Carma CO observations of three extremely metal-poor, star-forming galaxies

We present sensitive CO (J = 1 0) emission line observations of the three metal-poor dwarf irregular galaxies Leo P (Z ∼ 3% Zo), Sextans A (Z ∼ 7.5% Zo), and Sextans B (Z ∼ 7.5% Zo), all obtained with the Combined Array for Millimeter-wave Astronomy interferometer. While no CO emission was detected, the proximity of the three systems allows us to place very stringent (4σ) upper limits on the CO luminosity (LCO) in these metal-poor galaxies. We find the CO luminosities to be LCO < 2900 K km s-1 pc2 for Leo P, LCO < 12,400 K km s-1 pc2 for Sextans A, and LCO < 9700 K km s-1 pc2 for Sextans B. Comparison of our results with recent observational estimates of the factor for converting between LCO and the mass of molecular hydrogen, as well as theoretical models, provides further evidence that either the CO-to-H2 conversion factor increases sharply as metallicity decreases, or that stars are forming in these three galaxies very efficiently, requiring little molecular hydroge

PEGylated surfaces for the study of DNA–protein interactions by atomic force microscopy

DNA–protein interactions are vital to cellular function, with key roles in the regulation of gene expression and genome maintenance. Atomic force microscopy (AFM) offers the ability to visualize DNA–protein interactions at nanometre resolution in near-physiological buffers, but it requires that the DNA be adhered to the surface of a solid substrate. This presents a problem when working in biologically relevant protein concentrations, where proteins may be present in large excess in solution; much of the biophysically relevant information can therefore be occluded by non-specific protein binding to the underlying substrate. Here we explore the use of PLLx-b-PEGy block copolymers to achieve selective adsorption of DNA on a mica surface for AFM studies. Through varying both the number of lysine and ethylene glycol residues in the block copolymers, we show selective adsorption of DNA on mica that is functionalized with a PLL10-b-PEG113/PLL1000–2000 mixture as viewed by AFM imaging in a solution containing high concentrations of streptavidin. We show – through the use of biotinylated DNA and streptavidin – that this selective adsorption extends to DNA–protein complexes and that DNA-bound streptavidin can be unambiguously distinguished in spite of an excess of unbound streptavidin in solution. Finally, we apply this to the nuclear enzyme PARP1, resolving the binding of individual PARP1 molecules to DNA by in-liquid AFM

The Turn-Down of the Baryonic Tully-Fisher Relation at Low Galaxy Masses

The ratio of baryonic to dark matter in present-day galaxies constrains galaxy formation theories and can be determined empirically via the baryonic Tully-Fisher relation (BTFR), which compares a galaxy's baryonic mass (M$_{bary}$) to its maximum rotation velocity (V$_{max}$). The BTFR is well-determined at M$_{bary}>10^8$ M$_{\odot}$, but poorly constrained at lower masses due to small samples and the challenges of measuring rotation velocities in this regime. For 25 galaxies with high-quality data and M$_{bary}<\sim10^8$ M$_{\odot}$, we estimate M$_{bary}$ from infrared, optical, and HI observations and Vmax from the HI gas rotation. Many of the V$_{max}$ values are lower limits because the velocities are still rising at the edge of the detected HI disks; consequently, most of our sample has lower velocities than expected from extrapolations of the BTFR at higher masses. To estimate V$_{max}$, we map each galaxy to a dark matter halo assuming density profiles with and without cores, and find that the cored profiles match the data better. When we compare the V$_{max}$ values derived from the cored density profiles to our M$_{bary}$ measurements, we find a turndown of the BTFR at low masses that is consistent with CDM predictions and implying baryon fractions of 1-10% of the cosmic value. Although we are limited by the sample size and assumptions inherent in mapping measured rotational velocities to theoretical rotation curves, our results suggest that the galaxy formation efficiency drops at masses below M$_{bary}\sim10^8$ M$_{\odot}$, corresponding to M$_{200}\sim10^{10}$ M$_{\odot}$.Comment: 36 pages, 3 tables, 23 figure