3.1. The group and its environment
Hickson 96 is a hierarchical system composed of two large and two considerably smaller members. H96a (NGC 7674) is a luminous SBc (as classified by Williams & Rood 1987), while H96b (NGC 7675) is a giant E2, and H96c is a small Sa. H96d was classified by Hickson, Kindl & Auman (1989) as a dwarf Im, but as discussed in Sect. 3.5 we suggest its reclassification as Sm. NGC 7674 has been widely studied as a Seyfert galaxy, and classified as Seyfert 2 (Mirabel & Wilson 1984). We give a detailed study of this galaxy in Paper II where we discuss its possible Seyfert 1 nature.
The four galaxies show similar redshifts (see Table 2) with a mean heliocentric velocity of 8760 km s-1 and a velocity dispersion of = 160 km s-1.
Two long tails can be seen in Fig 1 emerging from the region between H96a and c. One extends more than to the NE and the other is cut by the edge of our frame at to the NW. A shorter tail ( ) is also present in the disk of H96a, N of its center. The beginning of a faint wide extension coming out from H96b toward the SW is detected in all 3 filters, and it is more clearly defined (as for the tails) after median filtering the images with a box size of . The effect of such a filtering can be seen for the V filter in Fig. 3a. This extension was not detected in previous observations of the group. It cannot be discarded that it extends outside the CCD field. There is also an indication of a bridge of matter between galaxies a and d, although deeper images would be needed in order to confirm it.
Fig. 3b shows a B-R colour index image of the group in a logarithmic grey scale representation. Galaxies H96a and d have blue colours, while b and c show redder ones. The measured and uncorrected colour indices of both long tails ((B-V) = 0.5-0.6) are consistent with those of the H96a disk ((B-V) = 0.51).
We have identified other galaxies in the field of Hickson 96, since many faint objects can be seen in Fig. 1. Radial profiles could be obtained for six of them, and allowed us to distinguish galaxies from stars by comparing the surface brightness profile of the objects detected in the field with that of a star (numbered as 0 in Fig. 1), which has a steeper light distribution. Six galaxies (numbered as 1-6 in Fig. 1a) were identified in this way, as shown in Fig. 4. We give their magnitudes and colour indexes in Table 3. No information on their redshifts is available, so we cannot say whether or not they belong to the group. Inspection of their colour indices indicates that galaxies 4 and 5 are quite red and therefore are probably background galaxies projected in the field. Galaxies 1,2,3 and 6 are possible dwarf galaxy members of Hickson 96. An excess of faint galaxies compared to the background has been reported by Carvalho et al. (1994) in Hickson groups. As is our case, the lack of velocity information prevents to state their membership to the group. Hunsberger, Charlton & Zaritsky (1996) report the presence of dwarf galaxies in tidal tails in Hickson groups, but this is not the case for the faint galaxies in Hickson 96.
Table 3. Magnitudes and colors of dwarf galaxies.
We have also investigated the neighborhood of Hickson 96. It is not located in a loose group or cluster (Rood & Struble 1994). After inspection of both the CfA catalog (Huchra et al. 1993) and NED 1 database we find that there is no galaxy with a magnitude comparable to that of H96a and H96b within 1 Mpc in distance and 1500 km s-1 in redshift. The closest galaxy, UGC 12630, is located at 780 kpc but has a magnitude of =15.4 mag, i.e. 1.5 mag fainter than H96a and b, and of the same order as H96c. The next galaxy with similar measured redshift is at a distance of 1.4 Mpc and has a magnitude of 16.5 (the same order as H96d). The remaining 7 objects (from NED database) within 1 Mpc do not have measured redshift and have fainter magnitudes than the members of the group, and probably correspond to background galaxies.
Fig. 5a shows the radial brightness profiles for H96a in the three filters as a function of the equivalent radius of the corresponding isophotal level. The signature of the Seyfert nucleus is visible at the central parts as a pronounced steepness that adds to the bulge and disk components. The humps at equivalent radii of and correspond to the spiral arms. The disturbed morphology of this galaxy, and its strong spiral arms prevent an accurate disk-bulge decomposition. We have fitted an exponential law to the disk and removed it from the profile. To the remaining emission a r law has been fitted. This procedure has been performed in an iterative way until convergence is achieved. The final decomposition gives in B colour
= 22.3 mag/arcsec2, = 13 7 and = 14.1 mag for the disk, and = 17.9 mag/arcsec2, = 0 5 and = 16.2 mag for the bulge.
