4 Properties of individual galaxies

4.1 VV 432 = IC 3105 = UGC 7326

The first long-slit spectrum of this chain-like galaxy has been obtained along the major axis by Arkhipova et al. ([1981]). They revealed high-excitation spectrum of H II-region in the NE region. The amplitude of the rotation curve has been found to be relatively small (less than about 60 km s^-1).

Our long-slit spectrophotometry allowed to detect the NE H II-region (knot "a'' in Fig. 1) [O III]-line $\lambda$4363 Å with a good S/N ratio in both spectra. The first spectrum, obtained on February 12, 1999 covered the range 3600-8000 Å and the spectra taken on April 24, 1999 with a dispersion 1.2 Å/pixel, covered two separate ranges 4000 - 5200 Å and 6000 - 7200 Å. Table 3 presents the data for all measured emission lines. According to this table, the relative intensities of strong oxygen and hydrogen lines in both spectra are the same within 3%, in accordance with their internal errors. For the abundance derivation we supposed that, for the April spectra, the relative intensity of the [O II]-line $\lambda$3727 Å and of the H$\beta$ line to be equal to those derived from the February spectrum.

Oxygen abundance determination gives consistent values for both data sets, so we accept the weighted mean of the two independent measurements. The resulting value of 12+log(O/H) = 7.58 $\pm$ 0.06 obtained for this H II-region appear to be very low.

Since the [O III]-line $\lambda$4363 Å was not detected in other emission-line knots, the oxygen abundances for knots "b'', "c'' and "d'' were estimated by the same empirical method mentioned in Sect. 2.3. The most uncertain parameter - the mean gas density - was assumed to be close to that found for knot "a'', what is about 1 atom cm^-3(the value, derived from the consistency of oxygen abundances, determined by classical and empirical methods). Oxygen abundances, found by the empirical method for this value of $N_{\rm e}$ for three knots, are consistent with the estimates, obtained for knot "a'', taking into account the uncertainties of parameter R₂₃ and possible variations of the mean density $N_{\rm e}$ within the factor of 2.

This galaxy is elongated (the axial ratio of b/a $\approx$ 1/3 according to LEDA), which is not untypical for dwarf galaxies. It looks like an edge-on disk, bent on both NE and SW edges, which probably indicates a tidal action from some other galaxy. The bright H II-region on the NE edge appears to be outside of the main body of the galaxy, and its connection to the disk is not evident, although a smooth velocity distribution along the galaxy favours its interpretation as a single object. Another possibility which cannot be excluded is that the outer knots are large SF regions at the ends of spiral arms, seen not exactly edge-on. However due to the rather small luminosity of VV 432, the presence of spiral arms is quite improbable.

Our long-slit spectrum with the dispersion of 1.2 Å/pixel allows to derive the rotation curve along the "disk'', using the H$\alpha$ line data (see Fig. 1b). A velocity gradient is well seen over the whole disk; there is no sign of flattening of the velocity curve on both NE and SW edges. The mean heliocentric velocity of the galaxy is -160 km s^-1. The amplitude of the rotation curve within the whole extent of H$\alpha$-emission of about 70 $^{\prime\prime}$ corresponds to the maximal rotational velocity of 40 $\pm$ 5 km s^-1. The expected inclination correction is very small, since the  disk is seen at the inclination angle $i > 70^{\circ}$.

The uncertainties of the data points of the Fig. 1b's velocity curve are small and remain in the range 4 to 15 km s^-1. The irregularities which are seen on the rotation curve can then be considered as real ones, probably connected with the regions of active SF.

The galaxy is situated in the direction of Virgo cluster (VC), and is also catalogued as VCC 0241. Its negative radial velocity does not contradict its membership to the cluster (Binggeli et al. [1993]). In this case, its distance is of $\approx$ 20.7 Mpc (distance modulus = 31.6 mag, Federspiel et al. [1998]). Its maximal diameter on the isophote $\mu_{B}$= 25 $^{{\rm m}}/\Box{\hbox{$^{\prime\prime}$ }}$ is 111 $\hbox{$^{\prime\prime}$ }$, according to LEDA, which corresponds to a linear size of about 11 kpc. The extent of the H$\alpha$ emission on our long-slit spectrum is of 70 $\hbox{$^{\prime\prime}$ }$, which gives the total extent of H II-emission to be $\approx$ 6.8 kpc.

If VV 432 is a member of VC it appears to be the most metal-deficient known galaxy of this cluster (see for comparison the metallicity data of the VC BCGs in Izotov & Guseva [1989]). Such low metallicity implies that the galaxy presumably experienced only one or two major SF episodes during its life. Since galaxy interactions in clusters are very important triggers, it is difficult to understand how VV 432 could sustain its very low metallicity in the dense VC environment. The existence of such metal-poor gas-rich galaxy poses serious questions on its evolution history, and thus gives additional arguments to make independent check of its distance, using, in particular, color-magnitude diagrams for resolved stars.

