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The ATLAS 3D project - XVI. Physical parameters and spectral line energy distributions of the molecular gas in gas-rich early-type galaxies

arXiv:1212.2630 · doi:10.1093/mnras/sts598

Abstract

[Abridged] We present a detailed study of the physical properties of the molecular gas in a sample of 18 molecular gas-rich early-type galaxies (ETGs) from the ATLAS $3 D s am pl e . O u r g o a l i s t o b e tt er u n d er s t an d t h es t a r f or ma t i o n p r ocessesocc u r r in g in t h ose g a l a x i es, s t a r t in g h er e w i t h t h e d e n ses t a r - f or min g g a s . W e u see x i s t in g in t e g r a t e d$ ^{12} $C O (1 - 0, 2 - 1),$ ^{13} $C O (1 - 0, 2 - 1), H C N (1 - 0) an d H C O$ ^{+} $(1 - 0) o b ser v a t i o n s an d p r ese n t n e w$ ^{12} $C O (3 - 2) s in g l e - d i s h d a t a . F r o m t h ese, w e d er i v e f or t h e f i r s tt im e t h e a v er a g e k in e t i c t e m p er a t u r e, H$ _{2} $v o l u m e d e n s i t y an d co l u mn d e n s i t y o f t h ee mi tt in g g a s, t hi s u s in g an o n - L T E t h eor e t i c a l m o d e l . S in ce t h e C O l in es t r a ce d i f f er e n tp h y s i c a l co n d i t i o n s t han o f t h ose t h eH C N an d H C O$ ^{+} $l in es, t h e tw ose t so f l in es a r e t r e a t e d se p a r a t e l y . W e a l soco m p a r e f or t h e f i r s tt im e t h e p r e d i c t e d C O s p ec t r a l l in ee n er g y d i s t r ib u t i o n s (S L E D s) an d g a s p r o p er t i eso f o u r m o l ec u l a r g a s - r i c h E T G s w i t h t h oseo f a s am pl eo f n e a r b y w e l l - s t u d i e dd i sc g a l a x i es . T h e g a se x c i t a t i o n co n d i t i o n s in 13 o f o u r 18 E T G s a pp e a r ana l o g o u s t o t h ose in t h ece n t r eo f t h e M i l k y W a y . S u c h r es u l t s ha v e n e v er b ee n o b t ain e d b e f or e f or E T G s an d o p e nan e w w in d o w t oe x pl or e f u r t h er s t a r - f or ma t i o n p r ocesses in t h e U ni v er se . T h eco n c l u s i o n s d r a w n s h o u l d n e v er t h e l ess b eco n s i d er e d c a r e f u l l y, a s t h ey a r e ba se d o na l imi t e d n u mb er o f o b ser v a t i o n s an d o na s im pl e m o d e l . I n t h e n e a r f u t u r e, w i t hhi g h er C O t r an s i t i o n o b ser v a t i o n s, i t s h o u l d b e p oss ib l e t o b e tt er i d e n t i f y t h e v a r i o u s g a sco m p o n e n t s p r ese n t in E T G s, a s w e l l a s m or e p r ec i se l y d e t er min e t h e i r a ssoc ia t e d p h y s i c a l co n d i t i o n s . T o a c hi e v e t h ese g o a l s, w es h o w h er e f r o m o u r t h eor e t i c a l s t u d y, t ha t mi d - J C O l in es (s u c ha s t h e$ ^{12}$CO(6-5) line) are particularly useful.

28 pages, Accepted in MNRAS, 7 figures, 6 tables