Sloan Digital Sky Survey Stripe Research Articles

Study of Double-mode RR Lyrae Stars in the Stripe 82 Region Using SDSS and ZTF

RRd stars are a subclass of RR Lyrae stars that pulsate simultaneously in the fundamental and the first-overtone radial modes. We report the analysis of 10 RRd stars discovered in the Sloan Digital Sky Survey (SDSS) Stripe 82 region of the Galactic halo using SDSS and Zwicky Transient Facility data. By combining the two data sets, we constructed ∼23 yr long lightcurves in gri bands, and conclude that out of 10, 9 were the classical RRd stars and 1 was the anomalous RRd star. We also observed the long-term variation, the Blazhko effect, in 3 classical RRd stars out of 10 stars, giving an incidence rate of 30%. The shortest Blazhko period observed was 226.4 ± 0.8 days and the longest was 2799 ± 75 days.

Tidal features and disc thicknesses of edge-on galaxies in the SDSS Stripe 82

ABSTRACT We examine deep optical images of edge-on galaxies selected from the Sloan Digital Sky Survey (SDSS) Stripe 82. The entire sample consists of over 800 genuine edge-on galaxies with spectroscopic redshifts out to $z\sim 0.2$. To discern the faintest details around the galaxies, we use three different data sources with a photometric depth of down to 30 mag arcsec$^{-2}$ in the r band: SDSS Stripe 82, Hyper Suprime-Cam Strategic Program, and DESI Legacy Imaging Surveys. Our analysis of the deep images reveals a variety of low surface brightness features. 49 galaxies exhibit prominent tidal structures, including tidal tails, stellar streams, bridges, and diffuse shells. Additionally, 56 galaxies demonstrate peculiar structural features such as lopsided discs, faint warps, and dim polar rings. Overall, we detect low surface brightness structures in 94 galaxies out of 838, accounting for 11 per cent of the sample. Notably, the fraction of tidal structures is only 5.8 per cent, which is significantly lower than that obtained in modern cosmological simulations and observations. Previous studies have shown that strongly interacting galaxies have stellar discs about 1.5–2 times thicker than those without apparent interactions. In an analysis where tidal features are carefully masked for precise disc axis ratio measurements, we show that discs of galaxies with tidal features are 1.33 times thicker, on average, than control galaxies that do not have visible tidal features. Furthermore, we find that edge-on galaxies with tidal structures tend to have a higher fraction of oval and boxy discs than galaxies without tidal features.

The Next Generation Virgo Cluster Survey. XXXVII. Distant RR Lyrae Stars and the Milky Way Stellar Halo Out to 300 kpc

RR Lyrae stars are standard candles with characteristic photometric variability and serve as powerful tracers of Galactic structure, substructure, accretion history, and dark matter content. Here we report the discovery of distant RR Lyrae stars, including some of the most distant stars known in the Milky Way halo, with Galactocentric distances of ∼300 kpc. We use time-series u*g′i′z′ Canada–France–Hawaii Telescope/MegaCam photometry from the Next Generation Virgo Cluster Survey (NGVS). We use a template light-curve fitting method based on empirical Sloan Digital Sky Survey Stripe 82 RR Lyrae data to identify RR Lyrae candidates in the NGVS data set. We eliminate several hundred suspected quasars and identify 180 RR Lyrae candidates with heliocentric distances of ∼20–300 kpc. The halo stellar density distribution is consistent with an r −4.09±0.10 power-law radial profile over most of this distance range with no signs of a break. The distribution of ab-type RR Lyrae in a period–amplitude plot (Bailey diagram) suggests that the mean metallicity of the halo decreases outward. Compared to other recent RR Lyrae surveys, like Pan-STARRS1, the High Cadence Transient Survey, and the Dark Energy Survey, our NGVS study has better single-epoch photometric precision and a comparable number of epochs but smaller sky coverage. At large distances, our RR Lyrae sample appears to be relatively pure and complete, with well-measured periods and amplitudes. These newly discovered distant RR Lyrae stars are important additions to the few secure stellar tracers beyond 150 kpc in the Milky Way halo.