The radial colour index profiles have been calculated from the individual B, V and R bands as follows. The images have been integrated over circular annuli with thickness of on the deprojected image of the galaxy (see below for the deprojection values). We have obtained the B-V and V-R colour indices from these values, and corrected them as explained in Sect. 2.1. A colour gradient from redder to bluer colours with increasing radii is seen (Fig. 6a). A colour gradient of 0.1 mg in B-V and in V-R is found in the disk of H96a. Disk gradients have been reported for spiral galaxies, but mostly studied in the NIR and have been assigned to extinction by dust in the blue bands (Peletier et al. 1994). H96a, although classified as unbarred in the RC3, was found to be barred by Williams & Rood (1987) from visual inspection of the POSS. We found for the bar a size of and a PA .
The B band image of H96a is shown in Fig. 7a in logarithmic grey scale representation. H96c is also seen separated by from H96a. We note that the outer isophotes of the disk in H96a are not centered on the nucleus but are shifted towards the side opposite H96c. In order to quantify this displacement we have fitted ellipses to a set of outer isophotes. Distorted portions of the isophotes closer to H96c have been excluded from the fit. One of the isophotes together with the fitted ellipse is plotted on the image. The fit shows, for all 3 filters, a displacement of the center of the disk of 0 3 to the SW, i. e. nearly perpendicular to the bar.
The outer isophotes in H96a are perturbed in the region closest to H96c. As noted in Sect. 3.1, a tail-like feature, already noticed by Arp (1966), can be seen in the three bands. It is located north of the H96a center. This tail has a size of and is bluer ((B-V)0 = 0.1) than the disk of the galaxy ((B-V)0 = 0.4). Opposite the nucleus and to the South, a similar, although less blue feature ((B-V)0 = 0.3), is observed. A smaller ( ) tail, also with colours similar to those of the disk, is seen to the NE of H96a.
H96a has two spiral arms that become broader with increasing radius. In Fig. 7b the arms are traced with dots, and we show also the contour up to which the galaxy can be considered bisymmetric. In order to enhance the small/intermediate scale structures we have constructed a "sharpened image" (Fig. 7c). We performed bayesian deconvolution with a softening parameter and subtracting one smoothed with , following the method developed by Molina et al. (1992). The bar in the center of NGC 7674 can be seen clearly in that image, as well as the beginning of both spiral arms. Figs. 7b and 7c show that the spiral arms start from the edges of the bar, and have a m symmetry out to r with a symmetry axis in the bar direction. At larger radii the arm in the vicinity of H96c disappears, and that extending to the SW of H96a is broadened.
From our images we obtain for H96a an inclination of to the line of sight and a PA of , assuming that the outer isophotes of the galaxy correspond to a pure disk component (Fig. 5a). The determination of the position angle could be however inaccurate since distortions of portions of the isophotes produced by the proximity of H96c prevent their use in the fit. The fact that the outer disk could be intrinsically non-circular due to interaction suggests the use of more internal isophotes. Those give a PA of and an i . However the shape of the isophotes for the inner disk is dominated by the spiral structure, and therefore should not be used for the deprojection of the galaxy.
The rotation curve in the direction joining the centers of H96a and H96c is shown in Fig. 8. It has been obtained from the high resolution spectrum that contains H , [SII] and the [NII] lines, with the cross-correlation technique proposed by Tonry & Davis (1979) using the central spectrum as template. The 0 value in abscissa corresponds to the continuum center position. Fig. 8a shows that the curve extends farther in the H96c opposite direction. We note that the kinematical center of the curve does not coincide with the emission one, both in H and continuum, but is shifted by in the direction of H96c, and has a velocity 30 km s-1 lower.
