Stil et al. (2006) AJ 132, 1158

20 Pages·2012·4.12 MB·English

Accepted forpublicationinTheAstronomicalJournal
PreprinttypesetusingLATEXstyleemulateapjv.11/26/04
THE VLA GALACTIC PLANE SURVEY
J. M. Stil1 and A. R. Taylor1, J. M. Dickey2,3 and D. W. Kavars3, P. G. Martin4,5, T. A. Rothwell5, and A. I.
Boothroyd4, Felix J. Lockman6, and N. M. McClure-Griffiths7
Accepted for publication inThe Astronomical Journal
ABSTRACT
The VLA Galactic Plane Survey (VGPS) is a survey of Hi and 21-cm continuum emission in the
Galactic plane between longitude 18(cid:14) and 67(cid:14) with latitude coverage from jbj < 1(cid:14):3 to jbj < 2(cid:14):3.
The survey area was observed with the Very Large Array (VLA) in 990 pointings. Short-spacing
information for the Hi line emission was obtained by additional observations with the Green Bank
Telescope (GBT). Hi spectral line images are presented with a resolution of 10 (cid:2)10 (cid:2)1:56 km s(cid:0)1
(FWHM) and rms noise of 2 K per 0:824 km s(cid:0)1 channel. Continuum images made from channels
0
without Hi line emission have 1 (FWHM) resolution. The VGPS imagesreveal structures of atomic
hydrogenand 21-cmcontinuum aslargeasseveraldegrees with unprecedented resolutionin this part
of the Galaxy. With the completion of the VGPS, it is now possible for the (cid:12)rst time to assess
the consistency between arcminute-resolution surveys of Galactic Hi emission. VGPS images are
compared with images from the Canadian Galactic Plane Survey (CGPS) and the Southern Galactic
PlaneSurvey (SGPS). In general, the agreementbetween these surveysis impressive, consideringthe
di(cid:11)erences in instrumentation and image processing techniques used for each survey. The di(cid:11)erences
between VGPS and CGPS images are small, .6 K (rms) in channels where the mean Hi brightness
temperatureinthe(cid:12)eldexceeds80K. AsimilardegreeofconsistencyisfoundbetweentheVGPSand
SGPS.The agreementwe(cid:12)nd between arcminute resolutionsurveysof the Galacticplaneis acrucial
step towards combining these surveysinto a single uniform dataset which covers90% of the Galactic
disk: the International Galactic Plane Survey(IGPS). The VGPS data will be made availableon the
World Wide Web through the Canadian Astronomy Data Centre (CADC).
Subject headings: ISM: atoms | Galaxy: disk | Surveys
1. INTRODUCTION quires observations which reveal parsec-scale structures.
For objects outside the solar neighborhood, resolving
The physical processesin the feedbackcycle of matter
parsec-scalestructuresrequiresarcminute-resolutionim-
betweenstarsandtheinterstellarmediumplayanimpor-
ages. Thedistributionofcoldatomicgasmayberevealed
tantroleintheevolutionofgalaxies,andinthewayprod-
by absorption of continuum emission or absorption of
ucts of stellar nucleosynthesis are dispersed. A quanti-
the Hi line emission itself (Hi self absorption). Results
tative description of these processes requires knowledge
from such observations depend strongly on the resolu-
of the poorly known topology of di(cid:11)erent phases of the
tion of the data (Dickey & Lockman 1990). To place
interstellar medium and the timescales and locations of
these processes into a Galactic context, the high reso-
transitions between these phases. Atomic hydrogen is
lution image should also show structure on large scales.
the link between the gas heated or expelled by massive
At 21 cm wavelength, images with arcminute resolution
starsand coldmoleculargasfrom whichnew starsform.
and a large spatial dynamic range can be obtained by
Hi is widely distributed and, within certain limits, eas-
aninterferometerincombinationwithalargesingledish
ily observableacrossthe Galaxythroughthe 21-cmline.
telescope to (cid:12)ll in large scale structure not detected by
As such, atomic hydrogen has been used to study the
the interferometer.
dynamics of the Galaxy and physical conditions in the
Previously the Canadian Galactic Plane Survey
di(cid:11)use interstellar medium.
(CGPS) (Taylor et al. 2003) and the Southern Galac-
To study the interstellar medium in transition, from
tic Plane Survey (McClure-Gri(cid:14)ths et al. 2001, 2005)
the cold phase to the warm phase or vice versa, or the
have provided high-resolution Hi images in the north-
dynamical e(cid:11)ect of stars on the interstellar medium, re-
ern sky (mainly the second Galactic quadrant) and the
1DepartmentofPhysicsandAstronomy,UniversityofCalgary, southernsky(thirdandfourthGalacticquadrants). The
(cid:14)
2500UniversityDriveNW,Calgary,AB,T2N1N4,Canada CGPSwillbeextendedtolongitude50 ,whiletheSGPS
2School of Mathematics and Physics-Private Bag 37, Univer- has been extended to longitude 20(cid:14). A large part of
sityofTasmania,Hobart,TAS7001,Australia
the (cid:12)rst Galactic quadrant is located near the celestial
3Department of Astronomy, University of Minnesota, 116
equator. This area of the sky cannot be observed at
ChurchStreet,SE,Minneapolis,MN55455,USA
4CanadianInstituteforTheoreticalAstrophysics,Universityof su(cid:14)cient angular resolution by the interferometers used
Toronto,60St. GeorgeStreet,TorontoON,M5S3H8,Canada in the CGPS and SGPS, because these interferometers
5Department of Astronomy and Astrophysics, University of
have exclusively or mostly east-west baselines. At the
Toronto,60St. GeorgeStreet,TorontoON,M5S3H8,Canada
6NationalRadioAstronomicalObservatory,P.O.Box2,Green extremesofthe CGPSandSGPS,theangularresolution
Bank,WestVirginia24944,USA of these surveys is degraded by a factor (cid:24) 3 in declina-
7Australia Telescope National Facility, CSIRO, P.O. Box 76, tion.
EppingNSW1710,Australia
The Very Large Array(VLA) can observe the equato-
2 Stil et al.
TABLE 1
VGPS parameters
Quantity Value
Surveyarea jbj<1(cid:14):3 18(cid:14)<l<46(cid:14)
jbj<1(cid:14):9 46(cid:14)<l<59(cid:14)
jbj<2(cid:14):3 59(cid:14)<l<67(cid:14)
Angularresolution(FWHM) 6000(cid:2)6000
Spectralresolution(FWHM) 1:56kms(cid:0)1
Channelwidth 0:824kms(cid:0)1
Noiseincontinuuma 0:3K
Noiseperchannela 2K
Tb=S(cid:23) 168K/Jy
aNoiselevelsmaybedi(cid:11)erentfromtheserepresentativeval-
uesdependingonlocationandvelocity(Hiline).
rial part of the Galactic plane with adequate resolution.
