楊辰濤

(中國科學(xué)院紫金山天文臺(tái)南京210008)

隨著亞毫米波望遠(yuǎn)鏡的發(fā)展,利用這些新的探測(cè)設(shè)備,人們?cè)趤喓撩撞ǘ伟l(fā)現(xiàn)了一類高紅移且富含塵埃的星系,將其稱為亞毫米星系.這類星系的發(fā)現(xiàn)革新了我們對(duì)星系的演化以及極端條件下的恒星形成過程的認(rèn)知.這些亞毫米星系是宇宙中最強(qiáng)的星暴星系,其中的恒星形成過程產(chǎn)生的能量接近愛丁頓極限.人們普遍認(rèn)為這類星系正是近鄰宇宙中那些大質(zhì)量星系的前身天體.但是，很難解釋其在高紅移為何具有較高的數(shù)密度.它們其中非常少的一部分會(huì)被處于視線方向上的大質(zhì)量星系通過引力透鏡作用放大變亮.盡管這類處于引力透鏡系統(tǒng)中的亞毫米星系十分稀少,但近來人們通過大視場(chǎng)河外巡天,發(fā)現(xiàn)了上百個(gè)此類天體,這為我們研究高紅移亞毫米星系中的星際介質(zhì)提供了一個(gè)令人振奮的新途徑.

我們從Herschel-ATLAS巡天獲得的源表中仔細(xì)地挑選出了一組受強(qiáng)引力透鏡作用的亞毫米星系的樣本.我們利用IRAM (Institut de Radioastronomie Millim′etrique)的干涉陣NOEMA (NOrthern Extended Millimeter Array)和30 m單天線望遠(yuǎn)鏡,從16個(gè)亞毫米星系中探測(cè)到了水分子(H2O)譜線.我們發(fā)現(xiàn)水分子譜線的光度和星系的紅外光度之間存在一個(gè)緊密的近似線性的相關(guān)性.我們利用水分子的遠(yuǎn)紅外激泵激發(fā)模型,從觀測(cè)數(shù)據(jù)中求出了水分子氣體和塵埃輻射的物理性質(zhì).我們發(fā)現(xiàn)這些由水分子氣體示蹤的致密熱分子氣體很可能跟劇烈的恒星形成活動(dòng)緊密關(guān)聯(lián).除了水分子氣體譜線之外,我們還在樣本中的3個(gè)星系中探測(cè)到了H2O+的若干條發(fā)射線.我們發(fā)現(xiàn)從近鄰的極亮紅外星系到高紅移的亞毫米星系,它們的H2O和H2O+發(fā)射線的光度之間呈現(xiàn)出緊密的線性關(guān)系.通過分析這兩者的譜線強(qiáng)度比,我們推測(cè)這些星系中星際介質(zhì)的氧化學(xué)過程很可能由其中劇烈的恒星形成活動(dòng)所產(chǎn)生的宇宙射線主導(dǎo).

除了水分子氣體,另外一個(gè)重要的分子氣體探針為一氧化碳(CO)分子氣體.因此,我們使用IRAM 30 m單天線毫米波射電望遠(yuǎn)鏡對(duì)亞毫米星系樣本中的多能級(jí)CO譜線進(jìn)行了觀測(cè).通過分析CO發(fā)射線的譜線輪廓,我們發(fā)現(xiàn)引力透鏡的非均勻放大效應(yīng)會(huì)使觀測(cè)到的譜線寬度被低估,這個(gè)低估最大可以到2倍左右.通過基于大速度梯度(LVG)的CO譜線輻射轉(zhuǎn)移模型,并利用結(jié)合了貝葉斯思想的馬爾可夫鏈蒙特卡洛方法,我們得到了多能級(jí)CO譜線所示蹤的分子氣體的物理性質(zhì),包括: 分子氣體的體密度、溫度以及CO分子的柱密度.通過對(duì)樣本整體的統(tǒng)計(jì)分析,我們發(fā)現(xiàn)星系整體的氣體熱壓力與其恒星形成效率呈緊密的線性相關(guān).同時(shí)還研究了星系的分子氣體與其整體恒星形成之間的關(guān)系,比如通過比較這些星系的氣塵比以及氣體耗散時(shí)標(biāo),發(fā)現(xiàn)樣本星系具有的這些性質(zhì)與其他亞毫米星系并無差異.最后,我們對(duì)比了樣本星系的CO譜線和H2O譜線的線寬,發(fā)現(xiàn)這兩者比較一致.這意味著這兩種分子氣體輻射分布的空間區(qū)域較為類似.

