arXiv subjects

Farhang Habibi

Publications and source records attributed to Farhang Habibi.

7 recordsLinked to original sources

arXiv2019

Impact of photometric redshifts on the galaxy power spectrum and BAO scale in the LSST survey

Imaging billions of galaxies every few nights during ten years, LSST should be a major contributor to precision cosmology in the 2020 decade. High precision photometric data will be available in six bands, from near-infrared to near-ultraviolet. The computation of precise, unbiased, photometric redshifts up to z = 2, at least, is one of the main LSST challenges and its performance will have major impact on all extragalactic LSST sciences. We evaluate the efficiency of our photometric redshift reconstruction on mock galaxy catalogs up to z=2.45 and estimate the impact of realistic photometric redshift (hereafter photo-z) reconstruction on the large-scale structures (LSS) power spectrum and the baryonic acoustic oscillation (BAO) scale determination for a LSST-like photometric survey. We study the effectiveness of the BAO scale as a cosmological probe in the LSST survey. We have performed a detailed modelling of the photo-z distribution as a function of galaxy type, redshift and absolute magnitude using our photo-z reconstruction code with a quality selection cut based on a Boosted decision tree (BDT). We have computed the fractional error on the recovered power spectrum which is dominated by the shot-noise at z>1 for scales k>0.1, due to the photo-z damping. The BAO scale can be recovered with a percent or better accuracy level from z = 0.5 to z = 1.5 using realistic photo-z reconstruction. Outliers can represent a significant fraction of galaxies at z>2, causing bias and errors on LSS power spectrum measurement. Although the BAO scale is not the most powerful cosmological probe in LSST, it can be used to check the consistency of the LSS measurement. Moreover we show that the impact of photo-z smearing on the recovered isotropic BAO scale in LSST should stay limited up to z=1.5, so as long as the galaxy number density balances the photo-z smoothing.

astro-ph.CO↗

arXiv2018

Peculiar velocity measurement in a clumpy universe

In this work we address the issue of peculiar velocity measurement in a perturbed Friedmann universe using the deviations from measured luminosity distances of standard candles from background FRW universe. We want to show and quantify the statement that in intermediate redshifts ($0.5< z < 2$), deviations from the background FRW model are not uniquely governed by peculiar velocities. Luminosity distances are modified by gravitational lensing. We also want to indicate the importance of relativistic calculations for peculiar velocity measurement at all redshifts. For this task we discuss the relativistic correction on luminosity distance and redshift measurement and show the contribution of each of the corrections as lensing term, peculiar velocity of the source and Sachs-Wolfe effect. Then we use the SNe Ia sample of Union 2, to investigate the relativistic effects we consider. We show that, using the conventional peculiar velocity method, that ignores the lensing effect, will result in an overestimate of the measured peculiar velocities at intermediate redshifts. Here we quantify this effect. We show that at low redshifts the lensing effect is negligible compare to the effect of peculiar velocity. From the observational point of view, we show that the uncertainties on luminosity of the present SNe Ia data prevent us from precise measuring the peculiar velocities even at low redshifts ($z<0.2$).

astro-ph.CO↗

arXiv2017

Supernovae anisotropy power spectrum

We contribute another anisotropy study to this field of research using Type Ia supernovae (SNe Ia). In this work, we utilise the power spectrum calculation method and apply it to both the current SNe Ia data and simulation. Using the Union2.1 data set at all redshifts, we compare the spectrum of the residuals of the observed distance moduli to that expected from an isotropic universe affected by the Union2.1 observational uncertainties at low multipoles. Through this comparison we find a dipolar anisotropy with tension of less that $2σ$ towards $l =171^\circ \pm 21^\circ$ and $b=-26^\circ \pm 28^\circ$ which is mainly induced by anisotropic spatial distribution of the SNe with $z > 0.2$ rather than being a cosmic effect. Furthermore, we find a tension of $\sim 4σ$ at $\ell=4$ between the two spectra. Our simulations are constructed with the characteristics of the upcoming surveys like the Large Synoptic Survey Telescope (LSST), which shall bring us the largest SNe Ia collection to date. We make predictions for the amplitude of a possible dipolar anisotropy that would be detectable by future SNe Ia surveys.

