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Register a new userThe growth rate of cosmic structure from peculiar velocities at low and high redshifts
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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.
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Measurement of peculiar velocities by combining redshifts and distance indicators is a powerful way to measure the growth rate of cosmic structure and test theories of gravity at low redshift. Here we constrain the growth rate of structure by comparing observed Fundamental Plane peculiar velocities for 15894 galaxies from the 6dF Galaxy Survey (6dFGS) and Sloan Digital Sky Survey (SDSS) with predicted velocities and densities from the 2M$++$ redshift survey. We measure the velocity scale parameter $beta equiv {Omega_m^gamma}/b = 0.372^{+0.034}_{-0.050}$ and $0.314^{+0.031}_{-0.047}$ for 6dFGS and SDSS respectively, where $Omega_m$ is the mass density parameter, $gamma$ is the growth index, and $b$ is the bias parameter normalized to the characteristic luminosity of galaxies, $L^*$. Combining 6dFGS and SDSS we obtain $beta= 0.341pm0.024$, implying that the amplitude of the product of the growth rate and the mass fluctuation amplitude is $fsigma_8 = 0.338pm0.027$ at an effective redshift $z=0.035$. Adopting $Omega_m = 0.315pm0.007$ as favoured by Planck and using $gamma=6/11$ for General Relativity and $gamma=11/16$ for DGP gravity, we get $S_8(z=0) = sigma_8 sqrt{Omega_m/0.3} =0.637 pm 0.054$ and $0.741pm0.062$ for GR and DGP respectively. This measurement agrees with other low-redshift probes of large scale structure but deviates by more than $3sigma$ from the latest Planck CMB measurement. Our results favour values of the growth index $gamma > 6/11$ or a Hubble constant $H_0 > 70$,km,s$^{-1}$,Mpc$^{-1}$ or a fluctuation amplitude $sigma_8 < 0.8$ or some combination of these. Imminent redshift surveys such as Taipan, DESI, WALLABY, and SKA1-MID will help to resolve this tension by measuring the growth rate of cosmic structure to 1% in the redshift range $0 < z < 1$.
How do peculiar velocities affect observed voids? To answer this question we use the VIDE toolkit to identify voids in mock galaxy populations embedded within an N-body simulation both with and without peculiar velocities included. We compare the resulting void populations to assess the impact on void properties. We find that void abundances and spherically-averaged radial density profiles are mildly affected by peculiar velocities. However, peculiar velocities can distort by up to 10% the shapes for a particular subset of voids depending on the void size and density contrast, which can lead to increased variance in Alcock-Paczynski test. We offer guidelines for performing optimal cuts on the void catalogue to reduce this variance by removing the most severely affected voids while preserving the unaffected ones. In addition, since this shape distortion is largely limited to the line of sight, we show that the void radii are only affected at the $sim$ 10% level and the macrocenter positions at the $sim$ 20% (even before performing cuts), meaning that cosmological probes based on the Integrated Sachs-Wolfe and gravitational lensing are not severely impacted by peculiar velocities.
This paper presents a compilation of clustering results taken from the literature for galaxies with highly enhanced (SFR [30-10^3] Msun/yr) star formation activity observed in the redshift range z=[0-3]. We show that, irrespective of the selection technique and only very mildly depending on the star forming rate, the clustering lengths of these objects present a sharp increase of about a factor 3 between z~1 and z~2, going from values of ~5 Mpc to about 15 Mpc and higher. This behaviour is reflected in the trend of the masses of the dark matter hosts of star-forming galaxies which increase from ~10^11.5 Msun to ~10^13.5 Msun between z~1 and z~2. Our analysis shows that galaxies which actively form stars at high redshifts are not the same population of sources we observe in the more local universe. In fact, vigorous star formation in the early universe is hosted by very massive structures, while for z~1 a comparable activity is encountered in much smaller systems, consistent with the down-sizing scenario. The available clustering data can hardly be reconciled with merging as the main trigger for intense star formation activity at high redshifts. We further argue that, after a characteristic time-scale of ~1 Gyr, massive star-forming galaxies at z>~2 evolve into z<~1.5 passive galaxies with large (Mstellar=[10^11 - 10^12] Msun) stellar masses.
We discuss how the interaction between the electrons in a relativistic jet and the Cosmic Microwave Background (CMB) affects the observable properties of radio-loud AGN at early epochs. At high z the magnetic energy density in the radio lobes of powerful radio-loud quasars can be exceeded by the energy density of the CMB (because of its (1+z)^4 dependance). In this case, relativistic electrons cool preferentially by scattering off CMB photons, rather than by synchrotron. Thus, sources sharing the same intrinsic properties have different extended radio and X-ray luminosities when located at different z: more distant sources are less luminous in radio and more luminous in X-rays than their closer counterparts. Instead, in compact regions where the local magnetic field still exceeds the CMB in terms of energy density, synchrotron radiation would be unaffected by the presence of the CMB. Such regions include the compact inner jet and the so-called hot spots in the radio lobes. The decrease in radio luminosity is larger in misaligned sources, whose radio flux is dominated by the extended isotropic component. These sources can fail detection in current flux limited radio surveys, and therefore they are possibly under-represented in the associated samples. As the cooling time is longer for lower energy electrons, the radio luminosity deficit due to the CMB photons is less important at low radio frequencies. Therefore objects not detected so far in current surveys at a few GHz could be picked up by low frequency deep surveys, such as LOFAR and SKA. Until then, we can estimate the number of high redshift radio-loud AGNs through the census of their aligned proxies, i.e., blazars. Indeed, their observed radio emission arises in the inner and strongly magnetized compact core of the relativistic jet, and not affected by inverse Compton scattering off CMB photons.
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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