Overview of my Research Interests
Summary
IONIZED GAS IN HIGH-REDSHIFT GALAXY HALOSDistant (high-redshift) galaxies
are faint
and small (on the sky), so studying them through direct imaging
is challenging, particularly from the ground. An alternative way to detect
such galaxies is to
analyze the spectra of luminous background objects (quasars and
gamma-ray bursts), and look for the tell-tale absorption lines that
arise due to foreground galaxies lying along the line-of-sight.
These lines contain a wealth of information on the chemical properties
(elemental abundances) and physical properties
(temperature, density, ionization state) of the absorbing gas.
The strongest absorbers, known as damped Lyman-alpha (DLA) systems,
are thought to trace gas in and around galaxy disks and protogalaxies.
I have led a program to survey the "high-ion" absorption in a large sample
of DLAs at z~2-3 observed with the UVES spectrograph on the VLT,
to investigate and characterize the warm-hot gas in these absorbers.
The lines studies are those of five-times-ionized oxygen
(O VI), four-times-ionized nitrogen (N V), and three-times-ionized
carbon (C IV). The strength and velocity spread of these high-ion lines
and their correlation with other DLA properties give important
observational constraints on the kinematics and structure of the DLA
galaxies. These surveys reveal that a significant fraction of metals
and baryons in DLAs exist in ionized gas (plasma).
The figure on the right shows that the high-ion (C IV) velocity widths in DLAs
are almost always broader than the low-ion (Si II) velocity widths
(from Fox et al. 2007, A&A, 473, 791). Galactic winds are one
possible energy source for the high ion kinematics.
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METAL ENRICHMENT IN THE HIGH-Z INTERGALACTIC MEDIUMThe intergalactic medium
(IGM), the filamentary web of plasma existing between
galactic structures (see figure at right), occupies the vast majority
of the volume of the Universe. The IGM is detected as a series of
hydrogen absorption lines (known
as the Ly-alpha forest) detected blueward of Ly-alpha emission in quasar
spectra. It has been known for several decades that the IGM
is metal-enriched, because metal lines are detected at the same redshift as
Ly-alpha forest lines, down to low H I column densities,
corresponding to low-density regions. What is less well known is how
homogeneous the metal enrichment is: do the metals exist in
relatively small, enriched "pockets", or are they widespread with a
large volume
filling factor? And how were the metals transported from their
sites of origin (in the cores of massive stars) to the IGM where
they are observed? Were most IGM metals produced at very high redshift
in an early generation of Population III stars, or are they dispersed later?
To address these questions, I'm involved in
the study of high signal-to-noise, high spectral resolution
optical quasar spectra (taken with VLT/UVES), focusing on the
detailed absorption-line profiles in individual high-redshift IGM
systems. In particular, studying pairs of quasars lying close together on the
sky (separations on the order of arcminutes) allows us to probe the transverse
structure of the IGM.
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INTERSTELLAR MATTER IN GAMMA-RAY BURST HOST GALAXIES Gamma-ray bursts (GRBs)
are the most energetic events in the Universe, typically releasing
as much energy in a few seconds as the Sun will emit in its entire 10
billion year lifetime. Aside from their
interest as the endpoints of massive star evolution, GRBs are ideal
background sources for
absorption-line spectroscopy, thanks to their enormous luminosities and
smooth power-law continua. Optical spectroscopy of GRB afterglows
(particularly the long-duration bursts, lasting for more than 2 seconds)
can be used to study both the intervening IGM along the
line-of-sight and the interstellar medium (ISM) of the host galaxy,
Using the UVES spectrograph on the VLT in Chile in rapid-response mode
(in which ongoing exposures are automatically interrupted to
slew the telescope to the GRB, which is then observed within minutes),
I am part of a team to determine physical conditions and chemical
abundances in the host galaxy's ISM. Certain exotic
absorption lines (from excited levels of singly-ionized iron and nickel)
show variation over short timescales (minutes to hours),
allowing us to track the evolving ionization and excitation level
of the gas in real time. In combination with photo-excitation
modeling, this allows one to derive the distance from the burst to
the absorbing cloud. The figure on the right shows such
time-variation in the Fe II* and Ni II* lines in the
spectrum of GRB 060418 (from Vreeswijk et al. 2007,
A&A, 468, 83).
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IONIZED GAS IN HIGH-VELOCITY CLOUDS (HVCs) The Milky Way is surrounded
by a network of high-velocity clouds (HVCs) that do not rotate with
the Galactic disk, but rather trace inflowing and outflowing gas
streams passing through the hot Galactic corona.
HVCs are studied in 21 cm emission from neutral hydrogen
(see map at right, courtesy Bart Wakker), and in UV and
optical absorption in the spectra of background sources.
The infalling HVCs play an important role in galaxy evolution,
as carriers of low-metallicity fuel to power future star formation in the disk.
Observational constraints on HVCs are important to understand these
physical processes. My work on HVCs has focused on
absorption-line studies of
high-ionization species (O VI, N V, C IV),
whose detection indicates the presence of hot plasma at temperatures of a
few hundred thousand Kelvin. These "high ions" are thought to
arise at the turbulent boundaries
between the clouds and the even hotter (million-degree) surrounding corona.
The long-term goal of this work is to determine the overall
ionization fraction in HVCs, allowing us to investigate whether most
accreting gas clouds make it safely into the Galactic disk, or whether they
become fully ionized and "evaporate" into the hot corona
(see Fox et al. 2010, ApJ, 718, 1046).
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BARYON BUDGET IN THE LOW-REDSHIFT IGM
Most of the baryonic (observable) matter in the Universe is outside galaxies,
so to detect and characterize the baryons, one has to
study the IGM. I am involved in absorption-line studies of the
low-redshift IGM
using the Hubble Space Telescope, with the goal of completing the
inventory of baryons, and studying the IGM absorbers' physical conditions,
chemical enrichment, proximity to galaxies, and relation to galaxy
evolution processes such as inflow and outflow.
The Cosmic Origins Spectrograph (COS) installed in May 2009 on HST
is addressing these questions in detail (see cartoon on right),
providing much higher sensitivity than previous space-based UV
spectrographs.
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