astro-ph.CO↗

arXiv2014

Probing the Local Bubble with Diffuse Interstellar Bands. II. The DIB properties in the Northern hemisphere

We present a new high signal to noise ratio spectroscopic survey of the Northern hemisphere to probe the Local Bubble and its surroundings using the $λ5780$ Å and $λ5797$ Å Diffuse Interstellar Bands (DIBs). We observed 432 sightlines to a distance of 200 pc over a duration of 3 years. In this study, we establish the $λ5780$ and $λ5797$ correlations with Na I, Ca II and E(B-V), for both inside and outside the Local Bubble. The correlations show that among all neutral and ionized atoms, the correlation between Ca II and $\lambda5780$ is stronger than its correlation with $\lambda5797$, suggesting that $\lambda5780$ is more associated with regions where Ca$^{+}$ is more abundant. We study the $\lambda5780$ correlation with $\lambda5797$, which shows a tight correlation within and outside the Local Bubble. In addition we investigate the DIB properties in UV irradiated and UV shielded regions. We find that, within and beyond the Local Bubble, $\lambda5797$ is located in denser parts of clouds, protected from UV irradiation, while $\lambda5780$ is located in the low density regions of clouds.

astro-ph.GA↗

arXiv2014

Unraveling the nature of Gravity through our clumpy Universe

We propose a new probe to test the nature of gravity at various redshifts through large-scale cosmological observations. We use our void catalog, extracted from the Sloan Digital Sky Survey (SDSS, DR10), to trace the distribution of matter along the lines of sight to SNe Ia that are selected from the Union 2 catalog. We study the relation between SNe Ia luminosities and convergence and also the peculiar velocities of the sources. We show that the effects, on SNe Ia luminosities, of convergence and of peculiar velocities predicted by the theory of general relativity and theories of modified gravities are different and hence provide a new probe of gravity at various redshifts. We show that the present sparse large-scale data does not allow us to determine any statistically- significant deviation from the theory of general relativity but future more comprehensive surveys should provide us with means for such an exploration.

astro-ph.CO↗

arXiv2013

Simulation of optical interstellar scintillation

Stars twinkle because their light propagates through the atmosphere. The same phenomenon is expected on a longer time scale when the light of remote stars crosses an interstellar turbulent molecular cloud, but it has never been observed at optical wavelengths. The aim of the study described in this paper is to fully simulate the scintillation process, starting from the molecular cloud description as a fractal object, ending with the simulations of fluctuating stellar light curves. Fast Fourier transforms are first used to simulate fractal clouds. Then, the illumination pattern resulting from the crossing of background star light through these refractive clouds is calculated from a Fresnel integral that also uses fast Fourier transform techniques. Regularisation procedure and computing limitations are discussed, along with the effect of spatial and temporal coherency (source size and wavelength passband). We quantify the expected modulation index of stellar light curves as a function of the turbulence strength --characterised by the diffraction radius $R_{diff}$-- and the projected source size, introduce the timing aspects, and establish connections between the light curve observables and the refractive cloud. We extend our discussion to clouds with different structure functions from Kolmogorov-type turbulence. Our study confirms that current telescopes of ~4m with fast-readout, wide-field detectors have the capability of discovering the first interstellar optical scintillation effects. We also show that this effect should be unambiguously distinguished from any other type of variability through the observation of desynchronised light curves, simultaneously measured by two distant telescopes.

astro-ph.IM↗

arXiv2004

Possibility of Magnetic Mass Detection by the Next Generation of Microlensing Experiments

We study the possibility of magnetic mass detection by the gravitational microlensing technique. Recently the theoretical effect of magnetic mass in the NUT space on the microlensing light curve has been studied. It was shown that in the low photometric signal to noise and sampling rate of MACHO experiment light curves, no signature of NUT factor has been found. In order to increase the sensitivity of magnetic mass detection, we propose a systematic search for microlensing events, using the currently running alert systems and complementary telescopes for monitoring the Large Magellanic Clouds stars. In this strategy of observation, we obtain the magnetic mass detection efficiency and also the lowest observable limit of the NUT factor. This method of survey for gravitational microlensing detection can also be used as a tool for searching other exotic space-times.

astro-ph↗