Inthispaperwepresentthe VLAGalacticPlaneSurvey
(VGPS). The VGPS is an Hi spectral line and 21-cm
continuum survey of a large part of the (cid:12)rst Galactic
quadrant. This survey was done with the Very Large
Array (VLA) and the 100 m Robert C. Byrd Green Fig. 1.|TheSensitivityFunctionoftheVGPS.Arepresentative
BankTelescope(GBT)oftheNationalRadioAstronom- section of the primarysurvey area isshown. Contours are drawn
atthe50%,80%,90%,95%,and98%levelsofthe maximumsen-
icalObservatory(NRAO).Shortspacinginformationfor
sitivityinthemosaic. Thisfunctionre(cid:13)ectstheGaussianprimary
the VGPS continuum images was provided by a contin- beamshapeoftheVLAandthegridofpointingcentersindicated
uum survey with the 100 m E(cid:11)elsberg telescope (Reich bythecrosses. Thelatitudecoverage ofthe surveywidensbytwo
& Reich 1986; Reich et al. 1990). The CGPS, SGPS, morerowsofpointingcentersforlongitudesgreaterthan45(cid:14):7,and
afurthertworowsforl>59(cid:14):7.
andVGPS will be combinedinto asingle data setwhich
provides arcminute-resolution images of Galactic Hi for 2.1.1. Mosaic Strategy
90% of the Galactic plane as part of the International
An interferometer survey using the mosaicking tech-
Galactic Plane Survey (IGPS). With the completion of
nique to combine many primary beam areas into a large
the VGPS, di(cid:11)erent parts of the IGPS overlap for the
map begins with the choice of the area to cover and the
(cid:12)rst time, allowing a detailed comparison of the results
telescopetimeavailable. Thesetwonumberssettheover-
from each survey. This paper describes the VGPS data,
all sensitivity or noise level of the survey, but this sensi-
and comparesthe VGPS spectral line images with those
tivity is not uniform over the area. The gain in a single
of the CGPS and SGPS in the areas where the surveys 0
VLA (cid:12)eld variesoverthe 32 (FWHM) primarybeam of
overlap.
the VLA antennas. When multiple (cid:12)elds are combined
2. OBSERVATIONSANDDATAREDUCTION in a mosaic, the spacing between pointing centers deter-
minesthecorrugationorspatialvariationofthesensitiv-
2.1. VLA observations
ityfunction. Acenteredhexagonalgeometryisoptimum
The main set of observations of this survey was done for (cid:13)attening the sensitivity function, for a given num-
with the Very Large Array (VLA) of the NRAO. The berofpointings,butthefunctioncanalwaysbe(cid:13)attened
technical speci(cid:12)cations of the array are described in de- furtherbydecreasingthespacingbetweenadjacentbeam
tailbyTaylor,Ulvestad&Perley(2003). TheVLAisan centers and so increasing the total number of pointings
interferometer with 27 elements, each 25 m in diameter observed. Theoverheadcostsintelescopedrivetimeand
(320 FWHM primary beam size at 21 cm). Several VLA the complexity of the data reduction are increased as
(cid:12)elds must be combined in a mosaic to image an appre- the number of pointings increases, so there is a compro-
ciable part of the Galaxy. The most compact con(cid:12)gura- mise requiredbetween (cid:13)attening the sensitivityfunction
tionofVLAantennas,theD-con(cid:12)guration,hasbaselines and reducing the number of pointings. For this survey
0
between 35 m and 1.03 km, and is the most suitable for we have used the relatively wide spacing of 25 between
imaging of widespread Galactic Hi emission. For short points,whichisnotmuchlessthanthefull-widthtohalf-
observations (snapshots), the largest angular scale that power point of the VLA beam at (cid:21) 21-cm of 320. This
canbeobservedreasonablywellis(cid:24)450arcsecondsat21 resultsinthesensitivityfunctionshowninFigure1. The
0
cm. The synthesized beam size is about 45 arcseconds 25 spacing choice is driven by the 20 second minimum
(FWHM) at 21cm. The total amount of observingtime lagtime betweenthe end ofdatatakingononescanand
attheVLAallocatedtothissurveywas260hoursinthe the beginning of the next, which is exacerbated by the
period July to September 2000. In addition to the VLA need to delete the (cid:12)rst 10 second data average of each
observations, a fast survey with the 100 m GBT of the scan. ThesefeaturesofthecurrentVLAdataacquisition
NRAO was done to obtain the necessary short-spacing systemmakeitarelativelyslowtelescopeformosaicking.
informationfortheHispectrallineimages. Table1lists There are other options available for data taking while
basic parameters of the VGPS. drivingthe telescope(modeOF), butthese provedtobe
impractical for this survey.
The D con(cid:12)guration of the VLA is subject to shad-
The VLA Galactic Plane Survey 3
Fig. 2.| Shadowing vs. hour angle and declination for the D
array of the VLA. Two elevations, 13(cid:14) and 20(cid:14), are plotted with
thin lines; two shadowing limits, zero shadowing and 10% shad-
owing, are plotted with thick lines. These limits were computed
usingtheAIPSsubroutinesGETANT,UVANT,andBLOCK,for
theantenna (cid:12)lecorrespondingtothearrayusedforallsurveyob-
servations. Allobservationsoftheprimarysurveyareaweretaken
with zero shadowing, i.e.,forhour anglesinthe area between the Fig. 3.| Hour angles of the observations of sample pointings.
solidcurves. Someoftheobservations ofthehigherlatitude (cid:12)elds The Galactic plane follows the center line, and the two lines on
((jbj>1(cid:14))weretaken withsomeshadowing,butnever morethan either side show latitudes (cid:0)1(cid:14) and +1(cid:14). Longitudes are marked
afewpercent. withshortbars,withlongitude20(cid:14) and25(cid:14) indicated. Thepoint-
ingcentersareshownastheverticesofseveralshortlinesegments
owing, i.e. blockage of one antenna by another, at low (\chicken feet"). The short segments each represent an observa-
tion,withthe directionofthesegment indicating thecorrespond-
elevations. Foramosaicsurvey,itisevenmoreimportant
inghourangle,with0hplottedasaverticalsegment,(cid:0)6hand+6h
than usual to avoid shadowing, because of its e(cid:11)ect on plotted horizontally to the leftand right. The observingschedule
the primary beam shape of the blocked antenna, which was chosen to optimize the uv coverage provided by awide range
compromises the estimate of the telescope response to a ofhourangles,i.e.,abroadfootprintforeachpointing.
