Submillimeter galaxies (SMGs) at z≳ 1 are luminous in the
far-infrared, and have star formation rates, SFR, of hundreds to
thousands of solar masses per year. However, it is unclear whether they
are true analogs of local ULIRGs or whether the mode of their star
formation is more similar to that in local disk galaxies. We target
these questions by using Herschel-PACS to examine the conditions in the
interstellar medium (ISM) in far-infrared luminous SMGs at z˜ 1-4.
We present 70-160 μm photometry and spectroscopy of the [O IV]26
μm, [Fe II]26 μm, [S III]33 μm, [Si II]34 μm, [O III]52
μm, [N III]57 μm, and [O I]63 μm fine-structure lines and the
S(0) and S(1) hydrogen rotational lines in 13 lensed SMGs identified by
their brightness in early Herschel data. Most of the 13 targets are not
individually spectroscopically detected; we instead focus on stacking
these spectra with observations of an additional 32 SMGs from the
Herschel archive—representing a complete compilation of PACS
spectroscopy of SMGs. We detect [O I]63 μm, [Si II]34 μm, and [N
III]57 μm at ≥slant 3σ in the stacked spectra, determining
that the average strengths of these lines relative to the far-IR
continuum are (0.36+/- 0.12)× {10}-3, (0.84+/-
0.17)× {10}-3, and (0.27+/- 0.10)×
{10}-3, respectively. Using the [O III]52 μm/[N III]57
μm emission line ratio, we show that SMGs have average gas-phase
metallicities ≳ {Z}⊙ . By using PDR modeling and
combining the new spectral measurements with integrated far-infrared
fluxes and existing [C II]158 μm data, we show that SMGs have average
gas densities, n, of ˜ {10}1-3 {{cm}}-3 and
FUV field strengths, {G}0˜ {10}2.2-4.5 (in
Habing units: 1.6× {10}-3 {erg} {{cm}}-2
{{{s}}}-1), consistent with both local ULIRGs and lower
luminosity star-forming galaxies.
Herschel is an ESA space observatory with science instruments provided
by European-led Principal Investigator consortia, and important
participation from NASA.