LAS Science Verification

1. People involved

1. Gavin Dalton
2. Jon Davies
3. Simon Dye
4. Simon Hodgkin
5. Phil James
6. Richard Jameson
7. Tim Kendall
8. Nicolas Lodieu
9. Jon Loveday
10. Richard Mc Mahon
11. Bob Nichol
12. Steve Phillipps
13. David Pinfield
14. Anthony Smith
15. Rory Smith
16. David Wake
17. Steve Warren
18. Steve Weatherley

2. Summary of LAS SV observations

(Current version 19/07/05 now verified by Simon Dye)

If you want any additional info added, contact Simon Dye or Steve Warren.

The following is a summary of the MSBs taken over the science verification phase for the LAS. All observations are LAS standard 40s in any band. Note that the dates of the MSBs are relevant. Currently it is planned to release the SV data in two phases. SV1 is all the data up to and including 18/4, and SV2 is the remainder.

The basic summary is that a total of 17 brown dwarfs, 4 cool white dwarfs, and 10 z>5.7 quasars were observed in YJHK. Nearly all of these were observed in a single pawprint, but in a small number of cases a tile was taken. In addition a set of 8 YJHK tiles in Virgo were taken, and a set of 8 tiles in HK only of the Millennium Galaxy Catalogue strip were taken (coords listed below).

Obs type: T=tile, P=pawprint

3. Lists of LAS SV tasks

Recall the list of SV tasks from the Minutes of SciVer meeting at Imperial College on Apr.01.2005. The ones specific to the LAS have been stripped out, and these are the ones on which we should present analysis at the two planned SV meetings (the first is now fixed for Aug 18 in Edinburgh, 9.30 to 4.30).

Please volunteer by putting your name against topics.

Technical topics

• Sensitivity: NL, RJ, RGM
• Sky brightness and sky limited:
• Photometry
  • overlap comparison: SW1, RGM
  • absolute: NL, RJ
• Astrometry
  • internal: NL, RJ
  • overlap comparison: NL, RJ
• Star/galaxy separation: SW1, RGM
• Large objects:
• Galaxy photometry: AS, JL
• Persistence:
• Cross talk:
• Bad pixels: SW1
• Image parameters: NL, RJ, SW1
• Dropouts: SW2, RGM

Science topics

• Star counts:
• Galaxy counts: AS, JL
• Source luminosity functions: AS, JL
• SDSS match/comparison: NL, RJ, SW2, AS, JL, RGM
• 2MASS match/comparison: AS, JL
• Synthetic colours: NL, RJ, SW1
• Photometric redshifts: AS, JL

4. SV reports, findings, warnings etc. (add your own)

4.1 SV report by Simon Dye

The latest version (currently 04/11/05) of the science verification report by Simon Dye can be found here.

This report details analysis of mainly LAS science verification data taken during April 2005. The analysis includes direct characterisation of the data, comparisons between manually reduced and pipeline reduced images and verification of WFCAM Science Archive photometry and astrometry. Problems identified in the commissioning data of November and December 2004 are re-tested in this new dataset.

4.2 Standards fields in the LAS SV1+2 catalogues, and M51 publicity

By a quirk of how the MSBs were set up for the LAS SV process a whole lot of observations of standards ended up in the LAS catalogues. You probably want to avoid these for most SV purposes because they don't go as deep, and because many don't have complementary SDSS data. Below is a list of the coordinates of the standard stars that remain in the full SV catalogues. (Most have now been removed from the merged catalogue.) These are not the same as the field centres (i.e. the standard appeared somewhere on one of the arrays), but by suitable choice of box size (see standards.sql) you should be able to excise all the objects in the LAS catalogue that are in fields of standards, without removing any of the LAS SV data (take a look at the WSA sky distribution plot of the fields).

In lasDetection you will also find some JHK (no Y) data for a field near 13h30m +47. These are the images taken for the colour picture of M51 used in the recent UKIDSS press release.

