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Optical and CO Radio Observations of Poor Cluster Zwicky 1615.8+3505

The cluster Zwicky 1615.8+3505 is considered to be a dynamically young poor cluster. To investigate the morphology and star-formation activity of galaxies under the environment of a dynamically young poor cluster, we have performed V, R, and I surface photometry, optical low-resolution spectroscopy, and 12CO (J=1-0) line observations for member galaxies. Our data show that more than 90% of the observed galaxies show regular morphologies and no star-formation activities, indicating that the environment does not affect these galaxy properties. Among sixteen galaxies observed, only NGC 6104 shows a significant star-formation activity, and shows a distorted morphology, indicating a tidal interaction. This galaxy contains double knots, and only one knot possesses Seyfert activity, though the sizes and luminosities are similar to each other; we also discuss this feature.


Introduction
The cluster Zwicky 1615.8+3505 is a poor cluster cataloged in CGCG (Catalogue of Galaxies and of Clusters of Galaxies; Zwicky et al. 1961Zwicky et al. -1968 and not cataloged as a rich cluster in Abell et al. (1989), and is also known as N45-389 in the X-ray field (e.g., Price et al. 1991). The cluster contains a peculiar galaxy; NGC 6109 is a well-developed head-tail radio source, B2 1615+35; Venkatesan et al. (1994) have argued that a poor cluster with such a radio source has a large internal motion of members, which indicates being dynamically young. In the bottom-up scenario, a dynamically young poor cluster is an early stage in the evolution of structures in the Universe. In the distant Universe, many galaxies with a distorted morphology and with star-formation activity are observed (e.g., Williams et al. 1996;Smail et al. 1997;Dressler et al. 1999). Although the environmental effects on galaxies have been studied, they are complicated and not yet clear (e.g., Irwin 1995;Takeuchi et al. 1999). To investigate the morphology and star-formation activity of galaxies under the environment of dynamically young poor cluster, we performed optical and CO radio observations of member galaxies. In this paper, we also present the basic optical and CO data in order to discuss such a dynamically young poor cluster.
We describe the observations in section 2. In section 3 we present the results, and a discussion is given in section 4. The characteristics of the cluster are summarized in table 1.

Observations
We carried out optical surface photometry, long-slit optical spectroscopy, and 12 CO (J = 1 − 0) line observations. The observed objects are listed in table 2 and the observation log is given in table 3.
Optical surface photometry in the V , R, and I bands for thirteen galaxies was performed in 1995 May at the Kiso Observatory (hereafter KISO) using the 1.05-m F 3.1 Schmidt telescope equipped with a single-chip CCD camera at the prime focus. The CCD chip has 1000 × 1018 pixels and one pixel size corresponds to 0. ′′ 752, giving a field-of-view of 12. ′ 5 × 12. ′ 7. For all galaxies, we took three frames in each band; the exposure time of one frame was 20 min. By combining three frames, we obtained a set of V , R, and I images with an integration time of 60 min for each galaxy. In the data reduction, we used IRAF in the usual manner (IRAF is the software developed in National Optical Astronomy Observatories, USA). We also used SPIRAL (Hamabe, Ichikawa 1992) in sky-background subtraction.
To obtain the standard Johnson-Kron-Cousins photometry system, we converted the observed CCD counts using following equations: where v, r, and i are the observed magnitudes, defined as −2.5 log (count) +26.0 for 300 s-exposure frames; V , R, and I are the magnitudes in the standard system, and F (z) is the air-mass function.
The standard stars were chosen from a list by Landolt (1992). Nine coefficients c with subscripts were determined every night by analyzing the standard star frames using the IRAF package APPHOT. The derived values of the coefficients are given in table 4. Using the coefficients, we measured the throughput of the telescope, which is shown in appendix 1. The seeing size was around 4 ′′ for all bands and nights.
Long-slit optical spectroscopy for eight galaxies was carried out in 1995 April and May at the Okayama Astrophysical Observatory (hereafter OAO) using the 1.88-m reflector. A spectrograph with a grating of 150 grooves mm −1 blazed at 5000Å and a Photometrics CCD of 516 × 516 pixels was equipped at the Cassegrain focus. We binned the spatial direction by two, and resultant one pixel size corresponds to 1. ′′ 5. The dispersion was 4.9Å pixel −1 , and the wavelength range covered between 4600 and 7100Å. For each galaxy, we observed one position, putting a slit with a position angle of 90 • (east -west direction), centered on the galactic center. The slit length was 4 ′ , and we obtained a sufficient region to subtract the sky emissions well. The exposure time of one frame was 20 min. Except for galaxy No. 1 (CGCG 1612.2+3500), we observed three times for each object, and combined these frames; we then obtained images with an integration time of 60 min. In the data reduction, we used IRAF in the usual manner. We also used SNGRED, a semi-automatic reduction package for spectroscopic data, being operated on IRAF. The flux was calibrated using the flux-standard star frames (HZ 44 and Ross 640); we obtained the relatively flux-calibrated spectral frames. We also measured the throughput of the telescope (appendix 1). The seeing size was around 3 ′′ . 12 CO (J = 1 − 0) line observations for six galaxies were made in 1995 January and March, and 4 1996 April at the Nobeyama Radio Observatory (hereafter NRO) using the 45-m telescope. The FWHM of the beam size was 16 ′′ , which corresponds to 9.5 kpc in physical size at an assumed angular distance of 122 Mpc (see table 1). We made on-off switching observations with the on-position centered on the galaxy centers. Only for galaxy No. 3 (NGC 6104), we observed another position off the center. An acousto-optical spectrometer was used, which had a velocity coverage of 650 km s −1 with 2048 channels.
The temperature calibration was made with an absorbing chopper at 297 K in front of the receiver. A reduction system, NEWSTAR at NRO, was used for the data reduction. Figure 1 shows R-band images for thirteen observed galaxies. All of the frames cover 103 × 103 pixels or 77 ′′ × 77 ′′ field-of-view. The length of one side of the frame corresponds to 46 kpc in physical size from the distance of the cluster.

