Sloan Digital Sky Survey Observations Research Articles

The Messy Nature of Fiber Spectra: Star–Quasar Pairs Masquerading as Dual Type 1 AGNs

Theoretical studies predict that the most significant growth of supermassive black holes (SMBHs) occurs in late-stage mergers, coinciding with the manifestation of dual active galactic nuclei (AGNs), and both major and minor mergers are expected to be important for dual AGN growth. In fact, dual AGNs in minor mergers should be signposts for efficient minor-merger-induced SMBH growth for both the more and less massive progenitor. We identified two candidate dual AGNs residing in apparent minor mergers with mass ratios of ∼1:7 and ∼1:30. Sloan Digital Sky Survey (SDSS) fiber spectra show broad and narrow emission lines in the primary nuclei of each merger while only a narrow [O iii] emission line and a broad and prominent Hα/[N ii] complex is observed in the secondary nuclei. The FWHMs of the broad Hα lines in the primary and secondary nuclei are inconsistent in each merger, suggesting that each nucleus in each merger hosts a Type 1 AGN. However, spatially resolved Large Binocular Telescope optical spectroscopy reveals rest-frame stellar absorption features, indicating the secondary sources are foreground stars and that the previously detected broad lines are likely the result of fiber spillover effects induced by the atmospheric seeing at the time of the SDSS observations. This study demonstrates for the first time that optical spectroscopic searches for Type 1/Type 1 pairs similarly suffer from fiber spillover effects as has been observed previously for Seyfert 2 dual AGN candidates. The presence of foreground stars may not have been clear if an instrument with more limited wavelength range or limited sensitivity had been used.

The Highly Accurate Relation Between the Radius and Mass of the White Dwarf Star From Zero to Finite Temperature

In this research, first considering the electron–electron interaction in the high-density Fermi electron gas at T = 0 K, this interaction causes the pressure 2/137 time less than the original value. However, the pressure of the Fermi electron gas should have something to do with temperature. Then, we estimate the temperature effect using statistical mechanics and find that the complicated form of the pressure p depends on temperature at the given particle number N and volume V. According to this, the central density–mass (ρc-M), central density–radius (ρc-R), and mass–radius (M-R) relations of the white dwarf star are obtained by considering the equation of state (EOS). Traditional formula gives the problematic mass–radius relation R∝M−1/3 for the low-density white dwarf stars because it leads to R→∞ and p→0 when M→0. We correct this relation and obtain two reasonable relations in the relativistic and nonrelativistic regions. In our EOS calculations, the central density is divided into the high-, middle-, and low-density regions. All three relations are almost unchanged until 108 K in the high-density region. The temperature effect mainly affects the middle- and low-density regions, and it becomes explicitly above 107 K. Our calculations can explain Sloan Digital Sky Survey observations where some white dwarf stars with a radius of more than 8 × 103 km have larger mass than the predictions by the relativistic EOS at T = 0 K. This result tells us that the temperature effect is important for the low and middle central-density white dwarf star and also useful to estimate the inner temperature of a white dwarf star.

The intrinsic SFRF and sSFRF of galaxies: comparing SDSS observation with IllustrisTNG simulation

The star formation rate function (SFRF) and specific star formation rate function (sSFRF) from observations are impacted by the Eddington bias, due to uncertainties in the estimated star formation rate (SFR). We develop a novel method to correct the Eddington bias and obtain the intrinsic SFRF and sSFRF from the Sloan Digital Sky Survey (SDSS) Data Release 7. The intrinsic SFRF is in good agreement with measurements from previous data in the literature that relied on UV SFRs but its high star-forming end is slightly lower than the corresponding IR and radio tracers. We demonstrate that the intrinsic sSFRF from SDSS has a bimodal form with one peak found at sSFR ∼ 10−9.7 yr−1 representing the star-forming objects while the other peak is found at sSFR ∼ 10−12 yr−1 representing the quenched population. Furthermore, we compare our observations with the predictions from the IllustrisTNG and Illustris simulations and affirm that the “TNG” model performs much better than its predecessor. However, we show that the simulated SFRF and CSFRD of TNG simulations are highly dependent on resolution, reflecting the limitations of the model and today’s state-of-the-art simulations. We demonstrate that the bimodal, two peaked sSFRF implied by the SDSS observations does not appear in TNG regardless of the adopted box-size or resolution. This tension reflects the need for inclusion of an additional efficient quenching mechanism in the TNG model.

