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Register a new userSoft X-ray and ultra-violet metal-line emission from the gas around galaxies
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Freeke van de Voort
Publication date
2012
fields
Physics
and research's language is
English
Authors
Freeke van de Voort
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(Abridged) A large fraction of the gas in galactic haloes has temperatures between 10^4.5 and 10^7 K. At these temperatures, cooling is dominated by metal-line emission if the metallicity Z>~0.1 Zsun. We explore the detectability of several lines using large cosmological, hydrodynamical simulations. We stack surface brightness maps centred on galaxies to calculate the expected mean surface brightness profiles for different halo masses. Assuming a detection limit of 10^-1 photon s^-1 cm^-2 sr^-1, proposed X-ray telescopes can detect O VIII emission from z=0.125 out to 80% of the virial radius (Rvir) of groups and clusters and out to 0.4Rvir for haloes with masses Mhalo=10^12-13 Msun. Emission lines from C VI, N VII, O VII, and Ne X can be detected out to smaller radii, 0.1-0.5Rvir. With a detection limit of 10^-20 erg s^-1 cm^-2 arcsec^-2, future UV telescopes can detect C III emission out to 0.2-0.6Rvir at z=0.25. C IV, O VI, Si III, and Si IV can be seen out to 0.1-0.2Rvir for Mhalo>10^12 Msun. Optical HI H-alpha emission is comparable in strength to C III emission. At z=3 it may be possible to observe C III out to 0.2-0.3Rvir and other rest-frame UV lines out to ~0.1Rvir for Mhalo>10^11 Msun with upcoming optical instruments. Metal-line emission is typically biased towards high density and metallicity and towards the temperature at which the emissivity curve of the corresponding metal line peaks. The bias is similar for the different soft X-ray lines considered, whereas it varies strongly between different UV lines. Active galactic nucleus (AGN) feedback can change the inner surface brightness profiles significantly, but it generally does not change the radius out to which the emission can be observed. Metal-line emission is a promising probe of the warm and hot, enriched gas around galaxies and provides a unique window into the interactions between galaxies and their gaseous haloes.
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We present an analysis of the diffuse soft X-ray emission from the nuclear region of M51 combining both XMM-Newton RGS and Chandra data. Most of the RGS spectrum of M51 can be fitted with a thermal model with a temperature of $sim0.5$ keV except for the OVII triplet, which is forbidden-line dominated. The Fe L-shell lines peak around the southern cloud, where the OVIII and NVII Lya lines also peak. In contrast, the peak of the OVII forbidden line is about 10$$ offset from that of the other lines, indicating that it is from a spatially distinct component. The spatial distribution of the OVII triplet mapped by the Chandra data shows that most of the OVII triplet flux is located at faint regions near edges, instead of the southern cloud where other lines peak. This distribution of the OVII triplet is inconsistent with the photoionization model. Other mechanisms that could produce the anomalous OVII triplet, including a recombining plasma and charge exchange X-ray emission, are discussed.
Past X-ray observations of the nearby luminous quasar PDS 456 (at $z=0.184$) have revealed a wide angle accretion disk wind (Nardini et al. 2015), with an outflow velocity of $sim-0.25c$, as observed through observations of its blue-shifted iron K-shell absorption line profile. Here we present three new XMM-Newton observations of PDS 456; one in September 2018 where the quasar was bright and featureless, and two in September 2019, 22 days apart, occurring when the quasar was five times fainter and where strong blue-shifted lines from the wind were present. During the second September 2019 observation, three broad ($sigma=3000$ km s$^{-1}$) absorption lines were resolved in the high resolution RGS spectrum, which are identified with blue-shifted OVIII Ly$alpha$, NeIX He$alpha$ and NeX Ly$alpha$. The outflow velocity of this soft X-ray absorber was found to be $v/c=-0.258pm0.003$, fully consistent with iron K absorber with $v/c=-0.261pm0.007$. The ionization parameter and column density of the soft X-ray component ($logxi=3.4$, $N_{rm H}=2times10^{21}$ cm$^{-2}$) outflow was lower by about two orders of magnitude, when compared to the high ionization wind at iron K ($logxi=5$, $N_{rm H}=7times10^{23}$ cm$^{-2}$). Substantial variability was seen in the soft X-ray absorber between the 2019 observations, declining from $N_{rm H}=10^{23}$ cm$^{-2}$ to $N_{rm H}=10^{21}$ cm$^{-2}$ over 20 days, while the iron K component was remarkably stable. We conclude that the soft X-ray wind may originate from an inhomogeneous wind streamline passing across the line of sight and which due to its lower ionization, is located further from the black hole, on parsec scales, than the innermost disk wind.
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