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Register a new userConnecting Galaxy Evolution, Star Formation and the X-ray Background
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Added by
David R. Ballantyne
Publication date
2005
fields
Physics
and research's language is
English
Authors
D.R. Ballantyne
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As a result of deep hard X-ray observations by Chandra and XMM-Newton a significant fraction of the cosmic X-ray background (CXRB) has been resolved into individual sources. These objects are almost all active galactic nuclei (AGN) and optical followup observations find that they are mostly obscured Type 2 AGN, have Seyfert-like X-ray luminosities (i.e., L_X ~ 10^{43-44} ergs s^{-1}), and peak in redshift at z~0.7. Since this redshift is similar to the peak in the cosmic star-formation rate, this paper proposes that the obscuring material required for AGN unification is regulated by star-formation within the host galaxy. We test this idea by computing CXRB synthesis models with a ratio of Type 2/Type 1 AGN that is a function of both z and 2-10 keV X-ray luminosity, L_X. The evolutionary models are constrained by parameterizing the observed Type 1 AGN fractions from the recent work by Barger et al. The parameterization which simultaneously best accounts for Bargers data, the CXRB spectrum and the X-ray number counts has a local, low-L_X Type 2/Type 1 ratio of 4, and predicts a Type 2 AGN fraction which evolves as (1+z)^{0.3}. Models with no redshift evolution yielded much poorer fits to the Barger Type 1 AGN fractions. This particular evolution predicts a Type 2/Type 1 ratio of 1-2 for log L_X > 44, and thus the deep X-ray surveys are missing about half the obscured AGN with these luminosities. These objects are likely to be Compton thick. Overall, these calculations show that the current data strongly supports a change to the AGN unification scenario where the obscuration is connected with star formation in the host galaxy rather than a molecular torus alone. The evolution of the obscuration implies a close relationship between star formation and AGN fueling, most likely due to minor mergers or interactions.
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We estimate the contribution of AGNs and of their host galaxies to the infrared background. We use the luminosity function and evolution of AGNs recently determined by the hard X-ray surveys, and new Spectral Energy Distributions connecting the X-ray and the infrared emission, divided in intervals of absorption. These two ingredients allow us to determine the contribution of AGNs to the infrared background by using mostly observed quantities, with only minor assumptions. We obtain that AGN emission contributes little to the infrared background ($<$5% over most of the infrared bands), implying that the latter is dominated by star formation. However, AGN host galaxies may contribute significantly to the infrared background, and more specifically 10--20% in the 1--20$mu$m range and $sim$5% at $lambda<60mu m$. We also give the contribution of AGNs and of their host galaxies to the source number counts in various infrared bands, focusing on those which will be observed with Spitzer. We also report a significant discrepancy between the expected contribution of AGN hosts to the submm background and bright submm number counts with the observational constraints. We discuss the causes and implications of this discrepancy and the possible effects on the Spitzer far-IR bands.
We present a new model for the X-ray properties of the intracluster medium that explicitly includes heating of the gas by the energy released during the evolution of cluster galaxies. We calculate the evolution of clusters by combining the semi-analytic model of galaxy formation of Cole et al. with a simple model for the radial profile of the intracluster gas. We focus on the cluster X-ray luminosity function and on the relation between X-ray temperature and luminosity. We show that cooling alone is not enough to account for the flatness of the observed T-L relation or for the lack of strong redshift evolution in the observed X-ray luminosity function. Gas heating, on the other hand, can solve these two problems: in the Lambda-CDM cosmology, our model reproduces fairly well the T-L relation and the X-ray luminosity function. Furthermore, it predicts only weak evolution in these two properties out to z=0.5, in agreement with recent observational data. A successful model requires an energy input of 1--2 x 10^49 ergs per solar mass of stars formed. This is comparable to the total energy released by the supernovae associated with the formation of the cluster galaxies. Thus, unless the transfer of supernovae energy to the intracluster gas is very (perhaps unrealistically) efficient, additional sources of energy, such as mechanical energy from AGN winds are required. However, the amplification of an initial energy input by the response of the intracluster medium to protocluster mergers might ease the energy requirements. Our model makes definite predictions for the X-ray properties of groups and clusters at high redshift which may soon be tested with data from the Chandra and Newton satellites.
