# Presentation: 664396

## Chandra Detection of an X-ray Cluster Around UM 425 at z=1.47

**ABSTRACT**
- We report on the serendipitous discovery of a candidate cluster of galaxies at z=1.47, based on *Chandra* observations of the close quasar pair UM 425.  Diffuse X-ray emission of about 17” extent is detected with high significance.  If the emission originates at the redshift of the quasar pair, this corresponds to a physical size of ~140 kpc, kT > 0.8 keV and a 2-10 keV luminosity of 3x1043 erg/s.  Due to its proximity to the brighter member of the quasar pair and good agreement with the Mulchaey & Zabludoff (1998) LX-T relation, it is unlikely that this emission is the long-sought gravitational lens invoked to explain the unusual properties of UM 425 A&B.  Although we cannot infer cosmological constraints from this measurement, it does have interesting implications for quasar triggering and future searches for high-redshift clusters.

*Chandra* Detection of an X-ray Cluster Around UM 425 at z=1.47
- Smita Mathur and Rik J. Williams (The Ohio State University)

- Background
- UM 425 A&B is a close pair of broad absorption line quasars (BALQSOs) at z=1.47, initially discovered by Meylan & Djorgovski (1989).  There has been debate over the years as to whether this is an actual quasar pair or simply lensed images of the same quasar.  The optical spectra of UM 425 A&B are similar enough that this explanation is plausible; indeed, BALQSOs are rare enough that such a pair is unlikely to arise by chance.  UM 425A is much brighter than typical BALQSOs, further reinforcing the lensing hypothesis.  Although there is a slight overdensity of galaxies in this region, optical imaging has not shown a foreground cluster or galaxy with sufficient mass and proximity to produce the observed image splitting.  Recently, a *Chandra * observation originally undertaken to study the BALQSOs has revealed diffuse X-ray emission associated with the quasar pair, which we interpret as hot intracluster gas at the redshift of the quasars.  Here we report on this finding and its implications.

**References**
- Aldcroft, T.L. & Green, P.J.  2003, ApJ, 592, 710
- Fabian, A.C., et al.  2001, MNRAS, 322, L11
- Meylan, G. & Djorgovski, S.  1989, ApJ, 338, L1
- Mulchaey, J., & Zabludoff, A.  1998, ApJ, 496, 73
- Williams, R.J., & Mathur, S.  2003, ApJ, 589, L1

**Figure 1.***  (left)*  *Chandra* soft-band (0.3-3 keV) image of the UM 425 field.  UM 425A is at the center of the image, and UM 425B is the faint object just up and to the right.  The hot intracluster medium is seen as faint emission extending to the lower right of UM 425A.  The faint object near the top of the frame is a foreground galaxy at z=0.1265 (Meylan & Djorgovski 1989).
*(right)*  Adaptively smoothed, exposure-corrected version of the soft-band *Chandra* image.

**Figure 2.**  Chandra ACIS-S spectra of the diffuse emission and background.  The lower panel shows the background spectrum with corresponding power-law fit.  The top panel shows the diffuse emission + background data along with the best-fit model (Raymond-Smith thermal plasma with kT=2.1 keV, 0.1 solar metallicity, z=1.47, fX(0.1-2.5 keV)=4.6x10-15 erg/cm2/s.

- Observations and Data Analysis
- We observed UM 425 with *Chandra* for 110 ksec at the nominal aimpoint of ACIS-S.  The data were reduced with the standard procedure using CIAO v2.3.  Figure 1 shows the soft-band image of the UM 425 field, and the same image with exposure-correction and adaptive smoothing applied.  UM 425A was clearly detected with ~5000 counts and UM 425B with ~20 counts.  In addition, a faint diffuse component extending to the lower right (southwest) of UM 425A is seen, most clearly in the smoothed image.  This emission is detected with an estimated 115 +/- 19 counts.
- Using CIAO, we extracted spectra for both quasars and the diffuse emission, as well as a background region well outside of the regions of interest.  The spectrum of the diffuse emission, seen in Figure 2, is well-fit by a Raymond-Smith thermal plasma with kT=2.1 keV (with a 2 lower bound of 0.8 keV).

- Evidence for a Cluster
- Although diffuse emission around the quasar pair is detected with high significance, the question still remains as to whether the emission is due to a foreground gravitational lens, resulting in the apparent quasar pair.  Several factors suggest that this is *not* the case.  As pointed out by Meylan & Djorgovski (1989), a lens would appear closer to the fainter image; however, the observed diffuse emission is clearly centered around UM 425A.  Furthermore, Aldcroft & Green (2003) note that in order to produce the observed image separation, a foreground lens would require a minimum temperature of kT=1.5 keV and observed flux 2.3x10-14 erg/cm2/s.  We cannot rule out this temperature, but the flux we measure is a factor of 5 lower than that required for such a lens.  Finally, the *Chandra* spectra of UM 425 A&B are inconsistent with each other, although this could in principle be due to different sightlines to the nucleus in the lensed case.  If at z=1.47, this cluster lies right on the Mulchaey & Zabludoff (1998) LX-T relation, which is surprising given our large observational uncertainties.

- Conclusions
- The diffuse X-ray emission around UM 425 is likely due to a group or cluster surrounding the quasars, making it the second-highest redshift cluster known (after the z=1.78 cluster found by Fabian et al. 2001).  Such high-z clusters are important for determining cosmological parameters and the mechanics of structure formation at high-redshifts; for instance, the existence of hot, high-z clusters can definitively rule out the m=1.0 universe.  This discovery also brings up the intriguing possibility of using high-z quasar pairs as signposts for finding clusters, and conversely studying how quasars such as these are affected by cluster environments.