Monday.
1301.2328
Molecular gas and star formation in nearby disk galaxies
Leroy et al
Compare molecular gas traced by 12CO(2-1) maps from HERACLES survey with tracers of the recent SFR across 30 nearby disk galaxies. Demonstrate first-order linear correspondence between Sig_mol and Sig_SFR but also find important second-order systematic variations in the apparent molecular gas depletion time, t_dep(mol) = Sig_mol/Sig_SFR. At 1kpc resolution, CO correlates closely with may tracers of the recent SFR. Weighting each line of sight equally and using a fixed, MW alpha_CO, data yield a molecular gas depletion time ~ 2.2 Gyr with 0.3 dex scatter, in good agreement with literature data [what are the other lit. data based on?]. Apply a forward-modeling approach to constrain the power-law index, N, that relates the SFR surface density and molecular gas surface density and find N=1pm0.15 for the full dataset (some variation for individual galaxies). Caution that a power law treatment oversimplifies the topic. The strongest of the simplifications are a decreased t_dep(mol) in low-mass, low-metallicity galaxies and a correlation of the kpc-scale t_dep(mol) with dust-to-gas ratio, D/G. These correlations can be explained by a CO-to-H2 conversion factor that depends on D/G in the theoretically expected way. After applying a D/G-dependent alpha_CO, some weak correlations between t_dep(mol) and local conditions persist. In particular, observe lower t_dep(mol) and enhanced CO excitation associated with some nuclear gas concentrations. These appear to reflect real enhancements in the SFR/H2 and t_dep appears multivalues at fixed Sig_mol, supporting the idea of "disk" and "SB" modes driven by environmental factors.
1301.2330
The velocity function of dark matter haloes at r=20 kpc: Remarkably little evolution since z~4
Weinmann, Franx, Dokkum, Brezanson
In the Millennium-II simulation. As the title says. Analyze the histories of the main progenitors of haloes, and find that the DM distribution within the central 20 kpc of massive halos has been in place since early times. This provides evidence for the inside-out growth of haloes. The constance of the central circular velocity of haloes may offer a natrual explanation for the observational finding that the galaxy circular velocity is an excellent predictor of various galaxy properties. Results also indicate that we can expect a significant number of galaxies with high circular velocities already at z=4. Finally, adding baryonic mass and using a simple model for halo adiabatic contraction, find remarkable agreement with the velocity dispersion functions inferred observationally by Bezanson+2011 up to z~1 and down to about 220 km/s.
1301.2338
HST proper motions of stars within globular clusters
Bellini, van der Marel, Anderson
HST is an excellent astrometric tool: diffraction-limited resolution allows measurements of positions and fluxes for stars all the way to the center of most globular clusters. Apart from small changes due to breathing, its PSFs and geometric distortion have been extremely stable over its 20-year lifetime. Now >20 globular clusters for which there exist two or more well-separated epochs in the archive, spanning up to 10+ years. Photometric and astrometric techniques have allowed measurements of 10s of 1000s of stars per cluster within 1" from the center, with typical proper-motion errors of ~0.02 mas/yr, which translates to 0.8 km/s for a typical cluster. THese high-quality measurements can be used to detect the possible presence of a central intermediate-mass BH and put constraints on its mass. In addition, they will provide a direct measurement of the cluster anisotropy and equipartition. Present preliminary results from this project, and discuss them in the context of what is already known from other techniques.
1301.2576
Decomposing CMB lensing power with simulation
Anderes
Reconstruction of CMB lensing potential is based on a Taylor expansion of lensing effects which is known to have poor convergence properties. For lensing of temperature fluctuations, an understanding of the higher order terms in this expansion accurate enough for current experimental sensitivity levels has been developed in Hanson+2010, as well as a slightly modified Okamoto+Hu quadratic estimator which incorporates lensed rather than unlensed spectra into the estimator weights to mitigate the effect of higher order terms. Extend these results in several ways: (1) generalize this analysis to the full set of quadratic temperature/polarization lensing estimators, (2) study the effect of higher order terms for more futuristic experimental noise levels, (3) show that the ability of the modified quadratic estimator to mitigate the effect of higher order terms relies on a delicate cancellation which occurs only when the true lensed spectra are known. Investigate the sensitivity of this cancellation to uncertainties in or knowledge of these spectra. Find that higher order terms in the Taylor expansion can impact projected error bars at experimental sensitivities similar to those found in future ACTpol/SPTpol experiments.
