An empirical relation between sodium absorption and dust extinction

Studies of the most luminous quasars at high redshift directly probe the evolution of the most massive black holes in the early universe and their connection to massive galaxy formation. However, extremely luminous quasars at high redshift are very rare objects. Only wide-area surveys have a chance to constrain their population. The Sloan Digital Sky Survey (SDSS) has so far provided the most widely adopted measurements of the quasar luminosity function at . However, a careful re-examination of the SDSS quasar sample revealed that the SDSS quasar selection is in fact missing a significant fraction of quasars at the brightest end. We identified the purely optical-color selection of SDSS, where quasars at these redshifts are strongly contaminated by late-type dwarfs, and the spectroscopic incompleteness of the SDSS footprint as the main reasons. Therefore, we designed the Extremely Luminous Quasar Survey (ELQS), based on a novel near-infrared JKW2 color cut using Wide-field Infrared Survey Explorer mission (WISE) AllWISE and 2MASS all-sky photometry, to yield high completeness for very bright ( ) quasars in the redshift range of . It effectively uses random forest machine-learning algorithms on SDSS and WISE photometry for quasar–star classification and photometric redshift estimation. The ELQS will spectroscopically follow-up ∼230 new quasar candidates in an area of ∼12,000 deg2 in the SDSS footprint to obtain a well-defined and complete quasar sample for an accurate measurement of the bright-end quasar luminosity function (QLF) at . In this paper, we present the quasar selection algorithm and the quasar candidate catalog.

We present the discovery of 17 double white dwarf (WD) binaries from our ongoing search for extremely low mass (ELM) < 0.3 M ⊙ WDs, objects that form from binary evolution. Gaia parallax provides a new means of target selection that we use to evaluate our original ELM Survey selection criteria. Cross-matching the Gaia and Sloan Digital Sky Survey (SDSS) catalogs, we identify an additional 36 ELM WD candidates with 17 < g < 19 mag and within the 3σ uncertainties of our original color selection. The resulting discoveries imply the ELM Survey sample was 90% complete in the color range −0.4 < (g − r)0 < −0.1 mag (approximately 9000 K < T eff < 22,000 K). Our observations complete the sample in the SDSS footprint. Two newly discovered binaries, J123950.370−204142.28 and J232208.733+210352.81, have orbital periods of 22.5 and 32 minutes, respectively, and are future Laser Interferometer Space Antenna gravitational-wave sources.

Diffuse interstellar bands (DIBs) have been discovered for almost a century, but their nature remains one of the most challenging problems in astronomical spectroscopy. Most recent work to identify and investigate the properties and carriers of DIBs concentrates on high-resolution spectroscopy of selected sight-lines. In this paper, we report detections of DIBs in the Sloan Digital Sky Survey (SDSS) low-resolution spectra of a large sample of Galactic stars. Using a template subtraction method, we have successfully identified the DIBs $\lambda$$\lambda$5780, 6283 in the SDSS spectra of a sample of about 2,000 stars and measured their strengths and radial velocities. The sample is by far the largest ever assembled. The targets span a large range of reddening, E(B-V) ~ 0.2 -- 1.0, and are distributed over a large sky area and involve a wide range of stellar parameters (effective temperature, surface gravity and metallicity), confirming that the carriers of DIBs are ubiquitous in the diffuse interstellar medium (ISM). The sample is used to investigate relations between strengths of DIBs and magnitudes of line-of-sight extinction, yielding results (i.e., EW(5780)= 0.61 x E(B-V) and EW(6283) = 1.26 x E(B-V)) consistent with previous studies. DIB features have also been detected in the commissioning spectra of the Guoshoujing Telescope (LAMOST) of resolving power similar to that of SDSS. Detections of DIBs towards hundreds of thousands of stars are expected from the on-going and up-coming large scale spectroscopic surveys such as RAVE, SDSS III and LAMOST, particularly from the LAMOST Digital Sky Survey of the Galactic Anti-center (DSS-GAC). Such a huge database will provide an unprecedented opportunity to study the demographical distribution and nature of DIBs as well as using DIBs to probe the distribution and properties of the ISM and the dust extinction.

