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Register a new userMaking Galaxies in a Cosmological Context: The Need for Early Stellar Feedback
صنع المجرات في سياق كوسمولوجي: الحاجة إلى ردود الفعل النجمية المبكرة
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G. Stinson
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نقدم برنامج Making Galaxies in a Cosmological Context (MaGICC) للمحاكاة بطريقة الذبابات الجزيئية المنعكسة (SPH). نصفح عن دراسة المعلمات لمحاكاة تشكيل المجرات لمجرة L* تستخدم الانفعال النجمي المبكر مع الانفعال النجوم الشعاعي لتطابق علاقة الكتلة النجمية والكتلة الهالية. على الرغم من أن الانفعال النجوم الشعاعي يمكن أن يقلل من التشكيل النجمي كافياً لتطابق علاقة الكتلة النجمية والكتلة الهالية، فإن المجرة تشكل الكثير من النجوم قبل z = 2 لتطابق التطور المرئي باستخدام التطابق الغذائي. يعمل الانفعال النجمي المبكر الخاص بنا بشكل حراري فقط وبالتالي يعمل كمصدر للايونيزاسيون كذلك كمصدر للضغط الإضافي من الإشعاع الناتج من النجوم الشابة الكبيرة. يحرك الانفعال المبكر الحرارة إلى > 10 ^ 6 K قبل التبريد إلى 10 ^ 4 K. ينشأ الضغط من هذا الغاز الساخن دفقاً أكثر واسعاً ويمنع المزيد من التشكيل النجمي قبل z = 1 من أجل الانفعال النجوم الشعاعي فقط. يحتوي الدفق الناتج على مجرة بدوران مستقر، وملخص سطح السطوع الأسيوي، ويطابق العديد من علاقات الدفق التالية. ينشأ الدفق من الداخل إلى الخارج مع طول مكعب الأسيوي الزائد كما يتطور المجرة. في المجمل، يساعد الانفعال النجمي المبكر في محاكاة المجاري التي تطابق النتائج المرئية في الأحمر الداكن والأحمر المتأخر.
We introduce the Making Galaxies in a Cosmological Context (MaGICC) program of smoothed particle hydrodynamics (SPH) simulations. We describe a parameter study of galaxy formation simulations of an L* galaxy that uses early stellar feedback combined with supernova feedback to match the stellar mass--halo mass relationship. While supernova feedback alone can reduce star formation enough to match the stellar mass--halo mass relationship, the galaxy forms too many stars before z=2 to match the evolution seen using abundance matching. Our early stellar feedback is purely thermal and thus operates like a UV ionization source as well as providing some additional pressure from the radiation of massive, young stars. The early feedback heats gas to >10^6 K before cooling to 10^4 K. The pressure from this hot gas creates a more extended disk and prevents more star formation prior to z=1 than supernovae feedback alone. The resulting disk galaxy has a flat rotation curve, an exponential surface brightness profile, and matches a wide range of disk scaling relationships. The disk forms from the inside-out with an increasing exponential scale length as the galaxy evolves. Overall, early stellar feedback helps to simulate galaxies that match observational results at low and high redshifts.
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Current data broadly support trends of galaxy surface brightness profile amplitude and shape with total stellar mass predicted by state-of-the-art Lambda-CDM cosmological simulations, although recent results show signs of interesting discrepancies, particularly for galaxies less massive than the Milky Way. Here I discuss how perhaps the largest contribution to such discrepancies can be inferred almost directly from how well a given model agrees with the observed present-day galaxy stellar mass function.
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We point out a natural mechanism for quenching of star formation in early-type galaxies. It automatically links the color of a galaxy with its morphology and does not require gas consumption, removal or termination of gas supply. Given that star formation takes place in gravitationally unstable gas disks, it can be quenched when a disk becomes stable against fragmentation to bound clumps. This can result from the growth of a stellar spheroid, for instance by mergers. We present the concept of morphological quenching (MQ) using standard disk instability analysis, and demonstrate its natural occurrence in a cosmological simulation using an efficient zoom-in technique. We show that the transition from a stellar disk to a spheroid can be sufficient to stabilize the gas disk, quench star formation, and turn an early-type galaxy red and dead while gas accretion continues. The turbulence necessary for disk stability can be stirred up by sheared perturbations within the disk in the absence of bound star-forming clumps. While gas stripping processes are limited to dense groups and clusters, and other quenching mechanisms like AGN feedback, virial shock heating and gravitational heating, are limited to halos more massive than 10^12 Mo, the MQ can explain the appearance of red ellipticals even in less massive halos and in the field. The dense gas disks observed in some of todays red ellipticals may be the relics of this mechanism, whereas red galaxies with quenched gas disks are expected to be more frequent at high redshift.
Massive galaxies today typically are not forming stars despite being surrounded by hot gaseous halos with short central cooling times. This likely owes to some form of quenching feedback such as merger-driven quasar activity or radio jets emerging from central black holes. Here we implement heuristic prescriptions for these phenomena on-the-fly within cosmological hydrodynamic simulations. We constrain them by comparing to observed luminosity functions and color-magnitude diagrams from SDSS. We find that quenching from mergers alone does not produce a realistic red sequence, because 1 - 2 Gyr after a merger the remnant accretes new fuel and star formation reignites. In contrast, quenching by continuously adding thermal energy to hot gaseous halos quantitatively matches the red galaxy luminosity function and produces a reasonable red sequence. Small discrepancies remain - a shallow red sequence slope suggests that our models underestimate metal production or retention in massive red galaxies, while a deficit of massive blue galaxies may reflect the fact that observed heating is intermittent rather than continuous. Overall, injection of energy into hot halo gas appears to be a necessary and sufficient condition to broadly produce red and dead massive galaxies as observed.
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