اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدPhysics of Non-Universal Larsons Relation
215
0
0.0
(
0
)
تأليف
Renyue Cen
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
From a new perspective, we re-examine self-gravity and turbulence jointly, in hopes of understanding the physical basis for one of the most important empirical relations governing clouds in the interstellar medium (ISM), the Larsons Relation relating velocity dispersion ($sigma_R$) to cloud size ($R$). We report on two key new findings. First, the correct form of the Larsons Relation is $sigma_R=alpha_v^{1/5}sigma_{pc}(R/1pc)^{3/5}$, where $alpha_v$ is the virial parameter of clouds and $sigma_{pc}$ is the strength of the turbulence, if the turbulence has the Kolmogorov spectrum. Second, the amplitude of the Larsons Relation, $sigma_{pc}$, is not universal, differing by a factor of about two between clouds on the Galactic disk and those at the Galactic center, evidenced by observational data.
قيم البحث
اقرأ أيضاً
We tested the validity of the three Larson relations in a sample of 213 massive clumps selected from the Herschel Hi-GAL survey and combined with data from the MALT90 survey of 3mm emission lines. The clumps have been divided in 5 evolutionary stages
to discuss the Larson relations also as function of evolution. We show that this ensemble does not follow the three Larson relations, regardless of clump evolutionary phase. A consequence of this breakdown is that the virial parameter $alpha_{vir}$ dependence with mass (and radius) is only a function of the gravitational energy, independent of the kinetic energy of the system, and $alpha_{vir}$ is not a good descriptor of clump dynamics. Our results suggest that clumps with clear signatures of infall motions are statistically indistinguishable from clumps with no such signatures. The observed non-thermal motions are not necessarily ascribed to turbulence acting to sustain the gravity, but they may be due to the gravitational collapse at the clump scales. This seems particularly true for the most massive (M$geq$1000 M$_{odot}$) clumps in the sample, where also exceptionally high magnetic fields may not be enough to stabilize the collapse.
We investigate dust obscuration as parameterised by the infrared excess IRX$equiv$$L_{rm IR}/L_{rm UV}$ in relation to global galaxy properties, using a sample of $sim$32$,$000 local star-forming galaxies (SFGs) selected from SDSS, GALEX and WISE. We
show that IRX generally correlates with stellar mass ($M_ast$), star formation rate (SFR), gas-phase metallicity ($Z$), infrared luminosity ($L_{rm IR}$) and the half-light radius ($R_{rm e}$). A weak correlation of IRX with axial ratio (b/a) is driven by the inclination and thus seen as a projection effect. By examining the tightness and the scatter of these correlations, we find that SFGs obey an empirical relation of the form $IRX$=$10^alpha,(L_{rm IR})^{beta},R_{rm e}^{-gamma},(b/a)^{-delta}$ where the power-law indices all increase with metallicity. The best-fitting relation yields a scatter of $sim$0.17$,$dex and no dependence on stellar mass. Moreover, this empirical relation also holds for distant SFGs out to $z=3$ in a population-averaged sense, suggesting it to be universal over cosmic time. Our findings reveal that IRX approximately increases with $L_{rm IR}/R_{rm e}^{[1.3 - 1.5]}$ instead of $L_{rm IR}/R_{rm e}^{2}$ (i.e., surface density). We speculate this may be due to differences in the spatial extent of stars versus star formation and/or complex star-dust geometries. We conclude that not stellar mass but IR luminosity, metallicity and galaxy size are the key parameters jointly determining dust obscuration in SFGs.
Different studies have reported a power-law mass-size relation $M propto R^q$ for ensembles of molecular clouds. In the case of nearby clouds, the index of the power-law $q$ is close to 2. However, for clouds spread all over the Galaxy, indexes large
r than 2 are reported. We show that indexes larger than 2 could be the result of line-of-sight superposition of emission that does not belong to the cloud itself. We found that a random factor of gas contamination, between 0.001% and 10% of the line-of-sight, allows to reproduce the mass-size relation with $q sim 2.2-2.3$ observed in Galactic CO surveys. Furthermore, for dense cores within a single cloud, or molecular clouds within a single galaxy, we argue that, even in these cases, there is observational and theoretical evidence that some degree of superposition may be occurring. However, additional effects may be present in each case, and are briefly discussed. We also argue that defining the fractal dimension of clouds via the mass-size relation is not adequate, since the mass is not {necessarily} a proxy to the area, and the size reported in $M-R$ relations is typically obtained from the square root of the area, rather than from an estimation of the size independent from the area. Finally, we argue that the statistical analysis of finding clouds satisfying the Larsons relations does not mean that each individual cloud is in virial equilibrium.
With ALMA making it possible to resolve giant molecular clouds (GMCs) in other galaxies, it is becoming necessary to quantify the observational bias on measured GMC properties. Using a hydrodynamical simulation of a barred spiral galaxy, we compared
the physical properties of GMCs formed in position-position-position space (PPP) to the observational position-position-velocity space (PPV). We assessed the effect of disc inclination: face-on (PPV_face) and edge-on (PPV_edge), and resolution: 1.5 pc versus 24 pc, on GMC properties and the further implications of using Larsons scaling relations for mass-radius and velocity dispersion-radius. The low-resolution PPV data are generated by simulating ALMA Cycle 3 observations using the CASA package. Results show that the median properties do not differ strongly between PPP and PPV_face under both resolutions, but PPV_edge clouds deviate from these two. The differences become magnified when switching to the lower, but more realistic resolution. The discrepancy can lead to opposite results for the virial parameters measure of gravitational binding, and therefore the dynamical state of the clouds. The power-law indices for the two Larsons scaling relations decrease going from PPP, PPV_face to PPV_edge and decrease from high to low resolutions. We conclude that the relations are not entirely driven by the underlying physical origin and therefore have to be used with caution when considering the environmental dependence, dynamical state, and the extragalactic CO-to-H2 conversion factor of GMCs.
We present a study of a star formation prescription in which star formation efficiency depends on local gas density and turbulent velocity dispersion, as suggested by direct simulations of SF in turbulent giant molecular clouds (GMCs). We test the mo
del using a simulation of an isolated Milky Way-sized galaxy with a self-consistent treatment of turbulence on unresolved scales. We show that this prescription predicts a wide variation of local star formation efficiency per free-fall time, $epsilon_{rm ff} sim 0.1 - 10%$, and gas depletion time, $t_{rm dep} sim 0.1 - 10$ Gyr. In addition, it predicts an effective density threshold for star formation due to suppression of $epsilon_{rm ff}$ in warm diffuse gas stabilized by thermal pressure. We show that the model predicts star formation rates in agreement with observations from the scales of individual star-forming regions to the kiloparsec scales. This agreement is non-trivial, as the model was not tuned in any way and the predicted star formation rates on all scales are determined by the distribution of the GMC-scale densities and turbulent velocities $sigma$ in the cold gas within the galaxy, which is shaped by galactic dynamics. The broad agreement of the star formation prescription calibrated in the GMC-scale simulations with observations, both gives credence to such simulations and promises to put star formation modeling in galaxy formation simulations on a much firmer theoretical footing.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