Morphology and size
The morphologies of the galaxies were determined using the R-band images (see figure 1). The second column of table 5 gives the morphology in the de Vaucouleurs-system and the third column gives the T index, as is in RC3 (de Vaucouleurs et al. 1991) with an error; inspections of morphology were made independently by two of the authors (A.T. and T.T.T.), and the error was estimated from the difference between the two inspections. Galaxy No. 3 (NGC 6104) is a peculiar galaxy having double knots, and can not be assigned the Hubble classification. We named knot A and knot B for the nuclear knot and the western knot, respectively (see figure 1a). Outer isophotal contours with µ R = 23.5 to 24 mag arcsec −2 show a water drop-like distorted shape, and a tail feature is seen from knot B to the lower right direction in figure 1a.
Except for this one peculiar galaxy, all galaxies have normal morphologies and are early-type galaxies.
We measured the shapes of the galaxies on the R-band images using the STSDAS package of ISOPHOTE. In measuring the shapes, we took a level of the R-band surface brightness of µ R = 23.5 mag arcsec −2 and fitted an ellipse. The CCD count for the surface brightness corresponds to about four-times the rms of the background noise in the V -and R-band frames, and about three times in the I-band frames. If taking B − R = 1.5, the surface brightness corresponds to µ B = 25.0 mag arcsec −2 , which has been used in many previous studies, such as in RC3.
The fourth, fifth, and sixth columns of table 5 give the major-axis diameter, ellipticity, and position angle of the major axis for the observed thirteen galaxies, respectively. The seeing blurring was not corrected in deriving the values. The errors were taken from an estimation given in the output through ISOPHOTE. Galaxy No. 10 (NGC 6109) was taken twice on different nights and in different CCD fieldsof-view. The difference in the two independent measurements is well within the error. This indicates that our observations and reduction procedures were successful.