model source that is needed for the maximum entropy
Generallythestrongestcontinuumsourcesthatcausethe
deconvolution. Thissurveyincludesmany(cid:12)eldsatnega-
worstdynamicrangeproblemsarelocatedintheprimary
tivedeclinations,forwhichtheavailablehouranglerange
area, so this strategy is appropriate. However it should
isminimal. Avoidingshadowingbecomesthe (cid:12)rstdriver
be kept in mind that the quality of the imaging in the
of the observing strategy. Figure 2 shows a rough guide
area of the survey with jbj > 1(cid:14) is degraded relative to
for the hour angle and declination range which is safe
the lower latitudes. Most (cid:12)elds (93%) were observed at
from shadowing for the D con(cid:12)guration. All scans for
the primary survey area (jbj < 1(cid:14), 18(cid:14) < l < 65(cid:14)) were leastthreetimesatdi(cid:11)erenthourangles,and38%ofthe
(cid:12)elds were observed four or more times. About 7% of
takenwithout shadowing,asweremostin the secondary
area (jbj>1(cid:14)). the (cid:12)elds were observed only 2 times. The theoretical
sensitivity of the spectral line mosaics is 8.4 mJy (1 (cid:27))
Besides avoiding shadowing and minimizing tele-
or 1.4 K per 0:824 km s(cid:0)1 channel for the beam size of
scope drive times, the most important consideration for
00
60 . The rms noise amplitude in the (cid:12)nal VGPS im-
scheduling was obtaining multiple scans on each (cid:12)eld at
agesis 1:8 K per channel on average. The actual spatial
widely spaced hour angles. This is the best way to min-
variation of the noise in the (cid:12)nal VGPS images deviates
imize sidelobes due to largeunsampled regionsof the uv
somewhat from the regular pattern shown in Figure 1
plane that compromise the dynamic range of the result-
because of di(cid:11)erences in the integration time per (cid:12)eld.
ingmaps. Theschedulingprocesswasdrivenbytheneed
Suchdi(cid:11)erencesexistbecauseofvariationinthenumber
tospreadtheobservationsofeach(cid:12)eldovertheavailable
of visits to a (cid:12)eld and because of a longer dwell time on
houranglerangeatthedeclinationofthat(cid:12)eld,andstill
the (cid:12)rst (cid:12)eld of a block of six.
(cid:12)t everything into about 25 sessions, each typically cov-
ering 14h < LST < 24h. The low longitude end, which
2.1.2. Spectrometer
is observable only from (cid:24) 16h to (cid:24) 20h LST, was hard
to (cid:12)t into approximately 25 sessions. The area north of Thissurveypushesthelimits ofthe VLAcorrelatorin
+10(cid:14) declination (l>45(cid:14)) is relatively easyto schedule. that we need high resolution in velocity (0:824 km s(cid:0)1
Forsimpli(cid:12)cation,eachrowofpointingsatconstantlon- = 3.90 kHz channel spacing) and broad bandwidth ((cid:24)
gitude (six beam areas, 20 minutes of integration time 300kms(cid:0)1 =1.4MHztotal). Thiswasimpossibletoob-
total) was scheduled in a block. The (cid:12)nal observing se- tainwith the existingspectrometerfortwo polarizations
quence gave hour angle coverages as shown in Figure 3. atonce. Tosacri(cid:12)ceoneofthetwocircularpolarizations
Toppriorityforschedulingwasgiventotheprimaryarea wouldbeequivalenttogivinguphalftheintegrationtime
(cid:14) (cid:14) (cid:14) (cid:14)
of the survey,latitude (cid:0)1 to +1 , longitude 18 to 65 . of the survey,sowechoseastrategythat keepsbothpo-
The (cid:12)elds at higher positive and negative latitudes were larizationsbut with the coarservelocitychannel spacing
observedwithsecondpriority,sotheiruvcoverageisless of 1:28 km s(cid:0)1. We then stagger the placement of the
evenly distributed over the available hour angle range. channelsbetweenthetwopolarizationsbyhalfachannel
4 Stil et al.
spacing, 0:64 km s(cid:0)1, so that the sampling on the spec-
tral axis may be increased when all data are combined
(Figure4). TheVGPSspectrallinedataaresampledon
the same 0:824 km s(cid:0)1 spectral channels as the CGPS
formaximumconsistencybetweenthetwodatasets. The
spectral resolution of the data (1:21(cid:2)1:28 km s(cid:0)1 =
1:56 km s(cid:0)1) is determined by the size of the time lag
window in the spectrometer. It is not changed by re-
sampling to narrower spectral channels. The centers
of the two polarizations are o(cid:11)set by +32:304 km s(cid:0)1
and (cid:0)31:460 km s(cid:0)1 from the nominal center velocity,
which is set at vc(l) = +80(cid:0)(1:6(cid:2)l) km s(cid:0)1 with l
the longitude in degrees. Combining these gives spec-
tra with velocity width 341 km s(cid:0)1 after dropping 20
channels on either side of the band due to the baseband
(cid:12)lter shape. The Hi line emission is unpolarized ex-
cept for tiny amounts due to the Zeeman e(cid:11)ect, which
are far below our sensitivity limit. But to avoid spuri-
ous spectral features arising from Hi absorption of lin-
early polarized continuum, which is common in the syn-
chrotron emission at low latitudes, we alternate the fre-
quency settings between the two polarizationsevery 100
seconds. Soasingleobservationofasurvey(cid:12)eldconsists
oftwoshortintegrations(100seach)withcomplementary
spectrometer settings. This gives enough bandwidth to
Fig. 4.| Spectrometer channel spacing. Observing frequency
cover the range of velocities in the (cid:12)rst Galactic quad- isindicatedonthebottom,witharepresentativespectrumshown
rant, +150 to (cid:0)80 km s(cid:0)1 at the lower longitudes, with inthe inset,with velocity shiftedbyatypical o(cid:11)set fromthe rest
0:824 km s(cid:0)1 channels throughout. The local oscillator frequency of 1420.4058 MHz due to terrestrial and solar motion.
Allobservations weredoneinpairs,withthe centerfrequenciesof
settings for the survey were (cid:0)3:2, 3590 with bandwidth the two polarizations switched as indicated. The channel center
code5(1.5625MHz,ofwhichtheinner(cid:24)85%isusable) frequencies are staggered as shown in the magni(cid:12)ed inset, so as
and correlator mode 2AD. No on-line Hanning smooth- tomakeitpossibletosamplethepro(cid:12)leshapewith0:824kms(cid:0)1
channels in spite of the necessity to take the data with a broader
ing was performed, and the single dish bandpass shape channel spacing (1:28 km s(cid:0)1). The center velocity was set at
was not used to normalize the spectra, as is sometimes vc(l)=+80(cid:0)(1:6(cid:2)l)foreachlongitude,l.
done as part of the correlationstep.
plingidenticalforallchannels. Loweramplitudeglitches
2.2. VLA Calibration
often had higher amplitude counterparts in other chan-
Calibration of the VLA data was carried out using nels. The policy to (cid:13)ag all channels was found to be
standard procedures within AIPS. The primary calibra- e(cid:11)ective in eliminating low-amplitude glitches as well.
tors 3C286 and 3C48 were used for (cid:13)ux and bandpass After the automated (cid:13)aggingprocedure,only afew inci-
calibration. Aftercalibration,thevisibilitydatawereim- dentalmanual(cid:13)aggingoperationswererequiredtomake
ported into MIRIAD for further processing. Editing out spectral line and continuum images free from noticeable
glitches in the largevolume of visibility data for the 990 e(cid:11)ects of bad data.
VLA(cid:12)eldswasdonewithanautomated(cid:13)aggingroutine. Preliminarycontinuummosaicswereconstructedfrom
Thevisibilitydataforthe VGPS(cid:12)elds weresearchedfor visibilitydataaveragedoverchannelsoutsidethevelocity
high-amplitude points relative to the median visibility range of Galactic Hi line emission. These mosaics were
amplitude for a particular baseline in a particular chan- analyzedwithanautomatedsourceextractionalgorithm
nel and for a particular spectrometer/polarization com- tocomparethe(cid:13)uxesofcompactcontinuumsourceswith
bination. The thresholds applied in this procedure were (cid:13)uxesinthe NVSSsurvey(Condonetal.1998). Sources
chosen after careful inspection of the data. Amplitudes were labeled as suspected variables and removed from
more than 20 Jy abovethe median amplitude in the du- consideration if the ratio of the absolute value of the
ration of the snapshot were (cid:13)agged. Also, scans with di(cid:11)erence between the NVSS and VGPS (cid:13)uxes and the
an overall median amplitude above 50 Jy were (cid:13)agged meanof these(cid:13)uxeswaslargerthan10timesthe formal
to eliminate saturated antennas. Special care was taken error. This comparison between the NVSS and VGPS
not to label legitimate signal on the shortest baselines showedthat(cid:13)uxesofcompactsourcesintheVGPSwere
as bad data. The snapshots are su(cid:14)ciently short that on average30% less than (cid:13)uxes listed in the NVSS. The
a constant visibility amplitude can be assumed for each origin of this discrepancy is not understood. It is be-
baseline in a single channel in this search. lieved to be the result of the higher system temperature
The (cid:13)agging procedure allowed streamlined visual in- in the VGPS observations, which is in part the result of
spectionofidenti(cid:12)edbaddataand,ifnecessary,ahuman bright Hi emission in the Galactic plane. The VLA has
veto before the actual (cid:13)agging. No false rejections were an automatic gaincontrol(AGC) system that scales the
foundbecausetherejectioncriteriaweresu(cid:14)cientlycon- signal with the system temperature, but visibility am-
servative. If visibilities for a particular combination of plitudes are corrected for this scaling. First we discuss
time,baselineandpolarizationwererejectedinonechan- variousfactorsthata(cid:11)ectthesystemtemperatureinthe
nel, all channels were rejected so as to keep the uv sam- VGPS. Later we derive a correction to the (cid:13)ux scale of
The VLA Galactic Plane Survey 5
itydata. Thescalaraverageamplitudeofthecontinuum-
subtracted visibilities (abbreviated here as ampscalar)
gives a spectrum that is proportional to the rms visi-
bility amplitude in each sample; it is noise dominated
(proportional to T ). It takes into account the edit-
sys
ing of data rejected by the automated (cid:13)agging routine,
and it is not necessarily averaged in frequency as is the
system temperature recorded with the data. After con-
tinuum subtraction, only the shortest baselines contain
some correlated signal because of the Galactic Hi line.
When averaging the ampscalar data over all antennas,
this remaining signal has a negligible e(cid:11)ect. This was
veri(cid:12)ed by comparing the result with ampscalar values
averagedoverbaselines longer than 1k(cid:21). The ampscalar
valuesare proportionalto the recordedsystem tempera-
ture averagedover the array.
We write the total system temperature as the sum
of the receiver temperature T (cid:25) 35 K, an elevation-
rec
dependent term T (h) which includes atmospheric
earth
emission but is usually dominated by spillover to the
ground, and the brightness temperature of cosmic radio
emissionTb(v) which depends onvelocity becauseof the
bright Galactic Hi line,
Fig. 5.| Relation between scalar averaged visibilityamplitude Tsys(v;h)=Tb(v)+Tearth(h)+Trec (1)
and sky brightness temperature (line + continuum averaged over The spectral line data of the VGPS allows separation of
theVLAprimarybeam)forthreesnapshotsofthe(cid:12)eldG65.3+1.4
the contribution of Galactic emission to T from other
takenwiththesamespectrometersettingsinasinglepolarization sys
(L)on2000September7,17,and29atelevations52(cid:14):3,63(cid:14):5,and contributions. This in turn allows us to derive a new
24(cid:14):2 respectively. Each point corresponds to a single frequency functionalformfortheelevationdependenceofT (h).
earth
channel. The noise per channel changes as the brightness of the The brightness temperature in each velocity channel,
Hilinevarieswithvelocity,andasigni(cid:12)cantvariationofthenoise
level with elevation is apparent. The lines represent linear least Tb(v), averaged over the VLA primary beam, was de-
squares (cid:12)ts used to separate the contribution of Galactic radio termined by smoothing the GBT maps and the E(cid:11)els-
emissionfromotherfactorscontributing tothenoise. berg maps to the resolution of the VLA primary beam.
Figure5 shows the relationbetween sky brightnesstem-
the VGPS to make it consistent with the NVSS.
perature (line + continuum) averaged over the primary
beam of the VLA, with the scalar averaged amplitude
2.2.1. Contributions to the system temperature
per channel. Three visits to the same (cid:12)eld on three dif-
Compared with the NVSS, there are two important ferent days are shown. The main di(cid:11)erence between the
enhanced contributionsto the system temperature. One snapshots in Figure 5 is the elevation of the (cid:12)eld at the
contribution is from bright Galactic Hi and continuum time of observation. Some (cid:12)elds wereobservedat nearly
emission, which can double the system temperature av- the same elevation on di(cid:11)erent days. Such observations
eraged over the inner 75% of the frequency band. The have nearly indistinguishable values of ampscalar. The
systemtemperaturechangesacrossthespectralbandbe- relation between ampscalar and brightness temperature
causethebrightnessoftheHilinechangeswithvelocity. was(cid:12)ttedwithalinearrelationtoallowextrapolationto
The other contribution is from spillover to the ground Tb = 0 K. The contribution of Galactic emission to the
when a (cid:12)eld is observedat low elevation. Emission from system temperature is eliminated by this extrapolation.
the atmosphere also depends on elevation, adding 2 to Werefertothisextrapolationasthescalar-averagedam-
4 K to the system temperature. The e(cid:11)ect of spillover plitude at Tb = 0 or ampscalar at Tb = 0, which is pro-
is an order of magnitude larger than this. The average portional to T (h)+T according to Equation (1).
earth rec
systemtemperatureoftheVLAantennasinthezenithis We(cid:12)ndthattheslopeoftherelationinFigure5increases
Tsys (cid:25) 35 K. The system temperature increases rapidly as the ampscalarat Tb =0 increases.
