Updated: 19 April 2016

MIT Kavli Institute for Astrophysics and Space Research

Probing the Extended Gaseous Halos of Galaxies with Quasar Absorption-Line Spectroscopy

Collaborator: Robert Simcoe, Department of Physics, MIT

Seeking the Lost Interstellar Medium of Red-Sequence Galaxies


In a typical galaxy color-magnitude diagram, there are two clear populations: a distinct red sequence and a diffuse blue cloud. The red-sequence galaxies (RSGs) generally have elliptical or spheroidal morphologies, have little or no neutral interstellar medium (ISM), and are not actively forming stars. RSGs tend to be more massive than the star-forming spiral galaxies that constitute the blue cloud. A crucial step to making massive, "red and dead" elliptical galaxies is to halt star formation, which allows the galaxies to evolve onto the red sequence. Likely mechanisms that shut down star formation are: (a) prevention of star formation through heating of the ISM; (b) complete consumption of the ISM; and/or (c) removal of the gas from the galaxy entirely.

The latter mechanism—the expulsion of gas through galactic feedback processes—is the focus of this study. Theoretical and observational evidence supports feedback as the process by which the ISM is removed from RSGs. Correlations between galaxy color and signatures of AGN activity make AGN-driven feedback more likely than supernova-driven feedback, but probing the halo gas directly with quasar absorption-line (QAL) spectroscopy will help distinguish the dominant feedback process. The heating or consumption of the ISM, (a) and (b), respectively, will not be addressed directly by the proposed research. However, both mechanisms likely result in feedback. The process that heats the gas often expels it (e.g., Type Ia supernovae), and supernova-driven winds correlate with intense bursts of star formation, which may occur during the merger that formed a RSG.

To understand how RSGs lose their ISM, a two-pronged attack is proposed: (1) probe the halos of a variety of red-sequence galaxies with QAL spectroscopy in search of T ≈ 104 K gas—the expelled ISM; and (2) characterize the ages, stellar abundances, environment, and AGN activity of the galaxies that do and do not have gaseous halos. A diverse sample of 50 0.25 < z < 0.5 RSG-QSO pairs will be surveyed with the Magellan Echellette Spectrograph, which allows for an efficient survey of the galaxies and quasars. Analysis will include correlating galaxy and halo-gas properties and applying the results to distinguish between different feedback mechanisms. The work will be done with Robert Simcoe at the MIT Kavli Institute for Astrophysics and Space Research (MKI).

The RSG-QSO pairs will be selected from the Sloan Digital Sky Survey and will have a range of impact parameters, environments, and masses. The redshift is chosen so that: H$\beta$ and the Lick indices Mgb, Fe5270, and Fe5335 are observable in the galaxy spectra and Mg II λλ2796, 2803 and Fe II λ2600 absorption lines are observable in the QSO spectra. These features will allow an apples-to-apples comparison of [α/Fe] of the stellar population and the halo gas of the RSG; [α/Fe] acts like a clock since Fe-enrichment, from Type Ia supernova, occurs later (t > 1 Gyr) than α-enrichment, from Type II supernova.


Alex Hedglen (astronomy & physics major, UH Hilo) worked with the RSG-QSO data for his Hawai`i/NASA Space Grant Consortium (HGSC) Traineeship for the 2015−2016 academic year (in addition to pro bono in summer 2015). His aim was to organize and process the spectra of 30 galaxy-quasar pairs. He wrote mase_non_gui.pro in the XIDL user library to non-graphically reduce MagE spectra with the MASE reduction software (also in XIDL). He presented posters at two HSGC Symposia: Fall 2015 and Spring 2016.

[The inspiration for this project came from a pilot study presented at The Cosmic Odyssey of the Elements (Aegina, Greece, June 2008); the abstract and presentation can be found here.]

Polluting the IGM: the Extended Gaseous Halos of z ≈ 2.5 LBGs


We aim to characterize the effect of galactic feedback processes on the gaseous halos of star-forming Lyman-break galaxies (LBGs) at z ≈ 2.5. This epoch probes the peak in the cosmic star-formation rate, when galaxies are potentially still young and polluting the IGM. We build upon a previous study that analyzed the absorption-line system closest (115 kpc) to a LBG in the high-resolution optical spectroscopy of a background, high-redshift quasar. From photoionization models, it was determined that the absorbing clouds were kiloparsec scale or smaller and had metallicities of 0.1 to 1 solar, less than 3 Gyr after the Big Bang. There was also evidence that the IGM was preferentially enriched by Type II supernovae, as indicated by the enhancement of silicon to carbon abundances. This could be interpreted as a "top-heavy" initial mass function. We present our analysis of two more rare, close LBG-absorber pairs, identified in our survey of LBGs in several quasar fields.