Current models of ΛCDM galaxy formation over-predict the masses of the most massive galaxies. It is widely accepted that some form of feedback is required to limit star formation in these systems. However, the nature of this feedback is still hotly debated. Radio jets emanating from massive galaxies can potentially expel gas and keep it sufficiently heated to prevent further star formation. Starburst winds driven largely by supernovae may also be responsible, as may the strong winds observed in Broad Absorption Line QSOs, although the origin of these winds remains uncertain. The GMTIFS IFS will progress understanding of galaxy feedback in at least two ways:
- It will provide direct observations of feedback occurring in young massive galaxies, and
- It will enable study of "red and dead" galaxies at high redshift that are the end products of galaxy feedback.
Direct observations of galaxy feedback
Bipolar outflows have been detected emanating from z ~ 2 radio galaxies along the radio jet axes. This may be evidence for Active Galactic Nuclei (AGN) feedback with the outflows being powered by the expanding radio jet. The data at present are not of sufficient quality to study the excitation and energetic in detail. Deeper observations with the GMTIFS IFS will allow this phenomenon to be studied in more objects with greater sensitivity and at higher angular resolution.
Similar outflows have also been seen using adaptive-optics-corrected near-infrared spectroscopy in some radio-quiet active galaxies. Here, intense starbursts drive the outflow through associated stellar winds or supernova explosions, not expanding radio lobes as for radio galaxies. It is not known which of these mechanisms is more influential in expelling gas from massive galaxies, and hence limiting their ultimate stellar mass. Observations with the GMTIFS IFS will resolve these issues.
Figure 1 - Simulation of jets outflowing through the dense interstellar medium of a young radio galaxy. Panel (a) shows the synthetic radio surface brightness map (in contour and colour-map); panel (b) shows the gas density; panel (c) shows a velocity slice for the warm ISM. The density map (panel b) indicates that the entrained hot ISM material and ablated cloud material divert the path of the jet. The velocity map (panel c) shows gas motions of hundreds of km/s and suggests that the ionised gas emission would be highly blueshifted and offset from the center.
"Red and dead" galaxies at high redshift
Red sequence galaxies have been identified out to z ~ 2. These massive (M > 1011 MSun) early galaxies appear from their spectra and colors to be in a post-starburst phase. It is likely that they are the end products of intense galaxy feedback events that remove much of the gas in massive young galaxies, either by conversion into stars or by expelling it from the system. The sub-millimeter galaxies may well be experiencing this intense starburst phase, probably triggered by a major merger of gas-rich galaxies.
Massive red galaxies at high redshift are a factor of ~ 5 smaller than nearby ellipticals, and must undergo significant size evolution if they are to evolve ultimately into nearby elliptical galaxy analogs. Understanding the nature of this growth of massive quiescent galaxies over cosmic time will be one of the challenges GMTIFS will address. Stellar velocity dispersion measurements are required to confirm that the high stellar masses inferred from their continuum luminosities and assumed mass-to-light ratios are indeed correct.
Despite their compact size, the stellar densities within a 1 kpc radius seem to be elevated by only a factor of 2-3 compared to nearby elliptical galaxies. This suggests an inside-out growth scenario involving minor mergers in which the compact high-redshift galaxies make up the cores of present-day massive elliptical galaxies. Determining size and mass estimates of samples of quiescent red galaxies at different redshifts between z > 2 and ~ 1 will clarify this evolution.
Figure 2 - Spectra extracted in elliptical annular apertures from a simulated 14 hr observation of the z = 2.1865 massive red galaxy, 1255-0 with the GMTIFS IFS. The extraction radii in units of the effective radius are indicated in each panel. Strong absorption features in the rest-frame optical spectrum are indicated. Regions of poor atmospheric transmission are not plotted, and regions around strong airglow emission lines are masked and indicated by bars at the bottom of each panel.