Galaxy Research Summary
Choose The Basics
(rated G for general audiences) ...or...
The Details (for the cognoscente)
How do galaxies evolve, and why do they look the way we see them today?
On our own
Local Group of Galaxies there are a wide variety
of galaxies, each composed of 1 to 100 billion stars.
There are...
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... large spiral galaxies, like the Andromeda Galaxy, M31, our nearest
large neighbor...
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...roundish, fuzzy, and rather small elliptical like galaxies...
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...and irregularly-shaped dwarf galaxies with bright regions
of young, massive stars just recently formed in the last few million
years.
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In a nutshell, galaxies begin as clouds of hydrogen and helium gas
which are turned into stars as the galaxy "ages". Stars generate
energy by the nuclear fusion process, transforming hydrogen and helium
into heavier elements like carbon, nitrogen, oxygen, and even iron.
When most stars exhaust their supply of nuclear fuel, they undergo
spectacular and violent phenomena we call nova or supernova explosions,
or planetary nebulae ejections. By these processes, stars return most
of their material, including the newly-synthesized heavy elements
essential for life as we know it, back into interstellar space
whereupon the whole process begins again.
As an observational astronomer, I study the stars and gas in galaxies
to understand the interplay between stars and gas that shape the
appearance of a galaxy. Facilities I've used include
optical and infra-red observatories at...
Using radio telescope facilities at...
...I have conducted studies of the atomic hydrogen and molecular
(carbon monoxide) gas in galaxies. Such observations show that
often collisions and interactions between galaxies trigger star
formation bursts and drastically alter their appearance.
Some observations cannot be made
from the surface of the earth and require orbiting satellite
observatories. The Hubble Space
Telescope has been crucial
for making observations in ultraviolet light that are not possible from
ground-based telescopes. My collaborators and I use
the orbitting Chandra X-Ray Observatory
to conduct X-ray observations of hot,
million degree gas heated by supernova explosions and galaxy
collisions. We also use the Spitzer Space Telescope.
Understanding the evolution of galaxies requires ...
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a detailed knowledge of individual galaxies in the nearby universe
such as our near neighbor, the dwarf galaxy
named the Small Magellanic Cloud located just 150,000 light years
from the Milky Way...
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...and an investigation of
galaxies at the edge of the observable universe which we
see as they were billions of years ago when galaxies
first began to form.
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Using a variety of observational techniques, I am seeking to
understand the physical processes that transform objects in the distant
universe into the galaxies all around us. My research program involves
understanding the chemical composition of stars and gas in
galaxies, the physics of star formation, and the dynamics of
stars and gas as galaxies are assembled and torn apart.
Students of all levels work with me on astronomical research projects at
telescopes here and around the world.
At WIRO
At Red Buttes
At Kitt Peak
The Evolution of Star-Forming Galaxies:
A Research Program in Star Formation and Galaxy Evolution
Understanding the origin of galaxies in the early universe and their
evolution to the present time is one of the fundamental astrophysical
challenges of this decade. A dominant paradigm is still emerging. Are
large galaxies built primarily by accretion of smaller proto-galaxies?
Was there a specific epoch when most galaxies formed? Are internal
(star formation) or external (cluster and environment) forces the
dominant influences on a galaxy's evolution? What are the conditions
and triggers that drive star formation?
My research programs are designed to answer these questions by
integrating our knowledge of the dynamical and chemical properties of
nearby star-forming regions with those in the distant universe. I
focus on three main areas relating to these origins themes. 1)
Galaxy kinematics and chemistry in the distant universe, 2)
the impact of star formation on the ISM of galaxies, and 3)
the formation of massive stars and star clusters. They make use
of the ultraviolet capabilities of the HST + STIS/Cosmic Origins
Spectrograph, the X-ray spectroscopic capabilities of Chandra/XMM, and
the new generation of instrumentation on large ground-based telescopes
like Gemini which I am eager to help develop so that this science may
proceed.
One fruitful approach to understanding how galaxies evolve is through
the study of the fundamental galaxy scaling relations: correlations
between luminosity, size, mass (as traced by velocity width), and
metallicity. The advent of 6-10 m class telescopes and infrared
spectrographs make chemical and kinematic analysis at high redshift a
potent way to probe the evolution of the universe
(
Kobulnicky, Kennicutt, & Pizagno 1998 ,
Kobulnicky & Phillips 2004
).
In order to connect the
chemical properties in the early universe inferred from QSO absorption
line studies with the chemical enrichment scenarios envisioned for
local galaxies, I have initiated a program using the Keck telescopes
with multi-object optical and infrared spectrographs to directly
measure the chemical properties of star-forming galaxies at high
redshifts using emission lines from H II regions. Some initial results
at intermediate redshifts (z=0.4) show that chemical enrichment is
strongly correlated with a galaxy's optical luminosity and mass, and
that chemical abundance ratios can constrain the future evolutionary
path of star-forming systems observed at earlier epochs (
Kobulnicky & Zaritsky 1999 ).