為了更進(jìn)一步了解高紅移亞毫米星系中分子氣體的性質(zhì)、結(jié)構(gòu)以及動(dòng)力學(xué)的性質(zhì),十分有必要利用高空間分辨率的觀測(cè)對(duì)其進(jìn)行研究.因此,我們利用目前最強(qiáng)大的(亞)毫米波干涉陣ALMA(Atacama Large Millimeter/sub-millimeter Array)和北半球最先進(jìn)的毫米波干涉陣NOEMA對(duì)樣本中最亮的兩個(gè)星系進(jìn)行了高空間分辨率的觀測(cè).我們對(duì)CO譜線和H2O譜線的高分辨率成圖進(jìn)行了對(duì)比,發(fā)現(xiàn)星系冷塵埃的輻射區(qū)域要小于CO和H2O的區(qū)域,但是后兩者的分布比較一致.通過構(gòu)架其動(dòng)力學(xué)模型我們發(fā)現(xiàn)這兩個(gè)星系的運(yùn)動(dòng)學(xué)圖像可以很好地用一個(gè)旋轉(zhuǎn)盤模型來解釋.據(jù)此,我們求出了其動(dòng)力學(xué)質(zhì)量以及有效半徑.

隨著未來NOEMA的發(fā)展以及ALMA的不斷運(yùn)行升級(jí),我們將不斷地?cái)U(kuò)大受強(qiáng)引力透鏡作用的高紅移亞毫米星系樣本,并在條件容許的情況下探測(cè)那些未經(jīng)過引力透鏡作用的星系.通過觀測(cè)這些星系中不同種類的分子氣體探針,我們將對(duì)這些星系中的星際介質(zhì)的物理化學(xué)狀態(tài)以及其與恒星形成的關(guān)系有更新的認(rèn)識(shí).

The discovery of a population of high-redshift dust-obscured submillimeter galaxies(SMGs)from ground-based submillimetre (submm)cameras has revolutionised our understanding of galaxy evolution and star formation in extreme conditions.They are the strongest starbursts in the Universe approaching the Eddington limit,and are believed to be the progenitors of the most massive galaxies today.However,theoretical models of galaxy evolution have even been unable to explain the large number of detections of high-redshift SMGs.A very few among them are gravitationally lensed by a foreground galaxy.Recent wide-area extragalactic surveys have discovered hundreds of such strongly lensed SMGs,opening new exciting opportunities for observing the interstellar medium in these exceptional objects.

We have thus carefully selected a sample of strongly gravitational lensed SMGs based on the submm flux limit from theHerschel-ATLAS sample.Using the IRAM (Institut de Radioastronomie Millim′etrique)telescopes,we have built a rich H2O-line-detected sample of 16 SMGs.We found a close-to-linear tight correlation between the H2O line and total infrared luminosity.This indicates the importance of far-IR pumping to the excitation of the H2O lines.Using a far-IR pumping model,we have derived the physical properties of the H2O gas and the dust.We showed that H2O lines trace a warm dense gas that may be closely related to the active star formation.Along with the H2O lines,several H2O+lines have also been detected in three of our SMGs.We also find a tight correlation between the luminosity of the lines of H2O and H2O+from the local ULIRGs(UltraLuminous Infrared Galaxies)to the high-redshift SMGs.The flux ratio between H2O+and H2O suggests that cosmic rays from strong star forming activities are possibly driving the related oxygen chemistry.

Another important common molecular gas tracer is the CO line.We have observed multiple transitions of the CO lines in each of our SMGs with the IRAM 30 m telescope.By analysing the CO line profile,we discovered a significant differential lensing effect that might cause underestimation of the linewidth by a factor of ～2.Using the LVG (Large Velocity Gradient)modelling and fitting the multi-J CO fluxes via a Bayesian approach,we derived gas densities and temperature,and the CO column density per unit velocity gradient.We then found a correlation between the gas thermal pressure and the star formation efficiency.We have also studied the global properties of the molecular gas and its relationship with star formation.We have derived the gas to dust mass ratio and the gas depletion time of our sample galaxies,which show no difference compared with other SMGs.With the detections of atomic carbon lines in our SMGs,we extended the local linear correlation between the CO and CI line luminosity.Finally,we compared the linewidths of the CO and H2O emission lines,which agree very well with each other.This suggests that the emitting regions of these two molecules are likely to be co-spatially located.

In order to understand the properties of molecular emission in high-redshift SMGs,and more generally,the structure and the dynamical properties of these galaxies,it is crucial to acquire high-resolution images.We thus observed two of our brightest sources with the ALMA (Atacama Large Millimeter/sub-millimeter Array)and NOEMA (NOrthern Extended Millimeter Array)interferometers using their high spatial resolution configuration.These images have allowed us to reconstruct the intrinsic morphology of the sources.We compared the CO,H2O,and dust emission.The cold dust emission has a smaller size compared with the CO and H2O gas,while the latter two are similar in size.By fitting the dynamical model to the CO data of the sources,we have shown that the sources can be modelled with a rotating disk,and the projected dynamical mass and the effective radius of those sources have been obtained.

With the future NOEMA and ALMA,we will be able to extend such kind of observations to a larger sample lensed SMGs and even to unlensed SMGs,to study various gas tracers,and to refresh our understanding of the physical conditions of the ISM and their relation to the star formation.