2016/09/06 by Selcuk Topal, Selçuk Topal, Martin Bureau +6 · 15 citations
Physics and Astronomy · #Astronomy #Astrophysics #Astrophysics and Star Formation Studies #Classical mechanics #Galaxies: Formation, Evolution, Phenomena #Galaxy #Geometry #Hubble sequence #Kinematics #Line (geometry) #Luminous infrared galaxy #Milky Way #Physics #Stellar kinematics #Stellar, planetary, and galactic studies #astro-ph.GA
paper · pdf · doi:10.1093/mnras/stw2257
published in Monthly Notices of the Royal Astronomical Society 463(4), 4121-4152 (Oxford University Press) · Accepted for publication in MNRAS, 34 pages, 21 figures, 7 tables
arxiv created 2016/09/06 · openalex publication_date 2016/09/08 · openalex created_date 2016/09/16 · arxiv updated 2016/09/21 · openalex updated_date 2026/08/05
Molecular clouds are the stellar nurseries of galaxies, and probing their physical properties in different galaxy types has the potential to answer many important open questions regarding star formation processes and galaxy evolution. Actively star-forming spiral galaxies, including our own Milky Way, are rich in cold gas and their molecular gas reservoirs have been studied for many years. However, since so-called ‘red and dead’ early-type galaxies (ETGs; lenticulars and ellipticals) are generally thought to be very poor in molecular gas, star formation within them is thought to have largely stopped. Nevertheless, roughly 10 years after the first detection of molecular gas in external spiral galaxies (Rickard et al. 1975; Solomon & de Zafra 1975), different phases of the interstellar medium (ISM) of ETGs were also studied, through observations of X-rays (e.g. Forman, Jones & Tucker 1985), optical emission lines (e.g. Caldwell 1984), H I (e.g. Knapp, Turner & Cunniffe 1985) and CO (e.g. Wiklind & Rydbeck 1986; Welch & Sage 2003; Sage, Welch & Young 2007). Young et al. (2011) carried out the most extensive survey of molecular gas [12CO(1–0)] in ETGs so far, in the 260 galaxies of the volume-limited ATLAS3D sample1 (Cappellari et al. 2011). They obtained a 22 per cent detection rate, with H2 masses ranging from 107 to 109 M⊙. The CO-rich ETGs in the ATLAS3D sample were further studied to probe the molecular gas properties in more details. Interferometric observations of 12CO(1–0) in 40 objects were presented in Alatalo et al. (2013), revealing a variety of CO morphologies (discs, rings, bars, and spiral arms), with sizes smaller than in spirals in absolute terms but similar when compared to their optical extent (Davis et al. 2013a). The molecular gas kinematics is generally regular (Davis et al. 2013a), allowing one to easily probe the Tully–Fisher (luminosity–rotational velocity; Tully & Fisher 1977) relation of ETGs (Davis et al. 2011b). However, gas–star kinematic misalignments indicate that the molecular gas has an external origin in at least one third of the systems, with significant field–cluster environmental differences (Davis et al. 2011a). The molecular gas can also be used to study the physical conditions (temperature, density, column density, opacity, excitation mechanism, etc.) within the dense cold gas of ETGs, where star formation takes place. For example, different transitions of a given molecule are good proxies for the gas temperature [e.g. 12CO(2–1)/ 12CO(1–0)], isotopologues probe the gas optical depth and column density (e.g. 13CO/ 12CO), and complex molecules (e.g. HCN, HCO+, HNC, and HNCO) require much larger critical densities (up to ncrit ≈ 106 cm−3) to be excited compared to simpler ones (typically ncrit ≈ 103 cm−3). More subtle effects also exist. HCN and its isotopomer HNC trace, respectively, the warm-dense and cool-slightly less dense parts of a cloud, while HCO+ traces even more tenuous regions (Huettemeister et al. 1995). HCO+ can also be enhanced in shocks associated with young supernova remnants (SNRs), due to cosmic rays (CRs) in the shocked material (Dickinson et al. 1980; Wootten 1981; Elitzur 1983), and is therefore also an important tracer of CR-dominated regions of the ISM. HNCO, on the other hand, is a good tracer of shocked gas (Meier & Turner 2005, 2012; Rodríguez-Fernández et al. 2010; Ott et al. 2014), and it correlates well with SiO, a well-known shock tracer (Zinchenko, Henkel & Mao 2000). Focusing on the physical conditions of the through observations of the and other et al. and et al. also et al. that the molecular of ETGs are generally similar to of spirals and but different from of and galaxies the are with other and stellar the gas absolute and stellar et al. The 12CO(1–0) of ETGs also to on et al. observations of galaxies and and and galaxies are and their gas is with the stellar that the gas is processes (e.g. stellar et al. is from the galaxy formation is a the at a of while is in a including spirals at a of However, since is in the of the and the and its is galaxies are to have significant with other galaxies to have been environmental The properties of the galaxies are in properties of and & Tully Young et al. et al. et al. et al. et al. and also to be for ‘red and dead’ systems, allowing the first study of molecular in an the of and in CO lines and probing tenuous molecular gas, well lines of more complex molecules and the lines for the first in probing The most an in galaxies, the of an & & & with gas in a within the and in an the of the and observations also a a in the of et al. that the and emission in in the and at on of it their The of is with its but that of is more given its the in have a different the of have to be Nevertheless, our and in of The first of our study is to the of the molecular the galaxy a of and to study the physical properties of the molecular gas in of and also the molecular with a et al. 2007). is to the star formation in ETGs to that in other types of For the are compared with at the of and other well with of molecular clouds in the spiral and regions of galaxies et al. et al. et al. et al. 2010; et al. The is the observations and while the