The correlations between optical/UV broad lines and X-ray emission for a large sample of quasars

ABSTRACT We present Chandra observations of 2106 radio-quiet quasars in the redshift range 1.7 ≤ z ≤ 2.7 from the Sloan Digital Sky Survey (SDSS), through data release 14 (DR14), that do not contain broad absorption lines in their rest-frame ultraviolet (UV) spectra. This sample adds over a decade worth of SDSS and Chandra observations to our previously published sample of 139 quasars from SDSS DR5 that is still used to correlate X-ray and optical/UV emission in typical quasars. We fit the SDSS spectra for 753 of the quasars in our sample that have high-quality (exposure time ≥ 10 ks and off-axis observation angle ≤ 10 arcmin) X-ray observations, and analyse their X-ray-to-optical spectral energy distribution properties (αox and Δαox) with respect to the measured C iv and Mg ii emission-line rest-frame equivalent width (EW) and the C iv emission-line blueshift. We find significant correlations (at the ≥99.99 per cent level) between αox and these emission-line parameters, as well as between Δαox and $\rm{C \, \small{IV}}$ EW. Slight correlations are found between Δαox and $\rm{C \, \small{IV}}$ blueshift, $\rm{Mg \, \small{II}}$ EW, and the $\rm{C \, \small{IV}}$ EW to $\rm{Mg \, \small{II}}$ EW ratio. The best-fitting trend in each parameter space is used to compare the X-ray weakness (Δαox) and optical/UV emission properties of typical quasars and weak-line quasars (WLQs). The WLQs typically exhibit weaker X-ray emission than predicted by the typical quasar relationships. The best-fitting relationships for our typical quasars are consistent with predictions from the disc–wind quasar model. The behaviour of the WLQs compared to our typical quasars can be explained by an X-ray ‘shielding’ model.

Stripping of the Hot Gas Halos in Member Galaxies of Abell 1795

Abstract The nearby cluster Abell 1795 is used as a testbed to examine whether hot gas in cluster galaxies is stripped by the ram pressure of the intracluster medium (ICM). The expected X-ray emission in and around Abell 1795 galaxies is likely dominated by the ICM, low-mass X-ray binaries, active galactic nuclei, and hot gas halos. In order to constrain these components, we use archival Chandra X-ray Observatory and Sloan Digital Sky Survey observations of Abell 1795 and identify 58 massive (M ⋆ > 1010 M ⊙) spectroscopic cluster members within 5′ of the Chandra optical axis. X-ray images at 0.5–1.5 and 4–8 keV were created for each cluster member and then stacked into two clustercentric radius bins: inner (0.25 < R clust/R 500 < 1) and outer (1 < R clust/R 500 < 2.5). Surface brightness profiles of inner and outer cluster members are fit using Markov chain Monte Carlo sampling in order to generate model parameters and measure the 0.5–1.5 keV luminosities of each model component. Leveraging effective total Chandra exposure times of 3.4 and 1.7 Ms for inner and outer cluster members, respectively, we report the detection of hot gas halos, in a statistical sense, around outer cluster members. Outer members have 0.5–1.5 keV hot halo luminosities ( ) that are six times larger than the upper limit for inner cluster members (L X < 1.3 × 1039 erg s−1). This result suggests that the ICM is removing hot gas from the halos of Abell 1795 members as they fall into the cluster.

