We revisit the well known discrepancy between the observed number of Milky Way (MW) dwarf satellite companions and the predicted population of cold dark matter (CDM) subhalos, in light of the dozen ...new low-luminosity satellites found in imaging data from the Sloan Digital Sky Survey (SDSS) and our recent calibration of the SDSS satellite detection efficiency, which implies a total satellite population far larger than these dozen discoveries. We combine a detailed dynamical model for the CDM subhalo population with simple, physically motivated prescriptions for assigning a stellar content to each subhalo, then apply observational selection effects and compare to the current observational census. Reconciling the observed satellite population with CDM predictions still requires strong mass-dependent suppression of star formation in low-mass subhalos: models in which the stellar mass is a constant fraction F *( b / m ) of the subhalo mass M sat at the time it becomes a satellite fail for any choice of F *. However, previously advocated models that invoke suppression of gas accretion after reionization in halos with circular velocity V circ <= V crit 35 km s-1 can reproduce the observed satellite counts for -15 <= MV <= 0. Successful models require F * 10-3 in halos with V circ>V crit and strong suppression of star formation before reionization in halos with V circ 10 km s-1; models without pre-reionization suppression predict far too many satellites with -5 <= MV <= 0. In this successful model, the dominant fraction of stars formed after reionization at all luminosities. Models that match the satellite luminosity distribution also match the observed heliocentric radius distribution, and they reproduce the observed characteristic stellar velocity dispersion s* 5-10 km s-1 of the SDSS dwarfs given the observed sizes (~50-200 pc) of their stellar distributions. The model satellites have M(<300 pc) ~ 107 M as observed even though their present-day total halo masses span more than two orders of magnitude; the constancy of central masses mainly reflects the profiles of CDM halos. Our modeling shows that natural physical mechanisms acting within the CDM framework can quantitatively explain the properties of the MW satellite population as it is presently known, thus providing a convincing solution to the 'missing satellite' problem.
Abstract
We measure the quasar two-point correlation function over the redshift range 2.2 < z < 2.8 using data from the Baryon Oscillation Spectroscopic Survey. We use a homogeneous subset of the ...data consisting of 27 129 quasars with spectroscopic redshifts - by far the largest such sample used for clustering measurements at these redshifts to date. The sample covers 3600 deg2, corresponding to a comoving volume of 9.7 (h
−1 Gpc)3 assuming a fiducial Λ cold dark matter cosmology, and it has a median absolute i-band magnitude of −26, k-corrected to z= 2. After accounting for redshift errors we find that the redshift-space correlation function is fitted well by a power law of slope −2 and amplitude s
0 = (9.7 ± 0.5) h
−1 Mpc over the range 3 < s < 25 h
−1 Mpc. The projected correlation function, which integrates out the effects of peculiar velocities and redshift errors, is fitted well by a power law of slope −1 and r
0 = (8.4 ± 0.6) h
−1 Mpc over the range 4 < R < 16 h
−1 Mpc. There is no evidence for strong luminosity or redshift dependence to the clustering amplitude, in part because of the limited dynamic range in our sample. Our results are consistent with, but more precise than, previous measurements at similar redshifts. Our measurement of the quasar clustering amplitude implies a bias factor of b ≃ 3.5 for our quasar sample. We compare the data to models to constrain the manner in which quasars occupy dark matter haloes at z∼ 2.4 and infer that such quasars inhabit haloes with a characteristic mass of 〈M〉≃ 1012
h
−1 M⊙ with a duty cycle for the quasar activity of 1 per cent.
Axions are a viable candidate for cold dark matter, which should generically form minihalos of subplanetary masses from white-noise isocurvature density fluctuations if the Peccei-Quinn phase ...transition occurs after inflation. Despite being denser than the larger halos formed out of adiabatic fluctuations from inflation, axion minihalos have surface densities much smaller than the critical value required for gravitational lensing to produce multiple images or high magnification, and hence are practically undetectable as lenses in isolation. However, their lensing effect can be enhanced when superposed near critical curves of other lenses. We propose a method to detect them through photometric monitoring of recently discovered caustic transiting stars behind cluster lenses, under extreme magnification factors 103-104 as the lensed stars cross microlensing caustics induced by intracluster stars. For masses of the first gravitationally collapsed minihalos in the range ∼10−15-10−8 h−1 M , we show that axion minihalos in galaxy clusters should collectively produce subtle surface density fluctuations of amplitude ∼10−4-10−3 on projected length scales of ∼10-104 au, which imprint irregularities on the microlensing lightcurves of caustic transiting stars. We estimate that, inside a cluster halo and over the age of the universe, most of these minihalos are likely to avoid dynamic disruption by encounters with stars or other minihalos.
