2 Properties of NGC 7673

The overall structure and kinematics of NGC 7673 were discussed by Homeier & Gallagher (1999, hereafter HG), who argued that, despite the disturbed optical appearance of this system, it is a relatively unperturbed, rotating disk galaxy seen nearly face-on.

Initial investigations of NGC 7673 highlighted its disturbed and "clumpy'' optical appearance (Markarian & Lipoveteski 1971; Börngen & Kalloglian 1975; Casini & Heidmann 1976), and earned it a classification as a "clumpy, irregular'' galaxy. The spiral pattern that is prominent in our images also was noted in earlier studies (de Vaucouleurs et al. 1976; Huchra 1977; Coupinot et al. 1982). Optical spectra revealed the presence of a broad emission component underlying the narrow lines more commonly associated with H II regions, and a remarkably constant radial velocity across the galaxy (Duflot-Augarde & Alloin 1982; Taniguchi & Tamura 1987; Homeier & Gallagher 1999).

H I observations of NGC 7673 and its companion NGC 7677 ( $v_{\rm r} =$ 3554 km s^-1) show that they have extended gas disks (Nordgren et al. 1997). NGC 7673 is apparently face-on and fairly regular, with a mild asymmetry on the western edge; NGC 7677 has a small outer extension pointing towards NGC 7673. The H I mass of NGC 7673 is $4 \times 10^{9}~M_{\odot}$, the dynamical mass (rather uncertain due to the low inclination) inferred from this line profile is $3 \times 10^{10}~M_{\odot}$, with the resulting fractional H I mass to total mass of 2% (Pisano et al. 2001).

The starburst activity is located within the inner half of the optical radius, concentrated in large complexes, the "distinctive clumps'' in this galaxy (see Fig. 4). The outer disk is relatively smooth and nearly circular, but with ripples and wisps, likely remnants of a past interaction that could have triggered the starburst. The overall distribution of optical light, however, is highly asymmetric, with a rotational asymmetry value of A=0.60 (Conselice et al. 2000a). There is an optical shell $\sim$21 kpc west of the main body (Dettmar et al. 1984; Homeier & Gallagher 1999); such features most frequently are seen around spheroidal systems, but an optical shell is also present in the post-merger starburst galaxy NGC 3310 (Mulder & van Driel 1996). NGC 7673 contains two strong non-thermal radio emitting regions characteristic of type II SN, indicating intense star formation over the last $\sim$ $3 \times 10^{7}$ yr (Condon & Yin 1990).

Due to its starburst, NGC 7673 is bright in wavebands that are sensitive to populations of young, massive stars, including the radio (Condon & Yin 1990), far infrared (Klein et al. 1986; Hunter et al. 1989), optical emission lines (Gallagher et al. 1989; Guzmán et al. 1997), and ultraviolet (Benvenuti et al. 1982; Kinney et al. 1993). Its integrated SFR derived from the H$\alpha$ and far infrared luminosities is 10-20 $M_{\odot}$ yr^-1 for a Kroupa (2002b) parameterization of the stellar initial mass function extending from 0.07 to 100  $M_{\odot}$ (see Sect. 8.4). Selected parameters of NGC 7673 are summarized in Table 1.

 

 

Table 1: Parameters of NGC 7673.

Parameter	Value	Reference
$\alpha$, $\delta$ (J2000)	23:27:41.2, +23:35:21	Falco et al. (1999)
Redshift	3408 km s-1	Huchra et al. (1999)
Distance	49 Mpc	redshift and H0 = 70 km s-1 Mpc-1
HI Mass	$4.09 \pm 0.03 \times 10^{-9}~M_{\odot}$	Pisano et al. (2001)
$L_{\rm H\alpha}$	$26.3 \times 10^{41}$ ergs s-1	Gallego et al. (1997)
$m_{\it U}$	12.84	de Vaucouleurs et al. (1991)
$m_{\it B}$	12.87	de Vaucouleurs et al. (1991)
$m_{\it V}$	13.11	Huchra (1977)
MB	-20.7	above with E(B-V)= 0.04
IRAS 12 microns	0.14 $\pm$ 0.049 Jy	Soifer et al. (1989)
IRAS 25 microns	0.52 $\pm$ 0.042 Jy	Soifer et al. (1989)
IRAS 60 microns	4.98 $\pm$ 0.047 Jy	Soifer et al. (1989)
IRAS 100 microns	6.66 $\pm$ 0.321 Jy	Soifer et al. (1989)
$L_{{\rm FIR}}$	$6.3 \times 10^{43}$ erg s-1	IRAS 60 & 100 micron flux after Hunter et al. (1989)