The main focus of my research is the local intergalactic medium, the diffuse, "dark" gas which lies between galaxies and is thought to form a filamentary, web-like structure known as the Cosmic Web. This is an area of astrophysics which remains poorly understood and poorly connected to many other parts of the larger cosmological theory yet makes up most of the mass and volume of normal, baryonic matter in the universe and plays a crucial part in the formation of stars and galaxies.
A simulation of the Cosmic Web. Galaxies and gas lie along huge filaments. Clusters of galaxies lie at the nodes in the web while huge voides make up the underdense space between.
The cosmic web should follow the same large-scale framework laid down through gravitational collapse of dark matter halos. The IGM should provide the raw materials for new star formation in galaxies and should itself be enriched by heavy elements created in those stars. The IGM is heated by supernovae, stellar winds, and active galaxies and is ionized by energetic ultraviolet and X-ray radiation from galaxies. However, the extent of these interactions are poorly observed at best.
Unfortunately, the IGM is "dark"--not in the sense of the still-mysterious Dark Matter, but in that it doesn't emit any* detectable radiation. The most sensitive method of studying IGM and other diffuse, low-density material is through absorption-line spectroscopy using bright background objects (stars, quasars) and looking for spectral shadows that diffuse gas produces through absorption. Many of the strongest and most useful diagnostic lines are in the rest-frame ultraviolet (90-300 nm), which is blocked by the Earth's atmosphere. The early universe can be observed through absorption lines redshifted to optical wavelengths and thus accessible to ground-based telescopes. However, it is only in the last decades that space-based ultraviolet cameras and spectrographs such as those installed on the Hubble Space Telescope have revealed the same information in the low-redshift, modern universe.
Absorption spectroscopy can be used to detect diffuse gas between the observer and a bright background object. Each cloud of diffuse gas shows up as a narrow absorption line in the spectrum of the background object and, thanks to the expansion of the universe, the redshift of each absorber corresponds to its distance from us.
With these instruments, I study the low-redshift, "local" IGM. Here are some highlights:
- Best statistical study of cool, photoionized gas (the Lyman-alpha forest), and warm-hot, metal-enriched gas.
- Evolution of the IGM during the second half of the universe.
- Accounting for more than half of the baryons (normal matter made up of protons/neutrons) in the modern universe.
- Detailed studies of the relationship between galaxies and their surrounding IGM.
STScI Press Release on my 2008 paper
- A Large HST/COS Survey of the Low-z IGM, Danforth et al. 2014 (in prep.)
- A Dection of Gas Associated with the M31 Stellar Stream, Danforth, et al. 2014 in prep.
- The Low-redshift IGM as Seen in Archival Legacy HST/STIS and FUSE DataTilton et al. 2012 ApJ, 759, 112
- The Baryon Census in a Multiphase IGM: 30% of the Baryons May Still be MissingShull, Smith, & Danforth 2012 ApJ, 759, 23
- An HST/COS Search for Warm-hot Baryons in the Mrk 421 Sight Line, Danforth et al. 2011 ApJ, 743, 18
- Hubble/COS Observations of the Lya Forest Toward the BL Lac Object 1ES 1553+113, Danforth et al. 2010 ApJ, 720, 976
- Broad HI Absorbers as Metallicity-independent Tracers of the Warm-Hot IGM, Danforth, Stocke, & Shull 2010 ApJ, 710, 613
- The Low-z IGM. III. HI and Metal Absorbers at z < 0.4, Danforth & Shull 2008 ApJ, 679, 194
- The Low-z IGM. II. Lya, OVI, and CIII Forest, Danforth et al. 2006 ApJ, 640, 716
- The Low-z IGM. I. OVI Baryon Census, Danforth & Shull 2005 ApJ, 624, 555
Active Galactic Nuclei
Much of my research uses Active Galactic Nuclei (AGN, sometimes called "quasars" although this is incorrect) as background sources in the study of the IGM. However, the AGN themselves are also quite interesting. The standard model for an AGN is a supermassive black hole (106-109 solar masses) at the center of a galaxy surrounded by a very hot accretion disk of material falling into the galaxy and, farther out, but different gas clouds in a circum-nuclear region. Since this is one of the most energetic phenomena in the universe, it's a great way to study the physics of environments we can't reproduce in the lab on Earth.
