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arXiv · 1712.03511

Revealing HI gas in emission and absorption on pc to kpc scales in a galaxy at z ~ 0.017

N. Gupta·R. Srianand·J. S. Farnes·Y. Pidopryhora·M. Vivek·Z. Paragi·P. Noterdaeme·T. Oosterloo·P. Petitjean
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Abstract

We present a detailed study of the quasar-galaxy pair: J1243+4043 - UGC07904. The sight line of the background quasar ( $z_q$= 1.5266) passes through a region of the galaxy ($z_g$=0.0169) at an impact parameter of 6.9 kpc with high metallicity (0.5 Z$_\odot$) and negligible dust extinction. We detect HI 21-cm absorption from the foreground galaxy at arcsecond and milliarcsecond scales. For typical cold neutral medium (CNM) temperatures in the Milky Way, this 21-cm absorber can be classified as a damped Ly$\alpha$ absorber (DLA). We infer the harmonic mean spin temperature of the gas to be $\sim$400 K and for a simple two-phase medium we estimate the CNM-fraction to be $f_{\rm CNM}$ = 0.27. This is remarkably consistent with the CNM fraction observed in the Galaxy and less than that of high-redshift DLAs. The quasar exhibits a core-jet morphology on milliarcsecond scales, corresponding to an overall extent of $\sim$9 pc at $z_g$. We show that the size of CNM absorbing clouds associated with the foreground galaxy is $>$5 pc and they may be part of cold gas structures that extend beyond $\sim$35 pc. Interestingly, the rotation measure of quasar J1243+4043 is higher than any other source in samples of quasars with high-$z$ DLAs. However, we do not find any detectable differences in RMs and polarization fraction of sight lines with or without high-$z$ ($z\ge2$) DLAs or low-$z$ ($z\le0.3$) 21-cm absorbers. Finally, the foreground galaxy UGC07904 is also part of a galaxy group. We serendipitously detect HI 21-cm emission from four members of the group, and a $\sim$80 kpc long HI bridge connecting two of the other members. The latter, together with the properties of the group members, suggests that the group is a highly interactive environment.

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N. Gupta, R. Srianand, J. S. Farnes, Y. Pidopryhora, M. Vivek, Z. Paragi, P. Noterdaeme, T. Oosterloo, P. Petitjean. 2017-12-10. Revealing HI gas in emission and absorption on pc to kpc scales in a galaxy at z ~ 0.017. https://doi.org/10.1093/mnras%2Fsty384

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