRGB magnitude corresponding to a distance of $4.33\pm 0.49$ Mpc. The derived TRGB distances of UGC 8320 and UGC 8308 agree with each other within the uncertainties.

NGC 5204. NGC 5204 is an irregular galaxy of Magellanic type, which is located at the northern edge of the CVn I cloud. Its distance, $4.9\pm 1.0$ Mpc, was estimated by Karachentsev et al. (1994) via the brightest blue and red stars. The CM diagram (Fig. 2) shows a mixed stellar population with a prominent RGB. The derived TRGB distance, $4.65\pm 0.53$ Mpc, is in good agreement with the previous estimate.

UGC 8833. This blue irregular galaxy looks like a binary system because of several regions with ongoing intense star formation. UGC 8833 is situated at the eastern edge of the CVn I cloud. According to Makarova et al. (1998) its distance derived from its brightest stars is $3.2\pm 0.6$ Mpc. Our photometry of the WFPC2 images yields a TRGB distance of $3.19\pm 0.21$ Mpc, which confirms the previous distance estimate.