Magnitudes and colors
In the following analyses of magnitudes and colors, we used frames after removing foreground stars using the IRAF task of IMEDIT. We measured the isophotal magnitudes and colors by integrating the region within the ellipsoid of µ R = 23.5 mag arcsec −2 . In table 6, the isophotal V -band magnitudes (V 23.5 ), V −R and V −I colors [(V −R) 23.5 and (V −I) 23.5 ] are presented in the second, third, and fourth columns, respectively. Galaxy No. 10 has two independent frames, and by inspecting the consistency between two measurements, we estimated the error of the isophotal magnitudes and colors to be 0.02 mag. The frame qualities for galaxies Nos. 12 and 14 were relatively more poor than others, and we estimated the errors for them to be 0.1 mag.
The asymptotic total V -band magnitudes (V T ) were measured following the descriptions in Photometric Atlas of Northern Bright Galaxies (Kodaira et al. 1990). We made growth curves of the Vband magnitudes and fitted the observed growth curves to the templates of the growth curves given in Kodaira et al. (1990). The templates are given for two categories divided by an axial ratio at µ B = 25.0 mag arcsec −2 of log R 25 = 0.30. We estimated the axial ratio as , where e is the ellipticity at µ R = 23.5 mag arcsec −2 , given in table 5. In most cases, we could successfully fit to templates with an appropriate morphology and axial ratio. If it was not successful, we used another template which fit the observed growth curve best. The fitting was made by two of the authors (A.T. 6 and T.T.T.), and the results were consistent with each other; we estimated the error of measurement to be 0.03 mag. Because the original frames for galaxies Nos. 12 and 14 were poor, the errors for them were estimated to be 0.1 mag. The fifth column of table 6 gives the measured V T .
The total asymptotic colors in V − R and V − I [(V − R) T and (V − I) T ] were measured following the descriptions in Buta, Crocker (1992). We used template growth curves in the B − V and U − B colors given in RC2 (de Vaucouleurs et al. 1976) for templates in the V − R and V − I colors, respectively, fixing the B-band effective radius r e . We estimated r e using observed effective radius in the V -band r e (V ), and the data of the template growth curves in the B − V color, depending on the morphology. We obtained log r e /r e (V ) to be 0.01 for galaxies earlier than Sa, 0.02 for Sa, 0.03 for Sab and Sb, and 0.04 for Sbc. The last column of table 5 lists the estimated r e ; The Hubble-type morphology for galaxy No. 3 (NGC 6104) is unknown, and in calculating r e for galaxy No. 3 we assigned T = 4, which is the latest among the other galaxies. The fitting was made by two of the authors (A.T. and T.T.T.), and the results were consistent with each other; we estimated the error of the measurement to be 0.03 mag. Because the original frames for galaxies Nos. 12 and 14 were poor, the errors for them were estimated to be 0.  Buta and Williams (1995); the correction considers the extinctions by our Galaxy and the galaxies themselves and the redshift effect (k-correction). RC3 gives the Galactic extinction in B-band A g for seven galaxies (galaxies Nos. 3, 6, 8, 10, 12, 14, and 15) of 0.00 or 0.01 mag. We neglected the Galactic extinction in the V , R, and I-bands toward the region of the cluster. The color excess in B − V due to internal extinction E(B − V ) was obtained by equation (63) (1995). Using the relation between R V and R(V − R) or R(V − I) given in Buta and Williams (1995), we obtained R(V − R) = 0.52 and R(V − I) = 1.20. 7 We did not correct the internal extinction for galaxy No. 3 (NGC 6104); the intrinsic Hubble-type is unknown and the extinction may be small, because of a small apparent inclination. Following Buta and Williams (1995), we obtained a k-correction by referring to the data given in Schneider et al. (1983); we took k(V − R) = 0.05 and k(V − I) = 0.05 for all galaxies. The last three columns of table 6 give the The error is estimated to be 0.1 mag, except for galaxies Nos. 12 and 14, 0.2 mag.
T diagram for the thirteen galaxies observed at KISO. Buta and Williams (1995) gives the standard colors for each galaxy morphology. It is found that all of the observed galaxies have intrinsic red colors, which correspond to those for galaxies earlier than or equal to Sa.
Except for galaxy No. 3 (NGC 6104), the spectra show few signs for emission lines, and the spectral energy distributions (SED) resemble those of early-type galaxies (e.g., Kennicutt 1992), which is consistent with the KISO color analysis mentioned in subsubsection 3.1.2. The strong sky emission line of the sodium D line caused a spurious line at about 5900Å in the reduced spectra, which is serious in spectra for galaxies Nos. 1, 7, and 14. The heliocentric radial velocity of galaxy No. 3 was derived to be 8313 ± 100 km s −1 after a correction of the Earth's motion; this is consistent with value in NED of 8 8382 ± 50 km s −1 .
Galaxy No. 3 (NGC 6104) has a complex spatial structure; the slit position was placed at the bar structure penetrating two knots. The region used for figure 3c was divided equally into three (5 pixels in OAO data, 7. ′′ 5, note that this is larger than the seeing size of 3 ′′ at OAO); from west to east, we call the regions knot B (western knot), knot A (nuclear knot), and Eastern Region, respectively. Figure 4 shows the spectra of (a) knot A, (b) Table 7 summarizes the characteristics of the Hα lines in three regions, and also gives the regional colors derived from the KISO data; we took circular regions with a radius of 3. ′′ 75 (5 pix) in the KISO data. At knot A, the Hα profile was decomposed into broad and narrow components by the fitting with two Gaussian curves. The Hα luminosities were calculated from the observed Hα surface brightness of the narrow component multiplied by the areas of the circular regions with a radius of 3. ′′ 75.
In other galaxies, we estimated the upper limits of the Hα fluxes. Assuming that the observed Hα line width of 7Å, which is the case for the Hα narrow component in galaxy No. 3, and upper limit of peak Hα emission is three-times the rms of noise, we calculated the upper limits of the surface brightnesses of the Hα emission. Multiplying by slit aperture areas for the spectra of figure 3, we obtained the upper limits of the Hα luminosities, though the slit aperture did not cover the entire galaxy. Table 8 lists the upper limits; following Kennicutt (1983), we derived the upper limits of the star-formation rates from the Hα data. 9 Figure 5 shows the resultant spectra of the 12 CO (J = 1 − 0) line for six observed galaxies. The observed positions were set to the galactic centers for all galaxies, except for galaxy No. 3, for which we also observed the Eastern Region, 7. ′′ 5 east of the nucleus as well as knot A (nuclear knot). The beam size of 16 ′′ , corresponding to 9.5 kpc, is about half the sizes of the galaxies listed in the fourth column of table 5; this means that the observed beams covered most parts of the galaxies. The center frequency was set to the redshifted 12 CO (J = 1 − 0) frequency, and the redshifts were obtained from the NED, which are listed in table 2. The spectra were smoothed every 64 channels, resulting in a velocity resolution of 21 km s −1 . The rms noise temperatures after smoothing are 3.9, 3.9, 3.3, 3.6, 3.6, 4.7, and Region is 8189 ± 100 km s −1 , which is not consistent with the radial velocity of the CO line, 8428 km s −1 , measured from figure 5b; the feature seems to be noise.

