An Optical-UV Survey of the North Celestial Cap
aa r X i v : . [ a s t r o - ph . I M ] J a n An Optical-UV Survey of the North Celestial Cap
Evgeny Gorbikov • Noah Brosch Abstract
We present preliminary results of an optical-UV survey of the North Celestial Cap (NCCS) based on ∼
5% areal coverage. The NCCS will provide good pho-tometric and astrometric data for the North CelestialCap region (80 ◦ ≤ δ ≤ ◦ ). This region, at galac-tic latitudes from 17 ◦ . b . ◦ , is poorly covered bymodern CCD-based surveys. The expected number ofdetected objects in NCCS is ∼ Keywords surveys, catalogs: optical, UV; astrometry;Galaxy: structure; (ISM:) dust, extinction; galaxies:statistics
From the dawn of humankind people were interestedin studying the surrounding world. In ancient times as-tronomy, and sky mapping in particular, had not only aworld-description function, but had also practical pur-poses. For example, sky maps were used for navigationand orientation as well as for predicting celestial phe-nomena.Two inventions contributed significantly to sky map-ping: the telescope and the permanent photography.Photographic catalogs, produced from the 1890s to the2000s, play an important role even today. The firsthigh-quality digital all-sky survey, the
Digitized SkySurvey (DSS) and its extension DSS-II, were pro-duced by scanning photographic survey plates (POSS-I,POSS-II, ESO/SERC) with specific photometric cali-brations. The POSS-I, POSS-II and ESO/SERC pho-tographic surveys provided high-precision astrometry
Evgeny GorbikovNoah Brosch The Wise Observatory and the Raymond and Beverly SacklerSchool of Physics and Astronomy, the Faculty of Exact Sciences,Tel Aviv University, Tel Aviv 69978, Israel used in catalogs such as
USNO-A1.0 , USNO-A2.0 , USNO-B1.0 , etc. USNO-A and USNO-B are all-sky high-precision astrometric catalogs including alsophotometric data from the DSS and DSS-II. In manycases, the USNO DSS-based catalogs are the only op-tical high-precision data available for a particular skyarea.However, photographic catalogs suffer from signifi-cant photometric and astrometric systematic and sta-tistical errors due to shortcomings of the photographicemulsion, such as low sensitivity, limited dynamic rangeand non-linearity (Monet et al. 2003). These weresolved with the introduction of the charge-coupled de-vice (CCD) to astronomy. CCDs eliminated not onlythe photographic emulsion shortcomings, but solvedalso other problems, such as providing an effective rawdata storage, yielding a short delay between the rawdata collection and the final data extraction, and beinga reusable photosensitive element. All modern opticalsky surveys are CCD-based.The modern optical sky survey in general use isthe
Sloan Digital Sky Survey (SDSS), which cov-ers about 10,000 deg of the sky and provides high-precision photometric and astrometric data in fiveSloan bands (Abazajian et al. 2009).An important extension to the optical catalogs areUV observations. One of the prominent availableUV instruments is the Galaxy Evolution Explorer (GALEX), performing both imaging and low-resolutionspectroscopy in two bands: near-UV (NUV). GALEXconducts several pioneering UV sky surveys aimed pri-marily to understand galaxy evolution, and its publicly-available dataset covers by now about 3/4 of sky(Morrissey et al. 2007).
North Celestial Cap Survey (NCCS) con-ducted at Wise Observatory, Israel, will obtain goodphotometric and astrometric data for the North Celes-tial Cap (NCC) region (80 ◦ ≤ δ ≤ ◦ ). A significantportion of the data collection is performed at Wise Ob-servatory with the 1-meter Ritchey-Chr´etien telescopeand the Large Area Imager for the Wise Observatory(LAIWO) camera (Gorbikov et al. 2010). The R and Iband imaging is ∼
90% complete and will be completedin about six months. The current coverage is shown inFig.1 a . Based on preliminary results, the final opticalcatalog will include & . m . m .2 and I = 19 m .1, and the cat-alog will be complete to these magnitudes. The astro-metric solution is derived using the third US NavalObservatory CCD Astrograph Catalog (UCAC3,Zacharias et al. 2010) and the demonstrated accuracyis ∼ ′′ .1–0 ′′ .3 for the both α and δ . The NCCS point-extended source separation (PESS) was checked againstthe SDSS and the PESS accuracy was found to be >
90% accurate (Gorbikov et al. 2010). a b
Fig. 1
Panel a : NCC coverage by the survey in R andI bands as for July, 27, 2010 (red) and by SEGUE-SDSS(brown).
Panel b : NCC coverage by GALEX (blue) andthe selected area (red)
Another important part of the project is collectingdata also in Sloan g’ band to extend the wavelength cov-erage and to fill the gap between the red optical filtersand UV filters. We expect to add Sloan g’ band obser-vations with the 48-inch Schmidt telescope of PalomarObservatory equipped with the
Palomar TransientFactory (PTF, Law et al. 2009) camera.The third part of the survey, the GALEX data(GR4/GR5), was extracted from the GALEX web site.These originate from the AIS survey, since yet there isno MIS coverage in the NCC region. The AIS cover-age in the NCC region is ∼
50% and is shown in Fig.1 b .There are ∼ × more sources detected in NUV than inFUV. 2.2 Motivation and PurposesThe NCCS project was originally planned to supportthe Tel Aviv University UV Experiment (TAU-VEX) mission by extending the wavelength base whereeach source was measured from the UV to the opti-cal (Gorbikov et al. 2010). In the last planned ver-sion, TAUVEX was expected to observe objects byscanning around the North and the South CelestialPoles (Almoznino 2007). These celestial cap regionsare poorly covered by modern optical surveys, but thenorthern sky patch can be easily observed from theWise Observatory.The DSS-based all-sky photographic catalogs, suchas USNO-B1.0, do cover the NCC region. However,they suffer from significant photometric statistical andsystematic errors, as mentioned above. SDSS is deeperand its photometry is more accurate than that ofUSNO-B1.0, but it covers only a tiny fraction of theNCC region (Fig.1 a ). The NCCS was planned to sur-vey the NCC region and provide photometry better by ∼ × relative to USNO-B1.0 for stars at the limitingmagnitude and even more accurate for brighter objects.Although NCCS is shallower than SDSS by ∼ ∼
50% of the NCC region are covered bythe All Sky Survey (AIS) of GALEX in the current datarelease, as shown in Fig.1 b , although more coverage isexpected to become available at the next data release.Using GALEX data is a good option, since its data aresufficiently deep and the NCC coverage by GALEX isfairly complete.The NCCS will fill the time gap produced betweenthe completion of the SDSS project and the beginningof next-generation major sky surveys, such as Pan-STARRS and LSST. The survey location is of highinterest since it is located at intermediate galactic lati-tudes (17 ◦ ≤ b ≤ ◦ ) where both the Milky Way (MW)stellar and interstellar dust structures, and extragalac-tic objects, can be studied. The primary scientific pur-poses of the survey are (a) a study of the MW stellarstructure, and (b) of galactic extinction at intermediatelatitudes, (c) the detection of fast-moving objects, (d)the identification of white dwarf candidates, QSOs andAGNs, and (e) the study of galaxy bimodality in a skyregion not previously surveyed for this. region imaged during a single night (January, 7, 2010) andshown in Fig.1 b . The objects were photometrically cal-ibrated relative to Landolt (2009) standards observedin the same night. More than 100,000 sources were de-tected in each band by the pipeline. Sources detectedin two bands were matched and ∼ m .2 and I = 19 m .1.The pipeline classified ∼
55% of the objects as extended.The optical detections were matched with GALEXsources. The GALEX coverage in the selected regionis ∼ E B − V values transformed to extinction values usingthe Schlegel et al. (1998) relations for A R , A I and theSeibert et al. (2005b) relations for A NUV and A F UV .The relative extinction in four bands at each locationwas calculated for the comparison.The median reddening in the selected region fromSchlegel et al. (1998) is ˜ E B − V ∼ m .
16 with a maximalvalue of 0.51 mag, consistent with the reddening forthe entire survey region. In general, our results agreewith Schlegel et al. Most extinction values are within1 σ of those predicted by Schlegel et al., although thereseem to be some exceptions. The extinction law for theselected region cannot be derived using the currentlyavailable data, however, the extinction law seems to becloser to the normal MW law than to grey extinction.3.3 Stellar StructureThe survey region is located at intermediate galacticlatitudes, where, in principle, both thick disk and halostellar populations can be observed.Objects in the selected region defined as pointsources by the NCCS pipeline were de-reddened using a b Fig. 2
Panel a : The colour-apparent magnitude stellardistribution of the selected region.
Panel b : Colour-colourdensity diagram of the galaxies in the selected region. Theintensity is a galaxy count log per squared colour bin the Schlegel et al. (1998) reddening map and assum-ing foreground dust, as done by de Jong et al. (2010).The colour-apparent magnitude diagram for the stellarcontent of the selected region are shown in Fig.2 a . Wedivided the stellar distribution into two ares - the blueand the red one, as shown in Fig.2 a . The distances forthe stars in the blue and the red regions were calculatedassuming main-sequence stars; this is valid for ∼ ∼ ∼ b .3.5 Exotic ObjectsWhile mining the preliminary dataset a few exotic ob-jects of high interest were identified. One such object isa star located at ( α, δ ) = (03 h m s .
7, 82 ◦ ′ ′′ . This star was noted for its blue colours: R = 17 m .44, I= 17 m .44, NUV = 18 m .09, FUV = 18 m .71, correspond-ing to an A-type star. The star is located in a relativelyhigh-extinction region: E B − V = 0 m .29 (Schlegel et al.1998), thus it may be of an even earlier spectral type. a b c Fig. 3
Panel a : DSS-I image of a high-proper-motion star.Epoch: 1954.89.
Panel b : Combined three-colour image ofthe same star from DSS-II. Mean epoch: 1995.41.
Panel c :The same star on an I band NCCS image. Epoch: 2010.02.All the images are aligned with each other
Comparing the star coordinates from the NCCS withthose of DSS showed that the star is a high-proper mo-tion object. Fig.3 shows images of the star from dif-ferent epochs. We compared the NCCS coordinates ofthe star with those measured on historical photographicsurveys and estimated its proper motion to be (∆ α , ∆ δ )= (+14 ±
8, -56 ±
6) mas/yr, consistent with the esti-mates from NOMAD (Zacharias et al. 2004): (∆ α , ∆ δ )= (+34 ±
22, -60 ±
1) mas/yr.The star is too faint to be included in the PPM andUCAC catalogs. From its colour, distance, extinctionand general astrophysical considerations we concludetentatively that this star is probably a white dwarf lo-cated .
200 pc from the Sun. For a final determinationof its nature spectroscopic data is required.Another example of exotic objects to be mined fromthe NCCS is QSOs and AGNs. We downloaded theentire V´eron-Cetty & V´eron (2010) catalog of quasarsand AGN in the survey area, and one of the objects(out of 60) was found in the selected area. The object islocated at ( α, δ ) = (05 h m s .
2, 82 ◦ ′ ′′ . B − V = 0 m .06(Schlegel et al. 1998), and it is relatively bright: R =15 m .12, I = 14 m .71, NUV = 18 m .64, FUV = 19 m .42.The object was confirmed spectroscopically as an AGNat a redshift z = 0.05 (Xu et al. 2001). The object wasidentified as an extended source by NCCS pipeline.To demonstrate our capability to distinguish be-tween AGNs and hot stellar objects we simulatedcolour-colour diagrams. AGN colours were simulatedusing the modified composite quasar spectrum fromVanden Berk et al. (2001). Stellar colours were sim-ulated using the Bruzual-Persson-Gunn-StrykerAtlas , which is an extension of Gunn & Stryker (1983)optical stellar spectra atlas, the
Pickles (1998) Stel-lar Spectral Flux Library and the
CALSPEC (Bohlin 2003) atlas, which is used for the calibrationof the HST instruments. These three atlases provide asample of ∼
250 stellar spectra encompassing all spec-tral types and luminosity classes from super-giants tobrown dwarfs and covering also a wide range of metal-licity.The AGN spectrum was redshifted from restframeto z ≤ α forest, Lyman limit and damped Ly- α sys-tems) using the transmission function calculated fromMøller & Jakobsen 1990. AGN and stellar spectra wereconvolved with the response functions of the FUV,NUV, g’, R and I filters to derive object flux. Themagnitudes were calculated relative to Vega.We conclude, that optical colours alone do not pro-vide a good AGN-stellar separation. The use of NUV-based and FUV-based colours provides an ideal sepa-ration for all the redshifts 0 < z < < z < . They se-lected 222 AGN candidates at redshifts < r mag-nitude. Assuming that the Johnson R magnitude issimilar to the Sloan r magnitude and using Figure2 in Bianchi et al. (2005), we estimate the numberof AGN candidates brighter than our limiting mag-nitude R ≤ . ∼
204 candidates. However,this number should be corrected for the area of thesurvey: × . ≈ candidates should be also corrected for the differ-ent galactic latitude, since the SDSS was performedmostly in high galactic latitudes, while the NCCSis an intermediate-latitude survey. Therefore, theNCCS would detect less AGN candidates per totalnumber of sources than the SDSS, as shown in Fig-ure 4 of Richards et al. (2009). However, it is notclear from Bianchi et al. (2005) where their match-ing area is located.2. Richards et al. (2009) analyzed the catalog of 1,172,157AGN candidates selected from the SDSS by colours.Figure 18 of Richards et al. (2009) shows the distri-bution of candidate counts per solid angle per mag-nitude as a function of the Sloan i magnitude for theredshifts 0 . < z < .
2. Assuming that the John-son I magnitude is similar to the Sloan i magnitude, we estimate the density of AGN candidates brighterthan our limiting magnitude I ≤ . ∼ . Thus the NCCS area of ∼ implies the estimation of ∼ candidates. How-ever, this number is (1) not corrected for the surveylatitude similar to the previous estimation and (2)is valid for a slightly different redshift range, thanthat of the NCCS AGN candidate selection.3. Figure 2 of Richards et al. (2009) demonstrates thedistribution of the SDSS AGN candidate counts asa function of the Sloan i magnitude. The number ofAGN candidates brighter than our limiting magni-tude I ≤ . ∼ - SDSS,313.6 deg - NCCS): × . ≈ . × ≈ candidates.4. Figure 4 of Richards et al. (2009) shows the ratioof AGN candidates to all sources as a function ofgalactic latitude b . The ratio for the NCCS location(17 ◦ < b < ◦ ) can be estimated from it as ∼ & , , × . ≈ ∼ ≤ . , , × ≈ candidates.We estimated the NCCS AGN yield using four almostindependent methods (the fourth method is partiallydependent on the estimation of the third one), eachone of which is biased differently. However, all the esti-mation methods produced remarkably similar results,allowing to calculate the average NCCS AGN yield ∼ ± candidates. We have shown that there is valuable new science thatcan be derived from the NCCS data set, apart fromimproving the global knowledge about the sky at highdeclinations. It is possible, therefore, to consider possi-ble extensions of this rather limited project, as definedabove.We can relatively easily extend the surveyed regionfor five more degrees of declination, increasing the sur-veyed region to more than 700 deg . This will increase the overlap with SDSS-surveyed regions, allowing animproved cross-calibration between the two surveys.Alternatively, we could repeat the observations of thealready surveyed region in one of the R and I bands tocheck the variability of the detected sources.We could merge the NCCS data with 2MASS ex-tending the wavelength base to IR. Though 2MASSis relatively shallow, it could provide additional infor-mation about NCCS objects brighter than ∼ m –17 m .Spectroscopic follow-up of the promising NCCS sourcescould be performed at the Wise Observatory for objectsbrighter than ∼ m or at a larger telescope for the faintones. Acknowledgements
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