3 Results for H I detected galaxies

In this section we briefly describe the H I morphology of detected galaxies, and the remarkable features concerning galaxy dynamics as revealed by the H I.

IC3913, KUG1255+275, and Mrk057

These three galaxies were detected in the SW of Coma, between 17$^{\prime}$ and 25$^{\prime}$ south of the cD galaxy NGC4839. They display several common features: they are H I rich galaxies with regular gas distributions, where H I disks extend well beyond the optical (Figs. 1a, 2 and 3a).

Figure 1a: H I density distribution of IC 3913, superposed on a DSS B-band gray scale image. The contours are 0.3 (2.5 $\sigma$), 1.3, 2.7, 4.0, and $5.4 \times 10^{20}$ cm-2. The FWHM is indicated by the circle, $30.7^{\prime \prime } \times 27.2 ^{\prime \prime }$.

Figure 1b: Intensity weighted mean velocity field of IC 3913. The optical center of the galaxy is indicated with a cross. The numbers indicate heliocentric velocity in km s-1. The FWHM is indicated by the circle, $30.7^{\prime \prime } \times 27.2 ^{\prime \prime }$.

Figure 2: H I density distribution of KUG 1255+275, superposed on a DSS B-band gray scale image. The contours are 0.3 (2.5 $\sigma$), 0.8, 1.4, 2.0, and $2.6 \times 10^{20}$ cm-2. The FWHM is indicated by the circle, $30.7^{\prime \prime } \times 27.2 ^{\prime \prime }$.

Figure 3a: H I density distribution of Mrk 057, superposed on a DSS B-band gray scale image. The contours are 0.6 (2.5 $\sigma$), 2.3, 3.4, 4.6, 5.7, 6.8 and $8.0 \times 10^{20}$ cm-2. The FWHM is indicated by the circle, $30.7^{\prime \prime } \times 27.2 ^{\prime \prime }$.

Figure 3b: Intensity weighted mean velocity field of Mrk 057. The optical center of the galaxy is indicated with a cross. The numbers indicate heliocentric velocity in km s-1. The FWHM is indicated by the circle, $30.7^{\prime \prime } \times 27.2 ^{\prime \prime }$.

Systemic velocities for these galaxies are between 7400 km s^-1 and 7650 km s^-1, close to the value of the NGC4839 group, 7339 km s^-1 (Colless & Dunn 1996). Because of their position, lying far from the cluster center and outside the X-ray emission, no strong interaction with the ICM is expected. Although these three galaxies are likely to be part of the infalling NGC4839 group (Neumann et al. 2001), we see no signs of a gravitational interaction either optically or in H I or the radio emission. Velocity fields of IC3913 and Mrk057 are shown in Figs. 1b and 3b respectively.

NGC4848

This Scd galaxy is one of the most interesting objects in our sample. NGC4848 is a blue disk galaxy showing a very intriguing H I distribution (see Fig. 4),

Figure 4: H I density distribution of NGC 4848, superposed on a DSS B-band gray scale image. The contours are 0.3 (2.5 $\sigma$), 1.0, 1.7, and $2.3 \times 10^{20}$ cm-2. The FWHM is indicated by the circle, $30.5^{\prime \prime } \times 27.5 ^{\prime \prime }$.