The occurrence rate of galaxies with polar structures may be significantly underestimated

Polar-ring galaxies are photometrically and kinematically decoupled systems that are highly inclined to the major axis of the host. These galaxies have been explored since the 1970s, but the rarity of these systems has made such systematic studies difficult. However, over 250 good candidates have been identified. In this work, we examine a sample of over 18 000 galaxies from the Sloan Digital Sky Survey (SDSS) Stripe 82 for the presence of galaxies with polar structures. Using deep SDSS Stripe 82, DESI Legacy Imaging Surveys, and Hyper Suprime-Cam Subaru Strategic Program, we selected 53 good candidate galaxies with photometrically decoupled polar rings, 9 galaxies with polar halos, 6 galaxies with polar bulges, and 34 possibly forming polar-ring galaxies, versus 13 polar-ring candidates previously selected in Stripe 82. Our results suggest that the occurrence rate of galaxies with polar structures may be significantly underestimated, as revealed by the deep observations, and may amount to 1–3% of non-dwarf galaxies.

The LSST AGN Data Challenge: Selection Methods

Development of the Rubin Observatory Legacy Survey of Space and Time (LSST) includes a series of Data Challenges (DCs) arranged by various LSST Scientific Collaborations that are taking place during the project's preoperational phase. The AGN Science Collaboration Data Challenge (AGNSC-DC) is a partial prototype of the expected LSST data on active galactic nuclei (AGNs), aimed at validating machine learning approaches for AGN selection and characterization in large surveys like LSST. The AGNSC-DC took place in 2021, focusing on accuracy, robustness, and scalability. The training and the blinded data sets were constructed to mimic the future LSST release catalogs using the data from the Sloan Digital Sky Survey Stripe 82 region and the XMM-Newton Large Scale Structure Survey region. Data features were divided into astrometry, photometry, color, morphology, redshift, and class label with the addition of variability features and images. We present the results of four submitted solutions to DCs using both classical and machine learning methods. We systematically test the performance of supervised models (support vector machine, random forest, extreme gradient boosting, artificial neural network, convolutional neural network) and unsupervised ones (deep embedding clustering) when applied to the problem of classifying/clustering sources as stars, galaxies, or AGNs. We obtained classification accuracy of 97.5% for supervised models and clustering accuracy of 96.0% for unsupervised ones and 95.0% with a classic approach for a blinded data set. We find that variability features significantly improve the accuracy of the trained models, and correlation analysis among different bands enables a fast and inexpensive first-order selection of quasar candidates.

Testing the Radio-selection Method of Dual Active Galactic Nuclei in the Stripe 82 Field

We test the merger-induced dual active galactic nuclei (dAGNs) paradigm using a sample of 35 radio galaxy pairs from the Sloan Digital Sky Survey Stripe 82 field. Using Keck optical spectroscopy, we confirm 21 pairs have consistent redshifts, constituting kinematic pairs; the remaining 14 pairs are line-of-sight projections. We classify the optical spectral signatures via emission line ratios, equivalent widths, and excess of radio power above star formation predicted outputs. We find six galaxies are classified as LINERs and seven are AGN/starburst composites. Most of the LINERs are retired galaxies, while the composites likely have AGN contribution. All of the kinematic pairs exhibit radio power more than 10× above the level expected from just star formation, suggestive of a radio AGN contribution. We also analyze high-resolution (0.″3) imaging at 6 GHz from the NSF’s Karl G. Jansky Very Large Array for 17 of the kinematic pairs. We find six pairs (two new, four previously known) host two separate radio cores, confirming their status as dAGNs. The remaining 11 pairs contain single AGNs, with most exhibiting prominent jets/lobes overlapping their companion. Our final census indicates a dAGN duty cycle slightly higher than predictions of purely stochastic fueling, although a larger sample (potentially culled from VLASS) is needed to fully address the dAGN fraction. We conclude that while dAGNs in the Stripe 82 field are rare, the merger process plays some role in their triggering and it facilitates low to moderate levels of accretion.