No evidence for modifications of gravity from galaxy motions on cosmological scales

The recent discovery of gravitational waves marks the culmination of a sequence of successful tests of the general theory of relativity (GR) since its formulation in 1915. Yet these tests remain confined to the scale of stellar systems or the strong gravity regime. A departure from GR on larger, cosmological scales has been advocated by the proponents of modified gravity theories as an alternative to the Cosmological Constant to account for the observed cosmic expansion history. While indistinguishable in these terms by construction, such models on the other hand yield distinct values for the linear growth rate of density perturbations and, as a consequence, for the associated galaxy peculiar velocity field. Measurements of the resulting anisotropy of galaxy clustering, when spectroscopic redshifts are used to derive distances, have thus been proposed as a powerful probe of the validity of GR on cosmological scales. However, despite significant effort in modelling such redshift space distortions, systematic errors remain comparable to current statistical uncertainties. Here, we present the results of a different forward-modelling approach, which fully exploits the sensitivity of the galaxy velocity field to modifications of GR. We use state-of-the-art, high-resolution N-body simulations of a standard GR and a compelling f(R) model, one of GR's simplest variants, to build simulated catalogues of stellar-mass-selected galaxies through a robust match to the Sloan Digital Sky Survey observations. We find that, well within the uncertainty of this technique, f(R) fails to reproduce the observed redshift-space clustering on scales 1-10 Mpc/h. Instead, the standard LCDM GR model agrees impressively well with the data. This result provides a strong confirmation, on cosmological scales, of the robustness of Einstein's general theory of relativity.

Narrow absorption lines complex I: one form of broad absorption line

We discover that some of the broad absorption lines (BALs) are actually a complex of narrow absorption lines (NALs). As a pilot study of this type of BAL, we show this discovery through a typical example in this paper. Utilizing the two-epoch observations of J002710.06-094435.3 (hereafter J0027-0944) from the Sloan Digital Sky Survey (SDSS), we find that each of the C IV and Si IV BAL troughs contains at least four NAL doublets. By resolving the Si IV BAL into multiple NALs, we present the following main results and conclusions. First, all these NALs show coordinated variations between the two-epoch SDSS observations, suggesting that they all originate in the quasar outflow, and that their variations are due to global changes in the ionization condition of the absorbing gas. Secondly, a BAL consisting of a number of NAL components indicates that this type of BAL is basically the same as the intrinsic NAL, which tends to support the inclination model rather than the evolution model. Thirdly, although both the C IV and Si IV BALs originate from the same clumpy substructures of the outflow, they show different profile shapes: multiple absorption troughs for the Si IV BAL in a wider velocity range, while P-Cygni for the C IV BAL in a narrower velocity range. This can be interpreted by the substantial differences in fine structure and oscillator strength between the Si IV {\lambda}{\lambda}1393, 1402 and C IV {\lambda}{\lambda}1548, 1551 doublets. Based on the above conclusions, we consider that the decomposition of a BAL into NALs can serve as a way to resolve the clumpy structure for outflows, and it can be used to learn more about characteristics of the clumpy structure and to test the outflow model, when utilizing high-resolution spectra and photoionization model.

Group galaxy number density profiles far out: Is the ‘one-halo’ term NFW out to >10 virial radii?

Abstract While the density profiles (DPs) of Lambda cold dark matter haloes obey the Navarro, Frenk & White (NFW) law out to roughly one virial radius, rvir, the structure of their outer parts is still poorly understood, because the one-halo term describing the halo itself is dominated by the two-halo term representing the other haloes picked up. Using a semi-analytical model, we measure the real-space one-halo number DP of groups out to 20rvir by assigning each galaxy to its nearest group above mass Ma, in units of the group rvir. If Ma is small (large), the outer DP of groups falls rapidly (slowly). We find that there is an optimal Ma for which the stacked DP resembles the NFW model to 0.1 dex accuracy out to 13 virial radii. We find similar long-range NFW surface DPs (out to 10rvir) in the Sloan Digital Sky Survey observations using a galaxy assignment scheme that combines the non-linear virialized regions of groups with their linear outer parts. The optimal Ma scales as the minimum mass of the groups that are stacked to a power 0.25–0.3. The NFW model thus does not solely originate from violent relaxation. Moreover, populating haloes with galaxies using halo occupation distribution models must proceed out to much larger radii than usually done.