We present the discovery of nine quasars at identified in the Sloan Digital Sky Survey (SDSS) imaging data. This completes our survey of quasars in the SDSS footprint. Our final sample consists of 52 quasars at , including 29 quasars with mag selected from 11,240 deg2 of the SDSS single-epoch imaging survey (the main survey), 10 quasars with selected from 4223 deg2 of the SDSS overlap regions (regions with two or more imaging scans), and 13 quasars down to mag from the 277 deg2 in Stripe 82. They span a wide luminosity range of . This well-defined sample is used to derive the quasar luminosity function (QLF) at . After combining our SDSS sample with two faint ( mag) quasars from the literature, we obtain the parameters for a double power-law fit to the QLF. The bright-end slope β of the QLF is well constrained to be . Due to the small number of low-luminosity quasars, the faint-end slope α and the characteristic magnitude are less well constrained, with and mag. The spatial density of luminous quasars, parametrized as , drops rapidly from to 6, with . Based on our fitted QLF and assuming an intergalactic medium (IGM) clumping factor of C = 3, we find that the observed quasar population cannot provide enough photons to ionize the IGM at ∼90% confidence. Quasars may still provide a significant fraction of the required photons, although much larger samples of faint quasars are needed for more stringent constraints on the quasar contribution to reionization.

Galaxy clusters are powerful probes for cosmological models. Next-generation, large-scale optical and infrared surveys are poised to reach unprecedented depths and, thus, they require highly complete and pure cluster catalogs, with a well-defined selection function. We have developed a new cluster detection algorithm named YOLO for CLuster detection (YOLO–CL), which is a modified version of the state-of-the-art object detection deep convolutional network named You only look once (YOLO) that has been optimized for the detection of galaxy clusters. We trained YOLO–CL on the red-sequence Matched-filter Probabilistic Percolation (redMaPPer) cluster catalog, based on Sloan Digital Sky Survey (SDSS) color images. We find that YOLO–CL detects 95–98% of the redMaPPer clusters, with a purity of 95–98%, that is calculated by applying the network to SDSS blank fields. When compared to the Meta-Catalog of X-Ray Detected Clusters of Galaxies 2021 (MCXC2021) X-ray catalog in the SDSS footprint, YOLO–CL recovers all clusters at LX ≳ 2–3 × 1044 erg s−1, M500 ≳ 2–3 × 1014M⊙, R500≳0.75–0.8 Mpc and 0.4 ≲ z ≲ 0.6. When compared to the redMaPPer detection of the same MCXC2021 clusters, we find that YOLO–CL is more complete than redMaPPer, which means that the neural network has indeed improved the cluster detection efficiency of its training sample. In fact, YOLO–CL detects ~98% of the MCXC2021 clusters with an X-ray surface brightness of IX,500 ≳ 20 × 10−15 erg s−1 cm−2 arcmin−2 at 0.2 ≲ z ≲ 0.6 and ~100% of the MCXC2021 clusters with IX,500 ≳ 30 × 10−15 erg s−1 cm−2 arcmin−2 at 0.3 ≲ z ≲ 0.6; while redMaPPer detects ~98% of the MCXC2021 clusters with IX,500 ≳ 55 × 10−15 erg s−1 cm−2 arcmin−2 at 0.2 ≲ z ≲ 0.6 and ~100% of the MCXC2021 clusters with IX,500 ≳ 20 × 10−15 erg s−1 cm−2 arcmin−2 at 0.5 ≲ z ≲ 0.6. The YOLO–CL selection function is approximately constant with redshift, with respect to the MCXC2021 cluster X-ray surface brightness. YOLO–CL exhibits a high level of performance when compared to traditional detection algorithms applied to SDSS. Deep learning networks display a strong advantage over traditional galaxy cluster detection techniques because they do not require the galaxy’s photometric and photometric redshift catalogs. This eliminates systematic uncertainties that may be introduced during source detections and photometry, as well as photometric redshift measurements. Our results show that YOLO–CL is an efficient alternative to traditional cluster detection methods. In general, this work shows that it is worth exploring the performance of deep convolution networks for future cosmological cluster surveys, such as the Rubin/Legacy Survey of Space and Time (Rubin/LSST), Euclid, and Roman Space Telescope surveys.

