Intergalactic Medium (IGM)
In the beginning, matter was spread more-or-less uniformly throughout the universe. In the billions of years the followed, denser regions attracted matter from less dense regions through gravity, forming stars and galaxies. The intergalactic medium (IGM) is what is left over mixed with what's been ejected from galaxies. The IGM is big and dark and mostly empty space. But it makes up more than 90% of the normal matter in the universe.
Studying the IGM can tell us a lot about both the initial conditions in the universe and the inner workings of galaxies. Perversely, it's a lot easier to study the distant IGM from the dawn of time than the early universe than that in our own back yard (in a relative sense) However, with the advent of ultraviolet (UV) astronomy satellites, we can trace the "local" IGM through absorption line spectra; looking for the shadows of gas against more distant light sources. The light sources in this case are distant quasars and active galaxies and the shadows are cast by huge clouds of hydrogen (and trace amounts of other materials such as helium, carbon, oxygen, etc) in the IGM.
Traditional means of studying the IGM involve looking for neutral hydrogen which traces relatively cool gas. But once the gas is heated beyond about 100,000 degrees, practically no hydrogen is left unionized. Instead, I use ionized lines of heavier elements such as O+++++ (five-times-ionized oxygen or OVI) to trace this "warm-hot ionized medium" (WHIM) phase. OVI is plentiful at temperatures of a few hundred thousand degrees and can serve as a proxy for hot, ionized (and largely invisible) hydrogen. Similarly, other ions such as CIV and NV can be used to trace this phase as well, but each has its disadvantages. These systems are rare and hard to detect, but we are making progress in mapping their extent and distinguishing features.
|My most recent work centers on a large survey of ~30 sight lines toward quasars in the local universe (z<0.4) in which we measure 83 OVI clouds among 650 neutral hydrogen clouds. This is the largest such survey to date and we use it to perform statistical analysis of the local cosmos. The distribution of neutral hydrogen and hot OVI are significantly different, as seen in the figures to the right. From this difference, we infer that they occupy different phases and probably represent shocked and unshocked material. In the larger picture, by making certain assumptions, we see that about 10% of the normal matter in the universe can be accounted for in the WHIM phase and 30% can be accounted for in neutral hydrogen. If another 10% is contained in stars and galaxies, that means that there's only another 50% of the matter in the universe still tobe accounted for! These quantities also appear to change as a function of time which is something we're going to investigate soon.|
|My results can also be compared against simulations of cosmic evolution and galaxy growth. My observations, shown as blue dots in the figure to the left, match well with the models in which galaxies generate strong winds from supernovae and star formation. These winds "pollute" the local IGM with enriched material, some of which I then detect in my spectra. The simulation without galaxy winds (pink) does not agree with my observations implying that it is not representative of the real universe.|
STScI Press Release on my latest paper (May 20, 2008)
- The Low-z Intergalactic Medium. III. HI and Metal Absorbers at z<0.4, Danforth & Shull 2008, ApJ, 679, 194
- The Low-z Intergalactic Medium. II. LyB, OVI, and CIII Forest, Danforth, Shull, Rosenberg, & Stocke 2006, ApJ, 640, 716
- The Low-z Intergalactic Medium. I. OVI Baryon Census, Danforth & Shull, 2005, ApJ, 624, 555.
Interstellar Medium (ISM)
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, Sankrit, Blair, Howk, & Chu, 2003, ApJ, 586, 1179
- An Atlas of FUSE Sight Lines Toward the Magellanic Clouds,
Danforth, Howk, Fullerton, Blair, & Sembach, 2002, ApJS, 139, 81-189
See my Publications for other recent work.
Other Recent Research
I have interests in Supernova Remnants (SNRs), hot gas, stellar evolution, galactic structure, ISM, the Magellanic Clouds, and the interaction between hot stars and their environments. Two regions in the Magellanic Clouds in particular--N66, a bright HII region in the SMC, and N70, a canonical superbubble in the LMC--have occupied my attention lately. I've also worked on an optical/radio search for SNRs in the SMC.