Near infrared imaging spectroscopy of cD galaxy NGC1275

Abstract

We present H and K band imaging spectroscopy of NGC 1275 taken with the MPE 3-D instrument which show an enormous variety of diagnostics which can be exploited to further constrain the star formation and excitation mechanisms in the galaxy's nucleus. These include H_2 lines from 1-0, 2-1, and 3-2 transitions, Br#gamma#, "1"2CO bandheads, [Fe II], He I, [Fe III], and Br#delta# lines. We match the observed spectra with synthetic spectra obtained by combining stellar, AGN and hot dust emission. At the nucleus, the infared light from stars is only 10% of that due to AGN and hot dust emission, whereas 1 kpc from the nucleus, the light is dominated by stars. The H_2 emission is strongly peaked at the nucleus, but shows broad, low level extensions. We attribute this geometry to two separate sources of ionization in NGC 1275: the AGN nucleus, which excites the vast majority of the radiation, and which produces the compact emission core seen in our maps, and the extended region of moderate star formation which excites the highly extended, weak H_2 envelope via shocks. The total H_2 line emission in H and K bands is equivalent to L_H_2#approx#8x10"4"0 erg s"-"1. The H_2 emission shows a rotation of 75#+-#15 km s"-"1 about the #approx#SW-NE axis at a distance of 1'' (#approx#350 pc) from the nucleus. The Br#gamma# emission is highly asymmetric both in line profile and in spatial distribution. The spatial elongation is coincident with the PA=160 angle of the Fanaroff-Riley type I radio lobes, suggesting that shocks from the jets produce star formation along their axis. The HeI_2_._0_6/Br#gamma# ratio suggests that the mean ionizing stars are late type O, although this depends on the nuclear AGN emission and several other factors. The [Fe II] emission dominates the H band spectrum and consists of a broad (1060 km s"-"1) as well as narrow (290 km s"-"1) component. The CO_s_p index shows a ''hole'' at the NGC 1275 nucleus due to the extra contributions of hot dust and AGN emission to the continuum. When the CO_s_p index is undiluted, the result is again consistent with an underlying K5III stellar population. The H_2 line-to-continuum map shows a tongue of emission extending #approx#3'' West of the nucleus, which is aligned with the CO emission ring seen in mm wave maps. (orig.)Available from TIB Hannover: RN 9303(348) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman