An Optical-UV Survey of the North Celestial Cap

0.32 MB·English

by  Evgeny Gorbikov

An Optical-UV Survey of the North Celestial Cap
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Evgeny Gorbikov • Noah Brosch
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Abstract Wepresentpreliminaryresultsofanoptical- used in catalogs such as USNO-A1.0, USNO-A2.0,

] UVsurveyoftheNorthCelestialCap(NCCS)basedon USNO-B1.0, etc. USNO-A and USNO-B are all-
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∼5%arealcoverage. TheNCCSwillprovidegoodpho- sky high-precision astrometric catalogs including also
I tometric and astrometric data for the North Celestial photometric data from the DSS and DSS-II. In many
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h Cap region (80◦ ≤ δ ≤ 90◦). This region, at galac- cases, the USNO DSS-based catalogs are the only op-
p tic latitudes from 17◦ . b . 37◦, is poorly covered by tical high-precision data available for a particular sky
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o modern CCD-based surveys. The expected number of area.
tr detected objects in NCCS is ∼1,500,000. We discuss However, photographic catalogs suffer from signifi-
s issues of galactic structure, extinction, and the galaxy cant photometric and astrometric systematic and sta-
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[ clustering in the colour-colour diagrams. tistical errors due to shortcomings of the photographic

Keywords surveys,catalogs: optical,UV;astrometry; emulsion,suchaslowsensitivity,limiteddynamicrange
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v Galaxy: structure; (ISM:) dust, extinction; galaxies: and non-linearity (Monet et al. 2003). These were
4 statistics solved with the introduction of the charge-coupled de-
0 vice (CCD) to astronomy. CCDs eliminated not only
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1 Introduction the photographic emulsion shortcomings, but solved
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. From the dawn of humankind people were interested also other problems, such as providing an effective raw
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instudyingthesurroundingworld. Inancienttimesas- data storage, yielding a short delay between the raw
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1 tronomy,andskymappinginparticular,hadnotonlya datacollectionandthefinaldataextraction,andbeing
1 world-description function, but had also practical pur- a reusable photosensitive element. All modern optical
v: poses. Forexample, skymaps wereused fornavigation sky surveys are CCD-based.
i and orientation as well as for predicting celestial phe- The modern optical sky survey in general use is
X
nomena. the Sloan Digital Sky Survey (SDSS), which cov-
ar Twoinventionscontributedsignificantlytoskymap- ers about 10,000 deg2 of the sky and provides high-
ping: the telescope and the permanent photography. precision photometric and astrometric data in five
Photographiccatalogs,producedfromthe 1890sto the Sloan bands (Abazajian et al. 2009).
2000s, play an important role even today. The first An important extension to the optical catalogs are
high-quality digital all-sky survey, the Digitized Sky
UV observations. One of the prominent available
Survey (DSS) and its extension DSS-II, were pro-
UV instruments is the Galaxy Evolution Explorer
ducedbyscanningphotographicsurveyplates(POSS-I,
(GALEX),performingbothimagingandlow-resolution
POSS-II, ESO/SERC) with specific photometric cali-
spectroscopy in two bands: near-UV (NUV). GALEX
brations. The POSS-I, POSS-II and ESO/SERC pho-
conducts severalpioneering UV sky surveys aimed pri-
tographic surveys provided high-precision astrometry
marilytounderstandgalaxyevolution,anditspublicly-
available dataset covers by now about 3/4 of sky
EvgenyGorbikov (Morrissey et al. 2007).
NoahBrosch
1The Wise Observatory and the Raymond and Beverly Sackler
SchoolofPhysicsandAstronomy,theFacultyofExactSciences,
TelAvivUniversity,TelAviv69978,Israel
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2 The North Celestial Cap Survey 2.2 Motivation and Purposes
2.1 Overview The NCCS project was originally planned to support
The North Celestial Cap Survey (NCCS) con- the Tel Aviv University UV Experiment (TAU-
ducted at Wise Observatory, Israel, will obtain good VEX)missionbyextending the wavelengthbasewhere
photometric and astrometric data for the North Celes- each source was measured from the UV to the opti-
tial Cap (NCC) region (80◦ ≤ δ ≤ 90◦). A significant cal (Gorbikov et al. 2010). In the last planned ver-
portionofthe datacollectionisperformedatWiseOb- sion, TAUVEX was expected to observe objects by
servatory with the 1-meter Ritchey-Chr´etien telescope scanning around the North and the South Celestial
and the Large Area Imager for the Wise Observatory Poles (Almoznino 2007). These celestial cap regions
(LAIWO) camera (Gorbikov et al. 2010). The R and I are poorly covered by modern optical surveys, but the
band imagingis ∼90%complete and willbe completed northern sky patch can be easily observed from the
in about six months. The current coverageis shown in Wise Observatory.
Fig.1a. Based on preliminary results, the final optical The DSS-based all-sky photographic catalogs, such
catalog will include &1,500,000 distinct objects. The as USNO-B1.0, do cover the NCC region. However,
photometric accuracy is ∆m . 0m.1 for point sources they suffer from significant photometric statistical and
brighter than R = 20m.2 and I = 19m.1, and the cat- systematic errors,as mentioned above. SDSS is deeper
alog will be complete to these magnitudes. The astro- and its photometry is more accurate than that of
metric solution is derived using the third US Naval USNO-B1.0, but it covers only a tiny fraction of the
Observatory CCD Astrograph Catalog (UCAC3, NCC region (Fig.1a). The NCCS was planned to sur-
Zacharias et al. 2010) and the demonstrated accuracy vey the NCC regionand provide photometry better by
is ∼0′′.1–0′′.3 for the both α and δ. The NCCS point- ∼4× relative to USNO-B1.0 for stars at the limiting
extendedsourceseparation(PESS)wascheckedagainst magnitudeandevenmoreaccurateforbrighterobjects.
the SDSS and the PESS accuracy was found to be Although NCCS is shallower than SDSS by ∼1.5–2.0
>90% accurate (Gorbikov et al. 2010). mag, it will fully cover the entire NCC region.
Since the TAUVEX UV data is not available,
GALEX data (GR4/GR5) was used for the UV mag-
nitudes. Only ∼50%of the NCC regionare coveredby
theAllSkySurvey(AIS)ofGALEXinthecurrentdata
release, as shown in Fig.1b, although more coverage is
expected to become available at the next data release.
Using GALEX data is a goodoption, since its data are
sufficiently deep and the NCC coverage by GALEX is
fairly complete.
The NCCS will fill the time gap produced between
a b
the completion of the SDSS project and the beginning
Fig. 1 Panel a: NCC coverage by the survey in R and of next-generation major sky surveys, such as Pan-
STARRS and LSST. The survey location is of high
I bands as for July, 27, 2010 (red) and by SEGUE-SDSS
(brown). Panel b: NCC coverage by GALEX (blue) and interest since it is located at intermediate galactic lati-
theselected area (red) tudes(17◦ ≤b≤37◦)whereboththeMilkyWay(MW)
stellar and interstellar dust structures, and extragalac-
Another important part of the project is collecting tic objects, can be studied. The primary scientific pur-
dataalsoinSloang’bandtoextendthewavelengthcov- poses of the survey are (a) a study of the MW stellar
erage and to fill the gap between the red optical filters structure,and(b)ofgalacticextinctionatintermediate
and UV filters. We expect to add Sloan g’ band obser- latitudes, (c) the detection of fast-moving objects, (d)
vations with the 48-inch Schmidt telescope of Palomar the identification ofwhite dwarfcandidates,QSOs and
Observatory equipped with the Palomar Transient AGNs, and (e) the study of galaxy bimodality in a sky
Factory (PTF, Law et al. 2009) camera. region not previously surveyed for this.
The third part of the survey, the GALEX data
(GR4/GR5), wasextractedfromthe GALEXwebsite.
3 Preliminary Results
These originate from the AIS survey, since yet there is
no MIS coverage in the NCC region. The AIS cover- 3.1 Selected Area
ageinthe NCC regionis ∼50%andis showninFig.1b.
To demonstrate the expected yield from the NCCS we
Thereare∼10×moresourcesdetectedinNUVthanin present below preliminary results for a 14 deg2 region
FUV.
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imaged during a single night (January, 7, 2010) and
showninFig.1b. The objectswerephotometricallycal-
ibrated relative to Landolt (2009) standards observed
in the same night. More than 100,000sources were de-
tected in each band by the pipeline. Sources detected
in two bands were matched and ∼77,000 of them were
found to be brighter than R = 20m.2 and I = 19m.1.
Thepipelineclassified∼55%oftheobjectsasextended.
The optical detections were matched with GALEX
a b
sources. The GALEX coverage in the selected region
is ∼50%, similar to the entire survey region. 12,788 Fig. 2 Panel a: The colour-apparent magnitude stellar
opticalobjectshaveaNUVcounterpartand1,433have distribution of the selected region. Panel b: Colour-colour
a FUV counterpart. density diagram of the galaxies in the selected region. The
intensity is a galaxy count log persquared colour bin
3.2 Galactic Extinction
To measure the galactic extinction in the test area we
the Schlegel et al. (1998) reddening map and assum-
usethe Wolf(1923)diagramcomparingthe cumulative
ing foreground dust, as done by de Jong et al. (2010).
star count distributions in an examined and in a refer-
The colour-apparentmagnitude diagramfor the stellar
enceregion,normalizedtothesamesolidangle,andthe
content of the selected region are shown in Fig.2a. We
’Bull’seye’method(Gorbikov & Brosch2010)compar-
divided the stellar distribution into two ares - the blue
ingthedistributionsintwoconcentricareas,takingthe
and the red one, as shown in Fig.2a. The distances for
inner region as the examined one and the outer region
thestarsintheblueandtheredregionswerecalculated
asthereference. Thesemethodscanonlydeterminethe
assuming main-sequence stars; this is valid for ∼90%
relativeextinctionbetweentworegions,thus weaimto
of the stars (Mengel et al. 1979), and using the Cox
detect only localized, small-scale dust features.
(2000) HR diagram.
The selected region was scanned with an inner ra-
The stellar distances for the blue regionconcentrate
dius of 1/4 deg and an outer radius of 1/2 deg in
at ∼2.4 kpc, consistent with a thick disk population,
each of the available bands, excluding the FUV band,
whilethestellardistancedistributionfortheredregion
since GALEX did not detect sufficient FUV sources
is moreextendedandconcentratesat∼3.9kpc, consis-
in this region. Only objects defined by the NCCS
tent with a halo population. It is still difficult to sepa-
pipeline as point sources were used for this study. The
rate the two populations, since they mutually contam-
results were compared with the Schlegel et al. (1998)
inate and usually one colour is not sufficient to deter-
EB−V values transformed to extinction values using mine unequivocally the object distance. For instance,
the Schlegel et al. (1998) relations for A ,A and the
R I Gorbikov & Brosch (2010) used two optical colours for
Seibert et al. (2005b) relations for A and A .
NUV FUV a more accurate distance estimation.
The relative extinction in four bands at each location
was calculated for the comparison. 3.4 Galaxy Colour Bimodality
The median reddening in the selected region from A clear galaxy bimodality in colour-colour space was
Schlegel et al.(1998)isE˜B−V ∼0m.16withamaximal firstdemonstratedbySeibert et al.(2005a)usingGALEX
value of 0.51 mag, consistent with the reddening for NUV-basedcolor-colorplots. Theyalsofoundthatdif-
the entire survey region. In general, our results agree ferent types of galaxies (ellipticals, spirals, starburst
with Schlegel et al. Most extinction values are within galaxies,Seyferts andLINERs) residein different loca-
1σ of those predicted by Schlegel et al., although there tions on this diagram.
seemtobesomeexceptions. Theextinctionlawforthe All the galaxies from our preliminary sample were
selected region cannot be derived using the currently selected and de-reddened for MW dust assuming
availabledata, however,the extinction law seems to be Schlegel et al. (1998) foreground reddening. Although
closer to the normal MW law than to grey extinction. we use different colours, the results are similar to
those of Seibert et al. (2005a) and a clear bimodality
3.3 Stellar Structure
in galaxy colours is present, see Fig.2b.
The survey region is located at intermediate galactic
3.5 Exotic Objects
latitudes, where, in principle, both thick disk and halo
stellar populations can be observed. While mining the preliminary dataset a few exotic ob-
Objects in the selected region defined as point jectsofhighinterestwereidentified. Onesuchobjectis
sources by the NCCS pipeline were de-reddened using a star located at (α, δ)= (03h27m11s.7, 82◦15′47′′.9).
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This star was noted for its blue colours: R = 17m.44,I (Bohlin 2003) atlas, which is used for the calibration
=17m.44,NUV=18m.09,FUV=18m.71,correspond- of the HST instruments. These three atlases provide a
ingtoanA-typestar. Thestarislocatedinarelatively sample of ∼ 250 stellar spectra encompassing all spec-
high-extinction region: EB−V = 0m.29 (Schlegel et al. tral types and luminosity classes from super-giants to
1998), thus it may be of an even earlier spectral type. brown dwarfs and covering also a wide range of metal-
licity.
The AGN spectrum was redshifted from restframe
to z≤3 and corected for the absorption by Lyman sys-
tems (Ly-α forest, Lyman limit and damped Ly-α sys-
tems) using the transmission function calculated from
Møller & Jakobsen1990. AGNandstellarspectrawere
convolved with the response functions of the FUV,
a b c
NUV, g’, R and I filters to derive object flux. The
Fig.3 Panela: DSS-Iimageofahigh-proper-motionstar. magnitudes were calculated relative to Vega.
Epoch: 1954.89. Panel b: Combined three-colour image of We conclude, that optical colours alone do not pro-
the same star from DSS-II.Mean epoch: 1995.41. Panel c: vide a good AGN-stellar separation. The use of NUV-
Thesame staron anI bandNCCS image. Epoch: 2010.02.
based and FUV-based colours provides an ideal sepa-
All theimages are aligned with each other
ration for all the redshifts 0<z<3 tested here, though
practically, only very few objects are detected in FUV
ComparingthestarcoordinatesfromtheNCCSwith
by GALEX, as mentioned above. However, the effec-
those ofDSS showedthatthe starisa high-propermo-
tive AGN colour selection can be performed using the
tion object. Fig.3 shows images of the star from dif-
NUV-alone-based colour-colourdiagram at 0<z<2.
ferent epochs. We compared the NCCS coordinates of
WeestimatedtheAGNyieldoftheprojectusingthe
thestarwiththosemeasuredonhistoricalphotographic
four following methods:
surveysandestimateditspropermotiontobe(∆α,∆δ)
= (+14±8, -56±6) mas/yr, consistent with the esti- 1. Bianchi et al. (2005) matched GALEX sources de-
matesfromNOMAD(Zacharias et al.2004): (∆α,∆δ) tected by the All-sky Imaging Survey (AIS) with
= (+34±22, -60±1)mas/yr. SDSS sources over an area of 92 deg2. They se-
The star is too faint to be included in the PPM and lected 222 AGN candidates at redshifts < 2 us-
UCAC catalogs. From its colour, distance, extinction ing their Sloan and GALEX colours. Figure 2 of
and general astrophysical considerations we conclude Bianchi et al. (2005) shows the distribution of the
tentatively that this star is probably a white dwarf lo- AGN candidates as a function of the Sloan r mag-
cated.200pcfromtheSun. Forafinaldetermination nitude. Assuming that the Johnson R magnitude is
of its nature spectroscopic data is required. similar to the Sloan r magnitude and using Figure
Another example ofexotic objectsto be mined from 2 in Bianchi et al. (2005), we estimate the number
the NCCS is QSOs and AGNs. We downloaded the of AGN candidates brighter than our limiting mag-
entire V´eron-Cetty & V´eron (2010) catalog of quasars nitude R ≤20.2 mag as ∼204 candidates. However,
and AGN in the survey area, and one of the objects this number should be corrected for the area of the
(outof60)wasfoundintheselectedarea. Theobjectis survey: 204×313.6 ≈ 695. The estimation of 695
92
located at (α, δ)= (05h36m07s.2, 82◦23′14′′.5), which candidates should be also corrected for the differ-
is a region of low galactic extinction: EB−V = 0m.06 ent galactic latitude, since the SDSS was performed
(Schlegel et al. 1998), and it is relatively bright: R = mostly in high galactic latitudes, while the NCCS
15m.12, I = 14m.71, NUV = 18m.64, FUV = 19m.42. is an intermediate-latitude survey. Therefore, the
The objectwasconfirmedspectroscopicallyasanAGN NCCS would detect less AGN candidates per total
at a redshift z = 0.05 (Xu et al. 2001). The object was number of sources than the SDSS, as shown in Fig-
identified as an extended source by NCCS pipeline. ure 4 of Richards et al. (2009). However, it is not
To demonstrate our capability to distinguish be- clear from Bianchi et al. (2005) where their match-
tween AGNs and hot stellar objects we simulated ing area is located.
colour-colour diagrams. AGN colours were simulated 2. Richards et al.(2009)analyzedthecatalogof1,172,157
using the modified composite quasar spectrum from AGNcandidatesselectedfromthe SDSS bycolours.
Vanden Berk et al. (2001). Stellar colours were sim- Figure 18 of Richards et al. (2009) shows the distri-
ulated using the Bruzual-Persson-Gunn-Stryker bution of candidate counts per solid angle per mag-
Atlas,whichisanextensionofGunn & Stryker(1983) nitudeasafunctionoftheSloanimagnitudeforthe
optical stellar spectra atlas, the Pickles (1998) Stel- redshifts 0.3 < z < 2.2. Assuming that the John-
lar Spectral Flux Library and the CALSPEC sonImagnitude issimilarto the Sloanimagnitude,
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we estimate the density ofAGN candidates brighter the overlap with SDSS-surveyed regions, allowing an
than our limiting magnitude I ≤ 19.1 mag as ∼1.8 improved cross-calibration between the two surveys.
counts/deg2. Thus the NCCS area of ∼313.6 deg2 Alternatively, we could repeat the observations of the
implies the estimation of ∼564 candidates. How- already surveyedregion in one of the R and I bands to
ever, this number is (1) not correctedfor the survey check the variability of the detected sources.
latitude similar to the previous estimation and (2) We could merge the NCCS data with 2MASS ex-
is valid for a slightly different redshift range, than tending the wavelength base to IR. Though 2MASS
that of the NCCS AGN candidate selection. is relatively shallow, it could provide additional infor-
3. Figure 2 of Richards et al. (2009) demonstrates the mation about NCCS objects brighter than ∼16m–17m.
distribution of the SDSS AGN candidate counts as Spectroscopicfollow-upofthepromisingNCCSsources
a function ofthe Sloani magnitude. The number of couldbeperformedattheWiseObservatoryforobjects
AGN candidates brighter than our limiting magni- brighterthan∼16m oratalargertelescopeforthefaint
tude I ≤ 19.1 mag can be determined from Figure ones.
2 as ∼23000 counts. However, this number should
Acknowledgements Based on observations made
be corrected for the survey area (8417 deg2 - SDSS,
withtheNASAGalaxyEvolutionExplorer. GALEXis
313.6 deg2 - NCCS): 23000×313.6 ≈ 857.The estima-
8417 operatedfor NASAby the CaliforniaInstitute ofTech-
tionof857candidatesis(1)notcorrectedforthesur-
nology under NASA contract NAS5-98034.
vey latitude and (2) is contaminated by higher red-
This study makes use of data from the SDSS
shift AGNs. However, Figure 18 of Richards et al.
(http://www.sdss.org/collaboration/credits.html).
(2009) shows that the contamination of the sample
byhigherredshiftAGNsis.20%,yieldingthe final References
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4 Conclusions and Further Work V´eron-Cetty, M.P.,&V´eron,P.2010, VizieROnlineDataCat-
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We have shown that there is valuable new science that
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We can relatively easily extend the surveyed region
for five more degrees of declination, increasing the sur- This2-columnpreprintwaspreparedwiththeAASLATEXmacros
veyed region to more than 700 deg2. This will increase v5.2.