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A Balmer-line Broad Absorption Line Quasar

Patrick B. Hall

TL;DR

This paper reports the discovery of blueshifted Balmer-line broad absorption troughs in SDSS J125942.80+121312.6, a low-ionization BAL quasar that also shows Ca II, Mg II, and excited Fe II absorption. The author argues that the strong $n=2$ population required for Balmer absorption is best explained by Ly$\alpha$ trapping in a substantial, yet Compton-thin $N_{HI}$ column, leading to a derived $N_{HI}\approx8.1\times10^{18}\ { m cm^{-2}}$, and an upper bound on electron density $n_e<1.6\times10^{13}\ { m cm^{-3}}$. This mechanism implies the absorber is relatively dust-poor and not Compton-thick, with Balmer BAL troughs being rare due to the specific geometric and physical conditions required along the line of sight. The study highlights the potential of Balmer absorption as a diagnostic of quasar winds and calls for follow-up X-ray, high-resolution UV/optical, and near-IR observations to tightly constrain the absorber’s properties and distance from the ionizing source.

Abstract

I report the discovery of blueshifted broad absorption line (BAL) troughs in at least six transitions of the Balmer series of hydrogen (Hbeta to H9) and in CaII, MgII and excited FeII in the quasar SDSS J125942.80+121312.6. This is only the fourth active galactic nucleus known to exhibit Balmer absorption, all four in conjunction with low-ionization BAL systems containing excited Fe II. The substantial population in the n=2 shell of H I in this quasar's absorber likely arises from Ly-alpha trapping. In an absorber sufficiently optically thick to show Balmer absorption, soft X-rays from the quasar penetrate to large τ_Lyαand ionize H I. Recombination then creates Ly-alpha photons that increase the n=2 population by a factor τ_Lyαsince they require about τ_Lyαscatterings to diffuse out of the absorber. Observing Ly-alpha trapping in a quasar absorber requires a large but Compton-thin column of gas along our line of sight which includes substantial H I but not too much dust. Presumably the rarity of Balmer-line BAL troughs reflects the rarity of such conditions in quasar absorbers.

A Balmer-line Broad Absorption Line Quasar

TL;DR

This paper reports the discovery of blueshifted Balmer-line broad absorption troughs in SDSS J125942.80+121312.6, a low-ionization BAL quasar that also shows Ca II, Mg II, and excited Fe II absorption. The author argues that the strong population required for Balmer absorption is best explained by Ly trapping in a substantial, yet Compton-thin column, leading to a derived , and an upper bound on electron density . This mechanism implies the absorber is relatively dust-poor and not Compton-thick, with Balmer BAL troughs being rare due to the specific geometric and physical conditions required along the line of sight. The study highlights the potential of Balmer absorption as a diagnostic of quasar winds and calls for follow-up X-ray, high-resolution UV/optical, and near-IR observations to tightly constrain the absorber’s properties and distance from the ionizing source.

Abstract

I report the discovery of blueshifted broad absorption line (BAL) troughs in at least six transitions of the Balmer series of hydrogen (Hbeta to H9) and in CaII, MgII and excited FeII in the quasar SDSS J125942.80+121312.6. This is only the fourth active galactic nucleus known to exhibit Balmer absorption, all four in conjunction with low-ionization BAL systems containing excited Fe II. The substantial population in the n=2 shell of H I in this quasar's absorber likely arises from Ly-alpha trapping. In an absorber sufficiently optically thick to show Balmer absorption, soft X-rays from the quasar penetrate to large τ_Lyαand ionize H I. Recombination then creates Ly-alpha photons that increase the n=2 population by a factor τ_Lyαsince they require about τ_Lyαscatterings to diffuse out of the absorber. Observing Ly-alpha trapping in a quasar absorber requires a large but Compton-thin column of gas along our line of sight which includes substantial H I but not too much dust. Presumably the rarity of Balmer-line BAL troughs reflects the rarity of such conditions in quasar absorbers.

Paper Structure

This paper contains 4 sections, 1 figure.

Table of Contents

  1. Background
  2. Observations
  3. Interpretation
  4. Conclusions

Figures (1)

  • Figure 1: Full SDSS spectrum of SDSS J125942.80+121312.6, smoothed by a 7 pixel boxcar, with $F_\lambda$ in units of 10$^{-17}$ erg s$^{-1}$ cm$^{-2}$ Å$^{-1}$ and $\lambda$ in Å. Uncertainties in the smoothed flux are shown by the thinner line along the bottom of the plot. Observed wavelengths are shown on the bottom axis, while rest frame wavelengths at the narrow-line emission redshift of $z=0.7517$ are shown along the top axis. Dotted lines show the expected location of emission lines at the emission redshift, while dashed lines show absorption at the peak absorption redshift of $z=0.7345$. The dot-dashed line shows the fit to the effective continuum.