CO Observations
We measured the upper limit of the molecular mass. The molecular mass M (H 2 ) and observed CO intensity I(CO) have the relation where a is the CO-to-H 2 conversion factor, and here we take the Galactic value of 4.5 M • ⊙ (K km s −1 pc 2 ) −1 (e.g., Sanders et al. 1984;Bloemen et al. 1986;note Arimoto et al. 1996); Ω is a solid angle of the telescope beam, and we take 16 ′′ aperture, 4.726 × 10 −9 str; d is the luminosity distance, and we take 130 Mpc (see We assumed that a potential emission line has a width of 100 km s −1 . The rms in 100 km s −1 -bin, σ 100 , was estimated from the rms in figure 5 (in 21 km s −1 -bin), σ, as σ 100 = (21/100) 1/2 σ. We took the upper limit of the CO intensity as I(CO) lim = 3 × σ 100 [K] ×100 km s −1 . The upper limits of M (H 2 ) within a central 9.5 kpc-diameter aperture for observed six galaxies are summarized in table 8.

Though galaxy No. 3 (NGC 6104) is not cataloged in
Atlas of Peculiar Galaxies by Arp (1966), it is an Arp-like galaxy. Sofue et al. (1993)  about ten-times ours. The molecular clouds may exist in an outer region of the galaxy. We should note that the CO spectrum of NGC 6104 by Maiolino et al. (1997) has a poor signal-to-noise ratio, and they wrote that the baseline fitting was relatively poor.