Integrated and Resolved Dust Attenuation in Clumpy Star-Forming Galaxies at 0.07 < z < 0.14

Dust attenuation in galaxies has been extensively studied nearby, however, there are still many unknowns regarding attenuation in distant galaxies. We contribute to this effort using observations of star-forming galaxies in the redshift range z = 0.05-0.15 from the DYNAMO survey. Highly star-forming DYNAMO galaxies share many similar attributes to clumpy, star-forming galaxies at high redshift. Considering integrated Sloan Digital Sky Survey observations, trends between attenuation and other galaxy properties for DYNAMO galaxies are well matched to star-forming galaxies at high redshift. Integrated gas attenuations of DYNAMO galaxies are 0.2-2.0 mags in the V-band, and the ratio of stellar E(B-V) and gas E(B-V) is 0.78-0.08 (compared to 0.44 at low redshift). Four highly star-forming DYNAMO galaxies were observed at H-alpha using the Hubble Space Telescope and at Pa-alpha using integral field spectroscopy at Keck. The latter achieve similar resolution (~0.8-1 kpc) to our HST imaging using adaptive optics, providing resolved observations of gas attenuations of these galaxies on sub-kpc scales. We find < 1.0 mag of variation in attenuation (at H-alpha) from clump to clump, with no evidence of highly attenuated star formation. Attenuations are in the range 0.3-2.2 mags in the V band, consistent with attenuations of low redshift star-forming galaxies. The small spatial variation on attenuation suggests that a majority of the star-formation activity in these four galaxies occurs in relatively unobscured regions and, thus, star-formation is well characterised by our H-alpha observations.

A MONTE CARLO METHOD FOR MAKING THE SDSS u-BAND MAGNITUDE MORE ACCURATE

ABSTRACT We develop a new Monte Carlo-based method to convert the Sloan Digital Sky Survey (SDSS) u-band magnitude to the south Galactic Cap of the u-band Sky Survey (SCUSS) u-band magnitude. Due to the increased accuracy of SCUSS u-band measurements, the converted u-band magnitude becomes more accurate compared with the original SDSS u-band magnitude, in particular at the faint end. The average u-magnitude error (for both SDSS and SCUSS) of numerous main-sequence stars with increases as the g-band magnitude becomes fainter. When g = 19.5, the average magnitude error of the SDSS u is 0.11. When g = 20.5, the average SDSS u error rises to 0.22. However, at this magnitude, the average magnitude error of the SCUSS u is just half as much as that of the SDSS u. The SDSS u-band magnitudes of main-sequence stars with and are converted, therefore the maximum average error of the converted u-band magnitudes is 0.11. The potential application of this conversion is to derive a more accurate photometric metallicity calibration from SDSS observations, especially for the more distant stars. Thus, we can explore stellar metallicity distributions either in the Galactic halo or some stream stars.

MULTI-EPOCH SPECTROSCOPY OF DWARF GALAXIES WITH AGN SIGNATURES: IDENTIFYING SOURCES WITH PERSISTENT BROAD Hα EMISSION

ABSTRACT We use time-domain optical spectroscopy to distinguish between broad emission lines powered by accreting black holes (BHs) and stellar processes (i.e., supernovae) for 16 galaxies identified as active galactic nucleus (AGN) candidates by Reines et al (2013). Our study is primarily focused on those objects with narrow emission line ratios dominated by star formation, for which the origin of the broad Hα emission was unclear. Based on follow-up spectroscopy, we find that the broad Hα emission has faded or was ambiguous for all of the star-forming objects (14/16), over baselines ranging from 5–14 years, suggesting a transient stellar process was responsible for the broad emission in previous Sloan Digital Sky Survey observations. For the two objects in our follow-up sample with narrow-line AGN signatures (RGG 9 and RGG 119), we find persistent broad Hα emission consistent with an AGN origin. Additionally, we use high spectral resolution observations to measure stellar velocity dispersions for 15 objects in the Reines et al. (2013) sample, all with narrow-line ratios indicating the presence of an AGN. Stellar masses range from to M ⊙, and we measure in the range of . These correspond to some of the lowest-mass galaxies with optical signatures of AGN activity. We show that RGG 119, the one object that has both a measured and persistent broad Hα emission, falls near the extrapolation of the relation to the low-mass end.