(cid:14)
atlowelevation,toapproximately70Katelevation30 . Figure 6 shows the relation between the scalar-
VGPSobservationsweremadeoverawiderangeofhour averagedvisibilityamplitudeandelevationofthe(cid:12)eldat
anglestoobtainadequatesamplingintheuvplane. Asa the time of observation for all snapshots taken on 2000
result, (cid:12)elds wereregularlyobservedfarfrom the merid- September 15. A poor correlation is found between the
ian at low elevation. In contrast, when the NVSS was raw band-averaged ampscalar and elevation. After cor-
made, its (cid:12)elds were observed close to the meridian in rection for the contribution by Galactic emission, a very
order to minimize ground noise. This strategy is more tight relation is found between the ampscalar at Tb = 0
suitable for a continuum survey which targets compact and elevation. The scatter in this relation is consistent
sources. with the estimated errorsintheextrapolationtoTb =0.
The system temperature for each antenna of the VLA This extrapolation is less accurate towards the bright-
isrecordedandstoredwith the visibilitydata. However, est continuum sources because the relation as shown in
for the present purpose it is more convenient to adopt a Figure 5 is not well de(cid:12)ned. The points which do not
di(cid:11)erentmeasureofT deriveddirectlyfromthevisibil- (cid:12)t on the relation represent snapshots with very bright
sys
6 Stil et al.
Fig. 6.|Rawbandaveragedampscalar(leftpanel)andampscalaratTb=0(rightpanel)asafunctionofelevationforsnapshotstaken
on2000September 15. This(cid:12)gure illustrates the magnitude ofcontributions tothe noisefromGalactic Hi andcontinuum emission,and
spillovertotheground. Lefthandpolarizationisshownasopensquares,righthandpolarizationas(cid:12)lledtriangles. Thebrightcontinuum
sourcesW49andW51wereobservedonthisdayatelevations50(cid:14) and69(cid:14) respectively. Thecurvesintherightpanelrepresent(cid:12)tsofthe
formA=acos4(h)+b,withhtheelevation ofthe(cid:12)eld.
Fig. 7.| Band averaged ampscalar as a function of position in the sky. Top panel: smallest ampscalar for each (cid:12)eld in gray scales.
Middle panel: smallest ampscalar per (cid:12)eld, corrected for elevation-dependent spillover to the ground. Bottom panel: VGPS continuum
image. Thepredominance oftheobservingpatternofblocksof6(cid:12)eldsintheupperpanelshowsthatspillovertothegroundisimportant
compared withGalacticemissionalmosteverywhereinthe VGPS.When theelevation-dependent contribution iseliminated,asdescribed
inthe text,aclearcorrelationwithbrightGalacticemissionisseen.
The VLA Galactic Plane Survey 7
hbL (cid:0)bRi = 0:13(cid:6)0:05. The di(cid:11)erence in ampscalar
TABLE 2 between the polarizationsis equivalent to a system tem-
Fits ofEquation(2)
perature that is approximately 12% higher in L than in
R. The rms residuals of the (cid:12)ts ((cid:27)L and (cid:27)R in Table 2)
Date aL bL (cid:27)L aR bR (cid:27)R are0.06inthemean. Inspectionofthe(cid:12)tsindicatedthat
07/23 1.26 1.033 0.088 ::: ::: ::: larger values of (cid:27)L and (cid:27)R indicate variation in the sys-
07/25 1.10 1.078 0.061 1.14 1.169 0.057 tem temperature during the day. Such variation may be
07/28 1.28 1.079 0.060 1.32 1.182 0.061
relatedtosolaractivity,inparticularinSeptemberwhen
07/29 1.52 1.075 0.094 1.54 1.216 0.093
08/02 1.37 1.021 0.052 1.41 1.135 0.049 the angular distance of the Sun was smaller. Inspection
08/04 1.38 1.133 0.055 1.44 1.276 0.051 of the (cid:12)ts showed that variations in aL and aR in Ta-
08/05 1.48 1.090 0.031 1.58 1.187 0.037 ble 2 appear to be related mainly to intra-dayvariation.
08/08 1.50 1.056 0.058 1.42 1.215 0.051
08/10 1.41 1.079 0.076 1.51 1.166 0.071 However,therangeofvaluesofbL andbR representsreal
08/14 1.31 1.121 0.061 1.31 1.246 0.059 variations in the receiver temperature Trec during the
08/17 1.46 1.102 0.063 1.55 1.231 0.066 period of observations.
08/19 1.30 1.055 0.026 1.31 1.141 0.029 The(cid:12)tsinTable2allowustomakeanelevationcorrec-
08/22 1.26 1.166 0.055 1.28 1.228 0.054
tion for the noise amplitude for each snapshot to obtain
08/24 1.42 1.110 0.051 1.45 1.272 0.061
08/29 0.89 1.118 0.090 0.91 1.228 0.093 a prediction of the noise level if the (cid:12)eld had been ob-
08/31 1.41 1.115 0.039 1.51 1.262 0.040 servedatthe zenith. Figure7showsthedistributionper
09/05 1.40 1.143 0.042 1.54 1.287 0.046 VGPS (cid:12)eld of the smallest value of the band averaged
09/07 1.64 1.115 0.058 1.75 1.260 0.058
ampscalar of all snapshots contributing to a (cid:12)eld. This
09/11 1.39 1.132 0.034 1.54 1.272 0.034
09/14 1.17 0.933 0.097 1.18 1.243 0.109 maptypicallyshowstheampscalarfortheobservationof
09/15 1.41 1.147 0.066 1.42 1.314 0.061 each(cid:12)eld atthe highestelevation. Mostof thestructure
09/17 1.29 1.011 0.041 1.40 1.127 0.041 inthedistributionofthebandaveragedampscalarcorre-
09/18 1.33 1.258 0.163 1.79 1.351 0.174
sponds to the VGPS observing sequences of six (cid:12)elds in
09/19 1.32 1.139 0.065 1.46 1.284 0.061
09/21 1.15 1.237 0.088 1.26 1.398 0.101 a row. A few bright continuum sources near the Galac-
09/24 1.15 0.996 0.044 1.12 1.083 0.055 tic plane can be identi(cid:12)ed as well. The observing pat-
09/29 1.34 1.146 0.054 1.51 1.233 0.050 ternvisibleinthedistributionoftheband-averagedamp-
09/30 1.32 1.173 0.060 1.32 1.273 0.065
scalar shows that almost everywhere in the survey area
elevation-dependentcontributionstoT areimportant.
sys
Elevation-dependent e(cid:11)ects and sky emission contribute
continuum. Similarresultswereobtainedforeachdayof roughly equal amounts to the total system temperature
VGPS observations. (Figure 6).