Prospects for future studies using deep imaging: analysis of individual Galactic cirrus filaments

ABSTRACT The presence of Galactic cirrus is an obstacle for studying both faint objects in our Galaxy and low surface brightness extragalactic structures. With the aim of studying individual cirrus filaments in Sloan Digital Sky Survey (SDSS) Stripe 82 data, we develop techniques based on machine learning and neural networks that allow one to isolate filaments from foreground and background sources in the entirety of Stripe 82 with a precision similar to that of the human expert. Our photometric study of individual filaments indicates that only those brighter than 26 mag arcsec−2 in the SDSS r band are likely to be identified in SDSS Stripe 82 data by their distinctive colours in the optical bands. We also show a significant impact of data processing (e.g. flat-fielding, masking of bright stars, and sky subtraction) on colour estimation. Analysing the distribution of filaments’ colours with the help of mock simulations, we conclude that most filaments have colours in the following ranges: 0.55 ≤g − r ≤ 0.73 and 0.01 ≤ r − i ≤ 0.33. Our work provides a useful framework for an analysis of all types of low surface brightness features (cirri, tidal tails, stellar streams, etc.) in existing and future deep optical surveys. For practical purposes, we provide the catalogue of dust filaments.

Low-redshift quasars in the SDSS Stripe 82 – III. MOS observations

ABSTRACT We present multi-object optical spectroscopy of the galaxies in the environment of 12 low-redshift (z < 0.5) quasars and of 11 inactive massive galaxies chosen to match the properties of the quasar host galaxies to probe physical association and possible events of recent star formation (SF). The quasars are selected from a sample of QSOs in the Sloan Digital Sky Survey (SDSS) Stripe 82 region for which both the host galaxy and the large-scale environments were previously investigated. The new observations complement those reported in our previous works on close companion galaxies of nearby quasars. For the whole data set, we find that for about half (19 out of 44) of the observed QSOs, there is at least one associated companion galaxy. In addition to the new spectroscopic observations, we add data from the SDSS data base for the full sample of objects. We find that the incidence of companion galaxies in the fields of QSO (17 per cent) is not significantly different from that of inactive galaxies (19 per cent) similar to quasar hosts in redshift and mass. Nevertheless, the companions of quasars exhibit more frequently emission lines than those of inactive galaxies, suggesting a moderate link between the nuclear activity and recent SF in their environments.

AGNet: weighing black holes with deep learning

ABSTRACT Supermassive black holes (SMBHs) are commonly found at the centres of most massive galaxies. Measuring SMBH mass is crucial for understanding the origin and evolution of SMBHs. Traditional approaches, on the other hand, necessitate the collection of spectroscopic data, which is costly. We present an algorithm that weighs SMBHs using quasar light time series information, including colours, multiband magnitudes, and the variability of the light curves, circumventing the need for expensive spectra. We train, validate, and test neural networks that directly learn from the Sloan Digital Sky Survey (SDSS) Stripe 82 light curves for a sample of 38 939 spectroscopically confirmed quasars to map out the non-linear encoding between SMBH mass and multiband optical light curves. We find a 1σ scatter of 0.37 dex between the predicted SMBH mass and the fiducial virial mass estimate based on SDSS single-epoch spectra, which is comparable to the systematic uncertainty in the virial mass estimate. Our results have direct implications for more efficient applications with future observations from the Vera C. Rubin Observatory. Our code, AGNet, is publicly available at https://github.com/snehjp2/AGNet.

The optical variability analysis of quasars based on fractal Brownian motion in Sloan digital sky survey

<p indent="0mm">The optical variability analysis of quasars based on fractal Brownian motion in the Sloan Digital Sky Survey (SDSS). The violent activities of the high energy variability can be observed in multiple wavelength bands of quasars, and studying quasar variability can contribute to a deeper understanding of the physical mechanism of the emission from the central region. Many studies have shown that the great majority of quasar variability is considered to originate in the accretion disk’s thermal emission, but some people do not agree with this view. The Hurst exponent of SDSS quasar variability is calculated using the rescaled range analysis method to investigate the fractal properties of variability in quasars from the SDSS stripe 82. The Hurst exponent results of variability show that the quasar variability is self-similar and long-range correlated. The variability obeys the biased random walk model, and its variation is similar to fractal Brownian motion. The power spectral exponent obtained from the Hurst exponent conversion shows that most of the variabilities can be approximately fitted by the damped random walk model, but a small number of samples have large deviations.

