English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

We have studied the molecular hydrogen energetics of the edge-on spiralgalaxy NGC\,891, using a 34-position map in the lowest three pure rotationalH$_2$ lines observed with the Spitzer Infrared Spectrograph. The S(0), S(1),and S(2) lines are bright with an extinction corrected total luminosity of$\sim2.8 \times 10^{7}$ L$_{\odot}$, or 0.09\% of the total-infrared luminosityof NGC\,891. The H$_2$ line ratios are nearly constant along the plane of thegalaxy -- we do not observe the previously reported strong drop-off in theS(1)/S(0) line intensity ratio in the outer regions of the galaxy, so we findno evidence for the very massive cold CO-free molecular clouds invoked toexplain the past observations. The H$_2$ level excitation temperatures increasemonotonically indicating more than one component to the emitting gas. More than99\% of the mass is in the lowest excitation (T$_{ex}$ $\sim$125 K) ``warm''component. In the inner galaxy, the warm H$_2$ emitting gas is $\sim$15\% ofthe CO(1-0)-traced cool molecular gas, while in the outer regions the fractionis twice as high. This large mass of warm gas is heated by a combination of thefar-UV photons from stars in photo-dissociation regions (PDRs) and thedissipation of turbulent kinetic energy. Including the observed far-infrared[OI] and [CII] fine-structure line emission and far-infrared continuum emissionin a self-consistent manner to constrain the PDR models, we find essentiallyall of the S(0) and most (70\%) of the S(1) line arises from low excitationPDRs, while most (80\%) of the S(2) and the remainder of the S(1) line emissionarises from low velocity microturbulent dissipation.

The Energetics of Molecular Gas in NGC 891 from H2 and FIR Spectroscopy