README FILE TO ACCOMPANY UDF GRAPES SPECTRA HIGH-LEVEL SCIENCE PRODUCTS ======================================================================= 1. General Information ---------------------- GRAPES is the Grism-ACS Program for Extragalactic Science which was conducted in the Hubble Ultra-Deep Field (HUDF) during HST Cycle 12, over the period 2004 September to 2005 January. The PI is Sangeeta Malhotra (san@stsci.edu), the proposal number is 9793 and 40 orbits were allocated. Slitless spectra in the Ultra-Deep field were obtained with the Advanced Camera for Surveys (ACS) with the Wide Field Camera (WFC) over the field of 204x204 arcsec. The G800L grism provides slitless spectra with a dispersion of about 40A per pixel, but the achieved spectral resolution depends on the size of the dispersing object. For a point source of width 1.5 pixels, the resolution is 130 at 8000A. The grism first order coverage is 5500-10500A with peak sensitivity around 7500A. Four epochs were observed in GRAPES at HST roll angles (PA_V3) of 126, 134, 217 and 231 degrees. Exposures at a roll angle of 117 degrees of the UDF field with the ACS WFC and the G800L grism were also obtained in program 9352 (PI A. Riess) and were included in the data set to be analysed. The total exposure time for the five epochs was 111462s. The strategy was to obtain a direct image in the F606W filter followed by a series of G800L exposures at the same roll angle but with small dithers. Thus at each roll angle there are several sequences of a F606W image and many G800L images. The earlier epoch (2002, October) direct images at roll angle 117 degrees were taken with the F850LP filter. The short direct images facilitated the accurate alignment with the much deeper HUDF images and catalogue. The log of the data taking, divided by epoch, is as follows: Dateset RA Dec Date Exp(s) Filter PA 117 J8G6M1010 03 32 37.53 -27 46 46.5 2002-10-01 2000 F850LP J8G6M1030 03 32 37.52 -27 46 46.5 2002-10-01 2400 G800L J8G6M1050 03 32 37.52 -27 46 46.2 2002-10-01 2400 G800L J8G6I3010 03 32 37.51 -27 46 45.8 2002-10-01 2280 G800L J8G6I3030 03 32 37.47 -27 46 46.8 2002-10-01 2280 G800L J8G6I3050 03 32 37.54 -27 46 47.2 2002-10-01 2280 G800L J8G6K2010 03 32 37.50 -27 46 46.6 2002-10-01 2400 G800L J8G6K2030 03 32 37.53 -27 46 46.8 2002-10-01 2400 G800L J8G6K2050 03 32 37.57 -27 46 46.3 2002-10-01 2400 G800L PA126 J8QQ10IXQ 03 32 39.26 -27 47 29.4 2003-09-22 354 F606W J8QQ10IKQ 03 32 39.26 -27 47 29.4 2003-09-22 1910 G800L J8QQ11JUQ 03 32 39.21 -27 47 29.2 2003-09-22 1132 G800L J8QQ11K0Q 03 32 39.26 -27 47 28.9 2003-09-22 1132 G800L J8QQ12KGQ 03 32 39.30 -27 47 29.6 2003-09-23 1132 G800L J8QQ12KRQ 03 32 39.25 -27 47 29.9 2003-09-23 1132 G800L J8QQ13JMQ 03 32 38.87 -27 47 26.3 2003-09-22 354 F606W J8QQ13JJQ 03 32 38.87 -27 47 26.3 2003-09-22 1910 G800L J8QQ14Q2Q 03 32 38.83 -27 47 26.1 2003-09-23 1132 G800L J8QQ14Q8Q 03 32 38.88 -27 47 25.8 2003-09-23 1132 G800L J8QQ15QOQ 03 32 38.91 -27 47 26.5 2003-09-24 1132 G800L J8QQ15QWQ 03 32 38.86 -27 47 26.8 2003-09-24 1132 G800L J8QQ16H2Q 03 32 39.02 -27 47 27.7 2003-09-26 1132 G800L J8QQ16H9Q 03 32 39.07 -27 47 27.3 2003-09-26 1132 G800L J8QQ17DVQ 03 32 39.10 -27 47 28.0 2003-09-25 1132 G800L J8QQ17E6Q 03 32 39.05 -27 47 28.4 2003-09-25 1132 G800L PA134 J8QQ20YPQ 03 32 39.25 -27 47 29.9 2003-10-01 354 F606W J8QQ20YHQ 03 32 39.25 -27 47 29.9 2003-10-01 1910 G800L J8QQ21ILQ 03 32 39.21 -27 47 29.6 2003-09-29 1132 G800L J8QQ21IQQ 03 32 39.26 -27 47 29.4 2003-09-30 1132 G800L J8QQ22JXQ 03 32 39.29 -27 47 30.1 2003-09-30 1132 G800L J8QQ22KCQ 03 32 39.24 -27 47 30.4 2003-09-30 1132 G800L J8QQ23F1Q 03 32 38.90 -27 47 26.1 2003-10-02 354 F606W J8QQ23EJQ 03 32 38.90 -27 47 26.1 2003-10-02 1910 G800L J8QQ24VJQ 03 32 38.86 -27 47 25.8 2003-10-04 1132 G800L J8QQ24VRQ 03 32 38.91 -27 47 25.6 2003-10-04 1132 G800L J8QQ25F8Q 03 32 38.94 -27 47 26.3 2003-10-02 1132 G800L J8QQ25FCQ 03 32 38.89 -27 47 26.6 2003-10-02 1132 G800L J8QQ26FSQ 03 32 39.04 -27 47 27.7 2003-10-02 1132 G800L J8QQ26G0Q 03 32 39.09 -27 47 27.5 2003-10-02 1132 G800L J8QQ27OEQ 03 32 39.11 -27 47 28.2 2003-10-04 1132 G800L J8QQ27ONQ 03 32 39.06 -27 47 28.5 2003-10-04 1132 G800L PA217 J8QQ30O6Q 03 32 38.97 -27 47 32.5 2003-12-05 354 F606W J8QQ30NVQ 03 32 38.97 -27 47 32.5 2003-12-05 2112 G800L J8QQ31A2Q 03 32 38.99 -27 47 31.9 2003-12-06 1233 G800L J8QQ31A8Q 03 32 39.01 -27 47 32.6 2003-12-06 1233 G800L J8QQ32P0Q 03 32 38.96 -27 47 33.0 2003-12-02 1233 G800L J8QQ32P6Q 03 32 38.93 -27 47 32.4 2003-12-02 1233 G800L J8QQ33MAQ 03 32 39.22 -27 47 27.4 2003-12-02 354 F606W J8QQ33M5Q 03 32 39.22 -27 47 27.4 2003-12-02 2112 G800L J8QQ34PZQ 03 32 39.23 -27 47 26.9 2003-12-02 1233 G800L J8QQ34Q8Q 03 32 39.25 -27 47 27.5 2003-12-02 1233 G800L J8QQ35QPQ 03 32 39.20 -27 47 28.0 2003-12-02 1233 G800L J8QQ35QYQ 03 32 39.18 -27 47 27.3 2003-12-02 1233 G800L J8QQ36RFQ 03 32 39.11 -27 47 29.4 2003-12-02 1233 G800L J8QQ36RPQ 03 32 39.13 -27 47 30.1 2003-12-02 1233 G800L J8QQ37S8Q 03 32 39.08 -27 47 30.5 2003-12-02 1233 G800L J8QQ37SDQ 03 32 39.05 -27 47 29.8 2003-12-02 1233 G800L J8QQ38B6Q 03 32 39.09 -27 47 30.0 2003-12-03 2624 G800L PA231 J8QQ40VJQ 03 32 38.91 -27 47 32.3 2004-01-17 354 F606W J8QQ40V5Q 03 32 38.91 -27 47 32.3 2004-01-17 2112 G800L J8QQ41WFQ 03 32 38.93 -27 47 31.8 2004-01-17 1233 G800L J8QQ41WLQ 03 32 38.95 -27 47 32.5 2004-01-17 1233 G800L J8QQ42X6Q 03 32 38.89 -27 47 32.8 2004-01-17 1233 G800L J8QQ42XAQ 03 32 38.88 -27 47 32.0 2004-01-17 1233 G800L J8QQ43C9Q 03 32 39.24 -27 47 28.2 2004-01-18 354 F606W J8QQ43C1Q 03 32 39.24 -27 47 28.2 2004-01-18 2112 G800L J8QQ44CZQ 03 32 39.26 -27 47 27.7 2004-01-18 1233 G800L J8QQ44DFQ 03 32 39.27 -27 47 28.4 2004-01-18 1233 G800L J8QQ45DOQ 03 32 39.22 -27 47 28.7 2004-01-18 1233 G800L J8QQ45E3Q 03 32 39.20 -27 47 27.9 2004-01-18 1233 G800L J8QQ46APQ 03 32 39.10 -27 47 29.7 2004-01-18 1233 G800L J8QQ46AWQ 03 32 39.11 -27 47 30.5 2004-01-18 1233 G800L J8QQ47YQQ 03 32 39.05 -27 47 30.7 2004-01-17 1233 G800L J8QQ47YXQ 03 32 39.04 -27 47 30.0 2004-01-17 1233 G800L J8QQ48ZKQ 03 32 39.07 -27 47 30.2 2004-01-18 2624 G800L J8QQ50NKQ 03 32 38.94 -27 47 27.3 2003-09-23 2421 G800L J8QQ51PMQ 03 32 39.18 -27 47 28.5 2003-09-23 2421 G800L J8QQ52NTQ 03 32 39.14 -27 47 28.9 2003-09-26 2421 G800L J8QQ53V4Q 03 32 39.02 -27 47 26.9 2003-10-04 2421 G800L J8QQ54CHQ 03 32 39.16 -27 47 29.0 2003-10-05 2421 G800L J8QQ55D2Q 03 32 38.99 -27 47 27.0 2003-10-05 2421 G800L The basic steps of data reduction of the ACS images were performed using the calacs pipeline. The spectra were extracted using the aXe package ( http://www.stecf.org/software/aXe/ ) to produce 1D spectra. Full details of the reduction procedure can be found in Pirzkal et al. 2004. 