Unveiling the Nature of Polar-ring Galaxies from Deep Imaging

General structural properties and low surface brightness tidal features hold important clues to the formation of galaxies. In this paper, we study a sample of polar-ring galaxies (PRGs) based on optical imaging from the Sloan Digital Sky Survey Stripe 82 and other deep surveys. We investigate the deepest images of candidates for PRGs to date. We carry out photometric decomposition on the host galaxies and associated polar structures that allows us to derive the structural properties of both components. We are able to detect very faint tidal structures around most PRGs in our sample. For several galaxies, we can directly observe the formation of the polar ring due to merging, which is manifested in debris of the victim galaxy and an arc-like polar structure made up of its material. In a few cases, we can discern signs of tidal accretion. The results obtained indicate that the gravitational interaction and merging of galaxies are the most plausible mechanisms for the formation of PRGs.

Examining AGN UV/Optical Variability beyond the Simple Damped Random Walk

We present damped harmonic oscillator (DHO) light-curve modeling for a sample of 12,714 spectroscopically confirmed quasars in the Sloan Digital Sky Survey Stripe 82 region. DHO is a second-order continuous-time autoregressive moving-average process, which can be fully described using four independent parameters: a natural oscillation frequency (ω 0), a damping ratio (ξ), a characteristic perturbation timescale (τ perturb), and an amplitude for the perturbing white noise (σ ϵ ). The asymptotic variability amplitude of a DHO process is quantified by σ DHO—a function of ω 0, ξ, τ perturb, and σ ϵ . We find that both τ perturb and σ ϵ follow different dependencies with rest-frame wavelength (λ RF) on either side of 2500 Å, whereas σ DHO follows a single power-law relation with λ RF. After correcting for wavelength dependence, σ DHO exhibits anticorrelations with both the Eddington ratio and the black hole mass, while τ perturb—with a typical value of days in the rest frame—shows an anticorrelation with the bolometric luminosity. Modeling active galactic nuclei (AGN) variability as a DHO offers more insight into the workings of accretion disks close to the supermassive black holes at the center of AGN. The newly discovered short-term variability (characterized by τ perturb and σ ϵ ) and its correlation with bolometric luminosity pave the way for new algorithms that will derive fundamental properties (e.g., Eddington ratio) of AGN using photometric data alone.

Interstellar Extinction in Galactic Cirri in SDSS Stripe 82

We have applied the method of star counts with Wolf diagrams to determine the interstellar extinction in five Galactic cirri in Sloan Digital Sky Survey (SDSS) Stripe 82. For this purpose, we have used the photometry of stars in the GALEX NUV filter and the photometry of red dwarfs in five SDSS bands and four SkyMapper Southern Sky Survey DR2 bands. We have identified the cirri as sky regions with an enhanced infrared emission from the Schlegel+1998 map. The extinction in them has been calculated relative to the nearby comparison regions with a reduced emission. The results for different filters agree well, giving the range of distances and the extinction law for each cirrus. The distances in the range 140--415 pc found are consistent with the 3D reddening maps. In the range between the $B$ and $V$ filters the extinctions found are consistent with the estimates from Schlegel+1998 for the Cardelli+1989 extinction law with $R_\mathrm{V}=3.1$. However, the extinctions found for all of the filters are best described not by the Cardelli+1989 extinction law with some $R_\mathrm{V}=3.1$, but by the inverse proportionality of the extinction and wavelength with its own coefficient for each cirrus. In one of the cirri our results suggest a very slight decrease in extinction with wavelength, i.e., a large contribution of gray extinction. In the remaining cirri a manifestation of gray extinction is not ruled out either. This is consistent with the previous measurements of the extinction law far from the Galactic midplane.

