Browsing by Author "Mohan, V."

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Analysis of possible anomalies in the QSO distribution of the Flesch & Hardcastle catalogue
(2008-01-02) M. L´opez-Corredoira; C. M. Guti´errez; Mohan, V.; Gunthardt, G. I.; Alonso, M. S.
Aims. A recent catalogue by Flesch & Hardcastle presents two major anomalies in the spatial distribution of QSO candidates: i/ an apparent excess of such objects near bright galaxies, and ii/ an excess of very bright QSO candidates compared to random background expectations in several regions of the sky. Because anyone of these anomalies would be relevant in a cosmological context, we carried out an extensive analysis of the probabilities quoted in that catalogue. Methods. We determine the nature and redshift of a subsample of 30 sources in that catalogue by analysing their optical spectra (another 11 candidates were identified from existing public databases). These have allowed us to statistically check the reliability of the probabilities QSO status quoted by Flesch & Hardcastle for their candidates. Results. Only 12 of the 41 candidates turned out QSOs (7 of which have been identified here for the first time). Conclusions. The probabilities of the QSOs’ being the candidates given by Flesch & Hardcastle are overestimated for mB ≤ 17 and for objects projected near (≤ 1 arcmin) bright galaxies. This is the cause of the anomalies mentioned above.
Constraining fundamental constants of physics with quasar absorption line systems
(2009-08-01) Petitjean, Patrick; Vivek, M.; Mohan, V.; et al.
We report long-slit spectroscopic observations of SDSS J092712+294344 carried-out at the recently commissioned 2m telescope in IUCAA Girawali Observatory, India. This AGN-like source is known to feature three sets of emission lines at zem = 0.6972, 0.7020 and 0.7128. Diﬀerent scenarios such as a recoiling black hole after asymmetric emission of gravitational waves, binary black holes and possible merging systems are proposed for this object. We test these scenarios by comparing our spectra with that fromthe Sloan Digital Sky Survey (SDSS), obtained 4 years prior to our observations. Comparing the redshifts of [Oiii]λλ4960,5008 we put a 3σ limit on the relative acceleration to be less than 32 km s−1 yr −1 between diﬀerent emitting regions. Using the 2D spectra obtained at diﬀerent position angles we show that the [Oiii]λ5008 line from the zem = 0.7128 component is extended beyond the spectral point spread function.We infer the linear extent of this line emitting region is ∼ 8 kpc.We also ﬁnd a tentative evidence for an oﬀset between the centroid of the [Oiii]λ5008 line at zem = 0.7128 and the QSO trace when the slit is aligned at a position angle of 299◦ . This corresponds to the zem = 0.7128 system being at an impact parameter of ∼1 kpc with respect to the zem = 0.6972 in the north west direction. Based on our observations we conclude that the binary black hole model is most unlikely. The spatial extent and the sizes are consistent with both black hole recoil and merging scenarios
Evolution of the cosmological mass density of neutral gas from Sloan Digital Sky Survey II - Data Release 7
(2009-08-01) Noterdaeme, P.; Vivek, M.; Mohan, V.
We present the results of a search for damped Lyman-α (DLA) systems in the Sloan Digital Sky Survey II (SDSS), Data Release 7.We use a fully automatic procedure to identify DLAs and derive their column densities. The procedure is checked against the results of previous searches for DLAs in SDSS.We discuss the agreements and diﬀerences and show the robustness of our procedure. For each system, we obtain an accurate measurement of the absorber’s redshift, the Hi column density and the equivalent width of associated metal absorption lines, without any human intervention. We ﬁnd 1426 absorbers with 2.15 < z < 5.2 with log N(Hi) ≥ 20, out of which 937 systems have log N(Hi) ≥ 20.3. This is the largest DLA sample ever built, made available to the scientiﬁc community through the electronic version of this paper. In the course of the survey, we discovered the intervening DLA with highest Hi column density known to date with log N(Hi) = 22.0 ± 0.1. This single system provides a strong constraint on the high-end of the N(Hi) frequency distribution now measured with high accuracy. We show that the presence of a DLA at the blue end of a QSO spectrum can lead to important systematic errors and propose a method to avoid them. This has important consequences for the measurement of the cosmological mass density of neutral gas at z ∼ 2.2 and therefore on our understanding of galaxy evolution over the past 10 billion years. We ﬁnd a signiﬁcant decrease of the cosmological mass density of neutral gas in DLAs, ΩDLA g , from z = 4 to z = 2.2, consistent with the result of previous SDSS studies. However, and contrary to other SDSS studies, we ﬁnd that ΩDLA g (z = 2.2) is about twice the value at z = 0. This implies that ΩDLA g keeps decreasing at z < 2.2.
IGO Discoveries
(2013-07-24) Mohan, V.
Quasars probing intermediate redshift star-forming galaxies
(2009-12-01) Noterdaeme, P.; Srianand, R.; Mohan, V.; et al.
We present a sample of 46 [Oiii]-emitting galaxies at z < 0.8 detected in the ﬁbre spectra of quasars from the Sloan Digital Sky Survey, Data Release 7 (SDSS-DR7) through an automatic search procedure. We also detect [Oii] and Hβ emission lines from most of these galaxies in the SDSS spectra. We study both the emission and absorption properties of a sub sample of 17 galaxies in the redshift range z = 0.4-0.7, where Mg ii lines are covered by the SDSS spectra. The measured lower-limits on the star-formation rates of these galaxies are in the range 0.2-20 M⊙ yr −1 . The emission line luminosities and (O/H) metallicities from R23 measured in this sample are similar to what is found in normal galaxies at these redshifts. Thus, this constitutes a unique sample of intermediate redshift star-forming galaxies where we can study the QSO absorber - galaxy connection. Strong Mg ii (Wλ2796> ∼ 1 Å) as well as Mg i absorption lines are detected in the QSO spectra at the redshift of most of these galaxies. Strong Fe ii (Wλ2600 > 1 Å) absorption lines are also generally detected whenever the appropriate wavelength ranges are covered. This suggests that most of these systems could be bona-ﬁde Damped Lyman-α systems. We investigate various possible relations between the Mg ii rest equivalent widths and the emission line properties.We ﬁnd a possible (2σ) correlation between the emission-line metallicity of the galaxies and the Mg ii rest equivalent width of the absorbers (log(O/H) + 12 = 0.1Wλ2796 + 8.27), which could be a consequence of an underlying mass-metallicity relation. However, [Oiii]-selected Mg ii systems represent only a minor fraction of the strongMg ii absorbers.We ﬁnd this cannot be attributed to biases related either to the spectral signal-to-noise ratio or to the brightness of the QSOs. We measure the average observed ﬂuxes (collected into the SDSS ﬁbre) of the [Oii] and [Oiii] lines associated to Mg ii-selected systems through stacking technique.We ﬁnd that the average lumiosities of emission lines are higher for systems with larger Wλ2796. The stacked luminosities are found to be below the typical detection limit in individual spectra, indicating that faint galaxies can contribute appreciably to the observed population of strong Mg ii absorbers at intermediate redshifts.We also present long-slit spectroscopic observations of SDSS J113108+202151, the most luminous line-emitting galaxy in our z ≥ 0.4 sample. Surprisingly, we ﬁnd that the line-emitting region does not coincide with the nearby extended bright galaxy with consistent photometric redshift seen in the SDSS image.
Structure and kinematical properties of the galaxy at intermediate galactic latitudes.
(2015-02-07) Ojha, D.K.; Bienayme, Olivier; Robin, A.C.; Creze, Michel; Mohan, V.