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Extremely Red NICMOS Objects in the Chandra Deep Fields

Extremely Red NICMOS Objects in the Chandra Deep Fields. James Colbert Spitzer Science Center Harry Teplitz & Lin Yan (SSC) Matthew Malkan (UCLA) Patrick McCarthy (OCIW). Faint X-ray Sources are Red. Among reddest optical/near-infrared objects

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Extremely Red NICMOS Objects in the Chandra Deep Fields

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  1. Extremely Red NICMOS Objects in the Chandra Deep Fields James Colbert Spitzer Science Center Harry Teplitz & Lin Yan (SSC) Matthew Malkan (UCLA) Patrick McCarthy (OCIW)

  2. Faint X-ray Sources are Red • Among reddest optical/near-infrared objects • Mushotsky et al. (2000), Barger et al. (2001), Alexander et al. (2001) • 10-20% of EROs are X-ray sources(Alexander et al. 2003, Roche et al. 2003) • Clear trend is • seen with optical magnitudes: • On average, the more optically faint, the redder the object. Alexander et al. (2001)

  3. This Project • Examine all extant NICMOS imaging in the Chandra Deep Fields • NIR has advantage of less dust vulnerability and sensitivity to high-z rest optical wavelengths • Includes NICMOS HDF-N, NICMOS UDF, and several NICMOS parallel fields. • Reach depths of H(AB)~27-27.5 [5-σ] • Parallel Fields H(AB)~26 • Covering almost 20 square arcminutes • Clearly vulnerable to small area/cosmic variance

  4. NICMOS UDF HST Program 9803 PI: R. Thompson NICMOS HDF-N Dickinson et al. (2000)

  5. NICMOS Detected All 42 Chandra X-ray Sources • 95% are brighter than H(AB)=24.5 • The two faint detections are H=25.3 and 27.3

  6. Near-infrared Colors • While bright sources have colors similar to field galaxies, a population of red J-H galaxies appear at the faint end. 5 of 14 (36%) faint sources have J-H>1.4 (approx. color of z=2 elliptical).

  7. Near-Infrared Extremely Red Objects: NEROs? • Reverse is also true: 5 of 10 (50%) red J-H objects are X-ray sources. In fact, X-ray source is reddest J-H object at every magnitude, down to H=24.5, where we run out of sources.

  8. Two major candidates for faint, red X-ray sources: 1) Obscured AGN • Dust could redden spectra and absorb much of AGN light • X-ray spectra at fainter X-ray flux levels appear, on average, harder Obscured AGN vs. Evolved High Redshift Galaxy

  9. However, not all red sources are hard

  10. 2) Evolved Galaxies at High Redshift Above z=1.5, 4000Å Balmer break moves into the near-infrared, placing the steeply falling UV continuum in the J-band and optical filters. To test this, we assembled all published spectroscopic information (35 sources) • Lowenthal et al. (1997), Cohen et al. (1999), Barger et al. (2003), Szokoly et al. (2004) • Supplemented with 7 photo-z’s • BPZ code (Benitez 2000)

  11. All 6 red J-H objects appear to lie at redshifts greater than z=1.5. • Suggests these are evolved galaxies with rest-wavelength uv/optical dominated by starlight.

  12. However, all have AGN-level X-ray luminosities • [0.5-2 keV] LX > 1042 ergs/sec No reason both possibilities can’t be occurring simultaneously, i.e. obscured AGN in evolved galaxies at high-z

  13. Question: If the cause of red X-ray sources is starlight at high-z, not dusty AGN activity, why do they make up such a large percentage of the red J-H sources? • Possible Answer: Because X-ray sources are in the brightest galaxies at high-z. • The fainter galaxies don’t become visible in large numbers until H>24.5.

  14. X-ray sources are the brightest galaxies at all redshifts. • Barger et al. (2003) produced a similar result, finding that most of their X-ray sources were > M*. Field galaxy redshifts taken from photo-z’s of Fernandez-Soto, Lanzetta, & Yahil 1999. This holds true at longer rest wavelengths as well, indicating X-ray sources are likely in the most massive galaxies as well.

  15. Future Work • NICMOS can be a powerful tool for identifying high-z AGN • Need to cover more area • 6 objects, 4 in UDF • NICMOS parallels cover 100s of square arcminutes across the sky. • Objects not ultra-faint, accessible to deepest ground surveys. • Combined with Spitzer Mid-IR should greatly help constrain dust and AGN contributions.

  16. NICMOS UDF HST Program 9803 PI: R. Thompson NICMOS HDF-N Dickinson et al. (2000)

  17. I-H EROs Clear H vs I-H relationship • Similar to previously seen down to fainter levels • 2 faintest objects are not very red. • Of 14 EROs • (I-H > 3.1), • 3 are X-ray sources • Smaller #’s, but similar to Alexander et al. (2003) and Roche et al. (2003)

  18. At faint X-ray flux levels new populations of X-ray objects appear Ratio of X-ray to Optical flux varies wildly • Starbursts • Even so-called “normal” Milky Way levels • Possibly more obscured AGN activity? Barger et al. (2003)

  19. Yan et al. (2003) Cowie et al. (2001) Previous studies have also seen some extreme near-infrared colors among optically faint X-ray sources.

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