6,206 research outputs found
The Evolving Faint-End of the Luminosity Function
We investigate the evolution of the faint-end slope of the luminosity
function, , using semi-analytical modeling of galaxy formation. In
agreement with observations, we find that the slope can be fitted well by
, with a=-1.13 and b=-0.1. The main driver for the evolution
in is the evolution in the underlying dark matter mass function.
Sub-L_* galaxies reside in dark matter halos that occupy a different part of
the mass function. At high redshifts, this part of the mass function is steeper
than at low redshifts and hence is steeper. Supernova feedback in
general causes the same relative flattening with respect to the dark matter
mass function. The faint-end slope at low redshifts is dominated by field
galaxies and at high redshifts by cluster galaxies. The evolution of
in each of these environments is different, with field galaxies
having a slope b=-0.14 and cluster galaxies b=-0.05. The transition from
cluster-dominated to field-dominated faint-end slope occurs roughly at a
redshift , and suggests that a single linear fit to the overall
evolution of might not be appropriate. Furthermore, this result
indicates that tidal disruption of dwarf galaxies in clusters cannot play a
significant role in explaining the evolution of at z< z_*. In
addition we find that different star formation efficiencies a_* in the
Schmidt-Kennicutt-law and supernovae-feedback efficiencies generally
do not strongly influence the evolution of .Comment: 4 pages, replaced with version accepted to ApJL, minor changes to
figure
Galaxy Mergers at z>1 in the HUDF: Evidence for a Peak in the Major Merger Rate of Massive Galaxies
We present a measurement of the galaxy merger fraction and number density
from observations in the Hubble Ultra Deep Field for 0.5<z<2.5. We fit the
combination of broadband data and slitless spectroscopy of 1308 galaxies with
stellar population synthesis models to select merging systems based on a
stellar mass of >10^10 M_sol. When correcting for mass incompleteness, the
major merger fraction is not simply proportional to (1+z)^m, but appears to
peak at z_frac~=1.3+-0.4. From this merger fraction, we infer that ~42% of
massive galaxies have undergone a major merger since z~1. We show that the
major merger number density peaks at z_dens~1.2, which marks the epoch where
major merging of massive galaxies is most prevalent. This critical redshift is
comparable to the peak of the cosmic star formation rate density, and occurs
roughly 2.6 Gyr earlier in cosmic time than the peak in the number density of
X-ray selected active galactic nuclei. These observations support an indirect
evolutionary link between merging, starburst, and active galaxies.Comment: Accepted to ApJ. 7 pages, 6 figures, 1 table. Uses and includes
emulateapj.cls. In the initial submission, Figures 1 and 2 where switche
Constraining the Distribution of L- & T-Dwarfs in the Galaxy
We estimate the thin disk scale height of the Galactic population of L- &
T-dwarfs based on star counts from 15 deep parallel fields from the Hubble
Space Telescope. From these observations, we have identified 28 candidate L- &
T- dwarfs based on their (i'-z') color and morphology. By comparing these star
counts to a simple Galactic model, we estimate the scale height to be 350+-50
pc that is consistent with the increase in vertical scale with decreasing
stellar mass and is independent of reddening, color-magnitude limits, and other
Galactic parameters. With this refined measure, we predict that less than 10^9
M_{sol} of the Milky Way can be in the form L- & T- dwarfs, and confirm that
high-latitude, z~6 galaxy surveys which use the i'-band dropout technique are
97-100% free of L- & T- dwarf interlopers.Comment: 4 pages, 4 figures, accepted to ApJ
Isolated OB Associations in Stripped HI Gas Clouds
HST ACS/HRC images in UV (F250W), V (F555W), and I (F814W) resolve three
isolated OB associations that lie up to 30 kpc from the stellar disk of the S0
galaxy NGC 1533. Previous narrow-band Halpha imaging and optical spectroscopy
showed these objects as unresolved intergalactic HII regions having Halpha
luminosities consistent with single early-type O stars. These young stars lie
in stripped HI gas with column densities ranging from 1.5 - 2.5 * 10^20 cm^-2
and velocity dispersions near 30 km s^-1. Using the HST broadband colors and
magnitudes along with previously-determined Halpha luminosities, we place
limits on the masses and ages of each association, considering the importance
of stochastic effects for faint (M_V >-8) stellar populations. The upper limits
to their stellar masses range from 600 M_sun to 7000 M_sun, and ages range from
2 - 6 Myrs. This analysis includes an updated calculation of the conversion
factor between the ionizing luminosity and the total number of main sequence O
stars contained within an HII region. The photometric properties and sizes of
the isolated associations and other objects in the HRC fields are consistent
with those of Galactic stellar associations, open clusters and/or single O and
B stars. We interpret the age-size sequence of associations and clustered field
objects as an indication that these isolated associations are most likely
rapidly dispersing. Furthermore, we consider the possibility that these
isolated associations represent the first generation of stars in the HI ring
surrounding NGC 1533. This work suggests star formation in the unique
environment of a galaxy's outermost gaseous regions proceeds similarly to that
within the Galactic disk and that star formation in tidal debris may be
responsible for building up a younger halo component.Comment: 21 pages, 9 figures, 6 tables; accepted for publication in Ap
Cilia Have a Significant Role in Regulating Cell Size in Response to Fluid Flow Induced Shear Stress in a Flow Chamber
Cilia are hair-like protrusions on the apical surface of cells. Their function is to relay mechanical signals like shear stress from extracellular into intracellular environment and thereby maintain cellular homeostasis. Ciliary dysfunctions include polycystic kidney disease and new therapeutic interventions based on ciliary function are under investigation. The current study evaluates the use of a custom designed fluid flow chamber’s ability to study the role of cilia in regulating cell size in response to shear stress.
A fluid flow chamber that continually maintains laminar flow at different flow rates and temperature was designed. Endothelial wild type cells (ETWT) that have cilia and polycystic kidney disease cells (PKD) that lost their ciliary function are grown on different glass slides. Cells on each glass slide are then exposed to continuous flow of phosphate-buffered saline at 37oC in the flow chamber. The optimal flow rate and duration of flow were first determined by measuring the total protein concentration before and after exposing the cells. Cell radius and area before and after exposing them to flow are measured using the NIS Software available on the microscope.
The results from protein concentrations (n=12) indicate that cells are still attached at normal physiological flow rate 467 mL/min (2.8 µg/µL) and did not significantly differ from 60 mL/min (4.08 µg/µL) or 600 mL/min (2.73 µg/µL). The results for duration of fluid flow (n=22) show that 60 minutes (0.09 + 0.01 µg/µL) is optimal compared to 120 minutes (0.06 + 0.01 µg/µL) or 180 minutes (0.10 + 0.02 µg/µL). Under these optimal conditions, the average area of ETWT cells (n=300) measured from different slides before and after the flow is 4420.81+ 67.40 µm2 and 4678.17 + 87.15 µm2 (n=200) respectively. For PKD cells, the average area before and after the flow (n=300) is 5682.46 + 105.48 µm2 and 4173.74 + 263.97 µm2 (n=250).
These results are in agreement with the published literature on the ability of cilia to maintain cell size in ETWT cells in response to shear stress that is similar to normal blood flow. However, under similar conditions, PKD cells could not maintain their cell size as the mechano-chemical signaling pathway that communicates external signals to prepare appropriate intracellular response is disrupted. These results provide confirmation that the custom designed parallel plate fluid flow chamber is a reliable tool to investigate the specific targets in the mechano-chemical cell signaling pathways
- …