اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدDust May Be More Rare Than Expected in Metal Poor Galaxies
191
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
Normal galaxies observed at z>6, when the Universe was <1 billion years old, thus far show no evidence of the cold dust that accompanies star formation in the local Universe, where the dust-to-gas mass ratio is 1%. A prototypical example is Himiko (z=6.6), which a mere 840 Myr after the Big Bang is forming stars at a rate of 30-100 Msun/yr, yielding a mass assembly time M^{star}/SFR 150x10^6 yr. Himiko is estimated to have a low fraction (2-3% of the Solar value) of elements heavier than helium (metallicity), and although its gas mass cannot be asserted at this time its dust-to-stellar mass ratio is constrained to be <0.05%. The local galaxy I Zw 18, with a metallicity 4% solar and forming stars less rapidly than Himiko but still vigorously for its mass (M^{star}/SFR 1.6x10^9 yr), is also very dust deficient and perhaps one of the best analogues of primitive galaxies accessible to detailed study. Here we report observations of dust emission from I Zw 18 from which we determine its dust mass to be 450-1800 Msun, yielding a dust-to-stellar mass ratio approx 10^{-6}-10^{-5} and a dust-to-gas mass ratio 3.2-13x10^{-6}. If I Zw 18 is a reasonable analog of Himiko, then Himikos dust mass is approx 50,000 Msun, a factor of 100 below the current upper limit. These numbers are considerably uncertain, but if most high-z galaxies are more like Himiko than like the quasar host SDSS J114816.64+525150.3, then the prospects for detecting the gas and dust in them are much poorer than hitherto anticipated.
قيم البحث
اقرأ أيضاً
The $rmLambda$CDM cosmological model is remarkable: with just 6 parameters it describes the evolution of the Universe from a very early time when all structures were quantum fluctuations on subatomic scales to the present, and it is consistent with a
wealth of high-precision data, both laboratory measurements and astronomical observations. However, the foundation of $rmLambda$CDM involves physics beyond the standard model of particle physics: particle dark matter, dark energy and cosmic inflation. Until this `new physics is clarified, $rmLambda$CDM is at best incomplete and at worst a phenomenological construct that accommodates the data. I discuss the path forward, which involves both discovery and disruption, some grand challenges and finally the limits of scientific cosmology.
We have analyzed in this work the updated sample of neutron star masses derived from the study of a variety of 96 binary systems containing at least one neutron star using Bayesian methods. After updating the multimodality of the distributions found
in previous works, we determined the maximum mass implied by the sample using a robust truncation technique, with the result $m_{max} sim 2.5-2.6 , M_{odot}$. We have checked that this mass is actually consistent by generating synthetic data and employing a Posterior Predictive Check. A comparison with seven published $m_{max}$ values inferred from the remnant of the NS-NS merger GW170817 was performed and the tension between the latter and the obtained $m_{max}$ value quantified. Finally, we performed a Local Outlier Factor test and verified that the result for $m_{max}$ encompasses the highest individual mass determinations with the possible exception of PSR J1748-2021B. The conclusion is that the whole distribution already points toward a high value of $m_{max}$, while several lower values derived from the NS-NS merger event are disfavored and incompatible with the higher binary system masses. A large $m_{max}$ naturally accommodates the lower mass component of the event GW190814 as a neutron star.
Galaxies are complex systems the evolution of which apparently results from the interplay of dynamics, star formation, chemical enrichment, and feedback from supernova explosions and supermassive black holes. The hierarchical theory of galaxy formati
on holds that galaxies are assembled from smaller pieces, through numerous mergers of cold dark matter. The properties of an individual galaxy should be controlled by six independent parameters including mass, angular-momentum, baryon-fraction, age and size, as well as by the accidents of its recent haphazard merger history. Here we report that a sample of galaxies that were first detected through their neutral hydrogen radio-frequency emission, and are thus free of optical selection effects, shows five independent correlations among six independent observables, despite having a wide range of properties. This implies that the structure of these galaxies must be controlled by a single parameter, although we cannot identify this parameter from our dataset. Such a degree of organisation appears to be at odds with hierarchical galaxy formation, a central tenet of the cold dark matter paradigm in cosmology.
We present infrared (IR) spectral energy distributions (SEDs) of individual star-forming regions in four extremely metal poor (EMP) galaxies with metallicity Z around Zsun/10 as observed by the Herschel Space Observatory. With the good wavelength cov
erage of the SED, it is found that these EMP star-forming regions show distinct SED shapes as compared to those of grand design Spirals and higher metallicity dwarfs: they have on average much higher f70um/f160um ratios at a given f160um/f250um ratio; single modified black-body (MBB) fittings to the SED at lambda >= 100 um still reveal higher dust temperatures and lower emissivity indices compared to that of Spirals, while two MBB fittings to the full SED with a fixed emissivity index (beta = 2) show that even at 100 um about half of the emission comes from warm (50 K) dust, in contrast to the cold (~20 K) dust component. Our spatially resolved images further reveal that the far-IR colors including f70um/f160um, f160um/f250um and f250um/f350um are all related to the surface densities of young stars as traced by far-UV, 24 um and SFRs, but not to the stellar mass surface densities. This suggests that the dust emitting at wavelengths from 70 um to 350 um is primarily heated by radiation from young stars.
We have observed a sample of 19 carbon stars in the Sculptor, Carina, Fornax, and Leo I dwarf spheroidal galaxies with the Infrared Spectrograph on the Spitzer Space Telescope. The spectra show significant quantities of dust around the carbon stars i
n Sculptor, Fornax, and Leo I, but little in Carina. Previous comparisons of carbon stars with similar pulsation properties in the Galaxy and the Magellanic Clouds revealed no evidence that metallicity affected the production of dust by carbon stars. However, the more metal-poor stars in the current sample appear to be generating less dust. These data extend two known trends to lower metallicities. In more metal-poor samples, the SiC dust emission weakens, while the acetylene absorption strengthens. The bolometric magnitudes and infrared spectral properties of the carbon stars in Fornax are consistent with metallicities more similar to carbon stars in the Magellanic Clouds than in the other dwarf spheroidals in our sample. A study of the carbon budget in these stars reinforces previous considerations that the dredge-up of sufficient quantities of carbon from the stellar cores may trigger the final superwind phase, ending a stars lifetime on the asymptotic giant branch.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