2. Data Reduction ----------------- The main steps of the data reduction are as follows: a) standard CALACS pipeline reduction All the images, both direct and grism, were processed with CALACS using the available best reference files. Direct images were bias, flat fielded and gain corrected and the resulting data products, labelled j......._flt.fits were subsequently analysed. For the grism images only bias subtraction and gain correction was applied. b) Image alignment and cosmic ray removal The IRAF multidrizzle task was used to create combined drizzled direct and grism images for each of the five epochs. The purpose was not to analyse the drizzled combined images but to flag the cosmic rays in the individual images. The shallow direct images were aligned using measured positions of bright objects and the sets of grism images aligned using the the positions of the zeroth order spectra. More details can be found in Pirzkal et al. 2004. c) Image Catalogue The direct images are shallow and with the availability of the catalogue derived from the HUDF images a deeper catalogue could be used to drive the extraction of the spectra. Thus the nomenclature of the extracted spectra follows the HUDF object ID. d) Removal of grism background The grism images contain a background arising from the dispersed spectrum of the background. A super sky image was formed by median combining 84 ACS WFC grism images. This median sky image, represented in electrons/s, was scaled to the background of each grism image, avoiding the positions of all spectra, and subtracted from the grism images. The resulting images had a uniform background, close to zero within 10**-4 electrons/s/pixel. e) Extraction of spectra Spectra were extracted using the aXe software ( see http://www/stecf.org/software/axe/ ). No background subtraction was performed in aXe since the background had very effectively been removed by the scaled "super sky". 1D spectra were extracted at each position angle, with a box of height 1.0 times the object size in the cross dispersion direction, as given by the HUDF catalogue. Only the first order spectra are publically released. Since most of the objects are resolved, it is not a priori valid to average the spectra at the different position angles and it was decided to release the spectra extracted at each position angle separately. This allows the user the liberty to combine the spectra - the simplest option being an average of all position angle spectra. f) Contamination of spectra Since there are very many spectra, and often the spectra of adjacent nearby objects overlap, then contamination of a spectrum by that of its neighbours is very common. This contamination can either be in the form of the first order spectrum overlapping the first order spectrum of a neighbour (the first order spectra are the strongest), or other orders, typically the second, zeroth or minus 1st order, which are the next strongest. In particular since the zeroth order is only a few pixels in extent in the dispersion direction, then a zeroth order spectrum contaminating a first order spectrum has the appearance of an emission line. The level of contamination in a given spectrum was computed by assuming each spectrum is flat with a magnitude taken from the HUDF catalogue (i band, F775W filter) and using the throughputs of the ACS and the G800L spectral orders. The contamination at a given wavelength is then expressed as the fraction of the flux (as a percent) at a given wavelength contributed by the neighbouring spectra. 