NARROW-LINE X-RAY-SELECTED GALAXIES IN THE CHANDRA-COSMOS FIELD. II. OPTICALLY ELUSIVE X-RAY AGNs

ABSTRACT In the Chandra-COSMOS (C-COSMOS) survey, we have looked for X-ray-selected active galactic nuclei (AGNs), which are not detected as such in the optical, the so-called elusive AGNs. A previous study based on XMM-Newton and Sloan Digital Sky Survey observations has found a sample of 31 X-ray AGNs optically misclassified as star-forming (SF) galaxies at , including 17 elusive Sy2s. Using Chandra observations provides a sample of fainter X-ray sources and so, for a given X-ray luminosity, extends to higher redshifts. To study the elusive Sy2s in the C-COSMOS field, we have removed the NLS1s that contaminate the narrow-line sample. Surprisingly, the contribution of NLS1s is much lower in the C-COSMOS sample (less than 10% of the optically misclassified X-ray AGNs) than in Pons &amp; Watson. The optical misclassification of the X-ray AGNs ( ) can be explained by the intrinsic weakness of these AGNs, in addition to, in some cases, optical dilution by the host galaxies. Interestingly, we found the fraction of elusive Sy2s (narrow emission-line objects) optically misclassified as SF galaxies up to to be 10% ± 3% to 17% ± 4%, compared to the 6% ± 1.5% of the Pons &amp; Watson work (up to ). This result seems to indicate an evolution with redshift of the number of elusive Sy2s.

THE QUEST FOR CRADLES OF LIFE: USING THE FUNDAMENTAL METALLICITY RELATION TO HUNT FOR THE MOST HABITABLE TYPE OF GALAXY

The field of astrobiology has made huge strides in understanding the habitable zones around stars (Stellar Habitable Zones) where life can begin, sustain its existence and evolve into complex forms. A few studies have extended this idea by modelling galactic-scale habitable zones (Galactic Habitable Zones) for our Milky Way and specific elliptical galaxies. However, estimating the habitability for galaxies spanning a wide range of physical properties has so far remained an outstanding issue. Here, we present a "cosmobiological" framework that allows us to sift through the entire galaxy population in the local Universe and answer the question "Which type of galaxy is most likely to host complex life in the cosmos"? Interestingly, the three key astrophysical criteria governing habitability (total mass in stars, total metal mass and ongoing star formation rate) are found to be intricately linked through the "fundamental metallicity relation" as shown by SDSS (Sloan Digital Sky Survey) observations of more than a hundred thousand galaxies in the local Universe. Using this relation we show that metal-rich, shapeless giant elliptical galaxies at least twice as massive as the Milky Way (with a tenth of its star formation rate) can potentially host ten thousand times as many habitable (earth-like) planets, making them the most probable "cradles of life" in the Universe.

Narrow absorption lines with two observations from the Sloan Digital Sky Survey

We assemble 3524 quasars from Sloan Digital Sky Survey (SDSS) with repeated observations to search for variations of narrow C IV1548,1551 and Mg II2796,2803 absorption doublets in spectral regions shortward of 7000 Ang at the observed frame, which corresponds to time-scales of about 150 ~ 2643 days at quasar rest frame. In these quasar spectra, we detect 3580 C IV absorption systems with z_{abs} = 1.5188 ~ 3.5212, and 1809 Mg II absorption systems with z_{abs} = 0.3948 ~ 1.7167. In term of the absorber velocity (beta) distribution at quasar rest frame, we find a substantial number of C IV absorbers with beta<0.06, which might be connected to the absorptions of quasar outflows. The outflow absorptions peak at v~2000 km/s and drop rapidly below the peak value. Among 3580 C IV absorption systems, 52 systems (~ 1.5%) show obvious variations in equivalent widths at the absorber rest frame (Wr): 16 enhanced, 16 emerged, 12 weaken, and 8 disappeared systems, respectively. We find that changes in Wr548 are neither related to time-scales of the two SDSS observations, nor to absorber velocities at the quasar rest frame. Variable absorptions in low-ionization species are important to constraint the physical conditions of absorbing gas. There are two variable Mg II absorption systems measured from SDSS spectra detected by Hacker et al. However, in our Mg II$ absorption sample, we find that neither shows variable absorption with confident levels of >4sigma for lambda2796 lines and >3sigma for lambda2803 lines.

