Multi-wavelength View of the Central Region of NGC

Starburst ring is a common galactic feature in barred galaxies. The ring is an important tracer of the dynamics in the nuclei of host galaxies (e.g., Buta

Figure 1.12 The diagnostic diagram. The x- and y-axis are log([OIII]/Hβ) versus log( [NII]/Hα). The circles are the HII galaxies. The squares and the triangles are the Syefert galaxies and the LINERs, respectively. The star indicates the line properties of NGC 7552.

1986; Kohno et al. 1999; Schinnerer et al. 2000; Sarzi et al. 2007; Mazzuca et al. 2008; Espada et al. 2009), since the ring associates with dynamical resonances in a galaxy. NGC 7552 is well known for its circumnuclear star-burst ring (e.g., Forbes et al. 1994a, b; Schinnerer et al. 1997). The AGN activity is weak in this galaxy (Forbes et al. 1994b). Therefore it is possible to study the intrinsic properties of the circumnuclear region without any con-tamination of emission emitted from the AGN. In this section we introduce the main observational results in literatures of NGC 7552.

X-ray

The X-ray is detected in the nucleus of NGC 7552 (Charles & Phillips 1982) with the Einstein Observatory. NGC 7552 shows a point source in the X-ray observation (see Figure 1.13). The X-ray luminosity (LX = 8 × 10⁴⁰ erg s⁻¹) of NGC 7552 is ∼30 times larger than that of normal spiral galaxies such as M31. Weedman et al. (1980) suggested a starburst nuclear model for the nucleus of NGC 7714 can be accounted for the X-ray and radio properties of the nucleus of NGC 7552 rather than AGN (Charles & Phillips 1982). They compared NGC 7552 with the nuclear region of M82 (Rieke et al. 1980) and concluded that the ratio of 1415 MHz flux to the X-ray luminosity is identical to that of M82. This ratio can be explained by a few thousand supernova remnants (SNRs) scaling from typical galactic value.

Infrared

Infrared continuum are better at revealing bulk of stellar population than op-tical, which highlight luminous massive stars. However, massive stars make

Figure 1.13 Map of the Einstein Observatory observation. The images is 1 deg² field. The center of the image is NGC 7552. There are four detected sources. The data have been smoothed by a 32^## gaussian (Charles & Phillips 1982).

up only a small fraction of the total stellar mass. Infrared lines have less sub-ject to extinction, and the H2 traces hot molecular hydrogen. Schinnerer et al. (1997) presented JHK continuums, most of the prominent near-infrared K-band lines (Brγ, He, and H2 in emission and¹²CO, ¹³CO in absorption), and mid-infrared (N-band continuum) high angular resolution spectroscopic and image data of the nuclear region of NGC 7552. The observations were made by the ESO 3.5 m NTT on La Silla, Chile. The circumnuclear ring is resolved in the observations.

Their spectral synthesis of individuals knots in the ring suggested the existence of young stellar clusters. The analysis also shows that the starbursts in the prominent knots have age of ∼1.5 × 10⁷ year and the starburst activity is decaying in the ring. Furthermore, the bar within bar system is seen in the central region of the galaxy in the IRAC2 H-band image (see Figure 1.14).

Forbes et al. (1994b) also suggested a inner bar by their H2 observation. The inner bar-like structure is perpendicular to the primary bar.

Optical

Feistein et al. (1990) presented an optical morphology study of NGC 7552. It is the first work linking the observational galactic features with the dynamical resonances. First of all, there is a prominent bar and two dominant outer arms (see left panel in Figure 1.15). Secondly, there is a giant HII region located in the eastern bar is shown in Hα image (labeled as ”E” in the right panel of Figure 1.15). The HII region also contributes to the radio continuum (3-cm and 6-cm) emission in later observations, indicating the vast amount of supernova explosions. Feistein et al. (1990) suggested that

Figure 1.14 IRAC2 H-band map shows the presence of a bar within a bar.

The inner bar is perpendicular to the primary bar, which is in horizontal direction of the image ( Schinnerer et al. 1997).

the HII regions probably define the CR radius. Thirdly, the center of the galaxy is surrounded by a ring. The ring emits most of the far infrared flux.

The ring may suggests the location of ILR of NGC 7552.

The Hα and optical continuum observations made by Cerro Tololo Inter-American Observatory (CTIO) in Chile are showed in Hameed & Devereus (1999) (see Figure 1.16). The observations were made in 24 October 1997 with exposure time of 900 seconds. Hameed & Devereus (1999) suggested that the continuum images of NGC 7552 reveal a uncataloged dwarf galaxy at the end of the northern spiral arm appearing to be interacting with the larger galaxy. However, there is no detectable Hα emission from the dwarf companion.

CO Observation

Claussen & Sahai (1992) observed the ¹²CO (1-0) emission with Swedish-ESO15m Submillimeter Telescope (SEST) from NGC 7552. The intensity of

12CO (1-0) in the nucleus is five times larger than the rest of the galaxy, and ICO is stronger along the major axis than that of the minor axis, indicating a gaseous bar along the stellar bar.

Centimeter Radio Continuum Observation

Forbes et al. (1994a) discovered a circumnuclear ring with a size of only 1 kpc in 3-cm and 6-cm radio continuum emissions. The observation were carry out by the ATCA. There are five knots in the ring in 3-cm image, and two northerly knots have the strongest intensity (Figure 1.17). The ring is presented in both NIR and optical color maps (Forbes et al. 1994a). The

Figure 1.15 The left panel is the isophotal optical image of NGC 7552 (Feis-tein et al. 1990). The inner and outer white contours are 20.5 and 23.0 mag arcsec ⁻¹, respectively. North is at left. The arrow points the reference star for polarimetry. The field is 154^## × 251^##. The right panel is the Hα of NGC 7552 (Feistein et al. 1990). The orientation is the same as the left panel.

The E is indicated with an arrow (See details about E in text).

Figure 1.16 Red continuum and continuum-subtracted Hα image. The white bar in the bottom indicates 1 kpc (Hameed & Devereus 1999).

ring can also be seen in its Hα and Brγ line emission (Forbes et al. 1994b).