Bibcode
Vaduvescu, Ovidiu
Bibliographical reference
PhD Thesis, York University, Toronto, Ontario, Canada
Advertised on:
11
2005
Citations
4
Refereed citations
4
Description
Dwarf galaxies are the most common galaxies in the Universe. They are
systems believed to consist of matter in a near-primordial state, from
which giant galaxies probably form. As such, they are important probes
for studying matter in its near-primordial state.
In an effort to study the main physical and chemical properties of
dwarfs, the present thesis focuses upon the main physical properties of
dwarfs. Two classes of star forming dwarf galaxies are considered: dwarf
irregulars (dIs), and blue compact dwarfs (BCDs). A third class, dwarf
ellipticals (dEs), is studied based on its structural properties and
compared with dIs.
Possible evolutionary connections are addressed between dIs and BCDs. To
measure the luminosity, deep imaging in the near-infrared (NIR) is
considered. Compared with the visible, the NIR domain gives a better
gauge of the galaxy mass contained in the old stellar populations,
minimising the starburst contribution and also the effects of
extinction.
Two observing samples of star-forming dwarf galaxies are considered. The
first includes 34 dIs in the Local Volume closer than 5 Mpc. The second
sample includes 16 BCDs in the Virgo Cluster. In six observing runs
between 2001 and 2004, we acquired deep NIR images (J and K_s) using the
3.6m Canada-France-Hawaii-Telescope (CFHT) in Hawaii and the 2.1m
telescope at the National Astronomical Observatory ''San Pedro Martir''
(OAN-SPM) in Mexico. Deep spectrocopy was acquired in 2003 on the 8.1m
Gemini-North telescope in Hawaii. We completed the observed samples with
spectroscopic data from the literature, and photometry from the 2MASS
survey and GOLDMine database.
From a statistical study at CFHT, we derived some strategies necessary
to image optimally faint extended sources in the NIR. Due to the airglow
variation in the atmosphere and the thermal contribution of the dome,
telescope and the instrumentation, repeated observations of the sky must
be alternated every 3-4 minutes with the science images, in order to
achieve 1% accuracy in surface photometry. We discovered that the NIR
surface brightness profiles of dIs can be fitted with a hyperbolic
secant (sech) function with only two parameters: the central surface
brightness and the scale length. This led to the discovery that BCD
profiles could be fitted with a sech component to trace the diffuse
component responsible for most of the light, and a Gaussian for the
central starburst.
For 25 of the 34 observed dIs, we resolved stars as faint as M_K=-7.5
mag out to 5 Mpc. We show that the resolved component comprises more
than 50% of the light from star formation bursts within the last 3 Gyr.
By separating the resolved sources associated with each galaxy from the
unresolved component, we determined for the first time the contribution
from the resolved stellar component to the total light in the NIR. In
nearly all galaxies, the resolved population up to M_K=-7.5 mag
represents less than 5% of the total flux in K_s, with ratios in J 1.5-2
times larger. Compared with the visible, the small contribution of the
resolved flux allows us to consider the NIR a better domain to sample
the old stellar populations, and thus use it to gauge the stellar mass
of star-forming dwarf galaxies.
For 29 dIs, colour-magnitude diagrams (CMDs) for the resolved component
were derived. Three CMDs include more than 1000 stars in both K_s and J,
while another 15 CMDs have more than 100 stars. Most of the CMDs show a
main blue finger centered around J-K_s=1 mag. In some cases, a red tail
extends from the finger out to J-K_s =+2.5 mag. The colour profiles of
the unresolved components show a remarkably constant J-K_s = +0.8 to
+1.0 mag, which matches the colour of the main finger in the CMDs.
For both dIs and BCDs, we searched for correlations between galaxy size,
absolute magnitude, central surface brightness, colours, and the
resolved over total ratio (for dIs). Good linear correlations were found
between the scale length, and the sech magnitude, and between the
isophotal semimajor axis and the sech magnitude. Also, correlations were
found between the central surface brightness and the sech magnitude.
Overall, galaxies with more luminous old components are larger, redder,
and brighter in the centre. Thus, size, colour, and the extent of the
central plateau appear to be determined by the mass of the old
component.
For both dIs and BCDs, the Tully-Fisher relation shows considerable
scatter in K_s, especially at low luminosities. The scatter appears to
be tied to variations in surface brightness. A new ''fundamental plane''
was discovered for dIs which relates the sech absolute magnitude, the
central surface brightness, and the neutral hydrogen line-width. The
residuals are low enough (rms 0.4 mag), that it offers considerable
potential as a distance indicator for star-forming dwarfs. BCDs appear
to lie on the dI fundamental plane, but the scatter is larger, probably
due to uncertitudes in their line widths.
We used NIR images for 22 dEs in the Virgo cluster taken from the
GOLDMine database, and some additional data for 9 other dwarf
spheroidals in the Local Group, to examine how closely dEs fit into the
dI fundamental plane. Over a 9 mag interval in absolute magnitude, the
dEs fall in the plane defined by the dIs. The outstanding overlap
suggests a close evolutionary connection between dIs and dEs.
Using oxygen abundances of dIs and BCDs from the literature, we studied
correlations between metallicity, stellar mass, gas mass, baryonic mass,
and gas fractions. Although there is some scatter, metallicity
correlates with all four parameters in the sense that more massive
systems contain more metals. The oxygen abundance correlates very well
with the luminosity in K_s, for both dIs and BCDs. Nevertheless, the two
relations appear to be different, with more luminous BCDs being more
metal rich than dIs with similar absolute magnitudes. This shift can be
biased by not including gas. After the gas mass is considered, BCDs
allign with dIs on the same mass-metallicity relation, suggesting
similar evolutionary connections.
Overall, based on their structural and physical properties, closer links
emerge between dIs, BCDs, and dEs. Comparing their structural
properties, dIs and BCDs appear to share common origins, with the sech
component modeling all the NIR flux for dIs and most of the flux for
BCDs. On the fundamental plane, dIs and BCDs also show similar dynamical
properties. Thus, BCDs seem to be dIs observed in a bursting phase.
Taking into account their gas-to-mass fraction, BCDs cannot be
considered closed systems, their shifted position in respect to dIs
suggesting flows of gas such as infall of gas clouds. Comparing
structural properties, dEs lie in the fundamental plane of dIs,
suggesting an intimate link between the two systems, in the sense that
dEs could be consider the final outcome of dIs after all gas is removed
from the system.
Full thesis available online at http://aries.phys.yorku.ca/
ovidiuv/Thesis.pdf (7 MB, 247 pag)