To estimate physical parameters of VV 432 we accept further that this galaxy is in VC, at the distance 20.7 Mpc.

Since as we already noticed VV 432 resembles by its morphology an edge-on bent disk, it is natural to check probable companion galaxies, acting as strong enough disturbing bodies.

According to NED, the nearest galaxies in both the projected distance and the relative velocity are VCC 0200 (MCG +02-31-076) at the angular distance 41.4$^{\prime}$ and $V_{{\rm hel}}$ = 65 km s^-1, with B-magnitude = $14\hbox{$.\!\!^{\rm m}$ }69$, and NGC 4216 (UGC 7284) at the angular distance 51.8$^{\prime}$ and $V_{{\rm hel}}$ = 131 km s^-1, with B-magnitude = $10\hbox{$.\!\!^{\rm m}$ }99$. If they are members of VC, their respective projected distances relative to VV 432 are 240 and 312 kpc. NGC 4216 according to LEDA is a giant spiral with $M_{B}= -21\hbox{$.\!\!^{\rm m}$ }6$. If it is indeed on the same radial distance as VV 432, it really can exert strong enough tidal action on VV 432 to trigger gravitational instability and subsequent SF bursts. In particular, tidal generation of shocks in gaseous disk according to Icke ([1985]) mechanism can be responsible for observed SF bursts (see e.g. Pustilnik et al. [2000], for the estimates of tidal effect in similar situation).

Figure 2: From top-left to bottom: a) DSS-1 image of galaxy pair VV 543E and VV 543W with the position of long slit superimposed; b) Brightness profile of H$\alpha$ in relative intensities along the slit and the velocity curve.; c) 2-D spectrum of the pair VV 543; d) 1-D spectrum of the Western component

Figure 3: a) DSS-1 image of galaxy VV 747 with the position of long slit superimposed; b) brightness profile of H$\alpha$along the slit and the velocity curve of VV 747; c) 2-D spectrum of VV 747; d) 1-D spectrum of the brightest SW component

 

 

Table 4: Abundances in VV 432, VV 543W and VV 747

Value	VV 432 (a) (4.6 Å/pixel)	VV 432 (a) (1.2 Å/pixel)	VV 747 (a) (4.6 Å/pixel)	VV 543W (4.6 Å/pixel)
$T_{\rm e}$(O III)(K)	20680 $\pm$ 1720	18480 $\pm$ 3320	15380 $\pm$ 840	7500 $\pm$ 1500
$N_{\rm e}$(S II)(cm-3)	< 10	< 10	110 $\pm$ 239	< 10
12+log(O/H)	7.57 $\pm$ 0.06	7.62 $\pm$ 0.13	7.85 $\pm$ 0.05	8.5 $\pm$ 0.1(1)
log(N/O)	-1.41 $\pm$ 0.17	-1.50 $\pm$ 0.34	-1.47 $\pm$ 0.16	--
log(Ne/O)	-0.63 $\pm$ 0.12	--	-0.78 $\pm$ 0.10	--
(1) -- without [O III]-line $\lambda$4363 Å obtained by the method described in Sect. 2.2.

4.2 VV 543 = NGC 5275

This galaxy was considered as a candidate dwarf galaxy (M_B= $-15\hbox{$.\!\!^{\rm m}$ }5$) due to its catalog radial velocity V = 1395 km s^-1 (RC3, de Vaucouleurs et al. [1991], cited also in both LEDA and NED, and originally obtained by Arkhipova & Esipov [1979]). However colour indices of this object are rather typical of giant E galaxies than for dIrr systems (Zasov & Arkhipova 2000). According to our observations, it is evident that there was some misprint in the original work, caused the catalog velocity of this galaxy to be in error by the factor of ten. Its real velocity, measured from our spectrum of emission-line region on the western periphery (hereafter VV 543W) is 14100 km s^-1.

In fact, the spectrum of the central bright region of this galaxy shows, that we have in this case an optical pair in projection. Indeed the central part shows only the absorption lines, typical of elliptical galaxy (e.g. Pickles [1988]), but its radial velocity is 1620 $\pm$ 120 km s^-1 lower than that for the emission-line galaxy. Its luminosity M_B = $-20\hbox{$.\!\!^{\rm m}$ }9$evidences that this galaxy falls to the class of normal ellipticals. The apparent magnitude of VV 543W is $\approx$ 2 $.\!\!^{\rm m}$5 fainter ( $B\approx 17\hbox{$.\!\!^{\rm m}$ }7$) than that of VV 543E (this estimate follows from the comparison of the flux near $\lambda$4400 Å in both galaxies), what leads to $M_{B} \sim$ $-18\hbox{$.\!\!^{\rm m}$ }7$. Its linear size along the major axis $\sim$ 13 kpc is quite modest, and, owing to the typical spectrum of H II-region, this western component can be considered as a bright blue compact/H II galaxy. The compact object lying at about 20 $\hbox{$^{\prime\prime}$ }$ to NE from the absorption-line galaxy is a foreground star.