ARCHIVAL OBSERVATIONS OF ACTIVE ASTEROID 313P/GIBBS

We present prediscovery observations of active asteroid 313P/(2014 S4) Gibbs taken in 2003 and 2004. Sloan Digital Sky Survey observations from 2003 establish that, after 133P/Elst‐Pizarro and 238P/Read, 313P is only the third main-belt active asteroid to show mass loss in different orbits. This repetitive activity is consistent with mass loss driven by the sublimation of ice. A Finson-Probstein model is used to analyze the morphology of the object, showing that the activity was continuous but declining steadily over two months of observation in 2003. Ejected dust particles are dominated by relatively large sizes with radii from ! 25 to ! 600 ! m. We estimate from photometry the dust mass loss rate to be 10.2kg s ! 1 . In contrast, Subaru telescope observations from 2004 show no evidence for mass loss and suggest a bare nucleus. The color, nucleus size, and the mass loss rates of 313P measured in 2003 are consistent with published values obtained from data taken in 2014. However, the effective cross-section was larger than in 2014 by a factor of ! 2, indicating a decline in the activity between orbits. A new orbital solution gives a 5 upper limit to the transverse non-gravitational parameter q � A 1 2.45 10 2 8 AU day ! 2 .

Past and present cosmic structure in the SDSS DR7 main sample

We present a chrono-cosmography project, aiming at the inference of the four dimensional formation history of the observed large scale structure from its origin to the present epoch. To do so, we perform a full-scale Bayesian analysis of the northern galactic cap of the Sloan Digital Sky Survey (SDSS) Data Release 7 main galaxy sample, relying on a fully probabilistic, physical model of the non-linearly evolved density field. Besides inferring initial conditions from observations, our methodology naturally and accurately reconstructs non-linear features at the present epoch, such as walls and filaments, corresponding to high-order correlation functions generated by late-time structure formation. Our inference framework self-consistently accounts for typical observational systematic and statistical uncertainties such as noise, survey geometry and selection effects. We further account for luminosity dependent galaxy biases and automatic noise calibration within a fully Bayesian approach. As a result, this analysis provides highly-detailed and accurate reconstructions of the present density field on scales larger than ∼ 3 Mpc/h, constrained by SDSS observations. This approach also leads to the first quantitative inference of plausible formation histories of the dynamic large scale structure underlying the observed galaxy distribution. The results described in this work constitute the first full Bayesian non-linear analysis of the cosmic large scale structure with the demonstrated capability of uncertainty quantification. Some of these results will be made publicly available along with this work. The level of detail of inferred results and the high degree of control on observational uncertainties pave the path towards high precision chrono-cosmography, the subject of simultaneously studying the dynamics and the morphology of the inhomogeneous Universe.

THE MILKY WAY TOMOGRAPHY WITH SLOAN DIGITAL SKY SURVEY. V. MAPPING THE DARK MATTER HALO

We present robust constraints from the Sloan Digital Sky Survey (SDSS) on the shape and distribution of the dark matter halo within the Milky Way (MW). Using the number density distribution and kinematics of SDSS halo stars, we probe the dark matter distribution to heliocentric distances exceeding 10 kpc and galactocentric distances exceeding 20 kpc. Our analysis utilizes Jeans equations to generate two-dimensional acceleration maps throughout the volume; this approach is thoroughly tested on a cosmologically derived N-body+SPH simulation of a MW-like galaxy. We show that the known accelerations (gradients of the gravitational potential) can be successfully recovered in such a realistic system. Leveraging the baryonic gravitational potential derived by Bovy & Rix (2013), we show that the gravitational potential implied by the SDSS observations cannot be explained, assuming Newtonian gravity, by visible matter alone: the gravitational force experienced by stars at galactocentric distances of 20 kpc is as much as three times stronger than what can be attributed to purely visible matter. We also show that the SDSS data provide a strong constraint on the shape of the dark matter halo potential. Within galactocentric distances of 20 kpc, the dark matter halo potential is well described as an oblate halo with axis ratio qDM=0.7+/-0.1; this corresponds to an axis ratio qDM=0.4+/-0.1 for the dark matter density distribution. Because of our precise two-dimensional measurements of the acceleration of the halo stars, we can reject several MOND models as an explanation of the observed behavior.