We present the first results of the SOAR (Southern Astrophysical Research) Gravitational Arc Survey (SOGRAS). The survey imaged 47 clusters in two redshift intervals centered at $z=0.27$ and $z=0.55$, targeting the richest clusters in each interval. Images were obtained in the $g'$, $r'$ and $i'$ bands using the SOAR Optical Imager (SOI), with a median seeing of 0.83, 0.76 and 0.71 arcsec, respectively, in these filters. Most of the survey clusters are located within the Sloan Digital Sky Survey (SDSS) Stripe 82 region and all of them are in the SDSS footprint. Photometric calibration was therefore performed using SDSS stars located in our SOI fields. We reached for galaxies in all fields the detection limits of $g \sim 23.5$, $r \sim 23$ and $i \sim 22.5$ for a signal-to-noise ratio (S/N) = 3. As a by-product of the image processing, we generated a source catalogue with 19760 entries, the vast majority of which are galaxies, where we list their positions, magnitudes and shape parameters. We compared our galaxy shape measurements to those of local galaxies and concluded that they were not strongly affected by seeing. From the catalogue data, we are able to identify a red sequence of galaxies in most clusters in the lower $z$ range. We found 16 gravitational arc candidates around 8 clusters in our sample. They tend to be bluer than the central galaxies in the lensing cluster. A preliminary analysis indicates that $\sim 10%$ of the clusters have arcs around them, with a possible indication of a larger efficiency associated to the high-$z$ systems when compared to the low-$z$ ones. Deeper follow-up images with Gemini strengthen the case for the strong lensing nature of the candidates found in this survey.

Astrophysical tests of modified gravity theories in the nearby universe have been emphasized recently by Hui 2009 and Jain 2011. A key element of such tests is the screening mechanism whereby general relativity is restored in massive halos or high density environments like the Milky Way. In chameleon theories of gravity, including all f(R) models, field dwarf galaxies may be unscreened and therefore feel an extra force, as opposed to screened galaxies. The first step to study differences between screened and unscreened galaxies is to create a 3D screening map. We use N-body simulations to test and calibrate simple approximations to determine the level of screening in galaxy catalogs. Sources of systematic errors in the screening map due to observational inaccuracies are modeled and their contamination is estimated. We then apply our methods to create a map out to 200 Mpc in the Sloan Digital Sky Survey footprint using data from the Sloan survey and other sources. In two companion papers this map will be used to carry out new tests of gravity using distance indicators and the disks of dwarf galaxies. We also make our screening map publicly available.