and a of the the in and through the in is presented in and in the for in and The 12CO(1–0) and observations were carried out in with per with in the of a and of at The observations were carried out in with per with the of a of at The and observations were obtained with a of a in a observations of and were obtained in For of and per were used with the an of for of and used a for with the lines were with a of the lines on an optical of the galaxy from the of at the of The observations obtained with are with The sizes are also with with The on the the for and were et al. on the are from 10 to per cent of the in of 10 per The are to and on the are The molecular and are also in dense gas and were also the de de with and in the an of for with a of The observations of the dense gas were obtained with a of at and with to the galaxy the galaxy the of to The of the observations to a of and for the tenuous and dense gas respectively, while that of the observations to The for are in for and with of and the dense gas and were carried out the a of at and for the dense gas The with a of 12CO(1–0) observations were obtained at in the a of at were used with a of the of to The of the observations to a of and for and the dense gas respectively, while that of the observations to The for are in The were the and & 1995). for were and The of the to when the and were a a within of the The were and to the carried out the at (typically a the for and for a given were one and the is a with the to the different were with a of and for the CO lines and dense gas respectively, the The were to a to the of the in regions of The were and a of at to that of the and obtained for molecular that to the the and were the with the and the molecular gas so that the emission is The presented in and therefore the of the galaxies on the but for The are to and The were the and and and used for the including the and and the of the used to and with the and sizes were for further in and the of and the extent of the emission first be in the and of The were therefore first and with a to that of the The were at a is the of the and of emission for were the and the The emission for a and used to the of the and in that The are in and for the 12CO(1–0) is the most in galaxies, the emission for the other lines be more than 12CO(1–0) emission the of the galaxies, that be the for the other lines were also used to the extent of the emission when emission is in the and regions the on the emission in The in were of the emission regions in the a for molecular a the of the galaxies in the and in the the galaxy larger than the the emission is generally to the The of the lines in and are in and The of the of the the of the the and of et al. 2013a). of the lines in The are with and at while the at The for the observations of and is and respectively, while that for the observations of and is and The in and in are on the The molecular and the are also in but for The for the observations of and are and can be in and the of the CO lines in galaxies in and the of in with a and an can easily be in the of galaxy & for a & & and & for the kinematics of gas to on to the within the at the of the a a and to the in the also the of the a an and to the of the The to the is of gas, a the and and in the However, is material at least in where the of the 12CO(1–0) a of at in less so for other tenuous gas The emission from the is less at and since emission from the and to the at and kinematic and generally The of that it is while the of the kinematic the optical in and The kinematic first in & and in Alatalo et al. (2013), from 12CO(1–0) The has a extent of in and in while the has a of in and in and lines with and in were also with the For the lines are et al. and et al. while for are 12CO(1–0) & Sage and et al. the is smaller than the of and at and and are to the molecular gas of the galaxies the out the of the from the and the our and for the in our a of to that of the at the given on the galaxies, The for the and 22 and respectively, while that for the and lines the were in temperature temperature the the given in the associated on the For at the The and and more the emission the are in and for and also of molecular for lines the of our and with lines the from our and observations lines from the The and lines were so their are for the first but for and but a at their the of galaxy a in the of for used the and for used the a of at and of and at and were to a at and is but Alatalo et al. a of at a of at at and at molecular lines require different physical conditions to be so a of are to probe complex types of on the physical conditions of the gas the of and of the a of the physical conditions in kinematic of the galaxies and the of a of the galaxy a of for kinematic a a of physical a molecular and the density a given molecular critical density the excitation and dense gas excitation is with the most CO 12CO(1–0) and its have ncrit ≈ 103 and have critical densities to ncrit ≈ 106 that when the gas are within the in a of the critical density a and a excitation temperature than that due to with H2 our in different to probe the physical conditions of different phases of the molecular ISM. The of CO lines the and column density of the tenuous molecular of dense gas the density, and of the dense molecular gas (e.g. and stellar and supernova and of CO to dense gas the dense gas our observations of and in are roughly our observations of the the of lines for the of first for molecular have the of and and They are also to a of the in our the of the at The of to be the our from to the the so that and the of in For our first to were from and of were for in of the lines dense gas and CO dense gas For the tracer and lines that in regions of the a a on the The are in where the indicate of CO lines and dense gas tracer lines in of CO lines with the 12CO(1–0) of dense gas tracer lines with the are at indicate to the The in and in are on the but for the of CO to dense gas tracer lines in and but