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Register a new userThe Weak Outnumbering the Mighty: Normal Galaxies in Deep Chandra Surveys
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Added by
Ann E. Hornschemeier
Publication date
2002
fields
Physics
and research's language is
English
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Chandra is detecting a significant population of normal and starburst galaxies in extremely deep X-ray exposures. For example, approximately 15% of the sources arising in the 2 Ms Chandra Deep Field-North survey are fairly normal galaxies, where normal means Milky Way-type X-ray emission rather than simply exhibiting an optically normal spectrum. Many of these galaxies are being detected at large look-back times z=0.1-0.5, allowing the study of the evolution of X-ray binary populations over significant cosmological timescales. We are also detecting individual off-nuclear ultraluminous X-ray sources (e.g., X-ray binaries), providing the first direct constraints on the prevalence of lower-mass black holes at significantly earlier times. The X-ray emission from such normal galaxies may also be a useful star-formation rate indicator, based on radio/X-ray cross-identifications. We describe the contribution of normal galaxies to the populations which make up the X-ray background and present their directly measured X-ray number counts. We find that normal and starburst galaxies should dominate the 0.5--2 keV number counts at X-ray fluxes fainter than 7e-18 erg cm^{-2} s^{-1} (thus they will outnumber the mighty AGN). Finally, we look to the future, suggesting that it is important that the population of X-ray faint normal and starburst galaxies be well constrained in order to design the next generation of X-ray observatories.
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Galaxy counts from bright ultraviolet (UV) and deep optical spectroscopic surveys have revealed an unexpectedly large number of very blue galaxies. The colors and luminosities of these objects indicate that they are dwarf galaxies undergoing bursts of star formation. We use a galaxy evolution model (PEGASE, Fioc & Rocca-Volmerange 1997) to describe this population as galaxies undergoing cyclical bursts of star formation, thereby determining the luminosity function of these galaxies. When these bursting galaxies are added to normally evolving populations, the combination reproduces the UV number counts, color distributions and deep optical redshift distributions fairly well. Optical (including the Hubble Deep Field) and near-infrared number counts are fitted assuming an open or a flat, Lambda-dominated, Universe. The high amplitude of the angular correlation function of very blue galaxies discovered by Landy et al. (1996) is also recovered in this modelling. The number of bursting galaxies is only a small fraction of the total number of galaxies at optical and near-infrared wavelengths, even at faintest magnitudes. In our evolution modelling, normal galaxies explain most of the blue excess in a low-Omega Universe. The problem of the blue excess remains in a flat Universe without a cosmological constant.
We discuss results from sensitive, wide-field imaging of the millimeter extragalactic background using the Max-Planck Millimeter Bolometer array (MAMBO) at the IRAM 30m telescope.
(abridged) We have investigated 136 Chandra extragalactic sources without broad optical emission lines, including 93 galaxies with narrow emission lines (NELG) and 43 with only absorption lines (ALG). Based on fx/fo, Lx, X-ray spectral hardness and optical emission line diagnostics, we have conservatively classified 36 normal galaxies (20 spirals and 16 ellipticals) and 71 AGNs. We found no statistically significant evolution in Lx/LB, within the limited z range. We have built log(N)-log(S), after correcting for completeness based on a series of simulations. The best-fit slope is -1.5 for both S and B energy bands, which is considerably steeper than that of the AGN-dominated cosmic background sources, but slightly flatter than the previous estimate, indicating normal galaxies will not exceed the AGN population until fx ~ 2 x 10-18 erg s-1 cm-2 (a factor of ~5 lower than the previous estimate). A group of NELGs appear to be heavily obscured in X-rays, i.e., a typical type 2 AGN. After correcting for intrinsic absorption, their X-ray luminosities could be Lx > 10^44 erg s-1, making them type 2 quasar candidates. While most X-ray luminous ALGs (XBONG - X-ray bright, optically normal galaxy candidates) do not appear to be significantly absorbed, we found two heavily obscured objects, which could be as luminous as an unobscured broad-line quasar. Among 43 ALGs, we found two E+A galaxy candidates with strong Balmer absorption lines, but no [OII] line. The X-ray spectra of both galaxies are soft and one of them has a nearby close companion galaxy, supporting the merger/interaction scenario rather than the dusty starburst hypothesis.
We present initial results and the source catalog from the NuSTAR survey of the Extended Chandra Deep Field South (hereafter, ECDFS) - currently the deepest contiguous component of the NuSTAR extragalactic survey program. The survey covers the full ~30 arcmin x 30 arcmin area of this field to a maximum depth of ~360 ks (~220 ks when corrected for vignetting at 3-24 keV), reaching sensitivity limits of ~1.3 x 10^-14 erg/cm2/s (3-8 keV), ~3.4 x 10^-14 erg/cm2/s (8-24 keV) and ~3.0 x 10^-14 erg/cm2/s (3-24 keV). Fifty four (54) sources are detected over the full field, although five of these are found to lie below our significance threshold once contaminating flux from neighboring (i.e., blended) sources is taken into account. Of the remaining 49 that are significant, 19 are detected in the 8-24 keV band. The 8-24 keV to 3-8 keV band ratios of the twelve sources that are detected in both bands span the range 0.39-1.7, corresponding to a photon index range of Gamma ~ 0.5-2.3, with a median photon index of 1.70 +/- 0.52. The redshifts of the 49 sources in our main sample span the range z = 0.21-2.7, and their rest-frame 10-40 keV luminosities (derived from the observed 8-24 keV fluxes) span the range L(10-40 keV) ~ (0.7-300) x 10^43 erg/s, sampling below the knee of the X-ray luminosity function out to z ~ 0.8-1. Finally, we identify one NuSTAR source that has neither a Chandra nor an XMM-Newton counterpart, but that shows evidence of nuclear activity at infrared wavelengths, and thus may represent a genuine, new X-ray source detected by NuSTAR in the ECDFS.
The far-ultraviolet (UV) counts and the deep optical spectroscopic surveys have revealed an unexpected number of very blue galaxies (vBG). Using constraints from the UV and optical, we apply the galaxy evolution model PEGASE (Fioc & Rocca-Volmerange 1997, hereafter FRV) to describe this population with a cycling star formation. When added to normally evolving galaxy populations, vBG are able to reproduce UV number counts and color distributions as well as deep optical redshift distributions fairly well. Good agreement is also obtained with optical counts (including the Hubble Deep Field). The number of vBG is only a small fraction of the number of normal galaxies, even at faintest magnitudes. In our modelling, the latter explain the bulk of the excess of faint blue galaxies in an open Universe. The problem of the blue excess remains in a flat Universe without cosmological constant.
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