Effect of Cluster Environment
Galaxy No. 10 (NGC 6109) is known as a well-developed head-tail radio source, B2 1615+35, and has been studied in the radio continuum and X-ray regions (Ekers et al. 1978;Burns, Gregory 1982;Burns et al. 1987;Feretti et al. 1995). Head-tail radio sources are usually found in rich clusters with a high density of intracluster medium (ICM) and a high speed of member galaxies relative to ICM, which make a high ram pressure. Venkatesan et al. (1994) have argued that a poor cluster with developed head-tail radio sources must have a high relative speed, which means that the cluster is dynamically young. Ulrich (1978) also discussed that this cluster consists of two subclusters.
Out of thirteen galaxies observed at KISO, all galaxies except for galaxy No. 3 have regular morphologies. Out of the sixteen galaxies observed either at KISO or OAO or NRO, all galaxies except for galaxy No. 3 do not show any star-formation activities. From the KISO data, the colors of the fifteen galaxies are what can be expected from their morphologies (see figure 2), and there is no peculiar surface-color distribution in each galaxy. These facts indicate that in this cluster the environment of a dynamically young poor cluster does not affect the morphologies and star-formation activities of the member galaxies. Vigroux et al. (1989) discussed that star-formation activities of member galaxies are activated at early stages of cluster dynamical evolution, by showing galaxies with recent star-formation activities in a poor cluster, Pegasus I. Though the X-ray luminosity of Pegasus I is similar to that of cluster Zwicky 1615.8+3505 (Giovanelli, Haynes 1985), five out of ten early-type galaxies show recent star formation in Pegasus I, and this is in contrast to our result in Zwicky 1615.8+3505. The star-formation activity may be independent of the cluster dynamical stage (see also Tomita et al. 1996). The compilation of more data of star formation properties in various cluster galaxies is necessary to confirm the above-mentioned result.

A Peculiar Galaxy, NGC 6104 (galaxy No. 3)
Only galaxy No. 3 (NGC 6104) shows significant star-formation activity among the observed galaxies, and has a star-formation rate of about 0.1 M • ⊙ yr −1 (see table 8). The galaxy has the bluest total color among the thirteen galaxies observed at KISO, indicating present star-formation activity as well. Only this galaxy has a peculiar morphology among the observed galaxies, as shown in figure 1a. The tail feature and distorted outer isophotal contours indicate that this galaxy is under a tidal interaction, or possibly a merger.
Both knot A and knot B have similar radial velocities to each other, which means that the galaxy surely has double knots, not by a chance alignment; the velocity difference between the two knots is 170 km s −1 . Only one (knot A) of the double knots shows Seyfert activity, though the sizes and luminosities of the two knots are similar to each other; the size is 7. ′′ 5 or about 4 kpc in diameter (note that this is larger than the seeing size of 4 ′′ at KISO), and in an aperture with a diameter of 7. ′′ 5, I-band magnitudes are 14.8 and 15.1 for knot A and knot B, respectively, and thus, I-band luminosities differ by a factor of 1.3. Knot B may be a giant H II region or hot spot at the end of the bar structure induced by the tidal interaction. It is also possible that knot B is a remnant of a possible merger. According to Sanders et al. (1988), merger events would generate AGN activity accompanying the starburst. If the two knots are remnants of pre-existing galaxies, a question arise as to why only one knot possesses Seyfert activity. The size and luminosity are similar to each other, and considering that knot A contains the Seyfert, knot A may not be more massive than knot B. In order to study the above issue further, we need higher spatial 12 resolution imaging observations so as to investigate whether the double knots are merger remnants.
We would like to thank Mamoru Saitō and Nagako Miyauchi-Isobe for valuable discussion. We are grateful to thank the OAO, KISO, and NRO staff members for their hospitality during our stay. Franz  (see table 4 for coefficients), N e = 0.88 N ′ e (AD conversion of the CCD camera was 0.88 electrons ADU −1 ). In calculating the expected photon number from objects, we referred to the data given by Fukugita et al. (1995); the band flux for a 0.0-mag object was calculated to be f Vega λ,eff (converted for 0.0 mag) × FWHM, and the energy of a photon for a band was calculated as hc/λ eff . We took the aperture of a complete circle with a diameter of 1.05 m, and then obtained throughputs of 29, 18, and 14% in the V , R, and I bands, respectively.
The throughput of the OAO 1.88-m telescope, including air absorption, was derived as follows.
We obtained a relation between the count rate of the spectrograph CCD at 6000Å on one pixel C We took the aperture of a complete circle with a diameter of 1.88 m; then, the throughput of the telescope-spectrograph system at 6000Å, including air absorption, turned out to be about 1.5%.      1.09 * The V -band magnitude and V − R and V − I colors within an isophotal ellipsoid of µ R = 23.5 mag arcsec −2 . The errors of measurements were estimated to be 0.02 mag for the magnitudes and colors, except for galaxies Nos. 12 and 14, the errors of which were estimated to be 0.1 mag.