ALBERT

Description: Based on Sloan Digital Sky Survey (SDSS) g, r and South Galactic Cap of u-band Sky Survey (SCUSS) u photometry, we develop a photometric calibration for estimating the stellar metallicity from u – g and g – r colours by using the SDSS spectra of 32 542 F- and G-type main-sequence stars, which cover almost 3700 deg 2 in the south Galactic cap. The rms scatter of the photometric metallicity residuals relative to spectrum-based metallicity is 0.14 dex when g – r  〈 0.4, and 0.16 dex when g – r  〉 0.4. Because of the deeper and more accurate magnitude of SCUSS u band, the estimate can be used up to the faint magnitude of g  = 21. This application range of photometric metallicity calibration is wide enough so that it can be used to study metallicity distribution of distant stars. In this study, we select the Sagittarius (Sgr) stream and its neighbouring field halo stars in south Galactic cap to study their metallicity distribution. We find that the Sgr stream at the cylindrical Galactocentric coordinate of R  ~ 19 kpc, | z | ~ 14 kpc exhibits a relative rich metallicity distribution, and the neighbouring field halo stars in our studied fields can be modelled by two-Gaussian model, with peaks, respectively, at [Fe/H] = –1.9 and –1.5.

Description: The Milky Way is one of the very few spiral galaxies known to host large-scale magnetic field reversals. The existence of the field reversal in the first Galactic quadrant near the Sagittarius spiral arm has been well established, yet poorly characterized due to the insufficient number of reliable Faraday depths (FDs) from extragalactic radio sources (EGSs) through this reversal region. We have therefore performed broad-band (1–$2, { m GHz}$) spectropolarimetric observations with the Karl G. Jansky Very Large Array (VLA) to determine the FD values of 194 EGSs in the Galactic longitude range of 20°–52° within ±5° from the Galactic mid-plane, covering the Sagittarius arm tangent. This factor of five increase in the EGS FD density has led to the discovery of a disparity in FD values across the Galactic mid-plane in the Galactic longitude range of 40°–52°. Combined with existing pulsar FD measurements, we suggest that the Sagittarius arm can host an odd-parity disc field. We further compared our newly derived EGS FDs with the predictions of three major Galactic magnetic field models, and concluded that none of them can adequately reproduce our observational results. This has led to our development of new, improved models of the Milky Way disc magnetic field that will serve as an important step towards major future improvements in Galactic magnetic field models.

Description: To study the secular evolution of the activity of Centaur 166P/2001 T4 (Near Earth Asteroid Tracking, NEAT) and its physical properties, we present the results of optical observations of the Centaur taken on 2009 March 29 with the Keck 10-m telescope located atop Mauna Kea, Hawaii. It was still active at r h  = 11.9 au post-perihelion. An upper limit of the nucleus radius of a N  〈 14.32 km is derived. The colour index is B  –  R  = 1.59 ± 0.05. The Af value is measured to be 288 ± 19 cm and the corresponding dust production rate is 252 kg s –1 . Finally, a possible mechanism of activity is discussed.

Description: H i intensity mapping is an emerging tool to probe dark energy. Observations of the redshifted H i signal will be contaminated by instrumental noise, atmospheric and Galactic foregrounds. The latter is expected to be four orders of magnitude brighter than the H i emission we wish to detect. We present a simulation of single-dish observations including an instrumental noise model with 1/ f and white noise, and sky emission with a diffuse Galactic foreground and H i emission. We consider two foreground cleaning methods: spectral parametric fitting and principal component analysis. For a smooth frequency spectrum of the foreground and instrumental effects, we find that the parametric fitting method provides residuals that are still contaminated by foreground and 1/ f noise, but the principal component analysis can remove this contamination down to the thermal noise level. This method is robust for a range of different models of foreground and noise, and so constitutes a promising way to recover the H i signal from the data. However, it induces a leakage of the cosmological signal into the subtracted foreground of around 5 per cent. The efficiency of the component separation methods depends heavily on the smoothness of the frequency spectrum of the foreground and the 1/ f noise. We find that as long as the spectral variations over the band are slow compared to the channel width, the foreground cleaning method still works.

Description: We presentphotometric investigations of three distant active comets, 228P/LINEAR, C/2006 S3 Lowell Observatory Near-Earth-Object Search (LONEOS) and 29P/Schwassmann–Wachmann 1. The data were obtained with the 1-m optical telescope at Lulin Observatory in Taiwan on 2011 February 5 and 6. These comets were observed at heliocentric distances larger than 3 au, all of them appeared to be active. By cometary morphological and photometric studies, the upper limits of the nuclei radii were derived. Also, the surface brightness profiles, Af parameters, mass production rates and the coma colours were measured. Finally, we discussed possible driver of activity in comets.

Description: We present scale-dependent measurements of the normalized growth rate of structure f 8 ( k , z  = 0) using only the peculiar motions of galaxies. We use data from the 6-degree Field Galaxy Survey velocity sample together with a newly compiled sample of low-redshift ( z  〈 0.07) Type Ia supernovae. We constrain the growth rate in a series of k ~ 0.03 h Mpc –1 bins to ~35 per cent precision, including a measurement on scales 〉300 h –1 Mpc, which represents one of the largest scale growth rate measurement to date. We find no evidence for a scale-dependence in the growth rate, or any statistically significant variation from the growth rate as predicted by the Planck cosmology. Bringing all the scales together, we determine the normalized growth rate at z  = 0 to ~15 per cent in a manner independent of galaxy bias and in excellent agreement with the constraint from the measurements of redshift-space distortions from 6-degree Field Galaxy Survey. We pay particular attention to systematic errors. We point out that the intrinsic scatter present in Fundamental Plane and Tully–Fisher relations is only Gaussian in logarithmic distance units; wrongly assuming it is Gaussian in linear (velocity) units can bias cosmological constraints. We also analytically marginalize over zero-point errors in distance indicators, validate the accuracy of all our constraints using numerical simulations, and demonstrate how to combine different (correlated) velocity surveys using a matrix ‘hyperparameter’ analysis. Current and forthcoming peculiar velocity surveys will allow us to understand in detail the growth of structure in the low-redshift universe, providing strong constraints on the nature of dark energy.