Figure 6 illustrates the relative importance of factors The minimum ampscalar per (cid:12)eld tends to be higher
which raise the system temperature. At high elevations, atlowlongitudes,inpartbecausethese(cid:12)eldstransitthe
Galactic emission roughly doubles the system tempera- meridianatalowerelevation. Equation(2)allowsacor-
tureintheGalacticplane. Thisincreaseismostlydueto rection to be made for the elevation of the (cid:12)eld at the
the brightHi line, but the continuum alsocontributes 5 time of observation. Theparametersaandbweredeter-
to 20 K to the system temperature, depending on longi- mined for each day and for each polarization separately
tude. Thebrightestcontinuumsources(W49, andW51) by(cid:12)ttingEquation(2)totheavailabledataasillustrated
actuallycontributemoretothesystemtemperaturethan inFigure6. Theelevation-correctedband-averagedamp-
the Hi line when averagedover the spectral band. scalar was calculated by subtracting the excess in the
The tight correlation between ampscalar at Tb = 0 band-averaged ampscalar value resulting from the ele-
and elevation resembles the increase in T with eleva- vation of the (cid:12)eld. The map of the elevation-corrected
sys
tion from spillover to the ground measured by Taylor, ampscalar in Figure 7 shows a higher noise level in the
Ulvestad & Perley (2003), who applied a second-order Galactic plane, and toward bright continuum sources.
polynomial (cid:12)t to the data. It is di(cid:14)cult to interpret Some residualsof the elevation correction remain in this
the results of a polynomial (cid:12)t when comparing di(cid:11)er- map. These are likely the result of intra-day variations
entdaysofobservation,becausesomedayscoveredmore that are not traced by the daily averageof the elevation
(cid:12)elds at low elevation than others. Inspection of all the correction applied here.
data for each day separately showed that the elevation The distribution of the elevation-corrected ampscalar
dependence of ampscalaratTb =0isalsodescribedwell showsastrongresemblancetotheVGPScontinuumim-
with two free parameters by the form age. This resemblance is not inconsistent with the fact
that the Hi line usually contributes more to the level
A=acos4h+b (2)
of T than the continuum. The visualization in Fig-
sys
Equation(2)was(cid:12)ttedtoeachdayandpolarizationsep- ure7tendstoemphasizethevariationsbetweenadjacent
arately. Three (cid:12)elds centered on (l,b) = (43:3,(cid:0)0:1), (cid:12)elds. Field-to-(cid:12)eld variations in the velocity-integrated
(49:0,(cid:0)0:5), and (49:4,(cid:0)0:3) were excluded from these Hi brightness are fairly small compared with (cid:12)eld-to-
(cid:12)ts. These (cid:12)elds are most a(cid:11)ected by W49 and W51. (cid:12)eld variations in the continuum level.
Results of the (cid:12)ts are listed in Table 2.
2.2.2. Correction of the VGPS (cid:13)ux scale
Equation (2) provides an accurate (cid:12)t of the elevation-
dependent noise contribution for each day. Note that The remainder of this section describes a method to
the scatter around the (cid:12)ts in Figure 6 is much smaller adjust the (cid:13)ux scale of the VGPS to the (cid:13)ux scale of
than the di(cid:11)erence between the L and R polarization. the NVSS. The Canadian Galactic Plane Survey also
The mean di(cid:11)erence between the two polarizations is corrects its (cid:13)ux scale to the NVSS by comparing the
8 Stil et al.
a list of identi(cid:12)ed sources was compiled. Each of these
source lists contains continuum sources throughout the
VGPSsurveyareadetectedinsnapshotswith nearlythe
same band-averaged ampscalar. Figure 8 shows the ra-
tio of NVSS (cid:13)ux to VGPS (cid:13)ux as a function of the
band averaged ampscalar. A tight relation is found be-
tween the band averaged ampscalar and the (cid:13)ux ratio
NVSS/VGPS. This relationde(cid:12)nes the correction factor
for the (cid:13)ux scale of each snapshot based on its band av-
eraged ampscalar. The scatter in the relation shown in
Figure 8 is dominated by source number statistics. The
e(cid:11)ectofthesamplesizewasinvestigatedbyregenerating
the relationfor (cid:12)fty randomlyselected subsamples, each
halfthe sizeof the totalnumber ofsnapshots. The error
barsinFigure8showthe rmsvariationforeachbinover
the (cid:12)fty subsamples. Values of the band-averaged amp-
scalararealwayslargerthan1.4intheVGPS.Asecond-
orderpolynomialwas(cid:12)ttedtothedatatoprovideapre-
scriptionforthecorrectionfactor,asa(cid:12)rst-orderpolyno-
mial(cid:12)twasdeemedinsu(cid:14)cient. Asmallnumberof(cid:12)elds
thatincludeverystrongcontinuumemissionhaveaband
averagedampscalaroutsidethe rangewherethe relation
is de(cid:12)ned. We assume the relation to be constant for
bandaveragedampscalarmorethan3.8. All VGPScon-
Fig. 8.|ThecalibratingrelationfortheVGPS(cid:13)uxcorrection.
The (cid:13)ux ratio (NVSS/VGPS) is shown as a function of band av- tinuum mosaics were made again with this correctionto
eraged ampscalar, which is proportional to Tsys. The solid curve the(cid:13)uxscale. Thesenewmosaicsweresearchedforcom-
representsthe(cid:12)tthatisusedto(cid:12)ndtheappropriatescalefactorfor pact continuum sources and the (cid:13)uxes of these sources
individual snapshots. After this correction, the (cid:13)uxes of compact
were compared with (cid:13)uxes listed in the NVSS catalog.
continuum sources inallVGPS continuum mosaicswere foundto
be consistent with the NVSS to within 5%. A detailed compari- This comparison included many faint sources that were
sonoftheHispectrallineimageswiththeCGPSandSGPS(this notincludedinthederivationofthe(cid:13)uxcorrection. The
paper)furthercon(cid:12)rmstheintegrityofthe(cid:13)uxscaleafterthiscor-
(cid:13)ux scale of the VGPS mosaics was found to be consis-
rection. The smallercorrection forhigher system temperatures is
notunderstood. tent with the NVSS (cid:13)ux scale within 5%.
Thesuccessofapplyingthewell-de(cid:12)nedrelationinFig-
(cid:13)uxes of continuum sources in each CGPS (cid:12)eld as part ure 8 adds con(cid:12)dence to the initial assumption that the
of the (cid:12)eldregistrationprocessdescribedinTayloretal. di(cid:11)erenceinthe(cid:13)uxscalebetweenVGPSandNVSScon-
(2003). A direct comparison of source (cid:13)uxes per VGPS tinuumimageswasrelatedtothehighersystemtempera-
(cid:12)eld is not possible because, on average, only four com- tureintheVGPS.However,thenegativeslopeinthisre-
pact continuum sources are available per VGPS (cid:12)eld. lationimpliesthatthelargestcorrectiontothe(cid:13)uxscale
Bright extended continuum emission from Hii regions is necessaryfor VGPS snapshots with the lowest system
andsupernovaremnantsalso inhibits acomparisonwith temperature. Thisresultremainsunexplained. However,
NVSSsources. Asubsetofthe VGPS(cid:12)eldscontainsone we note that increased contributions to the system tem-
ormoreisolatedcompactcontinuumsourcesthatcanbe perature by bright continuum emission and by spillover
compared with corresponding entries in the NVSS cata- to the ground at smaller Galactic longitudes must also
log. We obtain an empirical relation between the NVSS haveoccurredto someextent in the NVSS observations.