for the from the for our to the of the kinematic in the and a of the at a of the one of one in the in of kinematic at a to the at on a kinematic were the of The used to the are a in the parts while a in the parts and a is in the parts a is of The of the were a of the The are in and are in that while the in are of the in are of of the the of and where the are and one in the and associated indicate where kinematic and are the of a and associated indicate where a is a The are one in to the the of the the 12CO(1–0) 12CO(1–0) and the at in the of the where a kinematic is the of for where kinematic are and for where kinematic are The and lines on the the to the emission of the and the respectively, while the lines the of the The and lines on the and indicate of with to the of the associated per cent of the emission of lines on the and indicate the galaxy lines on the the of the to the at that of CO lines and dense gas tracer lines a of in of CO lines the and of dense gas tracer lines the and are with The in are on the of but for the of CO to dense gas tracer lines in and but for the where is at least one where is the in the an to the of where is the in the of the and is the of the used to the at that The on the in the and the on the of are and for the CO lines and and for the dense gas tracer lines For our third probe the physical conditions of the molecular gas the the et al. 2007). The and the are at in The for are the molecular gas temperature H2 density and column densities the critical densities of the are of larger than of the CO the of lines and to the associated molecular gas dense and the of are tenuous and dense gas in the of tenuous gas in the of and dense gas in the of The are in in in the in a and and the most from the of the and and The are generally with the most but the on the are due to the of the and within a kinematic are at the for in more the H2 density is generally well for the tenuous and dense molecular gas at least in the the indicate that is larger and smaller in the dense gas than in the tenuous gas The temperature of the tenuous gas in the has a at 10 the of the in the with a at the in the most However, the most in the are are for the dense gas with much but the are while to be a for the dense gas to have a and smaller the is significant given the For the dense gas in the the the indicate a similar dense gas density, column density and temperature in the of the for are than for and However, in the of have at least one that is a the at are the the physical conditions of a molecular tenuous and dense gas the of the ETGs and probing the of molecular a of the galaxy and of molecular gas also the properties of and the that are their and their molecular gas compared to other our in also their star et al. used 22 from the to the star formation of ETGs in the ATLAS3D the of and are that of is of that of However, the star formation is than the and have in the per cent of the ATLAS3D et al. also other ETGs, and have star formation than of spiral and galaxies, with molecular gas of 10 and respectively, and than that of spirals with Nevertheless, the molecular the of galaxies in the to in the of spirals and that the molecular are to is star formation in our and the tenuous molecular gas to on of the in The tenuous gas is in the than in the of but while is also the in its is more to the the tenuous gas is more than the dense gas the galaxy least given the roughly in most the generally and to the emission in the of a more the molecular gas therefore that different physical conditions are to in the and the complex the in galaxies, at the of in a the in the of and a and is easily in the of galaxy the is associated with a within the while the can be associated with an and it is that and the tenuous have similar and from the the in is more the from the in the the 12CO(1–0) of the in is larger than that of the in to be molecular gas the in where the the of the be to be The emission at the of the in is due to the are However, other emission are the and in the and that the molecular gas in of is The molecular gas and be associated with regions of star The the and in the is easily a of gas on are of gas the shocks on the of the However, the at the in 12CO(1–0) and is but & on that emission in of gas in of the of it is that the molecular gas in the galaxies and is in but kinematic a and an the dense gas and are the in their the it is to the physical conditions to be similar within kinematic are but for the conditions to be different the The of the kinematic is the emission in the to with in the the of CO to dense gas tracer that the of the other lines in galaxies, is dense gas at the very of the in The at is much in and that a different excitation gas in the very of (e.g. the emission is in due to a The since while is a in is in the for and for the first the molecular lines were with the Young et al. and et al. in the and where the the CO extent is the observations larger The the out much of the tenuous molecular gas is a for the dense gas are in the and the and with a and are with other given the the CO tenuous molecular are larger than the and However, given their emission the and dense gas tracer are with in the of the from to can further indicate different gas physical conditions the of the galaxies a different for the dense gas of to a extent and The in relation to the is the that the of the tenuous molecular gas in are than the the of The the and is most due to a of the of the in the and per cent is for observations Alatalo et al. The of the from different lines are in while the of the a of for kinematic and the are in the for the tenuous and dense molecular gas in the and rings, are in and that for rings, the different to different within the rings, so significant with The its well be the of the of the 12CO(1–0) and lines that the is than but is smaller in the than the a temperature in the tenuous molecular gas of the is the of the the kinematic that at the the and is smaller than that at