3. Release Spectra ------------------ The object spectra are released as FITS binary tables. 1400 spectra are released which are the number of objects with with HUDF i band magnitude brighter than 27.0 for which spectra could be extracted. The value of 27th mag. was chosen as this corresponds to a cumulative signal-to-noise of 10 or more, which is equivalent to a detection at greater than 3 sigma (see Pirzkal et al. 2004, Section 8 for details). Since the first order spectrum is displaced from the position of the direct object, then objects at the right edge of the field will have spectra falling outside the detector area. Thus not all i<27 mag. objects in the HUDF catalogue will occur in the GRAPES released spectra. The spectra at the different position angles are presented in the FITS table extensions. Most spectra have five extensions representing spectra from the five position angles, but depending on their field orientation the spectrum at any one position angle may not be present (i.e. extracted) at all position angles. The file names are formed according to the following convention: h_udf_wfc_id???_spc.fits with ??? as the ID of the object in the UDF catalogue. The FITS header keywords are: FILENAME : the name of the file as above OBJNAME : the object name h_udf_wfc_id??? RA : the right ascension in J2000 coordinates DEC : the declination in J2000 coordinates OBSERVATORY : the name of the telescope INSTRUMENT : the name of the instrument FILTER : the name of the grism MINWAVE : the minimum expected wavelength in the spectra MAXWAVE : the maximum expected wavelength in the spectra N.B. For a spectrum which is placed near the detector edges, the actual spectral extent may be less than 5500-10500A. Other keywords are derived from the FITS header of the extracted spectrum images. The FITS table extensions have the following header keywords: EXTNAME : Name of the extracted spectrum beam OBJECTID : HUDF catalogue number of the object WIDTH : Extraction width in pixels in cross-dispersion direction ORIENT : Position angle (degrees) of the extraction box with respect to the detector array X axis POSANG : Position angle (PA_V3) of the grism image (degrees) EXPTIME : Exposure time in seconds for the grism image from which spectrum was extracted The object spectra are stored in eight table columns: LAMBDA : the wavelength in [ANGSTROM] COUNT : the background subtracted object in [ELECTRONS/S] ERROR : the error for the value in column 'COUNT' in [ELECTRONS/S] FLUX : the object flux in [ERG/CM^2/S/A] FERROR : the error for the value in column 'FLUX' in [ERG/CM^2/S/A] CONTAM : the percentage contribution to the flux from adjacent spectra DQ : the data quality of the wavelength-flux point (values > 0 affected by bad data points) References ---------- Pirzkal, N., Xu, C., Malhotra, S., Rhoads, J., 2004. GRAPES, Grism Spectroscopy of the Hubble Ultra Deep Field: Description and Data Reduction. ApJS, 154, 501. Acknowledgements ---------------- We thank Adam Riess for allowing the PA117 data taken in program 9352 to be included in the released spectra.