to VGPS (cid:13)ux ratio and the system temperature, includ- The response of the VLA to increased system tem-
ing all contributions, as measured by the band-averaged perature has been tested by following a (cid:13)ux calibrator
ampscalar. Thisrelationisusedtopredictaspeci(cid:12)ccor- during a time span of several hours as it approaches the
rection factor for each snapshot. horizon. These experiments indicate that the raw cor-
Tothisend,eachcontinuumsnapshotwasimagedand relator output is adequately corrected for the scaling of
cleaned individually. Cleaning VLA snapshot images is the signal by the AGC (R. A. Perley, private commu-
di(cid:14)cult because of the highlevel of sidelobes of the syn- nication). Calibrators observed for the VGPS are not
thesized beam. This is a particular concern for faint well suited to repeat this experiment because they cover
sources. Therefore, only sources with a (cid:13)ux density a very limited range in elevation. In particular, the pri-
larger than 100 mJy were considered. Sources identi- mary (cid:13)ux calibrators were consistently observed at high
(cid:12)ed in the snapshot images were matched with sources elevation. With limited signi(cid:12)cance, we con(cid:12)rmed that
in the NVSS catalog. A source was accepted if its po- the raw correlator output for the secondary calibrators
sition was within 1500 of the NVSS position, and if the did not change with elevation. Our analysis shows that
deconvolved size of the source in the NVSS catalog was a calibrator must be followed to very low elevations to
less than 6000. These requirements are aimed to avoid probe the system temperature regime of VGPS observa-
misidenti(cid:12)cation while retaining a su(cid:14)cient number of tions. ThebehaviorofampscalaratTb =0inFigure6is
sources to determine a relationship between the band representativeforacalibrator,becausetheTb(v)termin
averagedampscalarand the (cid:13)ux ratio NVSS/VGPS. Equation(1)isverysmallforacalibrator. Acomparison
The snapshot images were sorted into narrow ranges ofthe twopanelsofFigure6showsthatthesystemtem-
of the band averagedampscalar. Foreachnarrowrange, perature in VGPS target (cid:12)elds almost always exceeds
The VLA Galactic Plane Survey 9
the system temperature of a calibrator at an elevation better than a (cid:12)rst order polynomial (cid:12)t. Gridding of the
(cid:14)
of only 25 . However, observations of a calibrator as it data was done with the task IMAGEMS, using a BOX
(cid:14)
sets to elevationsaslow as10 (Taylor,Ulvestad & Per- griddingfunction. Thelatitudeandlongitudesizeofeach
0
ley 2003) cover the system temperature range of most cell was set to 3, equal to the longitude spacing of the
VGPS observations. observations. Each of the small Hi maps had a narrow
The relation in Figure 8 compares (cid:13)ux measurements overlap in longitude with the adjacent maps. After ap-
of fairly bright, relatively isolated, compact continuum plying the absolute brightness temperature calibration,
sources. These(cid:13)uxmeasurementsshouldnotbea(cid:11)ected theoverlapregionswerecomparedforconsistency. Ama-
by modest di(cid:11)erences in the uv coverage between the jorityoftheoverlapregionsshowedsmalldiscrepanciesof
VGPS and the NVSS. Since ampscalar values do not order1(cid:0)3K,butafewregionswerefoundtohavebright-
changeif the minimum baselineistakenaslongas1k(cid:21), nesstemperaturedi(cid:11)erencesofupto6(cid:0)8K.Theoverlap
the ampscalar values are also not a(cid:11)ected by di(cid:11)erences regions were used to scale the cubes observed on di(cid:11)er-
in uv coverage. Therefore, we believe that the behavior entdaysto acommoncalibration,which is consistentto
in Figure 8 is not related to di(cid:11)erences in uv coverage within3%. As asecondcheckofthe observedbrightness
between the VGPS and the NVSS. temperature consistency, a small portion of the galactic
Insummary,the VGPScalibrationfollowsthesesteps: plane (65(cid:14) (cid:20) l (cid:20)67(cid:14); (cid:0)1:3(cid:14) (cid:20) b (cid:20) +1:3(cid:14)) was observed
1. StandardgainandphasecalibrationwasdoneinAIPS twice, once during the 2002 November 21(cid:0)25 observing
for all snapshots. The gain and phase corrections were sessionandagainduringthe2003August29(cid:0)30session.
applied to the data before importing the visibilities into Only small di(cid:11)erences of a few Kelvin were found be-
MIRIAD for imaging. tween these maps.
2. Visibilitieswerechannel-averaged(continuumimages)
2.4. Imaging
orcontinuum-subtracted(spectrallineimages)asappro-
priate. The de(cid:12)nition of VGPS mosaic images is a direct ex-
3. The gain for each snapshot was adjusted by a single tension of the set of mosaic images of the CGPS. VGPS
factorderivedfromitsbandaveragedampscalarandthe imagesaresampled in position and velocityon thesame
relation shown in Figure 8. A small number of (cid:12)elds grid as CGPS images. Mosaics of 1024(cid:2) 1024 pixels
was selected for self-calibrationafter construction of the (5(cid:14):12 (cid:2) 5(cid:14):12) are centered at intervals of 4(cid:14) in longi-
mosaics (Section 2.4.1). tude. This provides signi(cid:12)cant overlap between mosaics
for coverage of objects near the mosaic boundary. The
2.3. GBT observations
velocityaxisofVGPSspectrallinecubesissampledwith
Short-spacinginformationfortheHilineemissionwas thesamechannelde(cid:12)nitionsasthe spectrallinecubesof
obtained using the Green Bank Telescope (GBT) of the the CGPS, but the velocity range covered by the VGPS
National Radio Astronomy Observatory (NRAO). The is di(cid:11)erent. There are three VGPS data products as de-
GBTisa100mdishwithano(cid:11)-axisfeedarmforanun- scribed below: continuum images which include short-
blockedaperturewhichreducestheradiosidelobes,radio spacingdata,continuum-subtractedHispectralline im-
frequency interference, spectral standing waves, and the ages which include short-spacing data, and continuum-
e(cid:11)ects of strayradiation. The extent of the GBT survey includedspectrallineimageswhichdonotincludeshort-
varies with longitude, covering jbj (cid:20) 1(cid:14):3 for longitudes spacing data. All VGPS data products will be made
18(cid:14) (cid:20)l(cid:20)45(cid:14) andjbj(cid:20)2(cid:14):3forlongitudes45(cid:14) (cid:20)l(cid:20)67(cid:14). availableonthe WorldWideWebthroughtheCanadian
Observations of these regions began on 2002 November Astronomy Data Centre (CADC).