as we only detect gas in the northern edge of the galaxy, 19 $^{\prime\prime}$ (6.5 kpc) NW from the optical center. We are probably missing some H I flux previously detected by single dish observations in the center of the galaxy (Gavazzi 1989). This object was also observed in H${\alpha}$ by Amram et al. (1992) who reported a double peak in the H${\alpha}$ profile, with the northern H${\alpha}$ peak at the same position than the H I. Interestingly, the brightest HII regions are in the south, where the H I gas has been totally depleted. The star forming activity is also evidenced by 20 cm radio continuum emission above a level of 16 mJy. This emission shows an elongation to the NW which is resolved by the VLA FIRST Survey in two separated sources, one coincident with the optical center of NGC4848 and a secondary peak 12 $^{\prime\prime}$ NW, roughly at the same position than the H I. The CO imaging carried out by Vollmer et al. (2001a) shows the maximum of the emission coincident with the optical galaxy center and with the empty H I zone. A secondary CO peak is detected in the NW, roughly coincident with the H I and radio continuum emission. Lavezzi et al. (1999) reported a normal $M_{\rm H2}$ content ( $2.56 \times 10^9$ $M_{\odot}$), which gives a very low fraction of neutral to molecular gas ratio: $M_{\rm HI}$/ $M_{\rm H2}$ ${\sim}$ 0.17 (NGC4848 is H I deficient by a factor of around 10).

Mergers do not account for the H I morphology or the star formation activity in NGC4848, because no obvious companion is seen in the DSS optical image (Fig. 4). However, the more detailed B-band CCD imaging by Gavazzi et al. (1990) shows a ring-like structure and blue bright zones in the NW, where the H I and the secondary peaks of CO, H${\alpha}$ and 20 cm radio continuum are found. The hypothesis of a dwarf system crossing the NGC 4848 disk is explored by Vollmer et al. (2001a) but further observations, both optical and higher resolution H I imaging, are needed to confirm it. N-body simulations by the same authors suggest that NGC4848 has already gone through the cluster core, 4$\times$10⁸ yr ago, and is now moving away from the cluster. They conclude that re-accretion of some of the stripped gas could explain the star formation burst.

Mrk58

This Sb, blue disk galaxy is projected onto the X-ray emission, some 20$^{\prime}$ (${\sim}$400 kpc) SW of NGC4874, in the zone lying between the main cluster and the SW group. It is gas deficient by a factor of 3. Its H I map (Fig. 5) displays a considerably asymmetry,

Figure 5: H I density distribution of Mrk 058, superposed on a DSS B-band gray scale image. The contours are 0.2 (2.5 $\sigma$), 0.7, and $1.1 \times 10^{20}$ cm-2. The FWHM is indicated by the circle, $30.3^{\prime \prime } \times 27.6 ^{\prime \prime }$.

with the gas swept from the NE. The H I is observed at an offset position of ${\sim}$12 $^{\prime\prime}$(4 kpc) SW from the optical disk. The observed H I distribution could be explained if we consider the supersonic velocity of Mrk58 relative to the cluster, 1575 km s^-1 (the sound speed in Coma is estimated around 1460 km s^-1 by Stevens et al. 1999); under these conditions the interaction with the ICM may produce ram pressure stripping to enhance the density behind the galaxy (Stevens et al. 1999).

CGCG160-058 and CGCG160-076

These late-type spiral galaxies lie in the nearby northern vicinity of Coma, outside the X-ray emission. They are H I rich galaxies displaying regular gas distributions (Figs. 6a and 7a). As expected, considering their position relative to the cluster, no strong environmental effects are seen: their H I disks are larger than the optical, and the H I and optical centroids are coincident. Velocity fields are given in Figs. 6b and 7b.

CGCG160-086

Figure 6a: H I density distribution of CGCG 160-058, superposed on a DSS B-band gray scale image. The contours are 0.4 (2.5 $\sigma$), 1.6, 3.2, 6.4, and $9.6 \times 10^{20}$ cm-2. The FWHM is indicated by the circle, $30.5^{\prime \prime } \times 27.5 ^{\prime \prime }$.

Figure 6b: Intensity weighted mean velocity field of CGCG 160-058. The optical center of the galaxy is indicated with a cross. The numbers indicate heliocentric velocity in km s-1. The FWHM is indicated by the circle, $30.5^{\prime \prime } \times 27.5 ^{\prime \prime }$.

Figure 7a: H I density distribution of CGCG 160-076, superposed on a DSS B-band gray scale image. The contours are 0.2 (2.5 $\sigma$), 0.8, 1.6, 3.3, 6.6 and $9.9 \times 10^{20}$ cm-2. The FWHM is indicated by the circle, $29.9^{\prime \prime } \times 26.9^{\prime \prime }$.

Figure 7b: Intensity weighted mean velocity field of CGCG 160-076. The optical center of the galaxy is indicated with a cross. The numbers indicate heliocentric velocity in km s-1. The FWHM is indicated by the circle, $29.9^{\prime \prime } \times 26.9^{\prime \prime }$.

This Sb, blue disk galaxy, projected onto the SE outskirts of the X-ray emission drawn by ROSAT, was only marginally detected in the present survey. The galaxy is not resolved in H I, as shown in Fig. 8.

Figure 8: H I density distribution of CGCG 160-086, superposed on a DSS B-band gray scale image. The contours are 0.4 (2.5 $\sigma$), 0.9, and $1.3 \times 10^{20}$ cm-2. The FWHM is indicated by the circle, $35.2^{\prime \prime } \times 33.0 ^{\prime \prime }$.

We report an H I deficiency by a factor of ${\sim}$3, the H I disk appearing truncated in its periphery, but the low resolution does not allow to get a more conclusive picture.

IC 4040

This Sdm blue disk galaxy is projected near the very center of Coma, at only 15$^{\prime}$ (300 kpc) NE from NGC4874. Its H I distribution (see Fig. 9a) shows the H I contours compressed in the NW,

Figure 9a: H I density distribution of IC 4040, superposed on a DSS B-band gray scale image. The contours are 0.2 (5$\sigma$), 0.4, 0.7, 1.1, and $1.4 \times 10^{20}$ cm-2. The FWHM is indicated by the circle, $35.5^{\prime \prime } \times 34.1 ^{\prime \prime }$.

Figure 9b: Intensity weighted mean velocity field of IC 4040. The optical center of the galaxy is indicated with a cross. The numbers indicate heliocentric velocity in km s-1. The FWHM is indicated by the circle, $35.5^{\prime \prime } \times 34.1 ^{\prime \prime }$.

and an offset in position of 9 $^{\prime\prime}$ (${\sim}$3 kpc), with the H I emission SE of the optical galaxy. Our detection confirms the double peak detection reported by Gavazzi (1989) at 7600 km s^-1 and 7850 km s^-1. The velocity field for this galaxy is shown in Fig. 9b.

NGC4907

This Sb galaxy was only marginally detected in H I (Fig. 10).

Figure 10: H I density distribution of NGC 4907, superposed on a DSS B-band gray scale image. The contours are 0.2 (2.5 $\sigma$), 0.5, and $0.9 \times 10^{20}$ cm-2. The FWHM is indicated by the circle, $39.8^{\prime \prime } \times 34.5 ^{\prime \prime }$.

It is H I deficient by a factor of ten and presents a shrunken H I disk (not resolved by the VLA beam) which is smaller than the optical disk. Two facts make an ICM-ISM interaction the most likely explanation for this picture: the high radial velocity relative to the cluster ( $v_{\rm rel}=1179$ km s^-1), and the projected location of NGC4907, which puts it in a very high density region, 20$^{\prime}$ (400 kpc) NE from the cluster center.

KUG1258+287, FOCA195 and CGCG160-098

Figure 11a: H I density distribution of KUG 1258+287, superposed on a DSS B-band gray scale image. The contours are 0.3 (2.5 $\sigma$), 1.1, 2.2, 4.5, 6.8, and $9.0 \times 10^{20}$ cm-2. The FWHM is indicated by the circle, $30.2^{\prime \prime } \times 26.7 ^{\prime \prime }$.

Figure 11b: Intensity weighted mean velocity field of KUG 1258+287. The optical center of the galaxy is indicated with a cross. The numbers indicate heliocentric velocity in km s-1. The FWHM is indicated by the circle, $30.2^{\prime \prime } \times 26.7 ^{\prime \prime }$.