-- SteveWarren - 14 Jul 2005
-- AnthonySmith - 23 Nov 2005

4.3 Matching to SDSS DR4

One of the useful things to do is to match LAS data to SDSS. Besides the standards fields, above, most of which are not within the SDSS DR4 footprint, two of the SV targets also are not in DR4. These are:

• SDSS J102109.69-030420.1 (brown dwarf) - in SV1
• J104433.04-012502.2 (high-redshift quasar) - in SV2

-- SteveWarren - 14 Jul 2005

I have looked into the SV observations of the 17 listed LT dwarfs of which only 12 are available through the archive at present. The cross-correlation with SDSS DR4 is attached, as is a plot of the 12 in Y-J,J-H.

I have also done a cross-correlation with 2MASS for the LAS, similar although not yet as detailed as the one I did for GCS. Compare the plots jdiff.gif, hdiff.gif and kdiff.gif on the GCS Wiki. The methodology here is the same: 2MASS points with anything flagged as an upper limit are removed. The subset plotted has the 2MASS error < 0.1 mag and excludes any noise points in the UKIDSS data (Merged Class!=0). Plots are all UKIDSS mag. vs. (UKIDSS-2MASS).There are 27864 points matched. It seems to me like there is extra scatter in J, not present in H and K. This makes the plot for J look much more ragged than the GCS equivalent. The numbers are: mean differences are -0.043+/-0.301 for J (a larger s.d. than GCS); 0.138+/-0.188 for H (similar) and 0.050+/-0.226 for K (also similar). Compare GCS numbers in stats.dat for ALL.

-- TrKendall - 08 Aug 2005

I have cross-correlated one WFCAM field-of-view with the SDSS DR4 release. This field was randomly chosen and is located at RA=10h48, dec=+45d. Figure las_sdss_astrometry.ps displays the number of stars per 0.05 arcsec bin corresponding to the difference in RA and dec between the LAS and SDSS DR4 astrometry. The left and right hand side plots show the RA and dec, respectively. This plot was made for all sources, not only for point-source but the result holds for point-sources. Three peaks appear especially in the RA plot. I am not quite sure about the reason. Can someone confirm this with this field or another one? It seems nevertheless that the astrometry of the LAS and the one from SDSS DR4 agree quite well.

- NicolasLodieu - 17 Aug 2005

The Virgo strip is in DR4 but not DR2 (no spectra in DR4). Matching the lasSource SV1 data to DR4 galaxies, excluding spurious regions, gave a total of 1662 matches within 2". Matching the same lasSource data to the entire DR4 spectroscopic catalogue gave 970 matches within 2".

-- AnthonySmith - 15 Sep 2005

4.4 Photometry: observed vs. models

I have plotted several colour-colour diagrams with observed colours from the UKIDSS LAS Science Verification phase and compared them to model predictions (Hewett et al. 2005, in prep.).

Symbols in all diagrams have the same meaning. Open symbols are known L & T dwarfs (red), and quasars (green) as observed with WFCAM during the Science Verification Phase. Field dots represents the models predictions: see Tables given in Hewett et al. (2005, in prep.). Blue symbols are for Helium & Hydrogen WDs, red for L & T dwarfs, purple for Y dwarfs, and green for high-redshift quasars. Black lines are tracks for M dwarfs. Note that not all tables are included to get the plots readable.

The small black dots are point-sources in one WFCAM tile (randomly chosen) which satisfy the following criteria: yClass=jClass=hClass=kClass=-1

The following colour-colour diagrams are attached:

• ccd_las_jkyj.ps: (J-K,Y-J) diag for observed and synthetic colours

• ccd_las_jhyj.ps: (J-H,Y-J) diag for observed and synthetic colours

• ccd_las_hkjh.ps: (H-K,J-H) diag for observed and synthetic colours

Main results:

The models predictions agree well with the observed colours for L & T dwarfs and quasars. The WDs observed during the SV phase do not have measured magnitudes in all bands. Thus, we can not plot those objects in the colour-colour diagrams.