the in the at the of the and but is due to the very emission associated the temperature the tenuous gas in the and of be our but the are have CO transitions to the the the the is the at are and and so that the be with a of The are to and a to temperature in the ≈ but much but can be for the to and its were in observations of lines in the regions of that is larger than in the regions of et al. that the CO gas at the of is than that in However, since the observations the emission from the and be with in the of to its is larger a of in the regions of the of and than in other regions parts of the and the in the where a is is of the and at the of the of CO and its and a of the is the at and of the of are at the in the is at that the in the of are to for and to and of the [e.g. the detection of and at the the the indicate tenuous molecular gas in the regions of the that of the is but of the However, the emission in the regions from the well the of the of a significant at to a 10 and while the at has associated with a of the is since it is a the optical of the associated The optical of the 12CO(1–0) and lines for the are one the CO column density to with its optical at the the 12CO(1–0) optical optical and CO column density in the are generally larger than in the to our on the However, the are of the and most at the of the the associated and column densities are the in smaller in the regions per cent with other regions in the However, the is and to be associated with a generally smaller in the compared to the more in the at the of the the of the and are in the the at larger in the The is at are within the et al. also the and in the regions of galaxies the kinematic our the and the of with the et al. within the our for the and the of is larger than that et al. is less than its molecule and is generally largely in the optical The larger the the the gas The in and more in is that the CO gas is in the than in the the is and be to the parts of the in within kinematic (e.g. in the of and in indicate of the physical conditions within and gas are the of stellar and therefore indicate more star are in spirals et al. and the can have in star-forming regions spiral et al. 2011). that star formation is more in the of our galaxies than their least in a with the of galaxies, early-type spirals (e.g. & also with the for the molecular gas in the to that in the in the observations to The of the of the dense gas are in and for and where HCN but the other The of the and are a of in and the dense gas are in the (typically and The to is the detection of in the of significant at and The and are larger than at and of a with that is the the dense gas and et al. also and in the regions of galaxies, ≈ with the for the from young and X-rays from and from supernova have effects on the molecular gas physical and an important in the and of the of the and the molecular the of clouds regions & et al. while X-rays and regions & & and can HCN, but since X-rays can the gas much can so more et al. The of & that the density a in while in for a molecular indicate that for the dense gas for galaxies and with the that in galaxies, therefore that are most the physical conditions the of the molecular in the of densities are also much larger than obtained for the tenuous molecular gas (typically of are also with the of in and However, since supernova are also to be and young are for the HCN supernova were a in the HCO+ have been enhanced to to in and spiral galaxies have the a in the of galaxies, but be from supernova in that have similar the and in the of are the are larger in the than in the different and in the kinematic The HCN to be HCO+, HNC and enhanced in the More observations of HCN, HCO+, HNC and in the of galaxies to the physical conditions of their dense molecular ISM. The of CO to tracer are in and where CO but the while the of the a of are in and also and from is a of the CO to tracer the and for and the are larger in the for the HCN and and the differences are to be even than that and the in the are for a in HCN in is for the HCN, HCO+ and HNC but well also for are smaller than the For example, the of and in the of are at least per cent than in its more than the of in the of are at least in its the of are at least per cent the the of dense molecular gas that the of dense gas is larger in the of the galaxies than in their is with dense gas generally in galaxies, and with the more star formation and CO in the for and that the and the of are with et al. for the regions of the while the and in the are smaller per the and the of are with et al. but the in the is smaller per and in et al. is generally that traces the molecular gas of galaxies, due to its critical density ≈ 103 cm−3). of the gas therefore be in star However, transitions of more complex molecules and have critical densities (up to ncrit ≈ 106 cm−3) of their larger molecules are therefore generally of molecular gas, more to star-forming regions than CO (e.g. & Solomon the of CO to dense gas therefore the dense gas of the for smaller our that the have a larger of dense gas than the Nevertheless, for the in the is an of HCO+ supernova the can that with CO to However, in the the are to be is in HCO+ with to HCN and other so the of in the and gas is to the in the the in the can further be the of HCN from young et al. that at densities than similar to the in the of that a dense gas with but is the most of physical conditions the and than in the of the Milky Way, and other the with & & Solomon et al. in a of the dense gas with in and are the in their of and are larger than in the of and the have are dense gas at in the galaxies and & also that the is to the the the that the is in the than in the is for the regions of galaxies, the of and are well within their and is with the that to larger dense gas the and in the of galaxies are similar for that the dense gas the of the gas in the of galaxies are that the in the of galaxies were the of of HCN X-rays from an that also the smaller