The galaxy NGC 3310, located in the vicinity of the Ursa Major cluster, lies at a distance of $\sim$ 13 Mpc and is classified as SABbc(r)pec (de Vaucouleurs et al. 1991). Van der Kruit & de Bruyn (1976) have investigated its group membership and its environment and have concluded that it has no companions. Its main global properties are listed in Table 1. NGC 3310 is a relatively small system undergoing a strong starburst (Telesco & Gatley 1984; Smith et al. 1996). The optical morphology is illustrated in Fig. 1. The bright inner region is dominated by a two-armed open spiral pattern in H $\alpha$ (van der Kruit & de Bruyn 1976; Balick & Heckman 1981; Mulder & van Driel 1996). The inner part of this well-developed pattern connects to a $\sim$ $900~\mbox{pc}$ diameter starburst ring, surrounding the blue compact nucleus. The circumnuclear regions show a moderately low metallicity, whereas the nucleus has solar abundances (Pastoriza et al. 1993). The Far UV and B-band surface brightness profiles of NGC 3310 are very similar and seem to follow an R^1/4 law outside the inner starburst ring (Smith et al. 1996). The outer parts of NGC 3310 are dominated by the so-called "bow-and-arrow'' structure (Walker & Chincarini 1967). The diffuse "bow'' at the western side is a ripple (at $\sim$ 6 kpc from the center) consisting primarily of late-type stars, possibly debris from an accreted disk (Schweizer & Seitzer 1988). A second fainter ripple can be seen farther out to the north-west. The "arrow'' (extending from 4 to 9 kpc from the center to the north-west) is a chain of bright knots consisting of stellar clusters most likely containing young massive stars and has a similar age as the central starburst ring (Smith et al. 1996). The "bow-and-arrow'' structure has been interpreted as a one-sided jet emanating from the nucleus (Bertola & Sharp 1984) or as the result of the accretion of a small gas-rich galaxy (Balick & Heckman 1981; Schweizer & Seitzer 1988; Mulder et al. 1995; Smith et al. 1996). Besides the "arrow'', other knots with recent star formation are seen at the northern and southern part of the disk (their Fig. 1 Van der Kruit & de Bruyn 1976).

Table 1: Properties of NGC 3310.

Quantity Units NGC 3310 Ref.¹

Names UGC 5786

ARP 217

PGC 31650

Hubble type SABbc(r)pec V91

$\alpha_{1950}$ $10^{\rm h}35^{\rm m}40.3^{\rm s}$ V91

$\delta_{1950}$ $53^{\rm d}45^{\rm m}45^{\rm s}$ V91

distance (adopted) Mpc 13.3

D₂₅ arcmin $3.1\pm0.1$ V91

kpc $11.9\pm0.4$

$B_{\rm T}^{0}$ mag 10.92 V91

L_B² $10^{10}\ {L}_{B,\odot}$ 1.18

$(B-V)_{\rm T}^{0}$ mag 0.32 V91

$L_{\rm FIR}$ $10^{10}\ {L}_{\rm\odot}$ 1.10 S96

$SFR_{\rm global}$ $M_{\rm\odot}$ yr^-1 8.5 S96

inclination angle (H $\alpha$ ) degrees $32\pm6$ K76

inclination angle (H I) degrees $52\pm2$ M95

position angle (H $\alpha$ ) degrees $172\pm4$ K76

position angle (H I) degrees $163\pm3$ M95

**Table 1:** Properties of NGC 3310.
Quantity	Units	NGC 3310	Ref.¹
Names		UGC 5786
		ARP 217
		PGC 31650
Hubble type		SABbc(r)pec	V91
$\alpha_{1950}$		$10^{\rm h}35^{\rm m}40.3^{\rm s}$	V91
$\delta_{1950}$		$53^{\rm d}45^{\rm m}45^{\rm s}$	V91
distance (adopted)	Mpc	13.3
D₂₅	arcmin	$3.1\pm0.1$	V91
	kpc	$11.9\pm0.4$
$B_{\rm T}^{0}$	mag	10.92	V91
L_B²	$10^{10}\ {L}_{B,\odot}$	1.18
$(B-V)_{\rm T}^{0}$	mag	0.32	V91
$L_{\rm FIR}$	$10^{10}\ {L}_{\rm\odot}$	1.10	S96
$SFR_{\rm global}$	$M_{\rm\odot}$ yr^-1	8.5	S96
inclination angle (H $\alpha$ )	degrees	$32\pm6$	K76
inclination angle (H I)	degrees	$52\pm2$	M95
position angle (H $\alpha$ )	degrees	$172\pm4$	K76
position angle (H I)	degrees	$163\pm3$	M95

1.: Acronyms; K76 - van der Kruit (1976), V91 - de Vaucouleurs et al. (1991), M95 - Mulder et al. (1995) and S96 - Smith et al. (1996).
2.: The B-band magnitude is converted to luminosity in solar units using ${M}_{B,\odot} = 5.48$ .

$\begin{figure} \par\includegraphics[width=8cm,clip]{H2758F1.ps} \end{figure}$

Figure 1: An H $\alpha$ image (taken by H.C. Arp) overlayed on a negative optical B-band image (both images are from van der Kruit & de Bruyn 1976). The length of the N-W indicators is 1 arcmin (= 3.9 kpc).