Reddening and the shape of the variable component of the continua of active galactic nuclei from the optical to the far-ultraviolet – I

ABSTRACT We analyse the photometric variability of 4611 active galactic nuclei (AGNs) from the Sloan Digital Sky Survey (SDSS) Stripe 82. We recover the spectral energy distribution (SED) of the variable flux as a function of wavelength. For rest wavelengths longer than ∼2200 Å, we find that the SED of the variable component of the bluest AGNs is consistent with the Fν ∝ ν+1/3 spectrum predicted for an externally illuminated accretion disc. We confirm there is some residual variable emission corresponding to the ‘small blue bump’ and other broad-line region variability. We interpret steeper optical spectra of the variable component as being due to intrinsic reddening. This is supported by the correlation of the Balmer decrement with the colour excess of the variable component. We find the median internal reddening of SDSS AGNs in Stripe 82 with z &amp;lt; 0.4 to be E(B − V) ≈ 0.10 in agreement with the reddening derived from the Balmer decrement. Individual AGNs in the sample can have E(B − V) &amp;gt; 0.4.

Exploring the connection between ultraviolet/optical variations and radio emission in radio-quiet quasars: clues about the origin of radio emission

ABSTRACT Radio emission in radio-quiet quasars (RQQs) has long been a mystery and its physical origin remains unclear. In previous work, we have found that quasars that are more variable in ultraviolet (UV)/optical have stronger X-ray emission, indicating a link between disc turbulence and X-ray corona heating. In this work, for the first time, we investigate the relation between UV/optical variability and the radio emission in RQQs selected from the Sloan Digital Sky Survey Stripe 82 survey and the Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) survey. We median stack the FIRST images and detect clear signals from RQQs in the co-added images of individually radio non-detected sources. Controlling the effects of other parameters, including redshift, black hole mass, bolometric luminosity and Eddington ratio, we find more variable RQQs, which are known to be relatively brighter in X-ray, and show tentatively weaker radio emission, contrary to the linear X-ray/radio correlation if the radio emission is from, or driven by, the corona. This discovery also suggests that if the radio emission in RQQs is driven by AGN activity (such as a weak jet), the underlying driving process is independent of the disc turbulence, which drives UV/optical variability and probably also corona heating. Alternatively, the radio emission could be a result of star formation in the host galaxies.

Photometric Recalibration of the SDSS Stripe 82 to a Few Millimagnitude Precision with the Stellar Color Regression Method and Gaia EDR3

Abstract By combining spectroscopic data from the LAMOST DR7, Sloan Digital Sky Survey (SDSS) DR12, and corrected photometric data from the Gaia EDR3, we apply the stellar color regression (SCR) method to recalibrate the SDSS Stripe 82 standard stars catalog of Ivezić et al. With a total number of about 30,000 spectroscopically targeted stars, we have mapped out the relatively large and strongly correlated photometric zero-point errors present in the catalog, ∼2.5% in the u band and ∼1% in the griz bands. Our study also confirms some small but significant magnitude dependence errors in the z band for some charge-coupled devices. Various tests show that we have achieved an internal precision of about 5 mmag in the u band and about 2 mmag in the griz bands, which is about five times better than previous results. We also apply the method to the latest version of the catalog (v4.2), and find modest systematic calibration errors of up to ∼1% along the R.A. direction and smaller errors along the decl. direction. The results demonstrate the power of the SCR method when combining spectroscopic data and Gaia photometry in breaking the 1% precision barrier of ground-based photometric surveys. Our work paves the way for the recalibration of the whole SDSS photometric survey and has important implications for the calibration of future surveys. Future implementations and improvements of the SCR method under different situations are also discussed.

Identifications of RR Lyrae Stars and Quasars from the Simulated Data of Mephisto-W Survey

We have investigated the feasibilities and accuracies of the identifications of RR Lyrae stars and quasars from the simulated data of the Multi-channel Photometric Survey Telescope (Mephisto) W Survey. Based on the variable sources light curve libraries from the Sloan Digital Sky Survey (SDSS) Stripe 82 data and the observation history simulation from the Mephisto-W Survey Scheduler, we have simulated the uvgriz multi-band light curves of RR Lyrae stars, quasars and other variable sources for the first year observation of Mephisto W Survey. We have applied the ensemble machine learning algorithm Random Forest Classifier (RFC) to identify RR Lyrae stars and quasars, respectively. We build training and test samples and extract ∼150 features from the simulated light curves and train two RFCs respectively for the RR Lyrae star and quasar classification. We find that, our RFCs are able to select the RR Lyrae stars and quasars with remarkably high precision and completeness, with purity = 95.4% and completeness = 96.9% for the RR Lyrae RFC and purity = 91.4% and completeness = 90.2% for the quasar RFC. We have also derived relative importances of the extracted features utilized to classify RR Lyrae stars and quasars.