Context. The goal of the Turn-Off Primordial Stars survey (TOPoS) project is to find and analyse turn-off (TO) stars of extremely low metallicity. To select the targets for spectroscopic follow-up at high spectral resolution, we relied on low-resolution spectra from the Sloan Digital Sky Survey (SDSS). Aims. In this paper, we use the metallicity estimates we obtained from our analysis of the SDSS spectra to construct the metallicity distribution function (MDF) of the Milky Way, with special emphasis on its metal-weak tail. The goal is to provide the underlying distribution out of which the TOPoS sample was extracted. Methods. We made use of SDSS photometry, Gaia photometry, and distance estimates derived from the Gaia parallaxes to derive a metallicity estimate for a large sample of over 24 million TO stars. This sample was used to derive the metallicity bias of the sample for which SDSS spectra are available. Results. We determined that the spectroscopic sample is strongly biased in favour of metal-poor stars, as intended. A comparison with the unbiased photometric sample allows us to correct for the selection bias. We selected a sub-sample of stars with reliable parallaxes for which we combined the SDSS radial velocities with Gaia proper motions and parallaxes to compute actions and orbital parameters in the Galactic potential. This allowed us to characterise the stars dynamically, and in particular to select a sub-sample that belongs to the Gaia-Sausage-Enceladus (GSE) accretion event. We are thus also able to provide the MDF of GSE. Conclusions. The metal-weak tail derived in our study is very similar to that derived in the H3 survey and in the Hamburg/ESO Survey. This allows us to average the three MDFs and provide an error bar for each metallicity bin. Inasmuch as the GSE structure is representative of the progenitor galaxy that collided with the Milky Way, that galaxy appears to be strongly deficient in metal-poor stars compared to the Milky Way, suggesting that the metal-weak tail of the latter has been largely formed by accretion of low-mass galaxies rather than massive galaxies, such as the GSE progenitor.

We present three new candidate AM CVn binaries, plus one confirmed new system, from a spectroscopic survey of color-selected objects from the Sloan Digital Sky Survey. All four systems were found from their helium emission lines in low-resolution spectra taken on the Hale telescope at Palomar, and the Nordic Optical Telescope and the William Herschel Telescope on La Palma. The ultra-compact binary nature of SDSS J090221.35+381941.9 was confirmed using phase-resolved spectroscopy at the Keck-I telescope. From the characteristic radial velocity `S-wave' observed in the helium emission lines we measure an orbital period of 48.31 +/- 0.08 min. The continuum emission can be described with a blackbody or a helium white dwarf atmosphere of T_eff ~ 15,000K, in agreement with theoretical cooling models for relatively massive accretors and/or donors. The absence in the spectrum of broad helium absorption lines from the accreting white dwarf suggests that the accreting white dwarf cannot be much hotter than 15,000K, or that an additional component such as the accretion disk contributes substantially to the optical flux. Two of the candidate systems, SDSS J152509.57+360054.5 and SDSS J172102.48+273301.2, do show helium absorption in the blue part of their spectra in addition to the characteristic helium emission lines. This, in combination with the high effective temperatures of ~18,000K and ~16,000K suggests both two be at orbital periods below ~40min. The third candidate, SDSS J164228.06+193410.0, exhibits remarkably strong helium emission on top of a relatively cool (T_eff~12,000K) continuum, indicating an orbital period above ~50min.

