The Evolving Faint-End of the Luminosity Function

We investigate the evolution of the faint-end slope of the luminosity function, $\alpha$, using semi-analytical modeling of galaxy formation. In agreement with observations, we find that the slope can be fitted well by $\alpha (z) =a+b z$, with a=-1.13 and b=-0.1. The main driver for the evolution in $\alpha$ 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 $\alpha$ 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 $\alpha(z)$ 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 $z_* \sim 2$, and suggests that a single linear fit to the overall evolution of $\alpha(z)$ 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 $\alpha(z)$ at z< z_*. In addition we find that different star formation efficiencies a_* in the Schmidt-Kennicutt-law and supernovae-feedback efficiencies $\epsilon$ generally do not strongly influence the evolution of $\alpha(z)$.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