The integrated X-ray luminosity ($L_{mathrm{X}}$) of high-mass X-ray binaries (HMXBs) in a galaxy is correlated with its star formation rate (SFR), and the normalization of this correlation increases with redshift. Population synthesis models suggest that the redshift evolution of $L_{mathrm{X}}$/SFR is driven by the metallicity ($Z$) dependence of HMXBs, and the first direct evidence of this connection was recently presented using galaxies at $zsim2$. To confirm this result with more robust measurements and better constrain the $L_{mathrm{X}}$-SFR-$Z$ relation, we have studied the $Z$ dependence of $L_{mathrm{X}}$/SFR at lower redshifts. Using samples of star-forming galaxies at $z=0.1-0.9$ with optical spectra from the hCOSMOS and zCOSMOS surveys, we stacked textit{Chandra} data from the COSMOS Legacy survey to measure the average $L_{mathrm{X}}$/SFR as a function of $Z$ in three redshift ranges: $z=0.1-0.25$, $0.25-0.4$, and $0.5-0.9$. We find no significant variation of the $L_{mathrm{X}}$-SFR-$Z$ relation with redshift. Our results provide further evidence that the $Z$ dependence of HMXBs is responsible for the redshift evolution of $L_{mathrm{X}}$/SFR. Combining all available $z>0$ measurements together, we derive a best-fitting $L_{mathrm{X}}$-SFR-$Z$ relation and assess how different population synthesis models describe the data. These results provide the strongest constraints to date on the $L_{mathrm{X}}$-SFR-$Z$ relation in the range of $8.0<$12+log(O/H)$<9.0$.
We have compared stacked spectra of galaxies, grouped by environment and stellar mass, among 58 members of the redshift z = 1.24 galaxy cluster RDCS J1252.9-2927 (J1252.9) and 134 galaxies in the z = 0.84 cluster RX J0152.7-1357 (J0152.7). These two clusters are excellent laboratories to study how galaxies evolve from star-forming to passive at z ~ 1. We measured spectral indices and star-forming fractions for our density- and mass-based stacked spectra. The star-forming fraction among low-mass galaxies (< 7 x 10^10 M_sun) is higher in J1252.9 than in J0152.7, at about 4 sigma significance. Thus star formation is being quenched between z = 1.24 and z = 0.84 for a substantial fraction of low-mass galaxies. Star-forming fractions were also found to be higher in J1252.9 in all environments, including the core. Passive galaxies in J1252.9 have systematically lower D_n4000 values than in J0152.7 in all density and mass groups, consistent with passive evolution at modestly super-solar metallicities.
Chandra observations of the Cartwheel galaxy reveal a population of ultraluminous X-ray sources (ULXs) with lifetimes < 10^7 yr associated with a spreading wave of star formation which began some 3 x 10^8 yr ago. A population of high-mass X-ray binaries provides a simple model: donor stars of initial masses M_2 > 15 Msun transfer mass on their thermal timescales to black holes of masses M_1 > 10 Msun. For alternative explanations of the Cartwheel ULX population in terms of intermediate-mass black holes (IMBH) accreting from massive stars, the inferred production rate > 10^-6 yr^-1 implies at least 300 IMBHs, and more probably 3 x 10^4, within the star-forming ring. These estimates are increased by factors eta^-1 if the efficiency eta with which IMBHs find companions of > 15 Msun within 10^7 yr is <1. Current models of IMBH production would require a very large mass ($ga 10^{10}msun$) of stars to have formed new clusters. Further, the accretion efficiency must be low (< 6 x 10^-3) for IMBH binaries, suggesting super-Eddington accretion, even though intermediate black hole masses are invoked with the purpose of avoiding it. These arguments suggest either that to make a ULX, an IMBH must accrete from some as yet unknown non-stellar mass reservoir with very specific properties, or that most if not all ULXs in star-forming galaxies are high-mass X-ray binaries.
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