The Milky Way hosts about 150 globular clusters, and at least 17 dwarf spheroidal galaxies. These satellites experience a constantly changing gravitational field on their orbits. Close encounters with the Galactic bulge and passages through the Galactic disk enhance the effect of the constantly changing tidal field. As a consequence satellite member stars can leave their host's gravitational potential. For globular clusters, internal mechanisms, such as 2-body relaxation are also resulting in a loss of stars. Hence, the globular clusters are constantly losing stars and are being dissolved. In this thesis I investigate 17 globular cluster for signs of dissolution. I.e., we are studying the two-dimensional distribution of (potential) cluster member stars on the sky using photometric data from the Sloan Digital Sky Survey. We use a color-magnitude weighted counting algorithm to count the stars around the globular clusters. We detect the known tidal tails of Pal 5 and NGC 5466. Further, we also confirm some previous finding of possible tidal features for NGC 5053 and NGC 6341. For NGC 4147, we observe for the first time complex two-dimensional features, resembling a multiple-arm morphology. For almost all clusters in our sample we observe a halo of extra tidal stars. We observe no new large scale tidal features for our sample of clusters containing stars brighter than ~22.5 mag. The lack of large scale tidal tails is compatible with theoretical predictions of the destruction timescales for the clusters in our sample. We also observe the two-dimensional distribution of stars around three dwarf spheroidal galaxies: Sextans, Leo II, and Ursa Minor. Each galaxy reveals a unique structure. The main, luminous body of Sextans is not filling the tidal radius. We observe an off-center peak of highest stellar density. For Leo II, we observe an almost symmetric structure, compatible with the theory that Leo II has never come close to the Milky Way. We detect the complex structure of Ursa Minor, with two off-center peaks. We observe no large scale structure emanating from this dwarf galaxy. We further investigate the possibility of a line-of-sight depth of Sextans and Ursa Minor. We study the thickness of the blue horizontal branch. For Sextans, we observe an increasing thickness with increasing radius, comparable with the photometric error. Only detailed modeling will be able to show the significance of this varying thickness. For Ursa Minor, the increase in horizontal branch thickness is negligible, compared to the photometric error. Hence, Ursa Minor shows no sign of a significant line-of-sight depth. The distribution of red and blue horizontal stars was investigated for Sextans. The ''red'' population is much more concentrated. The peak of the density of the two populations does not coincide. Further, we investigated one globular cluster in particular, Pal 14. This cluster is sparse and at a remote location in the Galaxy. We aim to answer the question whether Pal 14 is governed by classical or modified Newtonian dynamics. We measured the radial velocity of 17 red giant branch stars and (probable) AGB stars with UVES@VLT and the Keck I telescope. The resulting line-of-sight velocity dispersion is comparable to the theoretical predictions for the case of classical dynamics. The predicted value for modified dynamics is about twice as large as the observed value. With HST images we derived the cluster's mass function and computed its total mass. The main sequence mass function slope is flatter than the canonical value, the cluster seems to be depleted in lower mass stars. N-body simulations predict for a given mass of the cluster its line-of-sight velocity dispersion in modified dynamics. The measured mass for Pal 14 is requiring a much larger velocity dispersion in modified Newtonian dynamics than we have measured. This leads to the conclusion that if Pal 14 is on a circular orbit, modified dynamics cannot explain the low velocity dispersion and the measured mass simultaneously.

ABSTRACTThe Palomar Transient Factory (PTF) provides multiple epoch imaging for a large fraction of the celestial sphere. Here, we describe the photometric calibration of the PTF data products that allows the PTF magnitudes to be related to other magnitude systems. The calibration process utilizes Sloan Digital Sky Survey (SDSS) r ∼ 16 mag point-source objects as photometric standards. During photometric conditions, this allows us to solve for the extinction coefficients and color terms and to estimate the camera illumination correction. This also enables the calibration of fields that are outside the SDSS footprint. We test the precision and repeatability of the PTF photometric calibration. Given that PTF is observing in a single filter each night, we define a PTF calibrated magnitude system for the R band and g band. We show that, in this system, ≈59% (47%) of the photometrically calibrated PTF R-band (g-band) data achieve a photometric precision of 0.02–0.04 mag and have color terms and extinction coefficients that are close to their average values. Given the objects’ color, the PTF magnitude system can be converted to other systems. Moreover, a night-by-night comparison of the calibrated magnitudes of individual stars observed on multiple nights shows that they are consistent to a level of ≈0.02 mag. Most of the data that were taken under nonphotometric conditions can be calibrated relative to other epochs of the same sky footprint obtained during photometric conditions. We provide a concise guide describing how to use the PTF photometric-calibration data products, as well as the transformations between the PTF magnitude system and the SDSS and Johnson-Cousins systems.