21(cid:0)25 and continued on 2003 March 6(cid:0)9, 2003 May
2.4.1. Continuum
26(cid:0)27, and 2003 August 29(cid:0)30. The observing strat-
egy was to make small, (cid:1)l = 2(cid:14)(cid:0)5(cid:14), Hi maps ‘on the VGPSlineorcontinuumimagesaremosaicsof several
(cid:13)y.’ Using this technique, the telescope was driven at a VLApointings. As itisimpracticaltoprocesstheentire
(cid:14)
rate of 3 per minute with a sample written every sec- survey area at the same time, only (cid:12)elds with their cen-
(cid:14)
ond. After driving through the full latitude range, the trallongitudeswithin4 ofthecenterofthemosaicwere
0
telescope was stepped 3 in longitude. This process was included in the construction of each mosaic. Each VLA
continued until the particular map was completed. The (cid:12)eld was imaged to the 10% level of the primary beam,
data were taken in frequency-switching mode using the and (cid:12)eld images were combined with the VLA primary
GBTspectralprocessorwithatotalbandwidthof5MHz beam as weighting function. The primary beam model
across 1024 channels. The resulting channel spacing is fortheVLAisthesameasusedintheAIPStaskLTESS.
1:03 km s(cid:0)1 and the spectral resolution is 1:25 km s(cid:0)1 Figure 9 shows the sampling in the uv plane and the
(FWHM).Dataweretakenbyin-bandfrequencyswitch- synthesizedbeam for a representative(cid:12)eld near the cen-
ing yielding a total velocity coverageof 530 km s(cid:0)1 cen- ter of the survey, composed of snapshots taken at three
tered at +50km s(cid:0)1 LSR. IAU standardregionsS6 and di(cid:11)erent hour angles. A Gaussian weighting function in
S8 (Williams 1973) were observed and used for absolute the uv plane was applied to obtain a 6000 (FWHM) syn-
brightness temperature calibration. The angular resolu- thesizedbeamthroughoutthesurveyarea. Thestrongest
0
tionof the GBTdatais(cid:24)9. The(cid:12)nalGBTHispectra sidelobesofthe synthesizedbeamarespokesrunningra-
have an rms noise of (cid:24)0:3 K in emission-free channels. dially outward from the main lobe in several position
Imaging and data calibration were done with the angles. The pattern of the spokes depends on the hour
AIPS++ data reduction package. A (cid:12)rst order polyno- anglesatwhichthe(cid:12)eldwasobserved. Thepatternisdif-
mialwas(cid:12)ttedtotheo(cid:11)-linechannelstoremoveresidual ferentforeach(cid:12)eld, but(cid:12)elds thatwereobservedwithin
instrumentalbaselinestructure. Polynomial(cid:12)tsofhigher the same sequence of six (cid:12)elds (Section 2.1), have simi-
orderwerealsoattempted,butthesewerefoundtobeno lar but slightly rotated patterns. The amplitude of the
10 Stil et al.
Fig. 9.| Sampling of the uv plane and the synthesized beam for a representative (cid:12)eld in the center of the survey area. This (cid:12)eld is
centered on (l;b) =(42(cid:14):5;(cid:0)0(cid:14):1). It was observed three times at hour angles (cid:0)2h:7, (cid:0)0h:2, and +2h:7. Left: sampling function of the uv
plane. Right: the corresponding synthesized beam, with peak amplitude equal to 1. Gray scales increase linearly from (cid:0)0:05 to 0:30 in
stepsof0:05.
spokes in the dirty beam is typically 10% of the main from this visit were excluded from the initial imaging
lobe, but peaks up to 17% are usually present. The step. The deconvolutionof this small mosaicproduces a
VGPS images must be deconvolved to remove artifacts skymodelof\cleancomponents",whichisusedforphase
resulting from these sidelobes. Emission in the Galactic self-calibration on the central (cid:12)eld only, after multipli-
plane usually (cid:12)lls the (cid:12)eld of view. In this case, decon- cation with the primary beam model of the VLA. The
volution is preferred after mosaicking the (cid:12)elds. smallmosaicisthenmadewiththe improvedcalibration
Continuum images were constructed from visibility solution for the central (cid:12)eld. If any data were left out
data averaged over channels without discernible line of the (cid:12)rst imaging step, those data were included after
emission, but avoiding noisy channels near the edges of the (cid:12)rst round of self calibration. If necessary, adjacent
the frequency band. Deconvolution of the VGPS images (cid:12)eldsareself-calibratedsubsequently. Thecentral(cid:12)eldis
was done with the MIRIAD program MOSMEM which usuallyself-calibratedasecondtimeafterself-calibration
uses the maximum entropy method described by Corn- of the surrounding(cid:12)elds. After self-calibration,the (cid:12)nal
well&Evans(1985)andSaultetal.(1996). Experiments VLA continuum mosaic was constructed and regridded
showed that no signi(cid:12)cant improvement was made after to Galactic coordinates.
about 20 iterations of the algorithm, even if no formal The results of the self-calibration are usually satisfac-
convergence could be reached. A maximum of 50 itera- tory. A dynamic range of (cid:24)200 was obtained in the
tionswasadoptedinthedeconvolutionofthecontinuum continuum images after self calibration on sources that
mosaics. The criteria that de(cid:12)ne the convergence of the mostlya(cid:11)ectasingle(cid:12)eld. However,self-calibrationdoes
deconvolution,theentropyfunctionandthe(cid:31)2criterium, not provide a good solution if the source is too far from
include a summation over the entire image. Therefore, the (cid:12)eld center. The brightest continuum sources in the
theresultofthedeconvolutiondependssomewhatonthe VGPS, in particular W49 and W51, generate artifacts
area that was imaged in the construction of the mosaic. even at large distances from the (cid:12)eld center. These ar-
This causes small di(cid:11)erences between neighboring mo- tifactsremaininthe continuum images,mostlya(cid:11)ecting
saicsintheareawheretheyoverlap,withrmsamplitude (cid:12)elds surrounding these sources.
at or below the level of the noise. TheVLAisnotsensitivetoemissiononangularscales
0
Standard calibration alone results in images with a larger than (cid:24) 30 because structures on these scales are
maximal dynamic range of (cid:24)100, which is not su(cid:14)cient resolved even by the shortest projected baselines. The
to image bright continuum sources in the survey area missing continuum short-spacing information was pro-
without discernible image artifacts. The continuum mo- vided by the continuum survey of Reich & Reich (1986)
saics were inspected for residual sidelobes around bright and Reich et al. (1990) with the 100 m E(cid:11)elsberg tele-
sourcesafterthedeconvolutionstep. Self-calibrationwas scope. ThedeconvolvedVLAmosaicwascombinedwith
attemptedforselected(cid:12)eldswithstrongartifacts,begin- the E(cid:11)elsberg image with the MIRIAD program IM-
ning with the (cid:12)eld in which the source is located closest MERGE. The interferometer and single-dish images are
to the (cid:12)eld center. A small mosaic, typically including bothFouriertransformedandaddedintheuvplanewith
(cid:14)
all(cid:12)eldswithin(cid:24)1 ofthe source,wasmadeanddecon- baseline-dependentweightssuchthatthesumrepresents
volved. Ifaparticularvisittothe(cid:12)eldcouldbeidenti(cid:12)ed the visibilities for all angular scales weighted by a single
0
as the prime origin of the artifacts, the visibility data Gaussian weighting function de(cid:12)ned by the 1 synthe-