Discovery of OH absorption from a galaxy at z~0.05: implications for large surveys with SKA pathfinders

Abstract

We present the first detection of OH absorption in diffuse gas at $z>0$, along with another eight stringent limits on OH column densities for cold atomic gas in galaxies at $0<z<0.4$. The absorbing gas detected towards Q0248+430 ($z_q$=1.313) originates from a tidal tail emanating from a highly star forming galaxy G0248+430 ($z_g$=0.0519) at an impact parameter of 15 kpc. The measured column density is $N$(OH) = (6.3$\pm$0.8)$\times$10${}^{13}$($\frac{T_{\mathrm{ex}}}{3.5}$)($\frac{1.0}{f_c^{\mathrm{OH}}}$) cm${}^{-2}$, where $f_c^{\mathrm{OH}}$ and $T_{\mathrm{ex}}$ are the covering factor and the excitation temperature of the absorbing gas, respectively. In our Galaxy, the column densities of OH in diffuse clouds are of the order of $N$(OH)$\sim$10${}^{13-14}$ cm${}^{-2}$. From the incidence (number per unit redshift; $n_{21}$) of HI 21-cm absorbers at $0.5<z<1$ and assuming no redshift evolution, we estimate the incidence of OH absorbers (with log$N$(OH)$>$13.6) to be $n_{\mathrm{OH}}$ = $0.008_{-0.008}^{+0.018}$ at $z\sim 0.1$. Based on this we expect to detect 10${}_{-10}^{+20}$ such OH absorbers from the MeerKAT Absorption Line Survey. Using HI 21-cm and OH 1667 MHz absorption lines detected towards Q0248+430, we estimate ($\Delta F/F$) = (5.2$\pm$4.5)$\times 10^{-6}$, where $F\equiv g_p({\alpha }^2/\mu {)}^{1.57}$, $\alpha$ $-$ the fine structure constant, $\mu$ $-$ the electron-proton mass ratio and $g_p$ $-$ the proton gyromagnetic ratio. This corresponds to $\Delta \alpha /\alpha$($z=0.0519$) = (1.7 $\pm$ 1.4)$\times 10^{-6}$, which is among the stringent constraints on the fractional variation of $\alpha$.

arXiv comment6 pages, 3 figures, 2 tables, Accepted for Publication in ApJ Letters

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