-- NicolasLodieu - 17 Aug 2005

4.5 Report on LAS SV2 (Nicolas Lodieu; 13 Dec 2005)

I have attached a report on LAS SV2 (pdf file) where I investigated the following issues:

• Magnitudes of known L/T dwarfs: comparison between SV2 and 2MASS
• Distribution of stars and galaxies in various colour-colur diagrams
• Cross-correlation with SDSS

I would like to emphasize the following point: plotting any cclour-colour with only point-sources revealed the presence of spurious detections at the top left and botton right of the diagrams. This problem do not seem to occur in all WFCAM tiles though. I have checked some of those objects and there appear to be nothing at those positions (see attached finding charts in my report).

-- NicolasLodieu - 13 Dec 2005

4.6 SQL Query to get extract metadata from LasSource input files(Richard McMahon; 16-Jan-2006)

The SQL query

• r2_dqc_las_20060113.sql: SQL to get the DQC metadata

Notes on the output:

• 461 rows are returned; this implies that there are some framesets with less than 4 extensions;

• PhotZPerr are all nulls;

• Some Y frames have large numbers(>10 times the the norm) of sources and seeing=0;

• Pixel Scale is always 0.4arcs

• xPixSize and yPixSize takes into account interleaving and has range
  1. 2 to 0.4 [this can cause confusion]

• There are a mixture of interleaved and non-interleaved images in the Y band. Thus the Pixel Scales are always 0.4 but the PixSize can vary between 0.2 and 0.4. [see confusion mentioned in (iv)]

-- RichardMcMahon - 16 Jan 2006

4.7 WFCAM filters with kcorrect

I have attached a .tar file containing filter curves for use with kcorrect.

-- AnthonySmith - 25 Jan 2006

4.8 Report on late T- and Y-dwarf searches from the LAS EDR

Part 1: K-band detections

This is the first of a series of reports on my findings from the LAS EDR. This concerns T- and Y-dwarf candidates with K-band detections in the LAS. They were sought as conforming to the following criteria: mergedClass = -1 or -2, pStar > 0.8, Y-J (AperMag3 ) > 0.7, J-H (AperMag3 ) < 0.5, with all four bands defined ( mag > -100). This gave 615 rows with errors on YJH in general < 0.15 mag (maximum 0.25) and on K < 0.2 mag (maximum 0.35).

Candidates were investigated from the Sloan DR4, requiring errors on z' < 0.5 mag. psfMag was used throughout. Initially, 477 were found in DR4 with a primary match within 1.2". These were searched for any object with z'-J > 2.8, typical for a mid-L dwarf or later (Hawley et al. 2002). Only 2 points conformed, neither to an additional extra constraint that i'-z' > 2 (~L3). These are detailed here:

• kdet_2primary.html: Two primary detections z'-J > 2.8

LAS and DR4 J-band and z-band cutouts are available below (N up E left) with the LAS (green) and Sloan (blue) positions marked by 5" circles. Further investigation reveals that the brighter object (at left) is listed in 2MASS with J = 16.33, cf. LAS J = 14.26, so the extreme z'-J colour is probably spurious. The fainter, second object listed (right) suffers from bad pixels at J in the LAS, and this may have affected the J-band magnitude. The Sloan detection seems secure, but there is no observable proper motion between the LAS and DR4 positions, and this source is too bright to be a distant T-dwarf.

To allow for proper motion, a further Sloan search was performed to find all nearby objects within a 0.3' radius, from which any LAS detection with a primary Sloan detection was excluded. These are therefore secondary Sloan detections; six for four LAS detections. Thus the total number of LAS sources with any Sloan counterpart is 481. Inspection of the LAS J-band images shows they are good detections in this band. However none have z'-J or i'-z' colours (last two columns) conforming to the above colour criteria:

• kdet_4_secondary.html: Secondary Sloan detections to 4 LAS sources

For an explanation of Sloan primary and secondary sources see here.

Finally 118 other LAS sources were found in the DR4 footprint, and only 16 outside. 69 were bright galaxies which had been excluded from the initial primary search on the basis of the Sloan STAR/GALAXY separator, and brightness. 49 remained potentially bona-fide Sloan non-detections (dropouts). Hence all 615 = 481+69+49+16 LAS sources have been investigated. However all but four of the 49 were UKIDSS cross-talk artifacts. Sloan z' and LAS J-band images are shown below, with the UKIDSS positions marked with 5" circles. N is up, E left and the LAS images are 1' square. Sources are labelled 1 2 3 4 in RA order and as in the table below:

• kdet_4_fp.html: LAS EDR data for 4 potential Sloan dropouts with K-band detections

In fact, sources 1 2 and 4 did have some Sloan information, but this is absent from the photometric catalogue as flagged (i.e. not a primary or secondary detection. 1 and 2 are probably faint galaxies with i' = 20.27 and 25.57 (i.e. an i'-band dropout), z' = 19.84 and 20.12 respectively. 1 appears to be detected in the LAS but not in Sloan, 2 does not appear to be a real source in either survey. The status of 4 is uncertain whether both the LAS and Sloan detection corresponds to the bright object within the 5" circle. Sloan cross-correlates this object with an USNO star with R = 20.03 and a proper motion of 0.961"/yr. Sloan itself gives i' = 19.34 and z' = 19.19 and with J = 17.43 (if real) the object is not especially red. The USNO identification though is contradicted by the lack of motion between Sloan and UKIDSS images (Sloan epoch 1999/22/03). Finally, object 3 in this set appears not to be a real detection in the LAS EDR. There is no Sloan data.

To conclude, there is no good evidence for any good very cool brown dwarf candidates in this dataset.

Part 2: K-band non-detections

679 points were isolated using similar SQL as above, except of course K < -100 (undefined) while YJH > -100 (defined). pStar is again > 0.8. Errors on YJH (AperMag3 ) are typically 0.2, 0.15 and 0.25 mag with a few points ranging up to 0.5, 0.3 and 0.45 mag, respectively. Cross-correlation with Sloan (psfMag_z < 0.5) for a radius 0.3' (all nearby objects) yielded 3529 matches of which 2029 were Sloan STARs. Matching these 2029 with the 679 LAS sources yielded 2060 possible pairs, over twice as many as for the LAS K-detections. A further search, as above, for primary Sloan counterparts (including galaxies) within 1.2" yielded 644 possible matches representing 644 LAS sources. None of the primary matches conformed to the z'-J > 2.8 criterion. Further cross-correlation revealed 25 LAS sources (with a total of 53 possible Sloan counterparts) within a 0.3' radius which had no primary match. The remaining 10 LAS sources had no Sloan catalogue match. Thus all 679 = 644 + 25 + 10 LAS sources are accounted for.

The 25 LAS points with no primary match were then investigated in order to check for possible counterparts separated by more than 1.2" and up to 0.3' by proper motion. Only 2 of the possible matches had z'-J > 2.8. One turned out to be a LAS cross-talk artifact. The other, with z'-J = 2.89 is the third of four possible matches in the table below. The image below shows the J-band 1' cutout (left) and Sloan z'-band (right) for the region. N is up and E left. The possible Sloan counterparts (blue) to the UKIDSS (green) object are shown. The position of the red counterpart is the large blue circle. However it is clear that this is not a real candidate as there seems to be a faint Sloan detection at the LAS position, and indeed a faint LAS detection at the Sloan position. Hence the LAS and Sloan points are 2 different objects. Also, there are 2 Sloan detections at the relevant position (2 observations of the same object) only one of which is red. Yet, the possibility of detecting wide proper motion counterparts is illustrated.

• knon_1_from_53.html: One LAS K-band non detection with a possible z'-J > 2.8 counterpart

Ten LAS sources remain with no primary or secondary Sloan counterpart. Visual inspection of Sloan images via Sloan "Visual Tools" was used to find counterparts absent from the photometric catalogue. After removal of LAS cross-talk artifacts, one object remained of interest:

• final_candidate.html: A possible K non-detection Sloan dropout

This is a good detection at YJH (see 1' LAS images below, where the UKIDSS (green) and Sloan (blue) positions are indicated), and is an i'-band dropout in the catalogue, with a z'-magnitude indicating an extremely red z'-J colour. However it is clear from the Sloan images (below, conventionally oriented, Sloan position marked by the blue 5" circle) that there is no actual detection at i' or z' or any of the other bands. Thus the object is a genuine Sloan dropout, and not seen at K, as expected for a faint, cool brown dwarf. However, the Y-J and J-H colours are not unusual, and this object is perhaps a very faint M-dwarf interloper. One further such object was found mismatched to a bright Sloan star:

• zindef.html: Another Sloan dropout, originally mismatched

Again, the YJH colours are possibly those of an M-dwarf interloper. We conclude that there is not yet compelling evidence for any late T- or Y-dwarf discovery in the LAS EDR, but follow-up (perhaps methane on/off imaging) could reveal these last menationed i'z'K non-detections as at least late T-dwarfs.

-- TimKendall - 13 Feb 2006

4.9 Early T and L dwarf candidates in the UKIDSS LAS EDR

Here I report on the identification of earlier type dwarf candidates in the LAS EDR. In a search designed to yield L- and early T- candidates, criteria used were Y-J in the range 0.9 to 1.7, J-H in the range 0.5 to 1.5 (a separate, bounding region to the earlier T and Y searches) and H-K in the range 0.0 to 1.3. All objects were required to be YJHK detections with pStar > 0.9 and mergedClass = -1. 114 candidates were found and inspected on UKIDSS images. There are no cross-talk artifacts.

Sloan DR4 cross-correlation was performed as above with a 1.2" search for primary objects followed by a 0.3' radius search for secondary objects containing possible proper motion separated counterparts. 88 unique primary counterparts were found of which 30 conformed to a condition Z'-J > 2 and I'-z' > 1.5, corresponding to an M8 dwarf. All Sloan magnitudes are psfMag and for this sample of 30, errors on z' are < 0.3 and on i' < 0.35. Details of these objects are given below:

• primary_30.html: LAS-selected candidates with Sloan primary counterparts

z'-J for these candidates ranges up to ~3 equivalent to near the L/T boundary. Below in the Table is a plot of z' vs. z'-J for the 30 candidates (blue) compared to the whole set of primary Sloan matches. It can be seen that the candidates form a well-defined, separate population.

A small number of further candidates were revealed by the wide, 0.3' search, which yielded a total of 656 possible matches in the DR4 catalogue. After removal of matches with the 30 LAS sources already listed, 479 matches remained representing 81 further LAS points. 468 of these had all Sloan magnitudes defined (the others were all magnitudes undefined, there were no cases of defined z' and an i'-band dropout) and of these, 20 conformed to the "ultracool" optical/IR colour criteria defined above. However 12 already had a primary match in the sample of 88 Sloan counterparts, which had failed the selection criteria. These were rejected, leaving 8 possible matches to 6 LAS sources:

• secondary6.html: Six further possible ultracool candidates

Conventionally oriented 1' J-band LAS cutouts for these six are given in the Table, together with the Sloan z' band ones. LAS positions are marked by green 5" circles, Sloan, blue. Inspection reveals that for three candidates there are probably 2 sources visible, but two remain candidates (where Sloan and LAS positions coincide). From their optical/IR colours they are likely distant early L-dwarfs.

Three sources remained in the DR4 footprint but with no catalogue counterpart at all:

• 3_nodr4.html: Sloan dropout candidates

LAS J-band 1' cutouts and the equivalent Sloan z'-band images can be seen in the last 2 entries (currently) of the Table. Two of the Sloan images may show a very faint detection at z', although one might be a pair of hot pixels. If one plots the Sloan psfMag_z against psfMagErr_z for a large number of points it can be seen that an error on z ~ 0.4 mag corresponds to z' ~ 21.5. If this is taken as an approximate limit for a real z'-band detection, then the three dropouts here have z'-J ~ 3, 2.8 and 2.3 respectively, corresponding to T0, L6 and L0.

In summary, there are 38 ultracool dwarf candidates identified here and worthy of follow-up. LAS J-band finders are available from the Table.