We identify SDSS 011009.09+132616.1, SDSS 030308.35+005444.1, SDSS 143547.87+ 373338.5 and SDSS 154846.00+405728.8 as four eclipsing white dwarf plus main-sequence (WDMS) binaries from the Sloan Digital Sky Survey (SDSS), and report on follow-up observations of these systems. SDSS 0110+1326, SDSS 1435+3733 and SDSS 1548+4057 contain DA white dwarfs, while SDSS 0303+0054 contains a cool DC white dwarf. Orbital periods and ephemerides have been established from multiseason photometry. SDSS 1435+3733, with Porb= 3 h has the shortest orbital period of all known eclipsing WDMS binaries. As for the other systems, SDSS 0110+1326 has Porb= 8 h, SDSS 0303+0054 has Porb= 3.2 h and SDSS 1548+4057 has Porb= 4.4 h. Time-resolved spectroscopic observations have been obtained and the Hα and Ca ii λλ8498.02, 8542.09, 8662.14 triplet emission lines, as well as the Na i λλ8183.27, 8194.81 absorption doublet were used to measure the radial velocities of the secondary stars in all four systems. A spectral decomposition/fitting technique was then employed to isolate the contribution of each of the components to the total spectrum, and to determine the white dwarf effective temperatures and surface gravities, as well as the spectral types of the companion stars. We used a light-curve modelling code for close binary systems to fit the eclipse profiles and the ellipsoidal modulation/reflection effect in the light curves, to further constrain the masses and radii of the components in all systems. All three DA white dwarfs have masses of MWD∼ 0.4–0.6 M⊙, in line with the expectations from close binary evolution. The DC white dwarf in SDSS 0303+0054 has a mass of MWD≳ 0.85 M⊙, making it unusually massive for a post-common-envelope system. The companion stars in all four systems are M dwarfs of spectral type M4 and later. Our new additions raise the number of known eclipsing WDMS binaries to 14, and we find that the average white dwarf mass in this sample is 〈MWD〉=0.57 ± 0.16 M⊙, only slightly lower than the average mass of single white dwarfs. The majority of all eclipsing WDMS binaries contain low-mass (<0.6 M⊙) secondary stars, and will eventually provide valuable observational input for the calibration of the mass–radius relations of low-mass main-sequence stars and of white dwarfs.

We would like to propose that Fermilab participate in a three-year extension of operations of the Sloan Digital Sky Survey (SDSS). The SDSS has already provided a wealth of new information about astrophysical and cosmological phenomena, ranging from discoveries of some of the lowest mass stars near the sun to the most distant known quasars. Fermilab scientists have authored or coauthored numerous papers on topics such as the discovery of new structures in the Milky Way halo and detailed studies of galaxy clustering. The detection of the ''shadow of dark energy'' last year by the SDSS shared honors with results from the WMAP satellite as the Discovery of the Year by Science Magazine. Fermilab has had a pivotal role in the creation of the SDSS project and the construction of the data archive, which has already become a valuable resource to the scientific community and to the general public at large. By June of 2005, the SDSS will have completed its nominal five-year period of operations. There is a strong desire among many of the existing SDSS collaborators, including Fermilab, to pursue a three-year extension of operations, both to complete an unfinished portion of the survey area and to pursue new science programs inspired by the discoveries of the current survey. The extension is composed of three science programs that can share the observing time around the year. At the end of our five year program, the SDSS will have surveyed two large but disconnected regions of sky in the North Galactic cap, leaving one of the original goals of the survey uncompleted: to have surveyed a complete filled volume of the universe. The reason for the gap between these two regions is that the rate of collecting data was simply less than anticipated, largely due to the impact of weather conditions at the site. Thus, there is a desire to fill in the gap. The discovery of new structures in the Milky Way halo, coupled with the realization that the SDSS is an ideal instrument for probing these structures, has led to the development of a program called SEGUE--the Sloan Extension for Galactic Underpinnings and Evolution. This program will survey the sky in areas not included in the original plan and will pursue a multitude of questions regarding the formation of the Milky Way galaxy, including the role and impact of Dark Matter that seems to dominate its mass. The SDSS 2.5 meter telescope is well suited for surveying and detecting supernovae (SNe) in the redshift range 0.1 to 0.3, a range poorly covered by today's existing SNe surveys. Such data are important for determining the expansion rate of the nearby universe and are essential for interpreting data at high redshift from current and future space-based surveys such as the SNAP experiment that is being proposed for the NASA/DOE Joint Dark Energy Mission and of which several Fermilab scientists (many of whom are on this proposal) are already members. Fermilab scientists are extremely interested in all three of the core science goals of the extension. No major new hardware systems are needed, allowing us to take advantage of the existing infrastructure at the Apache Point Observatory (APO) in New Mexico. Only modest changes or enhancements are needed to the data processing systems. The data acquisition system will likely require upgrading to replace the oldest computers that are no longer under warranty. A new data processing system at APO will be needed to process the supernova survey data in near-realtime.

