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Register a new userPeculiar Transverse Velocities of Galaxies from Quasar Microlensing. Tentative Estimate of the Peculiar Velocity Dispersion at $zsim 0.5$
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We propose to use the flux variability of lensed quasar images induced by gravitational microlensing to measure the transverse peculiar velocity of lens galaxies over a wide range of redshift. Microlensing variability is caused by the motions of the observer, the lens galaxy (including the motion of the stars within the galaxy), and the source; hence, its frequency is directly related to the galaxys transverse peculiar velocity. The idea is to count time-event rates (e.g., peak or caustic crossing rates) in the observed microlensing light curves of lensed quasars that can be compared with model predictions for different values of the transverse peculiar velocity. To compensate for the large time-scale of microlensing variability we propose to count and model the number of events in an ensemble of gravitational lenses. We develop the methodology to achieve this goal and apply it to an ensemble of 17 lensed quasar systems. In spite of the shortcomings of the available data, we have obtained tentative estimates of the peculiar velocity dispersion of lens galaxies at $zsim 0.5$, $sigma_{rm pec}(0.53pm0.18)simeq(638pm213)sqrt{langle m rangle/0.3 M_odot} , rm km, s^{-1}$. Scaling at zero redshift we derive, $sigma_{rm pec}(0)simeq(491pm164) sqrt{langle m rangle/0.3 M_odot} , rm km, s^{-1}$, consistent with peculiar motions of nearby galaxies and with recent $N$-body nonlinear reconstructions of the Local Universe based on $Lambda$CDM. We analyze the different sources of uncertainty of the method and find that for the present ensemble of 17 lensed systems the error is dominated by Poissonian noise, but that for larger ensembles the impact of the uncertainty on the average stellar mass may be significant.
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We present an analysis of peculiar velocities and their effect on supernova cosmology. In particular, we study (a) the corrections due to our own motion, (b) the effects of correlations in peculiar velocities induced by large-scale structure, and (c) uncertainties arising from a possible local under- or over-density. For all of these effects we present a case study of their impact on the cosmology derived by the Sloan Digital Sky Survey-II Supernova Survey (SDSS-II SN Survey). Correcting supernova redshifts for the CMB dipole slightly over-corrects nearby supernovae that share some of our local motion. We show that while neglecting the CMB dipole would cause a shift in the derived equation of state of Delta w ~ 0.04 (at fixed matter density) the additional local-motion correction is currently negligible (Delta w<0.01). We use a covariance-matrix approach to statistically account for correlated peculiar velocities. This down-weights nearby supernovae and effectively acts as a graduated version of the usual sharp low-redshift cut. Neglecting coherent velocities in the current sample causes a systematic shift of ~2% in the preferred value of w and will therefore have to be considered carefully when future surveys aim for percent-level accuracy. Finally, we perform n-body simulations to estimate the likely magnitude of any local density fluctuation (monopole) and estimate the impact as a function of the low-redshift cutoff. We see that for this aspect the low-z cutoff of z=0.02 is well-justified theoretically, but that living in a putative local density fluctuation leaves an indelible imprint on the magnitude-redshift relation.
We study correlated fluctuations of Type~Ia supernova observables due to peculiar velocities of both the observer and the supernova host galaxies, and their impact on cosmological parameter estimation. We demonstrate using the CosmicFlows-3 dataset that at low redshifts the corrections for peculiar velocities in the JLA catalogue have been systematically underestimated. By querying a horizon-size N-body simulation we find that compared to a randomly placed observer, an observer in an environment like our local Universe will see 2-8 times stronger correlations between supernovae in the JLA catalogue. Hence the covariances usually employed assuming a typical observer are unphysical and underestimate the effects of coherent motion of the supernova host galaxies. Contrary to previous studies which asserted that this should have negligible effect on cosmological parameter estimation, we find that when peculiar velocities are treated consistently the JLA data favours significantly smaller values of the dark energy density than in the standard $Lambda$CDM model. A joint fit to simultaneously determine the cosmological parameters and the bulk flow indicates that the latter is around 250 km/s even beyond 200$h^{-1}$ Mpc. The local bulk flow is thus an essential nuisance parameter which must be included in cosmological model fitting when analysing supernova data.
