1906.07724
The assembly of the Virgo cluster, traced by its galaxy halos
Taylor, et al
Kinematic studies have produced accurate measurements of the total dark matter mass and mean dark matter density within the optical extent of galaxies, for large samples of objects. Here we consider theoretical predictions for the latter quantity, $\bar{\rho}_{dm}$, measured within the isophotal radius $R_{23.5}$, for isolated halos with universal density profiles. Through a combination of empirical scaling relations, we show that $\bar{\rho}_{dm}$ is expected to depend weakly on halo mass and strongly on redshift. When galaxy halos fall into larger groups or clusters they become tidally stripped, reducing their total dark matter mass, but this process is expected to preserve central density until an object is close to disruption. We confirm this with collisonless simulations of cluster formation, finding that subhalos have values of $\bar{\rho}_{dm}$ close to the analytic predictions. This suggests that $\bar{\rho}_{dm}$ may be a useful indicator of infall redshift onto the cluster. We test this hypothesis with data from the SHIVir survey, which covers a reasonable fraction of the Virgo cluster. We find that galaxies with high $\bar{\rho}_{dm}$ do indeed trace the densest regions of the cluster, with a few notable exceptions. Samples selected by environment have higher densities at a significance of 3.5-4$\sigma$, while samples selected by density are more clustered at 3-3.5$\sigma$ significance. We conclude that halo density can be a powerful tracer of the assembly history of clusters and their member galaxies.
The assembly of the Virgo cluster, traced by its galaxy halos
Taylor, et al
Kinematic studies have produced accurate measurements of the total dark matter mass and mean dark matter density within the optical extent of galaxies, for large samples of objects. Here we consider theoretical predictions for the latter quantity, $\bar{\rho}_{dm}$, measured within the isophotal radius $R_{23.5}$, for isolated halos with universal density profiles. Through a combination of empirical scaling relations, we show that $\bar{\rho}_{dm}$ is expected to depend weakly on halo mass and strongly on redshift. When galaxy halos fall into larger groups or clusters they become tidally stripped, reducing their total dark matter mass, but this process is expected to preserve central density until an object is close to disruption. We confirm this with collisonless simulations of cluster formation, finding that subhalos have values of $\bar{\rho}_{dm}$ close to the analytic predictions. This suggests that $\bar{\rho}_{dm}$ may be a useful indicator of infall redshift onto the cluster. We test this hypothesis with data from the SHIVir survey, which covers a reasonable fraction of the Virgo cluster. We find that galaxies with high $\bar{\rho}_{dm}$ do indeed trace the densest regions of the cluster, with a few notable exceptions. Samples selected by environment have higher densities at a significance of 3.5-4$\sigma$, while samples selected by density are more clustered at 3-3.5$\sigma$ significance. We conclude that halo density can be a powerful tracer of the assembly history of clusters and their member galaxies.
1906.07737
Cooling flow solutions for the circumgalactic medium
Stern, et al
In several models of galaxy formation feedback occurs in cycles or mainly at high redshift. At times and in regions where feedback heating is ineffective, hot gas in the galaxy halo is expected to form a cooling flow, where the gas advects inward on a cooling timescale. Cooling flow solutions can thus be used as a benchmark for observations and simulations to constrain the timing and extent of feedback heating. Using analytic calculations and idealized 3D hydrodynamic simulations, we show that for a given halo mass and cooling function, steady-state cooling flows form a single-parameter family of solutions, while initially hydrostatic gaseous halos converge on one of these solutions within a cooling time. The solution is thus fully determined once either the mass inflow rate $\dot{M}$ or the total halo gas mass are known. In the Milky Way (MW) halo, a cooling flow with $\dot{M}$ equal to the star formation rate predicts a ratio of the cooling time to the free-fall time of ~10, similar to some feedback-regulated models. This solution also correctly predicts observed OVII and OVIII absorption columns, and the gas density profile implied by OVII and OVIII emission. These results suggest ongoing heating by feedback may be negligible in the inner MW halo. Extending similar solutions out to the cooling radius however underpredicts observed OVI columns around the MW and around other low-redshift star-forming galaxies. This can be reconciled with the successes of the cooling flow model with either a mechanism which preferentially heats the OVI-bearing outer halo, or alternatively if OVI traces cool photoionized gas beyond the accretion shock. We also demonstrate that the entropy profiles of some of the most relaxed clusters are reasonably well described by a cooling flow solution.
1906.07750
The Breakthrough Listen search for intelligent life: Observations of 1327 nearby stars over 1.10-3.45 GHz
Price, et al
Breakthrough Listen (BL) is a ten-year initiative to search for signatures of technologically capable life beyond Earth via radio and optical observations of the local Universe. A core part of the BL program is a comprehensive survey of 1702 nearby stars at radio wavelengths (1-10 GHz). Here, we report on observations with the 64-m CSIRO Parkes radio telescope in New South Wales, Australia, and the 100-m Robert C. Byrd Green Bank radio telescope in West Virginia, USA. Over 2016 January to 2019 March, a sample of 1138 stars was observed at Green Bank using the 1.10-1.90 GHz and 1.80-2.80 GHz receivers, and 189 stars were observed with Parkes over 2.60{3.45 GHz. We searched these data for the presence of engineered signals with Doppler-acceleration drift rates between -4 to 4 Hz/s. Here, we detail our data analysis techniques and provide examples of detected events. After excluding events with characteristics consistent with terrestrial radio interference, we are left with zero candidates. Given the sensitivity of our observations, we can put an upper limit on the power of potential radio transmitters at these frequencies at 2x10^12 W, and 9x10^12 W for GBT and Parkes respectively. These observations constitute the most comprehensive search over 1.10-3.45 GHz for technosignatures to date for Kardashev Type I civilizations. All data products, totalling ~219 TB, are available for download as part of the first BL data release (DR1), as described in a companion paper (Lebofsky et. al., 2019)
1906.08174
Optical follow-up study of 32 high-redshift galaxy cluster candidates from Planck with the William Herschel Telescope
Zohren, Schrabback, et al
The Planck satellite has detected cluster candidates via the Sunyaev Zel'dovich (SZ) effect, but the optical follow-up required to confirm these candidates is still incomplete, especially at high redshifts and for SZ detections at low significance. In this work we present our analysis of optical observations obtained for 32 Planck cluster candidates using ACAM on the 4.2-m William Herschel Telescope. These cluster candidates were preselected using SDSS, WISE, and Pan-STARRS images to likely represent distant clusters at redshifts $z \gtrsim 0.7$. We obtain photometric redshift and richness estimates for all of the cluster candidates from a red-sequence analysis of $r$-, $i$-, and $z$-band imaging data. In addition, long-slit observations allow us to measure the redshifts of a subset of the clusters spectroscopically. The optical richness is often lower than expected from the inferred SZ mass when compared to scaling relations previously calibrated at low redshifts. This likely indicates the impact of Eddington bias and projection effects or noise-induced detections, especially at low SZ-significance. Thus, optical follow-up not only provides redshift measurements, but also an important independent verification method. We find that 18 (7) of the candidates at redshifts $z > 0.5$ ($z > 0.8$) are at least half as rich as expected from scaling relations, thereby clearly confirming these candidates as massive clusters. While the complex selection function of our sample due to our preselection hampers its use for cosmological studies, we do provide a validation of massive high-redshift clusters particularly suitable for further astrophysical investigations.
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