Dissecting the Universe's Great Power Stations
August 31, 2006
The powerful and distant quasar 3C 249.1 has one of the largest extended emission-line regions (EELRs) known, extending out over tens of kiloparsecs of surrounding space. Astronomers Hai Fu and Alan Stockton of the Institute for Astronomy (University of Hawai'i) recently constructed velocity maps of the gas in the EELR around the quasar (which is at a redshift of z=0.31) using the Gemini Multi-Object Spectrograph's integral field unit (IFU) on Gemini North. Using diagnostic emission-line ratios, they were able to map the electron temperatures and densities of the extended emission gas and study the mechanisms exciting the gas.
The EELR of 3C 249.1 exhibits rather complex global kinematics. Its gas velocities range from -400 to +600 km/sec and some knots have velocity widths as broad as 600 km/sec (FWHM) and greater. The total mass of the EELR (derived from the Hβ luminosity and electron density) of the ionized gas is a staggering 109 Msun. Fu and Stockton estimate that the bulk of the kinetic energy associated with the EELR is about 2.5 x 1057 ergs and the momentum of the moving gas represents about 1050 dyne-second. A relatively short dynamical timescale of ~10 Myr is inferred for an average gas velocity of 500 km/sec
One process that could be speeding up the gases in the massive EELR is a superwind generated by supernovae. These stellar explosions would be expected in regions where sudden bursts of massive star formation had taken place earlier in the object's history. In the case of quasar 3C 249.1, a maximum star-formation rate (SFR) ~ 30 Msun per year can be derived from various luminosity indicators. However, the observed mass outflow is an order of magnitude larger than the expected mass injection rate from the many supernovae that would result if we had a SFR ~ 30 Msun per year. Hence a collective supernovae-driven flow is energetically insufficient.
Instead, the authors suggest a mechanism in which the EELR outflow is directly driven by the quasar. Radiation from the quasar can, they argue, couple to the surrounding gas via various processes such as electron scattering and photoionization to accelerate the gas. The input momentum rate from radiation pressure is related to the mass accretion rate of the black hole. The accretion rate describes how much mass and how fast the central supermassive black hole is gobbling the surrounding stars and gas. In the case of 3C 249.1, a mass accretion rate of 2.5 Msun per year would be sufficient to inject enough momentum to the clouds in 10 Myr. Although this is an impressive rate, it is not unrealistic and thus a plausible explanation.
For more details, see the article "Integral Field Spectroscopy of the Extended Emission-Line Region of 3C 249.1", by Hai Fu and Alan Stockton, The Astrophysical Journal, 2006, in press
Left-archived HST/WFPC2 F656N image of 3C 249.1 (the redshifted [O III] 5007 falls into this filter), overlaid with contours of a VLA image at 5 GHz and 0.′′35 resolution (Gilbert et al. 2004). Contours at 0.5 x (1, 2, 4, 8) mJy/beam. The peak of the radio continuum has been registered to the position of the quasar. The two x-ray emission regions (xa & xb) are marked in red and labeled. The gray-scale images in this and the middle panel have been allowed to wrap around to show both low-surface-brightness detail and high-surface-brightness peaks.
Middle and Right -GMOS/IFU [O III] 5007 radial velocity broad-band channel maps with contours of the HST image and contours of the radio jet overlaid, respectively. The radial velocity range (relative to that of the nuclear narrow line region, z = 0.3117; negative velocities are blue shifted) is shown in each panel. The rectangles delineate the 3.′′5 x 5′′ FOV of GMOS/IFU. The crosses indicate the position of the quasar, which has been removed from the datacube.
Velocity field of 3C 249.1 EELR derived from the [O III] 5007 region in the GMOS/IFU data cube. The three columns, from left to right, are line intensity, radial velocity (relative to that of the nuclear narrow line region) and velocity width maps. To separate different clouds that are present along the same line of sight, the velocity field is shown in three separate velocity bands (−460 to −300 km/sec, −230 to +150 km/sec and +180 to +590 km/sec from the top row to the bottom row). The rectangles delineate the 3.′′5 x 5′′ FOV of GMOS/IFU. Pixels are 0.′′2 squares. The crosses indicate the position of the quasar, which has been removed from the datacube.
Extracted spectrum of 3C249.1 EELR-b with important emission lines labeled.