Figure 12a: H I density distribution of FOCA 0195, superposed on a DSS B-band gray scale image. The contours are 0.2 (2.5 $\sigma$), 0.9, 1.8, 2.6, 3.5, and $4.4 \times 10^{20}$ cm-2. The FWHM is indicated by the circle, $30.2^{\prime \prime } \times 26.7 ^{\prime \prime }$.

Figure 12b: Intensity weighted mean velocity field of FOCA 0195. The optical center of the galaxy is indicated with a cross. The numbers indicate heliocentric velocity in km s-1. The FWHM is indicated by the circle, $30.2^{\prime \prime } \times 26.7 ^{\prime \prime }$.

Figure 13a: H I density distribution of CGCG 160-098, superposed on a DSS B-band gray scale image. The contours are 0.3 (2.5 $\sigma$), 1.1, 2.2, and $4.4 \times 10^{20}$ cm-2. The FWHM is indicated by the circle, $30.2^{\prime \prime } \times 26.7 ^{\prime \prime }$.

Figure 13b: Intensity weighted mean velocity field of CGCG 160-098. The optical center of the galaxy is indicated with a cross. The numbers indicate heliocentric velocity in km s-1. The FWHM is indicated by the circle, $30.2^{\prime \prime } \times 26.7 ^{\prime \prime }$.

These objects are part of a group dominated by CGCG160-098 (see Biviano et al. 1996). As all the galaxies in Coma detected in H I outside the X-ray emission, this group shows a normal H I content and a regular gas distribution (Figs. 11a, 12a and 13a), with the H I more extended than the optical disk and coinciding with the optical position. Particular features of these galaxies are their systematic blue $m_{\rm UV}{-}b$ color (see Table 2 and Fig. 7 in Paper I), and their high radial velocity (between 8400 km s^-1 and 8880 km s^-1), indicating a fast accretion of the group towards the main cluster body. Normal rotation patterns are shown by the bluest objects KUG1258+287 and FOCA195 (see Figs. 11b and 12b). H I emission equivalent to $2.2 \times 10^{8}$ $M_{\odot}$ was detected at 1.4$^{\prime}$ W of KUG1258+287 but no optical counterpart is observed.

NGC4911

This is one of the two brightest spirals in Coma. The H I morphology of NGC4911 shows a shrunken disk with two central peaks and a normal surface density in the central region as predicted by ram-pressure stripping (Fig. 14a).

Figure 14a: H I density distribution of NGC 4911, superposed on a DSS B-band gray scale image. The contours are 0.3 (2.5 $\sigma$), 1.4, and $2.0 \times 10^{20}$ cm-2. The FWHM is indicated by the circle, $29.8^{\prime \prime } \times 27.7^{\prime \prime }$.

Figure 14b: Intensity weighted mean velocity field of NGC 4911. The optical center of the galaxy is indicated with a cross. The numbers indicate heliocentric velocity in km s-1. The FWHM is indicated by the circle, $29.8^{\prime \prime } \times 27.7^{\prime \prime }$.

A shift in position is shown between the H I and the optical, the former lying about 4 kpc south of the optical component. NGC4911 shows a very high total gas content, $M_{\rm HI}$ + $M_{\rm H2}$ ${\sim}$ $3.7 \times 10^9$ $M_{\odot}$ (Casoli et al. 1996), but only a small fraction ( $M_{\rm HI}$/ $M_{\rm H2}$ = 0.34) is in atomic form. Considering the CO observations by Lavezzi et al. (1999) this fraction is much lower: $M_{\rm HI}$/ $M_{\rm H2}$ = 0.22. The velocity field shows normal rotation of the inner disk (see Fig. 14b) along an axis oriented roughly SE-NW.