The gas seems to be perturbed in the outer parts, mostly toward H96c, where it shows increasing velocities. This area corresponds to the area where one of the arms gets disrupted. The rotation curve also shows several minima and maxima that correspond to the morphological structure of the galaxy. In fact, the position of the minima at - , - and from the kinematical center (Fig. 8a) coincide with the inter-arm regions. The points at - and - from the center correspond to two HII regions located in the spiral arm in the opposite direction of H96c.
Using both our rotation curve (PA = ) and those by Unger et al. (1987; PA = and ), we find that the kinematic axis of the galaxy must lie around a PA of . The best choice for the inclination of the galaxy is i = . These values for the PA and inclination are compatible with those derived from the analysis of the outer disk isophotes.
We have measured a semi-amplitude in the central parts of our rotation curve of about 60 km s-1. Taking into account the inclination of the galaxy, and the fact that the slit was placed at from the major axis, we obtain a semi-amplitude after deprojection of 200 km s-1, a normal value for the morphological type of this galaxy.
We have a also carried out observations of the CO(J = 2 1) and (J =1 0) rotational transition lines toward H96a with the 30-m telescope of the Institut de Radio Astronomie Millimétrique (IRAM) at Pico Veleta (Granada, Spain). We have obtained a total H2 mass of 2 1010 assuming a H2 to integrated CO flux ratio of 3.6 1020 cm-2 (K km s-1)-1 (Dickman et al. 1986). Both CO maps show enhanced emission that seems to be associated with the spiral arms, and the gas follows the overall rotation of the galaxy. A detailed description of these observations and results are given in a following contribution (hereafter Paper II), since the coverage includes only the central part of H96a.
In the image of the galaxy shown in Fig. 7b and c it can be seen that H96c shows a disk-bulge morphology with a two-armed spiral at the inner parts. One of the arms however disappears at 2 5 from the center of the galaxy in the direction of H96a. A bar could also be present, as suggested by the elongated structure seen in the sharpened image shown in Fig. 7c, but the high inclination of the galaxy together with its small size do not allow a sure identification.
The surface brightness profiles of H96c in the three filters are shown in Fig. 5b. The galaxy shows an excess of light produced by its spiral structure in an intermediate region ( ) and, although the bulge and disk can be clearly noticed in the profile, a reliable quantitative decomposition is not possible due its high inclination. Colour index profiles (Fig. 6b) have been obtained as for H96a, with the deprojection parameters given below. H96c shows a rather steep colour index gradient, becoming bluer at larger radii. In addition it has perturbed outer isophotes and two bluer protuberances are noted towards the direction opposite to H96a. Their colour indices are slightly bluer ((B-V) , (V-R) ) than those of the disk ((B-V) , (V-R) ).
Both the protuberances and the spiral structure of H96c make it difficult to determine accurately the isophote centers. However we have found no important isophotal off-centering either before the beginning of the spiral arms at re 1 7, or for the outer isophotes free from perturbations ( ). For larger radii the proximity of H96a prevents a fit. The ellipticity is constant in the central parts, with a value of 0.25, increasing to 0.4 in the outer parts, where only the disk is present. This shows the existence of an important bulge in the inner parts as expected for an early-type spiral and a possible hint of the presence of a central bar. We have used the outer unperturbed isophotes for the calculation of the deprojection parameters of H96c, obtaining an inclination of and a PA of .
The spectrum of the galaxy shows prominent absorption bands and lines, typical of an old stellar population with a large break at 4000Å. Following the indices by Pickles (1985) the galaxy has a spectral type between G0 and G8 and over-solar metallicity. Overlapped to the old stellar population the galaxy shows emission lines indicating that a burst of star formation is occurring as found by Laurikainen & Moles (1988). The emission lines like H and [NII] extend for about 6 kpc. In Fig. 8b we show the rotation curve obtained with the high resolution spectrum by using H , [NII] and [SII] lines. The direction of the slit was that of the line connecting the centers of H96a and H96c. The kinematical and geometrical centers are basically coincident within our spatial resolution. In fact, as can be seen when we symmetrize the curve, the section of maximum emission is located at 0 5 from the kinematical center. The curve is symmetric in the central parts, but some perturbations seem to occur for the outer ones. The relative velocities remain constant toward H96a, while they might decrease toward the opposite direction. The change in velocity, however, is within the error bars. The deprojected semi-amplitude of the velocity curve is 317 km s-1.
