Context :

Aims : We aim to understand the unexpected presence of mm-wave molecular absorption at -200 km s ^ { -1 } < { v } < -140 km s ^ { -1 } in a direction that is well away from regions of the Galactic bulge where CO emission at such velocities is prominent .

Methods : We compared 89 GHz Cycle 2 ALMA absorption spectra of \mathrm { HCO ^ { + } } , HCN , and HNC toward the extragalactic continuum source B1741-312 at l=-2.14 ^ { o } , b=-1.00 ^ { o } with existing CO , H I , and dust emission and absorption measurements .
We placed the atomic and molecular gas in the bulge and disk using circular and non-circular galactic kinematics , deriving N ( H I ) from a combination of 21cm emission and absorption and we derive N ( \mathrm { H _ { 2 } } ) from scaling of the \mathrm { HCO ^ { + } } absorption .
We then inverted the variation of near-IR reddening E ( J-K ) with distance modulus and scale E ( J-K ) to a total gas column density N ( H ) that may be compared to N ( H I ) and N ( \mathrm { H _ { 2 } } ) .

Results : At galactocentric radii R _ { gal } > 1.5 kpc , conventional measures such as the standard CO- \mathrm { H _ { 2 } } conversion factor and locally observed N ( \mathrm { HCO ^ { + } } ) /N ( \mathrm { H _ { 2 } } ) ratio separately imply that H I and \mathrm { H _ { 2 } } contribute about equally to N ( H ) , and the gas-derived N ( H ) values are in broad agreement with those derived from E ( J-K ) .
Within the Galactic bulge at R _ { gal } < 1.5 kpc , H I contributes less than 10 % of the material inferred from E ( J-K ) , so that the molecular absorption detected here is needed to understand the extinction .

Conclusions :