Lymphedema Monitor

Xmm-Newton

T.P. Sasseen1, I. Eisenman,1, K. Mason2, and the Optical Monitor Team

Abstract

We use galaxies detected in a deep ultraviolet XMM- Newton Optical Monitor image and a model that predicts UV galaxy counts based on local counts and evolution pa- rameters to constrain galaxy evolution to Z=1.2. The 17’ square 2000 ˚A (UVW2 filter) image was taken as part of

The Xmm-Om Team’S Guaranteed Time Program. We De-

tect sources in this image to a flux limit of 2.7 × 10−17 ergs cm−2 s−1 ˚A−1 (AB magnitude = 22). Since some of the sources may be stars, we perform a number of checks, in- cluding shape, color and implied distance to remove stars from the detected counts. We find galaxy number counts as a function of magnitude roughly in agreement Milliard et al. (1992), but again find these counts are in excess of evolution models. The excess counts at faint magni- tudes may provide evidence for either a new population of galaxies emerging around Z=0.7 or more dramatic evo- lution than some earlier predictions. The integrated light from the detected galaxies totals 32–36 ph cm−2 s−1 ˚A−1 sr−1, placing a firm lower limit on the integrated UV light from galaxies.

Figure: System Model & Simulation Flow for Lymphedema Monitor

Key words: galaxies: evolution – ultraviolet: galaxies

1. Introduction

Measurements of the integrated light from a galaxy at 2000 ˚A provides a fairly direct measure of the instanta- neous rate of star formation, since the massive stars that provide most of this radiation are short-lived compared with the age of the galaxy. Knowledge of the star forma- tion rate also gives a measure of the rate of heavy element production in a galaxy, or in the universe when a large sample of galaxies are measured (Madau et al. 1996). The integrated light from these galaxies contributes to the ex- tragalactic background light at ultraviolet wavelengths, whose main sources are hot stars and active galactic nu- clei. Measurements of galaxy number counts in the ultra- violet have been made by Milliard et al. (1992) using the FOCA balloon-borne UV telescope, Gardner et al. (2000) and Hill et al. (1997) using HST archival fields. These data have been interpreted with models that predict num- ber counts based on galaxy spectral energy distributions (SED’s) and luminosity functions, such as those of Ar- mand & Milliard (1994) and Granato et al. (2000). The to- tal far-ultraviolet extragalactic background has been mea- sured to be as high as 500 ph cm−2 s−1 ˚A−1 and as low as 30 ph cm−2 s−1 ˚A−1 (see review by Bowyer 1991). Predic- tions for the number of galaxies that might be detected in deep ultraviolet Optical Monitor (OM) images are given by Sasseen et al. (1997). In this paper, we detect galaxies in a deep UV image taken with the Optical Monitor (OM) and use these galaxy number counts to place constraints on galaxy luminosity evolution via a a galaxy evolution model similar to that of Armand & Milliard (1994). We also find a lower limit to the galaxy contribution to the extragalactic UV background.

Figure: System Model & Simulation Flow for Lymphedema Monitor

2. The Data

The OM 13 hr deep field (at J2000.0 13 34 37.00, +37 54 44.0) was observed for approximately 200 ks with XMM- Newton around June 22, 2001. Details of the OM expo- sures used in this study are shown in Table 1.

Table 1. OM images used in this study.

30

Several exposures of typically 7 ks were brought to a common astrometric reference frame and coadded. We searched each image for sources using Sextractor and made a catalog of the sources we found. We concentrate here on sources in the UVW2 image (Fig. 1) and use measure- ments in the other filters to differentiate between stars, galaxies and QSO’s. We also use a deep R band image (to R∼27) of this field taken with the 8m Subaru tele- scope on Mauna Kea (Fig. 2) to check for source shape and possible confusion. We perform two checks to discrim- Proc. Symposium ‘New Visions of the X-ray Universe in the XMM-Newton and Chandra Era’

2

inate stars from galaxies. First, we compare the SED of

Each Uvw2 Source (Determined From Om Photometry)

against stellar templates. Second, we compute an inferred distance, as if the source were a main sequence star, from U-B color and B magnitude, as shown in Fig. 3. We find these checks form reliable stellar discriminators for more than 90% of the sources brighter than AB=22. 1 We also find a number of QSO’s in the field that show UV excess and appear point-like in the OM and Subaru images. We categorize these separately in our galaxy number counts.

Further work remains to completely discriminate any re- maining stellar content and the QSO populations. We plot the detected galaxy counts as a function of magnitude in Fig. 4. Our counts are in approximate agree- ment with that of Milliard et al. 1992 (also shown in Fig. 4) in the range of overlap, and we extend these counts to

Ab=22.

Figure 1. The 17’ by 17’ UVW2 (2000 A) image of total exposure 31,450 s. The five sigma detection limit deter-

Mined With 5” Fwhm Psf Is 3×1015 Ergs Cm−2S−1. We

detected 122 sources above this limit. Source with further detail is indicated in box.

3. The Models

We have constructed a model is similar to that of Armand & Milliard (1994) and use it to predict galaxy counts at

1 The Ab Magnitude System Is Defined By

AB = −2.5log(Fν) −48.6 where Fν is given in ergs cm−2 s−1 Hz−1 (Oke 1971). Figure 2. Detail of an example source region in UVW2 im- age (upper) and Subaru R-band image (lower) with detec- tion limit R∼27. We examine multiple detections in one location in more detail, both in the direct images and in a surface plot using the Subaru image.

2000 ˚A as a function of apparent magnitude. The model uses a Schechter absolute luminosity distribution function for 6 different galaxy types at redshifts between zero and 1.2, along with K-corrections and a single parameter lu- minosity evolution factor for each galaxy type. We have normalized the Schechter function using observed counts at Bj=17, and set our evolution parameters to agree with the modeled galactic evolution of Rocca-Volmerange & Guiderdoni (1988), following Armand & Milliard (1994).

Our model implicitly includes the effects of dust absorp- tion and scattering because it is based on observed UV SED’s. Like Armand & Milliard, our model predicts fewer galaxies in each magnitude band than our measured num- ber counts, as shown in Figure 4. We also compare the observed counts with the model of Granato et al. (2000), whose model explicitly includes expected contributions to the observed galaxy counts from starburst galaxies and dust. Our model agrees well with the Granato et al. model that includes dust, but our observed counts are higher than both models that include dust.

4. Discussion

The summed the flux from non-stellar sources detected in the UVW2 image totals 32–36 ph cm−2 s−1 sr−1 ˚A−1, with the higher limit including the contribution from QSO’s and active galaxies. The integrated far-ultraviolet light

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