Peculiar velocities are an important probe of the growth rate of mass density fluctuations in the Universe. Most previous studies have focussed exclusively on measuring peculiar velocities at intermediate ($0.2 < z < 1$) redshifts using statistical redshift-space distortions. Here we emphasize the power of peculiar velocities obtained directly from distance measurements at low redshift ($z lesssim 0.05$), and show that these data break the usual degeneracies in the Omega_{m,0} -- $sigma_{8,0}$ parameter space. Using only peculiar velocity data, we find $Omega_{m,0} = 0.259pm0.045$ and $sigma_{8,0} = 0.748pm0.035$. Fixing the amplitude of fluctuations at very high redshift using observations of the Cosmic Microwave Background (CMB), the same data can be used to constrain the growth index $gamma$, with the strongest constraints coming from peculiar velocity measurements in the nearby Universe. We find $gamma = 0.619pm 0.054$, consistent with LCDM. Current peculiar velocity data already strongly constrain modified gravity models, and will be a powerful test as data accumulate.
Type Ia Supernovae (SNe Ia) are widely used to measure the expansion of the Universe. To perform such measurements the luminosity and cosmological redshift ($z$) of the SNe Ia have to be determined. The uncertainty on $z$ includes an unknown peculiar velocity, which can be very large for SNe Ia in the virialized cores of massive clusters. We determine which SNe Ia exploded in galaxy clusters. We then study how the correction for peculiar velocities of host galaxies inside the clusters improves the Hubble residuals. Using 145 SNe Ia from the Nearby Supernova Factory we found 11 candidates for membership in clusters. To estimate the redshift of a cluster we applied the bi-weight technique. Then, we use the galaxy cluster redshift instead of the host galaxy redshift to construct the Hubble diagram. For SNe Ia inside galaxy clusters the dispersion around the Hubble diagram when peculiar velocities are taken into account is smaller in comparison with a case without peculiar velocity correction, with a $wRMS=0.130pm0.038$ mag instead of $wRMS=0.137pm0.036$ mag. The significance of this improvement is 3.58 $sigma$. If we remove the very nearby Virgo cluster member SN2006X ($z<0.01$) from the analysis, the significance decreases to 1.34 $sigma$. The peculiar velocity correction is found to be highest for the SNe Ia hosted by blue spiral galaxies, with high local specific star formation rate and smaller stellar mass, seemingly counter to what might be expected given the heavy concentration of old, massive elliptical galaxies in clusters. As expected, the Hubble residuals of SNe Ia associated with massive galaxy clusters improve when the cluster redshift is taken as the cosmological redshift of the SN. This fact has to be taken into account in future cosmological analyses in order to achieve higher accuracy for cosmological redshift measurements. Here we provide an approach to do so.
The line-of-sight peculiar velocities of galaxies contribute to their observed redshifts, breaking the translational invariance of galaxy clustering down to a rotational invariance around the observer. This becomes important when the line-of-sight direction varies significantly across a survey, leading to what are known as `wide angle effects in redshift space distortions. Wide-angle effects will also be present in measurements of the momentum field, i.e. the galaxy density-weighted velocity field, in upcoming peculiar velocity surveys. In this work we study how wide-angle effects modify the predicted correlation function and power spectrum for momentum statistics, both in auto-correlation and in cross-correlation with the density field. Using both linear theory and the Zeldovich approximation, we find that deviations from the plane-parallel limit are large and could become important in data analysis for low redshift surveys. We point out that even multipoles in the cross-correlation between density and momentum are non-zero regardless of the choice of line of sight, and therefore contain new cosmological information that could be exploited. We discuss configuration-space, Fourier-space and spherical analyses, providing exact expressions in each case rather than relying on an expansion in small angles. We hope these expressions will be of use in the analysis of upcoming surveys for redshift-space distortions and peculiar velocities.
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