1301.2328
Molecular gas and star formation in nearby disk galaxies
Leroy et al
Compare molecular gas traced by 12CO(2-1) maps from HERACLES survey with tracers of the recent SFR across 30 nearby disk galaxies. Demonstrate first-order linear correspondence between Sig_mol and Sig_SFR but also find important second-order systematic variations in the apparent molecular gas depletion time, t_dep(mol) = Sig_mol/Sig_SFR. At 1kpc resolution, CO correlates closely with may tracers of the recent SFR. Weighting each line of sight equally and using a fixed, MW alpha_CO, data yield a molecular gas depletion time ~ 2.2 Gyr with 0.3 dex scatter, in good agreement with literature data [what are the other lit. data based on?]. Apply a forward-modeling approach to constrain the power-law index, N, that relates the SFR surface density and molecular gas surface density and find N=1pm0.15 for the full dataset (some variation for individual galaxies). Caution that a power law treatment oversimplifies the topic. The strongest of the simplifications are a decreased t_dep(mol) in low-mass, low-metallicity galaxies and a correlation of the kpc-scale t_dep(mol) with dust-to-gas ratio, D/G. These correlations can be explained by a CO-to-H2 conversion factor that depends on D/G in the theoretically expected way. After applying a D/G-dependent alpha_CO, some weak correlations between t_dep(mol) and local conditions persist. In particular, observe lower t_dep(mol) and enhanced CO excitation associated with some nuclear gas concentrations. These appear to reflect real enhancements in the SFR/H2 and t_dep appears multivalues at fixed Sig_mol, supporting the idea of "disk" and "SB" modes driven by environmental factors.
1301.2330
The velocity function of dark matter haloes at r=20 kpc: Remarkably little evolution since z~4
Weinmann, Franx, Dokkum, Brezanson
In the Millennium-II simulation. As the title says. Analyze the histories of the main progenitors of haloes, and find that the DM distribution within the central 20 kpc of massive halos has been in place since early times. This provides evidence for the inside-out growth of haloes. The constance of the central circular velocity of haloes may offer a natrual explanation for the observational finding that the galaxy circular velocity is an excellent predictor of various galaxy properties. Results also indicate that we can expect a significant number of galaxies with high circular velocities already at z=4. Finally, adding baryonic mass and using a simple model for halo adiabatic contraction, find remarkable agreement with the velocity dispersion functions inferred observationally by Bezanson+2011 up to z~1 and down to about 220 km/s.
1301.2338
HST proper motions of stars within globular clusters
Bellini, van der Marel, Anderson
HST is an excellent astrometric tool: diffraction-limited resolution allows measurements of positions and fluxes for stars all the way to the center of most globular clusters. Apart from small changes due to breathing, its PSFs and geometric distortion have been extremely stable over its 20-year lifetime. Now >20 globular clusters for which there exist two or more well-separated epochs in the archive, spanning up to 10+ years. Photometric and astrometric techniques have allowed measurements of 10s of 1000s of stars per cluster within 1" from the center, with typical proper-motion errors of ~0.02 mas/yr, which translates to 0.8 km/s for a typical cluster. THese high-quality measurements can be used to detect the possible presence of a central intermediate-mass BH and put constraints on its mass. In addition, they will provide a direct measurement of the cluster anisotropy and equipartition. Present preliminary results from this project, and discuss them in the context of what is already known from other techniques.
1301.2576
Decomposing CMB lensing power with simulation
Anderes
Reconstruction of CMB lensing potential is based on a Taylor expansion of lensing effects which is known to have poor convergence properties. For lensing of temperature fluctuations, an understanding of the higher order terms in this expansion accurate enough for current experimental sensitivity levels has been developed in Hanson+2010, as well as a slightly modified Okamoto+Hu quadratic estimator which incorporates lensed rather than unlensed spectra into the estimator weights to mitigate the effect of higher order terms. Extend these results in several ways: (1) generalize this analysis to the full set of quadratic temperature/polarization lensing estimators, (2) study the effect of higher order terms for more futuristic experimental noise levels, (3) show that the ability of the modified quadratic estimator to mitigate the effect of higher order terms relies on a delicate cancellation which occurs only when the true lensed spectra are known. Investigate the sensitivity of this cancellation to uncertainties in or knowledge of these spectra. Find that higher order terms in the Taylor expansion can impact projected error bars at experimental sensitivities similar to those found in future ACTpol/SPTpol experiments.
No comments:
Post a Comment