The galaxy H96b (NGC 7675), an early type object, is the second brightest galaxy in the group, and its center is located 2 3 to the SE away from the center of H96a. In Fig. 9a we show the azimuthally averaged surface brightness profiles obtained for the three photometric filters. The profiles of the outer regions are well fitted by a law. However in the central region, the galaxy exhibits sinusoidal deviation from the de Vaucouleurs profile with amplitude increasing towards the center. This is illustrated in Fig. 9b where we plot the difference between the observed brightness profile and the law obtained when fitting for distances larger than . Different effective radii have been obtained for each filter,
and = for B filter,
1 0 3 and =21.6 0.1 for V filter, and
0 0 2 and =20.5 0.1 for R filter.
The colour index profiles indicate that NCG 7675 becomes increasingly redder towards the center (Fig. 9c). The colour index images show a red structure that extends approximately along the major axis. (light central region in Fig. 3b) with an axial ratio of about 0.4. It could indicate the presence of dust.
We performed a quantitative analysis of the shape of the galaxy by least squares fitting of a set of isophotes to ellipses following the well known method proposed by Carter (1978) and widely applied by Bender et al. (1989). The results are displayed in Fig. 9d, where we plot the axial ratio as a function of the semimajor axis. Excluding radii smaller than 1 2, where the ellipticity is dominated by the seeing, there is a region between = 1 2 and where is nearly constant in all three filters with a value of . This region has a larger ellipticity than the rest of the galaxy, and is also the same region where the residuals with respect to the become more significant. This is opposite to the seeing effect, which makes constant ellipticity isophotes to appear rounder toward the center. Then an abrupt variation to rounder isophotes () is seen from to , to reach an almost constant value of at about from the center.
All the isophotes are concentric for all filters. Their position angle is nearly constant at PA for the inner and decreases to at . Its determination, however, becomes less accurate with increasing radius since the ellipticity is lower in the outer regions. The same applies when determining the fourth coefficient of the cosine term, A4, in the Fourier analysis of the residuals with respect to ellipses. Caon et al. (1990) determined that is necessary in order to obtain significant values for A4. In our case it was only possible to determine A4 out to from the center. We do not detect any significant trend until , and only in the region where changes rapidly is it noted that A4 becomes positive. For larger distances, however, this coefficient is not statistically significant in the Fourier expansion.
We have further analyzed the morphology of the galaxy (Fig. 10a) in two other ways. The first method was to smooth the image by filtering by a running median box of of side, and to subtract the result from the original image (Fig. 10b). The second approach was to subtract a model of a seeing convolved galaxy (Fig. 10c) with the parameters obtained from the external isophotes of the original galaxy, i.e. and PA . The subtraction is shown in Fig. 10d. In both cases there exist significant residuals in the central part of the galaxy, where we see a structure of 7 5 in size plus some residuals for the central , while the outer part of the galaxy vanishes, since it is well represented by a law. This inner feature is detected in the three filters.
A faint plume also appears when we analyze the median smoothed V image that is displayed in Fig. 3. It is also seen in the B and R filters. Another deep image of the group, but with a larger field of view, would be needed to characterize its properties, in particular its extension and colour indexes. A second tidal feature, a bridge of optical light, is also visible in the same image, joining H96b to H96d. It is also detected in the B image, and more strongly in the R image.
The spectra of the galaxy show only absorption lines with no signature of recent star formation. The velocities and velocity dispersion along the major and minor axes of H96b, were obtained by the Tonry & Davies (1979) method. The velocity, velocity dispersion and their errors were determined by cross-correlation of the spectra in each spatial section with those of 15 template stars. The final values for each spatial section were then obtained by weighting by the errors. In Fig. 11a and 11b we present the velocity curve along the slit for the major and minor axes respectively. For the minor axis, the radial scale was divided by the axial ratio in order to compare directly with the curve obtained along the major axis. The error bars in the figure correspond to the mean quadratic error for each point. As can be seen from the figure a rotation is observed in the major axis for the central where the S/N is high, and it amounts to 30-40 km s-1. The velocity curve is perturbed in the minor axis direction, being decoupled from the rest of the galaxy for the region within from the center.