More extensive studies of hundreds of galaxies over the redshift range
z=0.2 - 1.0 show that galaxies at 10 Gyr lookback times are
significantly brighter and less chemically enriched compared to local galaxies
of comparable mass and luminosity (
Kobulnicky et al. 2003;
Kobulnicky & Kewley 2004 )
At higher redshifts near z=0.8 to z~3,
there appears to be significant evolution of fundamental galaxy scaling
relations
( Kobulnicky & Koo 2001),
but so far only a few of the most luminous
objects have been studied. In the next few years, I plan to be
developing and using the capabilities of multi-object optical and
infrared spectrographs on ground-based telescopes like Gemini to
measure the chemical content and spatially-resolved kinematics of
distant galaxies. These kinds of observations constitute the best
tests of numerical and semi-analytic models of galaxy and structure
formation in the early universe.
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In the local universe, galaxy interactions and starburst-driven winds
are often invoked as feedback mechanisms for regulating star formation
rates, removing gas from galaxies, transforming one type of galaxy into
another, or expelling metal rich gas from galaxies with shallow
potential wells. These same mechanisms must play an even larger role in
driving the evolution of galaxies in the early universe, yet, their
impact in well-studied nearby galaxies is not yet clear. Local
starforming galaxies are chemically homogeneous on scales of <2 kpc (
Kobulnicky & Skillman 1996,
1997 ) despite the presence of concentrated starbursts which should
give rise to large localized chemical enhancements
(Kobulnicky et al. 1997 ). Starburst-driven winds drive
freshly-synthesized heavy elements on a long (100 Myr) excursion into
galactic halos before they are mixed back into the ISM and participate
in subsequent generations of star formation. How effective is the
feedback phenomenon that regulates the star formation rate in galaxies
and shapes their evolutionary paths?
In some galaxies we see clear evidence of freshly-produced heavy elements synthesized in
the current burst of star formation being expelled from galaxies
in a hot 1-million K phase
(Martin, Kobulnicky, & Heckman 2002) . This work was featured
in a Chandra X-Ray Observatory Press release
and by Astronomy Picture of the Day
My collaborators and I have also begun a campaign of HST/Cosmic
Origins Spectrograph observations and FUSE archival data of local
galaxies to probe the chemical content and ionization properties of gas
in the halos of nearby galaxies.
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Local galaxies with high star formation rates are often the products of
galaxy mergers. Tell-tale tidal tails are often seen in the neutral
hydrogen gas kinematics (
Kobulnicky & Skillman 1995 ) or in the
molecular gas dynamics. A galaxy's position
within a cluster may have a strong influence on its interaction
history, and on its ability to retain interstellar material.
The gas content of galaxies at higher redshifts is
particularly important since the amount of raw material for star
formation which remains determines the future evolutionary
path. We are engaged in a program to measure the neutral
hydrogen content of compact star-forming galaxies at $z$=0.05--0.2
to determine if they may be the progenitors to todays spheroidal
galaxies
(Kobulnicky & Gebhardt 2000 )
and (Pisano, Kobulnicky et al. 2001).
Using Hubble Space Telescope archive data and Keck telescope
spectroscopy, I am surveying the gaseous and chemical properties of
cluster galaxies over a range of redshift and cluster environments to
understand the impact of environment on galaxy evolution. Two sample
galaxies with the highest N/O ratios, NGC 5253 and II Zw 40, appear to
be among the youngest starbursts known, judging from the flat, thermal
nature of the radio continuum. Making use of new VLA high resolution
continuum imaging, and data from the VLA archives, I have been working
to reconstruct the burst history of the proto-typical Wolf-Rayet
galaxy, Henize 2-10 and determine the nature of its compact radio and
ultraviolet knots (
Kobulnicky et al. 1995 ).
The small-scale and thermodynamic structure in the
interstellar medium of the Milky Way and Magellanic Clouds provide
important clues for understanding the energy balance in galaxies at all
cosmic epochs. I conducted some of the first millimeter-wave molecular
absorption observations against extragalactic background sources
(
Kobulnicky, Dickey & Akeson 1995 ) through the plane of the Galaxy,
revealing remarkably similar structures on lines of sight that probe
scales from 1 pc to 1 AU! These data show no evidence for extremely
cold molecular gas at large galactocentric radii claimed by some
researchers, but they do show that the molecular gas seen in absorption
in the Cygnus rift is rotationally very cold (8 K). The Magellanic
Clouds also show cold gas condensations even in the tenuous Bridge
region, providing evidence that star formation may take place in a wide
range of environments (
Kobulnicky & Dickey 1999 ). A more extensive HI
and OH survey of the Galactic Plane and the Magellanic Bridge region is
underway.
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