We present optical spectroscopy and near-infrared photometry of 57 faint (g = 19–22) high proper motion white dwarfs identified through repeat imaging of ≈3100 deg2 of the Sloan Digital Sky Survey footprint by Munn et al. We use ugriz and JHphotometry to perform a model atmosphere analysis, and identify 10 ultracool white dwarfs with Teff < 4000 K, including the coolest pure H atmosphere white dwarf currently known, J1657+2638, with Teff = 3550 ± 100 K. The majority of the objects with cooling ages larger than 9 Gyr display thick disc kinematics and constrain the age of the thick disc to ≥11 Gyr. There are four white dwarfs in our sample with large tangential velocities (vtan > 120 km s−1) and UVW velocities that are more consistent with the halo than the Galactic disc. For typical 0.6M ⊙ white dwarfs, the cooling ages for these halo candidates range from 2.3 to 8.5 Gyr. However, the total mainsequence+ white dwarf cooling ages of these stars would be consistent with the Galactic halo if they are slightly undermassive. Given the magnitude limits of the current large-scale surveys, many of the coolest and oldest white dwarfs remain undiscovered in the solar neighbourhood, but upcoming surveys such as Gaia and the Large Synoptic Survey Telescope should find many of these elusive thick disc and halo white dwarfs.

We compare the most successful and widely used map of Galactic dust extinction, provided by Schlegel, Finkbeiner, and Davis (1998, hereafter SFD), to the galaxy number counts in the Sloan Digital Sky Survey (SDSS) photometric/spectroscopic DR4 sample. We divide the SDSS survey area into 69 disjoint subregions according to the dust extinction provided by SFD and compare the surface number density of galaxies in each subregion. As expected, the galaxy surface number density decreases with increasing extinction, but only for SFD extinction values above about 0.1 to 0.2 magnitudes (depending on the band). At lower values of the SFD extinction, we find that the sky surface density of galaxies increases with increasing extinction, precisely the opposite of the effect expected from Galactic dust. We also find that the average color of the SDSS photometric galaxy sample is slightly bluer at higher SFD extinctions in this regime, again the opposite of the effect expected from Galactic dust. Even though these anomalies occur only for sight-lines with low SFD extinction values, they affect approximately 68% of the high galactic latitude sky in which galaxies and their clustering properties are normally studied. Although it would be possible to explain these effects with a mysterious component of Galactic dust, which is anti-correlated with the 100$\mu$m flux on which the SFD extinction map is based, this model is not physically plausible. Moreover, we find that the surface number density of SDSS photometric quasars does not show any similar effect, as would be expected if the explanation were an unknown Galactic dust component. Considering these results, we suggest that the far infrared (FIR) brightness of the sky in regions of true low dust extinction is significantly “contaminated” by the FIR emission from background galaxies. We show that such an explanation is both qualitatively and quantitatively consistent with the available data. Based on this interpretation we conclude that systematic errors in the SFD extinction map due to extragalactic FIR emission are quite small, on the order hundredths of a magnitude, but nevertheless statistically detectable. Unfortunately, however, these errors are also entangled in a complex way with a signal of great interest to many “precision cosmology” applications, namely the large-scale clustering of galaxies.