Description: We investigate the effects of warm dark matter (WDM) on the cosmic 21-cm signal. If dark matter exists as WDM instead of cold dark matter (CDM), its non-negligible velocities can inhibit the formation of low-mass haloes that normally form first in CDM models, therefore delaying star formation. The absence of early sources delays the build-up of UV and X-ray backgrounds that affect the 21-cm radiation signal produced by neutral hydrogen. With use of the 21CMFAST  code, we demonstrate that the pre-reionization 21-cm signal can be changed significantly in WDM models with a free-streaming length equivalent to that of a thermal relic with mass m X of up to ~10–20 keV. In such a WDM cosmology, the 21-cm signal traces the growth of more massive haloes, resulting in a delay of the 21-cm absorption signature and followed by accelerated X-ray heating. CDM models where astrophysical sources have a suppressed photon-production efficiency can delay the 21-cm signal as well, although its subsequent evolution is not as rapid as compared to WDM. This motivates using the gradient of the global 21-cm signal to differentiate between some CDM and WDM models. Finally, we show that the degeneracy between the astrophysics and m X can be broken with the 21-cm power spectrum, as WDM models should have a bias-induced excess of power on large scales. This boost in power should be detectable with current interferometers for models with m X 3 keV, while next-generation instruments will easily be able to measure this difference for all relevant WDM models.

Description: Primordial, non-Gaussian perturbations can generate scale-dependent bias in the galaxy distribution. This in turn will modify correlations between galaxy positions and peculiar velocities at late times, since peculiar velocities reflect the underlying matter distribution, whereas galaxies are a biased tracer of the same. We study this effect, and show that non-Gaussianity can be constrained by comparing the observed peculiar velocity field to a model velocity field reconstructed from the galaxy density field assuming linear bias. The amplitude of the spatial correlations in the residual map obtained after subtracting one velocity field from the other is directly proportional to the strength of the primordial non-Gaussianity. We construct the corresponding likelihood function and use it to constrain the amplitude of the linear flow β and the amplitude of local non-Gaussianity $f^{\rm {local}}_{\rm {NL}}$ . Applying our method to two observational data sets, the Type-Ia supernovae (A1SN) and Spiral Field I -band (SFI++) catalogues, we obtain constraints on the linear flow parameter consistent with the values derived previously assuming Gaussianity. The marginalized 1D distribution of $\,|f^{\rm {local}}_{\rm {NL}}|$ does not show strong evidence for non-zero $f^{\rm {local}}_{\rm {NL}}$ , and we set 95 per cent upper limits $\,|f^{\rm {local}}_{\rm {NL}}| 〈 51.4$ from A1SN and $\,|f^{\rm {local}}_{\rm {NL}}| 〈 92.6$ from SFI++. These limits on $f^{\rm {local}}_{\rm {NL}}$ are as tight as any set by previous large-scale structure measurements. Our method can be applied to any survey with radial velocities and density field data, and provides an independent check of recent CMB constraints on $f^{\rm {local}}_{\rm {NL}}$ , extending these to smaller spatial scales.

Description: We combine the equations of motion that govern the dynamics of galaxies in the local volume with Bayesian techniques in order to fit orbits to published distances and velocities of galaxies within 3 Mpc. We find a Local Group (LG) mass 2.3 ± 0.7 10 12 M that is consistent with the combined dynamical masses of M31 and the Milky Way, and a mass ratio $0.54^{+0.23}_{-0.17}$ that rules out models where our Galaxy is more massive than M31 with ~95 per cent confidence. The Milky Way's circular velocity at the solar radius is relatively high, 245 ± 23 km s –1 , which helps to reconcile the mass derived from the local Hubble flow with the larger value suggested by the ‘timing argument’. Adopting Planck 's bounds on yields a (local) Hubble constant H 0  = 67 ± 5 km s –1 Mpc –1 which is consistent with the value found on cosmological scales. Restricted N -body experiments show that substructures tend to fall on to the LG along the Milky Way–M31 axis, where the quadrupole attraction is maximum. Tests against mock data indicate that neglecting this effect slightly overestimates the LG mass without biasing the rest of model parameters. We also show that both the time dependence of the LG potential and the cosmological constant have little impact on the observed local Hubble flow.

Description: A maximum-likelihood method, tested as an unbiased estimator from numerical simulations, is used to estimate cosmic bulk flow from peculiar velocity surveys. The likelihood function is applied to four observational catalogues (ENEAR, SFI++, A1SN and SC) constructed from galaxy peculiar velocity surveys and Type Ia supernovae data at low redshift ( z  ≤ 0.03). We find that the Spiral Field I-band catalogue constrains the bulk flow to be V  = 290 ± 30 km s –1 towards l  = 281° ± 7°, $b=8^{\circ +6^{\circ }}_{-5^{\circ }}$ on effective scales of 58 h –1 Mpc, which is the tightest constraints achievable at the present time. By comparing the amplitudes of our estimated bulk flows with theoretical prediction, we find excellent agreement between the two. In addition, directions of estimated bulk flows are also consistent with measurements in other studies.