The velocity dispersion profiles along the principal axes are shown in Fig. 11c, where the distance scale along the minor axis has been corrected as before. The agreement between the two axes has to be emphasized. In the central region the velocity dispersion may be considered roughly constant with a value of , but some structure appears. Then the velocity dispersion falls to about at a radius of and seems to increase again in the outer parts starting from a radius of from the center. The global behavior of the velocity dispersion profile, as well as the structure seen at the inner parts, seem to be real since they appear in the two axes and in spectra which were obtained in different conditions.
This galaxy, the bluest of all in the group ((B-V)0 = 0.17, (V-R)0 = 0.40), was previously classified as an irregular Im because only the knotty inner part was detected. However the outer isophotes that we detect here look quite symmetric and in fact the data are consistent with a late type spiral with an exponential disk. In Fig. 5c we present the surface brightness profiles in the three filters, and as it can clearly be seen from the figure, they agree well with an exponential disk with some perturbations due to the knots which are observable on the images. Assuming that the external isophotes correspond to an intrinsically circular disk we obtain for the position angle and inclination respectively and .
The innermost structure of H96d as seen in the three filters consists of 3 knots (named A, B and C in Fig. 12b), which are nearly aligned with the galaxy major axis. Knots A and B seem to be separated by a dust lane (Fig. 12a), seen as a lack of light in B, V and R, and with red colour indexes ((B-V)0 = 0.38, (V-R)0 = 0.53). Knot A shows redder colour indexes ((B-V)0 = 0.34, (V-R)0 = 0.50) than B and C, which have similar values ((B-V)0 = 0.18, (V-R)0 = 0.34). The eastern edge of knots B and C is surrounded by a bluer area ((B-V)0 = 0.00, (V-R)0 = 0.38). All of this indicates a very blue galaxy with a population of young stars. The redder colours of knots B and C and even more so of knot A are the consequence of the presence of very intense emission lines that contribute to the red part of the spectrum, reflecting a very recent burst of star formation.
For this galaxy we have two low resolution spectra at PA = and . In the first case the slit passes through knots B and C, but it was taken under bad weather conditions. For the second, the slit goes through the disk and covers basically the B and C knots and part of A. Since it was taken under not very good seeing it was not possible to spatially resolve the different regions. To derive physical parameters we added all the spatial sections along the slit. In this case we note the presence of a faint stellar continuum (Fig. 2d). The spectrum also shows a depression to the red of H due to a weak G-band. Since the 4000Å break is not clearly visible, the underlying dominant stellar population is earlier than G. This result is also supported by the presence of absorption for the hydrogen lines until H .
Over the stellar continuum we see emission lines of [OII], [OIII], [NII], [SII] and the Balmer lines H , H and H . In Table 4 we give the observed and de-reddened intensities of the lines. The values and the non existence of the [OI]6300Å line show that the gas is emitted by classical HII regions with a thermal origin. The value of 1.34 for the ratio of the [SII] lines corresponds to a density of 100 cm-3. By using the empirical relations by Pagel et al. (1979) and the curves by Edmunds & Pagel (1984) we found 10500 1500K and an abundance 1/3 solar. From these values we have calculated the absolute flux of the H continuum = 5.62 10-16 ergs-1 cm-2 A-1. For the adopted distance the total flux emitted by both knots B and C is 3.6 1039 ergs-1 and the Lyman photon flux is 6.7 1052 phs-1. Given the low value for the equivalent width for H , the observed star formation should have happened in a burst about 8 106 yrs ago. The effective temperature for the stars of the burst is 30000 K, corresponding to the of B0. These results indicate an important stellar formation in H96d. The condensations may represent different bursts of induced star formation that took place in this dwarf galaxy.
Table 4. Line parameters for H96d.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998