The extent of H$\alpha$ emission along the minor axis is traced within 8 $\hbox{$^{\prime\prime}$ }$. Even after the binning in 4 pixels (which corresponds to the seeing of 2 $\hbox{$^{\prime\prime}$ }$) the velocity curve does not indicate clear gradient. The full range of the radial velocity is about 80 km s^-1 with the mean value of 14100 $\pm$ 30 km s^-1.

According to NED, VV 543W has a probable companion galaxy NGP9 F324-0303806 at 5.8$^{\prime}$ with the radial velocity 13699 $\pm$ 126 km s^-1and B = $16\hbox{$.\!\!^{\rm m}$ }68$. Corresponding projection distance $\approx$ 300 kpc and velocity difference 401 $\pm$ 130 km s^-1 are in the range typical of wide pairs of binary galaxies (see e.g. the study by Chengalur et al. [1993] and Nordgren et al. [1998]). Some weak tidal action from this galaxy can be responsible for the enhanced SF in VV 543W (see e.g. Reshetnikov & Combes [1997], and Rudnick & Rix [1998]).

   
4.3 VV 747 = CG 798

This dwarf galaxy consists of two clearly separated regions embedded into the common envelope of low surface brightness. The brighter SW component has full size of about 15 $\hbox{$^{\prime\prime}$ }$ and the fainter NE can be traced down to about 8 $\hbox{$^{\prime\prime}$ }$ ($\approx$ 0.8 and 0.4 kpc respectively). Both regions show emission-line spectrum (see Fig. 3b). The high excitation spectrum with the observed [O III] line $\lambda$4363 Å of the SW component allows to determine oxygen abundance using the direct measurement of $T_{\rm e}$. Our O abundance is 12+log(O/H) = 7.85 $\pm$ 0.05 compared to the abundance 7.97 derived by Izotov & Thuan ([1999]). This low metallicity is not untypical for BCGs.

Smooth velocity curve along the slit shows a small but clear slope across the SW component in the region of high S/N ratio of H$\alpha$ with the total extent of about 15 $\hbox{$^{\prime\prime}$ }$ and the velocity range from 540 to 630 km s^-1, consistent with the maximal rotation velocity of about 45 km s^-1 and mean radial velocity of 585 km s^-1. The galaxy body is well elongated, so the inclination correction does not seem to be larger than 20 $\div$ 30%. In the NE component the velocity can be measured only in two independent points, showing significant scattering, consistent with their internal uncertainties. The mean velocity of NE component, taking from these two points, is about 690 $\pm$ 42 km s^-1.

Huchra et al. ([1995]) presented the values of the observed velocities for two components of this galaxy, designated as SW and NE, but their coordinates given in the paper, are the same for both of them. If we accept that they observed the same components, our results are consistent with theirs (V(SW) = 621 $\pm$ 32 and V(NE) = 665 $\pm$ 44 km s^-1) within the cited uncertainties.

Since the two separate velocity points for the NE component do not show any clear gradient, which would indicate its independent rotation, and the mean velocity of the NE component matches well the continuation of the velocity curve for the SW component, there is no reason to consider this system as two different galaxies in collision. The current data favour the interpretation of this system as a single galaxy with two super-giant H II-regions in different excitation stages. In this case the systemic velocity of this single galaxy is about 620 km s^-1 with the full velocity range of 540 to 710 km s^-1. Both these values are quite well consistent with the parameters of H I-profile of VV 747 described and shown in Sect. 3.2. The latter full velocity range corresponds to the maximal rotational velocity of 85 km s^-1.

The better knowledge of the velocity curve in the region of the NE component is necessary to exclude completely the hypothesis of merger of two dwarf galaxies. At least one indirect argument favours this interpretation. If TF relation holds for this galaxy, its H I mass and blue luminosity are more consistent with the narrower width (W_0.2 = 70-80 km s^-1), than with the observed one in the integrated H I-profile. It can indicate that we observe the sum of two "narrow'' H I-profiles displaced at about 40-50 km s^-1 each from other. Only 21 cm line mapping of H I velocity field and/or very high quality H$\alpha$ velocity data can help to make a final interpretation of this system.

The search in the NED resulted in the bright companion galaxy NGC 3432 (= Arp 206 = VV 011) at the angular distance 67$^{\prime}$ with B = $11\hbox{$.\!\!^{\rm m}$ }67$ and the radial velocity 616 km s^-1, very close to that of VV 747. This galaxy, classified as SB(s)m (LINER H II), has M_B = $-18\hbox{$.\!\!^{\rm m}$ }3$. At the projected distance $\approx$ 190 kpc the tidal action of this galaxy to VV 747 may also be strong enough to trigger SF in this tiny dwarf.

Figure 4: H I-profile of galaxy VV 747. Velocity resolution is 6.3 km s-1, W0.5 = 90 km s-1, W0.2 = 117 km s-1