Experiment for cryogenic large-aperture intensity mapping: instrument design

The experiment for cryogenic large-aperture intensity mapping (EXCLAIM) is a balloon-borne telescope designed to survey star formation in windows from the present to z = 3.5. During this time, the rate of star formation dropped dramatically, while dark matter continued to cluster. EXCLAIM maps the redshifted emission of singly ionized carbon lines and carbon monoxide using intensity mapping, which permits a blind and complete survey of emitting gas through statistics of cumulative brightness fluctuations. EXCLAIM achieves high sensitivity using a cryogenic telescope coupled to six integrated spectrometers employing kinetic inductance detectors covering 420 to 540 GHz with spectral resolving power R = 512 and angular resolution ≈4 arc min. The spectral resolving power and cryogenic telescope allow the survey to access dark windows in the spectrum of emission from the upper atmosphere. EXCLAIM will survey 305 deg2 in the Sloan Digital Sky Survey Stripe 82 field from a conventional balloon flight in 2023. EXCLAIM will also map several galactic fields to study carbon monoxide and neutral carbon emission as tracers of molecular gas. We summarize the design phase of the mission.

What is Important? Morphological Asymmetries are Useful Predictors of Star Formation Rates of Star-forming Galaxies in SDSS Stripe 82

Abstract The morphology and structure of galaxies reflect their star formation and assembly histories. We use the framework of mutual information (MI) to quantify the interdependence among several structural variables and to rank them according to their relevance for predicting the specific star formation rate (SSFR) by comparing the MI of the predictor variables with the SSFR and penalizing variables that are redundant. We apply this framework to study ∼3700 face-on star-forming galaxies (SFGs) with varying degrees of bulge dominance and central concentration and with stellar mass M ⋆ ≈ 109 M ⊙−5 × 1011 M ⊙ at redshift z = 0.02–0.12. We use the Sloan Digital Sky Survey (SDSS) Stripe 82 deep i-band imaging data, which improve measurements of asymmetry and bulge dominance indicators. We find that star-forming galaxies are a multiparameter family. In addition to M ⋆, asymmetry emerges as the most powerful predictor of SSFR residuals of SFGs, followed by bulge prominence/concentration. Star-forming galaxies with higher asymmetry and stronger bulges have higher SSFR at a given M ⋆. The asymmetry reflects both irregular spiral arms and lopsidedness in seemingly isolated SFGs and structural perturbations by galaxy interactions or mergers.

Stellar loci IV. red giant stars

In the fourth paper of this series, we present the metallicity-dependent Sloan Digital Sky Survey (SDSS) stellar color loci of red giant stars, using a spectroscopic sample of red giants in the SDSS Stripe 82 region. The stars span a range of 0.55 – 1.2 mag in color g – i, –0.3 – –2.5 in metallicity [Fe/H], and have values of surface gravity log g smaller than 3.5 dex. As in the case of main-sequence (MS) stars, the intrinsic widths of loci of red giants are also found to be quite narrow, a few mmag at maximum. There are however systematic differences between the metallicity-dependent stellar loci of red giants and MS stars. The colors of red giants are less sensitive to metallicity than those of MS stars. With good photometry, photometric metallicities of red giants can be reliably determined by fitting the u – g, g – r, r – i, and i – z colors simultaneously to an accuracy of 0.2 – 0.25 dex, comparable to the precision achievable with low-resolution spectroscopy for a signal-to-noise ratio of 10. By comparing fitting results to the stellar loci of red giants and MS stars, we propose a new technique to discriminate between red giants and MS stars based on the SDSS photometry. The technique achieves completeness of ∼70 per cent and efficiency of ∼80 per cent in selecting metal-poor red giant stars of [Fe/H] ≤ –1.2. It thus provides an important tool to probe the structure and assemblage history of the Galactic halo using red giant stars.