Recent observations of lensed galaxies at cosmological distances have detected individual stars that are extremely magnified when crossing the caustics of lensing clusters. In idealized cluster ...lenses with smooth mass distributions, two images of a star of radius R approaching a caustic brighten as and reach a peak magnification before merging on the critical curve. We show that a mass fraction ( ) in microlenses inevitably disrupts the smooth caustic into a network of corrugated microcaustics and produces light curves with numerous peaks. Using analytical calculations and numerical simulations, we derive the characteristic width of the network, caustic-crossing frequencies, and peak magnifications. For the lens parameters of a recent detection and a population of intracluster stars with , we find a source-plane width of for the caustic network, which spans on the image plane. A source star takes years to cross this width, with a total of crossings, each one lasting for with typical peak magnifications of . The exquisite sensitivity of caustic-crossing events to the granularity of the lens-mass distribution makes them ideal probes of dark matter components, such as compact halo objects and ultralight axion dark matter.
We have developed two independent methods for measuring the one-dimensional power spectrum of the transmitted flux in the Lyman-α forest. The first method is based on a Fourier transform and the ...second on a maximum-likelihood estimator. The two methods are independent and have different systematic uncertainties. Determination of the noise level in the data spectra was subject to a new treatment, because of its significant impact on the derived power spectrum. We applied the two methods to 13 821 quasar spectra from SDSS-III/BOSS DR9 selected from a larger sample of over 60 000 spectra on the basis of their high quality, high signal-to-noise ratio (S/N), and good spectral resolution. The power spectra measured using either approach are in good agreement over all twelve redshift bins from ⟨z⟩ = 2.2 to ⟨z⟩ = 4.4, and scales from 0.001 km s-1 to 0.02 km s-1. We determined the methodological andinstrumental systematic uncertainties of our measurements. We provide a preliminary cosmological interpretation of our measurements using available hydrodynamical simulations. The improvement in precision over previously published results from SDSS is a factor 2–3 for constraints on relevant cosmological parameters. For a ΛCDM model and using a constraint on H0 that encompasses measurements based on the local distance ladder and on CMB anisotropies, we infer σ8 = 0.83 ± 0.03 and ns = 0.97 ± 0.02 based on H i absorption in the range 2.1 < z < 3.7.
We develop semi-empirical models of the supermassive black hole and active galactic nucleus (AGN) populations, which incorporate the black hole growth implied by the observed AGN luminosity function ...assuming a radiative efficiency and a distribution of Eddington ratios λ. By generalizing these continuity-equation models to allow a distribution P(λ | M
BH, z), we are able to draw on constraints from observationally estimated λ distributions and active galaxy fractions while accounting for the luminosity thresholds of observational samples. We consider models with a Gaussian distribution of log λ and Gaussians augmented with a power-law tail to low λ. Within our framework, reproducing the high observed AGN fractions at low redshift requires a characteristic Eddington ratio λc that declines at late times, and matching observed Eddington ratio distributions requires a P(λ) that broadens at low redshift. To reproduce the observed increase of AGN fraction with black hole or galaxy mass, we also require a λc that decreases with increasing black hole mass, reducing the AGN luminosity associated with the most massive black holes. Finally, achieving a good match to the high-mass end of the local black hole mass function requires an increased radiative efficiency at high black hole mass. We discuss the potential impact of black hole mergers or a λ-dependent bolometric correction, and we compute evolutionary predictions for black hole and galaxy specific accretion rates. Despite the flexibility of our framework, no one model provides a good fit to all the data we consider; it is particularly difficult to reconcile the relatively narrow λ distributions and low duty cycles estimated for luminous broad-line AGN with the broader λ distributions and higher duty cycles found in more widely selected AGN samples, which typically have lower luminosity thresholds.
ABSTRACT
We detect a tidal stream generated by the globular cluster NGC 3201 extending over ∼140 deg on the sky, using the Gaia DR2 data, with the maximum-likelihood method we presented previously to ...study the M68 tidal stream. Most of the detected stream is the trailing one, which stretches in the southern Galactic hemisphere and passes within a close distance of 3.2 kpc from the Sun, therefore making the stream highly favourable for discovering relatively bright member stars, while the leading arm is further from us and behind a disc foreground that is harder to separate from. The cluster has just crossed the Galactic disc and is now in the northern Galactic hemisphere, moderately obscured by dust, and the part of the trailing tail closest to the cluster is highly obscured behind the plane. We obtain a best-fitting model of the stream which is consistent with the measured proper motion, radial velocity, and distance to NGC 3201, and show it to be the same as the previously detected Gjöll stream by Ibata et al. We identify ∼200 stars with the highest likelihood of being stream members using only their Gaia kinematic data. Most of these stars (170) are photometrically consistent with being members of NGC 3201 when they are compared to the cluster H–R diagram, only once a correction for dust absorption and reddening by the Galaxy is applied. The remaining stars are consistent with being random foreground objects according to simulated data sets. We list these 170 highly likely stream member stars.