My particular interest is in blazars and Fanaroff-Riley class 1 and 2 galaxies, a class of extremely energetic AGN consisting of a rapidly spinning black hole with little or no gas accretion. They produce extremely powerful, relativistic radio jets and photons of all energies from radio waves all the way to TeV-scale gamma rays. I'm also interested in time variability of AGN and in the properties of gas flowing into and out of their host galaxies.
Here are some highlights:
- Detection of weak Lya emission features in spectra of "featureless" BL Lacertae Objects (a subclass of blazars). We used this to constrain the mass accretion rate and other circumnuclear properties.
- Direct redshift measurement for several blazars which turn out to be at much larger distances than had been assumed. These large distances pose a problem for the "gamma-ray horizon" and current models of the extragalactic background light.
- The Firm Redshift Lower Limit of the Most Distant TeV-detected Blazar PKS1424+240, Furniss et al. 2013 ApJ, 768, L31
- On the Redshift of the Very High Energy Blazar 3C66A, Furniss et al. 2013 ApJ, 766, 35
- A Fast Flare and Direct Redshift Constraint in Far-UV Spectra of the Blazar S50716+714, Danforth et al. 2013 ApJ, 764, 57
- Broad Lya Emission from Three Nearby BL Lacertae Objects, Stocke, Danforth, & Perlman 2011 ApJ, 732, 113
Another of my interests is the structure and dynamics of the interstellar medium (ISM). The interaction of stars with their environment is important to understanding superbubbles, ionized nebulae, metal enrichment of the ISM, and the star-formation history of galaxies. The Magellanic Clouds are a great laboratory for ISM studies since they represent a large sample of ISM structures of all types located at a uniform but (relatively) short distance from us and uncomplicated by line-of-sight ambiguities. I use absorption line spectroscopy in the far-UV as well as optical emission line spectra to study superbubbles, supernova remnants, HII regions (ionized nebulae), and supergiant shells.
The superbubble N70 in the Large Magellanic Cloud shows an excess of hot OVI absorption compared to nearby field sight lines. Do objects such as this produce the hot gas in a galaxies halo?
My Ph.D. dissertation, Interstellar Matter Kinematics in the Magellanic Clouds, presents a large dataset of spectra and images of over one hundred different sight lines in the Magellanic Clouds, a pair of satellite galaxies to the Milky Way. Far-UV spectral lines probe the dynamics of hot, warm, and cold gas. Optical images in several prominent ISM emission lines help define the ISM morphology surrounding each sight line; for instance, does the sight line pass through a superbubble or HII region or does it lie in relatively 'empty' space? Optical long-slit spectra served to connect the morphology seen in the images with the dynamics seen in the absorption spectra. My focus was to look for kinematic and environmental differences between sight lines in different morphologies.
Since the completion of my thesis, I have been investigating subsets of the Magellanic Cloud dataset. Theory suggests that superbubbles are reservoirs of hot gas which help to produce and maintain the hot halo surrounding galaxies. Thus I have been examining sight lines toward stars in superbubbles, such as the spectacular N70 at left, in comparison with nearby "field" sight lines. Preliminary evidence suggests that there is a correlation between superbubbles and extra OVI absorption, but a larger study of additional objects is necessary.
- Possible Detection of OVI From the LMC Superbubble N70, Danforth & Blair, 2006, ApJ, 646, 205
- Far-Ultraviolet & Hα Spectroscopy of SNR0057-7226 in the SMC HII Region N66, Danforth et al. 2003, ApJ, 586, 1179
- An Atlas of FUSE Sight Lines Toward the Magellanic Clouds, Danforth et al. 2002, ApJS, 139, 81-189