The ionized gas shows large streaming motions (up to 60 km s^-1) along the arms (van der Kruit 1976; Grothues & Schmidt-Kaler 1991), which are consistent with the presence of a strong density wave (van der Kruit 1976; Mulder & van Driel 1996). The H $\alpha$ rotation curve in the nuclear region is one of the steepest rising rotation curves found in spiral galaxies (van der Kruit 1976; Grothues & Schmidt-Kaler 1991). Another peculiarity is the offset between the dynamical center and the stellar nucleus of $(96\pm19)$ pc (Walker & Chincarini 1967; van der Kruit 1976; Balick & Heckman 1981).

Radio continuum maps (van der Kruit & de Bruyn 1976; Balick & Heckman 1981; Duric et al. 1986) show extended, remarkably bright synchotron emission from the inner regions and strong sources coincident with the giant H II regions in the arms, in the inner ring, and in the nucleus. The strong IR emission correlates well with both the continuum and H $\alpha$ emission (Telesco & Gatley 1984).

An earlier study of the neutral hydrogen (Mulder et al. 1995) revealed an H I extension of the optical "arrow'' out to $\sim$ 24 kpc and extended H I at $\sim$ 37 kpc to the south of the nucleus with no optical counterpart. Another peculiarity was a "hole'' in the H I distribution centered on the south-eastern part of the optical ring (SN 1991N is situated near the center of this "hole''). The molecular gas shows a clumpy distribution and is primarily associated with the spiral arms (Kikumoto et al. 1993; Mulder et al. 1995). In the nucleus it is less clear: a small amount of molecular gas is not ruled out. The total H₂ mass, obtained using the Galactic CO-H₂conversion factor, is $\sim$ $2\times10^{8}~M_{\rm\odot}$ (Kikumoto et al. 1993; Mulder et al. 1995), a value typical for late-type galaxies.

In summary, many features of NGC 3310 - the unusual morphology, the starburst features, the streaming of ionized gas along the arms, the offset between the dynamical center and the nucleus, the ripples in the outer parts, the "arrow'' and its H I counterpart - indicate that some major disturbance has affected gas and stars and has led to massive star formation. These features together with the Far UV and B-band R^1/4 surface brightness profiles indicate that NGC 3310 may well be the result of a merger event.

New H I data, obtained with the Westerbork Synthesis Radio Telescope (WSRT) and combined with those of Mulder et al. (1995) are presented here. They give additional information concerning the origin of the starburst and of the optical and H I peculiarities.

Table 2: WSRT Observing Parameters.

Observation A

                12 hrs, 36 m 16 July 1997

                12 hrs, 72 m 11 June 1997

B

                12 hrs, 72 m 15 Jan. 1987

Field centers A

                 $\alpha_{1950}$ $10^{\rm h}35^{\rm m}40.00^{\rm s}$

                 $\delta_{1950}$ $53^{\rm d}43^{\rm m}0.01^{\rm s}$

B

                 $\alpha_{1950}$ $10^{\rm h}35^{\rm m}40.1^{\rm s}$

                 $\delta_{1950}$ $53^{\rm d}45^{\rm m}49^{\rm s}$

Central velocity (km s^-1) A 970.00

B 1000.00

Baselines (m) A 36:2736:36

B 36:2700:72

Bandwidth (MHz) 5

Number of channels 63

Channel separation (km s^-1) 16.6

Velocity weighting A Uniform

B Hanning

Synthesized beam (FWHM) C $14\hbox{$.\!\!^{\prime\prime}$ }2\times17\hbox{$.\!\!^{\prime\prime}$ }7$

Velocity resolution (km s^-1) (FWHM) C 33.3

Noise level (1 $\sigma$ ) (mJy beam^-1) C 0.8

(K) 1.92

Observation	A
12 hrs, 36 m		16 July 1997
12 hrs, 72 m		11 June 1997
	B
12 hrs, 72 m		15 Jan. 1987
Field centers	A
$\alpha_{1950}$		$10^{\rm h}35^{\rm m}40.00^{\rm s}$
$\delta_{1950}$		$53^{\rm d}43^{\rm m}0.01^{\rm s}$
	B
$\alpha_{1950}$		$10^{\rm h}35^{\rm m}40.1^{\rm s}$
$\delta_{1950}$		$53^{\rm d}45^{\rm m}49^{\rm s}$
Central velocity (km s^-1)	A	970.00
	B	1000.00
Baselines (m)	A	36:2736:36
	B	36:2700:72
Bandwidth (MHz)		5
Number of channels		63
Channel separation (km s^-1)		16.6
Velocity weighting	A	Uniform
	B	Hanning
Synthesized beam (FWHM)	C	$14\hbox{$.\!\!^{\prime\prime}$ }2\times17\hbox{$.\!\!^{\prime\prime}$ }7$
Velocity resolution (km s^-1) (FWHM)	C	33.3
Noise level (1 $\sigma$ ) (mJy beam^-1)	C	0.8
(K)		1.92

1 Introduction