NGC4911, projected 20$^{\prime}$SE of NGC4874 ( $v_{\rm rel}$ ${\sim}$ 1000 km s^-1), is thought to be the dominant galaxy of a group that recently crossed the cluster core (Biviano et al. 1996). Vikhlinin et al. (1997) reported an X-ray filamentary structure in this region, crossing over NGC4911 and culminating at the position of NGC4921. This cool spot has recently been confirmed with XMM (Arnaud et al. 2001), who consider the possibility of gas stripped from an infalling group, but not directly produced in the Coma center. They found that part of the X-ray excess in this zone is due to NGC4911. No other galaxies of this group were detected in H I, in support of the stripping hypothesis. However, the fact that none of the 14 catalogued galaxies (Biviano et al. 1996) have been classified as spirals weakens this argument somewhat.

NGC4922

This is the best known case of a merger in Coma, and the only pair detected in this survey; it is located in the northern most field, 1.4$^{\circ}$ from the cluster center. It is very bright in radio (27.5 mJy) and the brightest IR source in Coma (Mirabel & Sanders 1988). NGC4922 consists of two merging galaxies, one spiral in the north and one early type galaxy in the south. We detected H I in emission and absorption. The H I shown in Fig. 15,

Figure 15: H I density distribution showing the emission feature of NGC 4922, superposed on a DSS B-band gray scale image. Strong H I absorption is present which attenuates the emission. The contours are 0.2 (2.5 $\sigma$), 0.4, and $0.5 \times 10^{20}$ cm-2. The FWHM is indicated by the circle, $57.8^{\prime \prime } \times 53.4^{\prime \prime }$.

centered in the spiral galaxy, is strongly attenuated by the absorption. An H I emission equivalent to $5.8 \times 10^{7}$ $M_{\odot}$ was detected south of the merging system, but no optical counterpart is evident from the DSS image.

NGC4921

This is the brightest spiral in Coma, located 24$^{\prime}$ (${\sim}$0.5 Mpc) SE from NGC4874. It roughly coincides with a second order X-ray peak (Vikhlinin et al. 1997) also detected with XMM by Arnaud et al. (2001) and Briel et al. (2001). NGC4921 presents a very peculiar picture in 21 cm: a double peaked H I disk (Fig. 16a) which is considerably smaller than the optical one,

Figure 16a: H I density distribution of NGC 4921, superposed on a DSS B-band gray scale image. The contours are 0.3 (2.5 $\sigma$), 1.2, 1.7, and $2.3 \times 10^{20}$ cm-2. The FWHM is indicated by the circle, $39.8^{\prime \prime } \times 34.5 ^{\prime \prime }$.

Figure 16b: Intensity weighted mean velocity field of NGC 4921. The optical center of the galaxy is indicated with a cross. The numbers indicate heliocentric velocity in km s-1. The FWHM is indicated by the circle, $39.8^{\prime \prime } \times 34.5 ^{\prime \prime }$.

with a strongly asymmetric distribution, with most of the neutral gas distributed along the SE spiral arm. The gaseous disk presents a striking shift in position of ${\sim}$25 $^{\prime\prime}$ (some 8 kpc) SE from the optical. The velocity field is shown in Fig. 16b.

NGC4921 is perhaps the best example in Coma where several mechanisms are present simultaneously. (a) The shrunken H I disk and the shift between optical and H I positions look like clear signatures of ram-pressure stripping, which is also supported by the supersonic velocity of the galaxy relative to the cluster, 1521 km s^-1. (b) NGC4921 shows the largest cross section in our sample, which could produce, by viscous stripping, a mass loss rate value up to 20 $M_{\odot}$yr^-1. Another feature supporting viscous stripping is the relatively low surface gas density in the central region of NGC4921, previously classified as anemic by van den Bergh (1976). (c) The NW zone, where the brightest HII regions are seen (Amram et al. 1992), appears depleted of H I, similar to the case of NGC4848. Furthermore, NGC4921 displays a high total gas content, $M_{\rm HI}$ + $M_{\rm H2}$ = $2.65 \times 10^9$ $M_{\odot}$, with only a small fraction (0.36) in atomic form, suggesting that H I is actively converted to molecular gas. Gas re-accretion in the NW may also be present, triggering the HII regions along the spiral arm. As this galaxy does not show any optical distortion, processes involving gravitational effects are unlikely to be important.