Emergences of C iv narrow absorption troughs in the quasar SDSS J095254.10+021932.8

In this paper, we report on two C IV narrow absorption systems emerging from the spectrum of the quasar SDSS J095254.10 +021932.8, which are located at $z_{\rm abs}=2.0053$ and $z_{\rm abs}=1.8011$, respectively. These features have velocity offsets of $\rm \sim 13,800$ and $\rm \sim 34,700~km~s^{-1}$ with respect to the quasar, and show obviously partial coverage to the background emission sources. The two C IV absorption systems are imprinted on the spectrum obtained by the Sloan Digital Sky Survey (SDSS) III on 2011 March 25, which cannot be observed from the spectrum obtained by SDSS-I/II on 2000 December 30. The time interval of the two SDSS observations is 1186.4 d at the quasar rest frame. Because the continuum radiation, broad emission lines and mass accretion rate are stable for the two observations, together with the time interval of observations, we believe that the observed spectral variations are less likely to be caused by the changes in the ionization states of the absorbing gas or by a new outflow, arising from the situation where there was previously no outflow at all. Instead, they probably arise from the motions of multiple streaming gas across the sightline of the quasar.

High-redshift Fermi blazars observed by GROND and Swift

We observed 5 gamma-ray loud blazars at redshift greater than 2 with the X-Ray Telescope (XRT) and the UltraViolet and Optical Telescope (UVOT) onboard the Swift satellite, and the Gamma-Ray burst Optical Near-Infrared Detector (GROND) instrument. These observations were quasi simultaneous, usually within a few hours. For 4 of these blazars the near-IR to UV data show the presence of an accretion disc, and we could reliably estimate its accretion rate and black hole mass. One of them, PKS 1348+007, was found in an extraordinarily high IR-optical state, almost two orders of magnitude brighter than at the epoch of the Sloan Digital Sky Survey observations. For all the 5 quasars the physical parameters of the jet emitting zone, derived by applying a one-zone emission model, are similar to that found for the bulk of other gamma-ray loud quasars. With our observations we have X-ray data for the full sample of blazars at z>2 present in the Fermi 2-yrs (2LAC) catalog. This allows to have a rather complete view of the spectral energy distribution of all high-redshift Fermi blazars, and to draw some conclusions about their properties, and especially about the relation between the accretion rate and the jet power.

TESTING STELLAR POPULATION SYNTHESIS MODELS WITH SLOAN DIGITAL SKY SURVEY COLORS OF M31's GLOBULAR CLUSTERS

Accurate stellar population synthesis models are vital in understanding the properties and formation histories of galaxies. In order to calibrate and test the reliability of these models, they are often compared with observations of star clusters. However, relatively little work has compared these models in the ugriz filters, despite the recent widespread use of this filter set. In this paper, we compare the integrated colors of globular clusters in the Sloan Digital Sky Survey (SDSS) with those predicted from commonly used simple stellar population (SSP) models. The colors are based on SDSS observations of M31's clusters and provide the largest population of star clusters with accurate photometry available from the survey. As such, it is a unique sample with which to compare SSP models with SDSS observations. From this work, we identify a significant offset between the SSP models and the clusters' g-r colors, with the models predicting colors which are too red by g-r\sim0.1. This finding is consistent with previous observations of luminous red galaxies in the SDSS, which show a similar discrepancy. The identification of this offset in globular clusters suggests that it is very unlikely to be due to a minority population of young stars. The recently updated SSP model of Maraston & Stromback better represents the observed g-r colors. This model is based on the empirical MILES stellar library, rather than theoretical libraries, suggesting an explanation for the g-r discrepancy.