We construct evolutionary models of the populations of active galactic nuclei (AGNs) and supermassive black holes, in which the black hole mass function grows at the rate implied by the observed ...luminosity function, given assumptions about the radiative efficiency and the luminosity in Eddington units. We draw on a variety of recent X-ray and optical measurements to estimate the bolometric AGN luminosity function and compare to X-ray background data and the independent estimate of Hopkins et al. to assess remaining systematic uncertainties. The integrated AGN emissivity closely tracks the cosmic star-formation history, suggesting that star formation and black hole growth are closely linked at all redshifts. We discuss observational uncertainties in the local black hole mass function, which remain substantial, with estimates of the integrated black hole mass density rho - spanning the range 3-5.5 X 105 M Mpc-3. We find good agreement with estimates of the local mass function for a reference model where all active black holes have a fixed efficiency = 0.065 and L bol/L Edd 0.4 (shifting to = 0.09, L bol/L Edd 0.9 for the Hopkins et al. luminosity function). In our reference model, the duty cycle of 109 M black holes declines from 0.07 at z = 3 to 0.004 at z = 1 and 10-4 at z = 0. The decline is shallower for less massive black holes, a signature of 'downsizing' evolution in which more massive black holes build their mass earlier. The predicted duty cycles and AGN clustering bias in this model are in reasonable accord with observational estimates. If the typical Eddington ratio declines at z < 2, then the 'downsizing' of black hole growth is less pronounced. Models with reduced Eddington ratios at low redshift or black hole mass predict fewer low-mass black holes (M - 108 M ) in the local universe, while models with black hole mergers predict more black holes at M - > 109 M . Matching the integrated AGN emissivity to the local black hole mass density implies = 0.075 X ( rho -/4.5 X 105 M Mpc-3)-1 for our standard luminosity function estimate, or 25% higher for Hopkins et al.'s estimate. It is difficult to reconcile current observations with a model in which most black holes have the high efficiencies 0.16-0.20 predicted by MHD simulations of disk accretion. We provide electronic tabulations of our bolometric luminosity function and our reference model predictions for black hole mass functions and duty cycles as a function of redshift.
We examine a caustic-straddling arc at z = 0.9397 in the field of the galaxy cluster MACS J0416.1-2403 (z = 0.397) using archival multiband Hubble Space Telescope images and show that its surface ...brightness exhibits anomalies that can be explained by a single highly magnified star undergoing microlensing. First, we show that the surface brightness pattern is not perfectly symmetric across the cluster critical curve, which is inconsistent with a locally smooth lens model; the location of the candidate star exhibits the most significant asymmetry. Second, our analysis indicates that the asymmetric feature has ∼30% higher flux in the 2012 visits compared to the Frontier Fields program visits in 2014. Moreover, the variable asymmetric feature shows an anomalous color between the F814W and F105W filters in 2014. These anomalies are naturally explained by microlensing-induced variability of a caustic-transiting blue supergiant in a star-forming region, with a mean magnification factor around ∼ 200. We extend this study to a statistical analysis of the whole arc image and find tentative evidence of the increased mismatch of the two images in the proximity of the critical line. Robust detection of one or multiple caustic-transiting stars in this arc will enable detailed follow-up studies that can shed light on the small-scale structure of the dark matter inside the cluster halo.
ABSTRACT
A method to search for tidal streams and to fit their orbits based on maximum likelihood is presented and applied to the Gaia data. Tests of the method are performed showing how a simulated ...stream produced by tidal stripping of a star cluster is recovered when added to a simulation of the Gaia catalogue. The method can be applied to search for streams associated with known progenitors or to do blind searches in a general catalogue. As the first example, we apply the method to the globular cluster M68 and detect its clear tidal stream stretching over the whole North Galactic hemisphere, and passing within 5 kpc of the Sun. This is one of the closest tidal streams to us detected so far, and is highly promising to provide new constraints on the Milky Way gravitational potential, for which we present preliminary fits finding a slightly oblate dark halo consistent with other observations. We identify the M68 tidal stream with the previously discovered Fjörm stream by Ibata et al. The tidal stream is confirmed to contain stars that are consistent with the HR-diagram of M68. We provide a list of 115 stars that are most likely to be stream members, and should be prime targets for follow-up spectroscopic studies.
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