c Pleiades Publishing, Inc., 2015. ISSN 1063-7737, Astronomy Letters, 2015, Vol. 41, No. 12, pp. 704–711. Published in Russian in Pis’ma v Astronomicheski˘ı Zhurnal, 2015, Vol. 41, No. 12, pp. 764–771.
Extragalactic Survey Using GALEX–Spitzer Matching Fields* Lakshmi S. Bose1** , N. V. Sujatha2, K. Narayanankutty1 , and Jayant Murthy3 1
Amrita Vishwa Vidyapeetham, Kollam—690525, Kerala, India St. Xavier’s College for Women, Aluva—683101, Kerala, India 3 Indian Institute of Astrophysics, Bangalore—560034, Karnataka, India 2
Received June 8, 2015
Abstract—We have used Spitzer observations of galaxies in the ELAIS-N1 region centered at 16h 10m 01s, +54◦ 30 36 to extend the effective sensitivity of GALEX UV observations of the same region. Spitzer is more sensitive to external galaxies than GALEX and we have searched for UV sources at the positions of galaxies in the Spitzer catalog. This has allowed us to extend the sensitivity of the GALEX instrument to 27th magnitude. We have compared the derived number counts of galaxies in the two GALEX bands to standard models of galaxy evolution finding good agreement.The total contribution of these galaxies to the UV background is about 40 photons cm−2 sr−1 s−1 A˚ −1 , in agreement with previous determinations. DOI: 10.1134/S1063773715120026 Keywords: matching fields, number counts, UV magnitude, galactic models.
INTRODUCTION The European Large Area ISO (Infrared Space Observatory) Survey (ELAIS) fields were chosen specifically to observe extragalactic sources in a location where the Galactic cirrus emission is exceptionally low (Oliver et al. 2000). Of these, the N1 field (ELAIS-N1) has been intensively observed by a wide range of instruments over different wavelengths with optical associations of ELAIS sources in N1 region made using the Isaac Newton Telescope (INT) Wide Field Survey (Gonzalez-Solares et al. 2005). J and K surveys were made with using the STELIRCam ¨ anen ¨ instrument (Vais et al. 2002; Rowan-Robinson et al. 2004) with additional observations including a deep Hubble Space Telescope Advanced Camera for Surveys (HSTACS); a Very Large Array (VLA) radio survey at 20 cm (Ciliegi et al. 1999); Chandra observations of the central region of N1 (Manners et al. 2003); and partial coverage by the Sloan Digital Sky Survey (Adelman-McCarthy et al. 2008). The N1 field is a northern ELAIS field centered at 16h 10m 01s , +54◦ 30 36 . This field is also covered by Spitzer Space Infrared Telescope SWIRE survey. Observations from the Spitzer Space Infrared Telescope (Werner et al. 2004) detected a total of 114 658 objects in this field (Mauduit et al. 2012), most of which are galaxies. The ELAIS-N1 field was also ∗ **
The article was submitted by the authors in English. E-mail:
[email protected]
observed in two ultraviolet (UV) bands (FUV: 1350– ˚ by the Galaxy 1750 A˚ and NUV: 1750–2800 A) Evolution Explorer (GALEX). We have examined the positions of the Spitzer-detected galaxies both in order to understand their distribution with magnitude and to examine the contribution of these galaxies to the extragalactic background in the UV (Calura and Matteucci 2003). Number counts of field galaxies in different spectral regions may be used to test models of galactic evolution (Hoversten et al. 2009). The slope of the number count versus magnitude curve is a function of the galaxy distribution and redshift (z) (Pozzetti et al. 1998; Gardner et al. 2000a). UV observations are particularly interesting because the Lyman break is redshifted into the far and near ultraviolet for distant objects whose instantaneous rate of star formation may therefore be constrained. OBSERVATIONS The ELAIS is a key project of the ISO mission and covered about 12 square degrees split into several different fields. Spitzer observed one of these fields, the N1 field centered at 16h 10m 01s , +54◦ 30 36 as part of the Spitzer Extragalactic Representative Volume Survey (SERVS) with an average of 1400 s observation time per pixel (∼2 resolution). Point sources in the IRAC 3.6 and 4.5 μm (Infrared Array Camera: Mauduit et al. 2012) observations of the region have been extracted into a merged catalog (available from 704
EXTRAGALACTIC SURVEY USING GALEX–Spitzer
705
Table 1. GALEX observation log
Tilename
R.A. (deg)
Dec (deg)
Exp:Time (s) NUV
FUV
Visits
Observation (dd/mm/yy)
NUV
FUV
ELAISN1_00
243.3
54.9
26877
28704
30.03.06–13.07.07
24
26
ELAISN1_02
241.7
54.5
20762
6088
06.04.07–25.06.07
20
06
ELAISN1_04
243.2
53.8
17790
3222
06.04.07–23.06.07
17
06
ELAISN1_08
241.0
55.0
18242
5412
01.05.05–21.06.07
20
07
ELAISN1_09
241.6
53.5
20289
7062
02.05.05–11.08.04
16
05
PS-ELAISN1 MOS_12
242.8
55.0
26349
6929
05.09.05–10.11.06
21
05
PS-ELAISN1 MOS_14
243.9
54.1
28044
6977
09.09.05–10.11.06
22
05
PS-ELAISN1 MOS_15
242.3
53.9
26780
6887
09.09.05–10.11.06
21
05
PS-ELAISN1 MOS_16
241.3
54.4
26881
6951
09.09.05–10.11.06
21
05
the Spitzer archive1 ) containing 114 658 objects. We identified and removed stars from the source catalog using the prescription of Groenewegen et al. (2002) and Fadda et al. (2004) based on the stellarity index value (CLASS_STAR is 0 for a galaxy-like object), leaving 87 625 galaxies in the field. GALEX took 9 deep observations of the ELAISN1 region with a minimum exposure of 17 000 s in the NUV band and 3000 s for FUV band (Table 1). Both the GALEX and Spitzer observations are marked on a 100 μm image from the Infrared Astronomical Satellite (IRAS) mission in Fig. 1. The E(B − V ) variation in the field is 0.01–0.05m (Schlegel et al. 1998), which corresponds to an optical depth of 0.08–0.40 in the UV. The standard GALEX pipeline (Morrissey et al. 2007) provides a catalog of UV sources based on the detection and flux measurements with SExtractor (Bertin and Arnouts 1996) for each flux calibrated image but, given the GALEX point spread function of about 5 , source confusion limits the star/galaxy separation in deep fields (Xu et al. 2005; Hammer et al. 2010). A different catalog was produced for each of the GALEX observations and we combined the individual catalogs. If two sources were within the 5 GALEX spatial 1
http://sha.ipac.caltech.edu/applications/Spitzer/SHA/
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resolution, we identified them as duplicates and combined the fluxes weighting/averaging each by its exposure time. Note that there were no cases where the sources had very different fluxes, indicating different sources. This left a total of 58 245 sources of which 37 548 were identified as galaxies based on the STAR_CLASS parameter (