IC842 and IC4088

Figure 17a: H I density distribution of IC 842 superposed on a DSS B-band gray scale image. The contours are 0.3 (2.5 $\sigma$), 1.2, 2.5, 3.7, 4.9, 6.2, and $7.4 \times 10^{20}$ cm-2. The FWHM is indicated by the circle, $36.6^{\prime \prime } \times 35.4^{\prime \prime }$.

Figure 17b: Intensity weighted mean velocity field of IC 842. The optical center of the galaxy is indicated with a cross. The numbers indicate heliocentric velocity in km s-1. The FWHM is indicated by the circle, $36.6^{\prime \prime } \times 35.4^{\prime \prime }$.

Figure 18a: H I density distribution of IC 4088 and 3 neighbor dwarf systems, superposed on a DSS B-band gray scale image. The contours are 0.2 (2.5 $\sigma$), 1.0, 1.9, 2.9, 3.9, 5.8, 7.8, and $9.7 \times 10^{20}$ cm-2. The FWHM is indicated by the circle, $36.6^{\prime \prime } \times 35.4^{\prime \prime }$.

Figure 18b: Intensity weighted mean velocity field of IC 4088. The optical center of the galaxies are indicated with crosses. The numbers indicate heliocentric velocity in km s-1. The FWHM is indicated by the circle, $36.6^{\prime \prime } \times 35.4^{\prime \prime }$.

These galaxies lie in the far northern region of Coma, along the supercluster NE filament, and far from the X-ray emission. As expected, they display normal H I content and no effects of interaction with the ICM are seen (Figs. 17a and 18a). They also display normal rotation patterns (Figs. 17b and 18b). Three dwarf systems were detected in H I around IC4088, one of them ([GMP 83] 1866) lies at only 2$^{\prime}$ (${\sim}$40 kpc) north of the spiral, but no optical distortions are evident. The dwarfs show velocity dispersions between 43 km s^-1 and 173 km s^-1, and their H I masses range between 0.2 and $2.0 \times 10^9$ $M_{\odot}$. All the galaxies detected in this region, including the merger NGC4922, are likely part of a group falling towards to the cluster center (Paper I).

CGCG160-106

This blue disk galaxy is located in the SE outskirts of the X-ray emission. The ICM does not exert important effects on this galaxy because of the low ICM density at the galaxy position, the low value of $v_{\rm rel}$, and the small cross section. CGCG160-106 is not very H I poor (it is deficient by a factor of 1.5), and the H I morphology, barely resolved in Fig. 19,

Figure 19: H I density distribution of NGC 4926-A, superposed on a DSS B-band gray scale image. The contours are 0.3 (2.5 $\sigma$), 1.4, and $2.1 \times 10^{20}$ cm-2. The FWHM is indicated by the circle, $35.2^{\prime \prime } \times 33.0 ^{\prime \prime }$.

only shows a shrunken disk slightly bigger than the optical one.

We detect a clear shift in the H I position, lying 15 $^{\prime\prime}$ (some 5 kpc) SW from the optical disk, and an intriguing difference in velocity between the H I and optical: 6876 km s^-1 and 7188 km s^-1, respectively. Amram et al. (1992) observed this galaxy in H${\alpha}$ and reported a velocity of 7100 km s^-1 and an extension to the SW, coincident with the position of a dwarf companion separated by ${\sim}$20 $^{\prime\prime}$. We detect weak H I emission in the same zone, but higher resolution is needed to resolve the dwarf system.