-- TimKendall - 15 Feb 2006

5.0 Mid-T dwarf candidates in the UKIDSS LAS EDR

I have identified seven mid-T candidates in the EDR. Finders in the J-band and i' zYJH cutouts can be found on this page. Details are in the HTML below. The candidates are probable Sloan dropouts, with no Sloan catalogue counterpart, as can be seen in the cutouts, where the green (LAS) position is shown together with any Sloan DR4 detection within 0.3' (blue). All are LAS K-band non-detections. I have checked the LAS YJH zeropoints from the fits image headers, none are discrepant (typically zpY = 22.7, zpH = 24.5 and zpJ = 24.7). I have also confirmed ellipticities from the LAS merged catalogue which are maximum ~ 0.3 at Y and typically ~0.15 - 0.2 at JH. The seventh object is only in the EDRPLUS database. All are very faint (H~19) with Y-J and J-H colours > 0.9 (in most cases > 1) and J-H < 0.5, making them likely mid-T dwarfs, with one exception (#4) which is the reddest in both these colours and might be more likely a late-L or early T-dwarf. Adopting z'~ 21.5 as the Sloan limit (where errors on the z'-magnitude of Sloan detections are typically +/-0.3) then lower limits on z'-J are in the range 2.1 - 2.9. Errors on YJH are given in the HTML. These sources could be confirmed by methane imaging, and supersede those in Sections 4.8 above.

• t_finders.tar: 4' finders for T candidates in J-band

• seven.html: Seven T-dwarf candidates

-- TimKendall - 9 Mar 2006

5.1 Some follow-up results for LAS candidates

I have attached some of the more prominent new results from various spectroscopic and methane imaging follow-up efforts thus far. They can be found attached below:

1) subaru_tdwarf.gif: Confirmation of the first LAS T dwarf from Motohide Tamura and collaborators at Subaru. This is the centerpiece result so far. The spectrum is near T4. With J=18.7 and adopting M_J = 14.5 from Knapp et al. (2004) the distance is 70 pc. If M_J = 15.4 (Hawley et al. 2002) then 45 pc. In either case we have proved we can detect T dwarfs out to the expected distance for the LAS. This is the candidate from seven.html at 223.1816078, 6.928596.

Of the others from seven.html, 195.472, 0.345 and 196.322, 0.234 have been ruled out by Chris Tinney using methane imaging at IRIS2/AAT. 230.948, 5.480 was observed at Subaru and seems to be an interloper. Poor seeing and high humidity ruled out observations of 223.408, 6.265 at Subaru; this and the remaining two remain unobserved.

2) L0.eps: TNG/DOLORES spectrum of the primary_30.html candidate at 232.7870253, 6.0197558 from Eduardo Martin. The green line is the best fit comparison indicating L0. This agrees nicely with the Sloan colours i'-z' = 1.95 and z' - J = 2.44 which would point to L1. This is a bright object Y=16.83, J=15.81.

3) subaru_2.gif: Subaru spectrum of faint primary_30.html candidate at 190.6559508, 0.1069014. This was also observed by Eduardo at WHT/LIRIS; that spectum looked to be mid to late L but this spectrum seems to be late M or perhaps early L.

4) ch4_aat.ps: Methane imaging results from Chris Tinney at AAT/IRIS2 for the T dwarf. The methane detection is < 5 sigma, but the UKIDSS J-H colour of 0.21+/-0.17 intersects the methane index prediction (grey band in bottom figure) near T3-T4, providing crucial agreement between narrowband and broadband colours, as seen from the excellent Subaru result.

5) subaru_3.gif: primary_30.html candidate 223.614152 6.868468 (J=18.4) observed at Subaru. Strong FeH at 1.2 micron, probably early L.

6) Finally I note that the following objects from primary30.html have also been observed at TNG by Antonio Magazzu.

• 14 07 54.91 -00 06 49.9
• 14 50 54.71 +06 43 44.6
• 15 31 08.87 +06 01 11.2
• 15 47 22.82 -21 39 14.3
• 13 45 31.43 +00 15 51.2
• 14 07 36.34 -00 08 56.7

-- TimKendall - 21 Jun 2006