اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدThe Age-Metallicity Relation of the Universe in Neutral Gas: The First 100 Damped Lya Systems
185
0
0.0
(
0
)
تأليف
Jason X. Prochaska
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
We present accurate metallicity measurements for 121 damped Lya systems at 0.5<z<5 including ~50 new measurements from our recently published Echellette Spectrograph and Imager surveys. This dataset is analysed to determine the age-metallicity relation of neutral gas in the universe. Contrary to previous datasets this sample shows statistically significant evolution in the mean metallicity. The best linear fit rate to metallicity vs. redshift is -0.26 +/- 0.07 dex corresponding to approximately a factor of 2 every Gyr at z=3. The DLA continue to maintain a floor in metallicity of ~1/700 solar independent of observational effects. This metallicity threshold limits the prevalence of primordial gas in high redshift galaxies and stresses the correspondence between damped systems and star formation (i.e. galaxy formation). This floor is significantly offset from the metallicity of the Lya forest and therefore we consider it to be more related to active star formation within these galaxies than scenarios of enrichment in the very early universe. Finally, we comment on an apparent missing metals problem: the mean metallicity of the damped systems is ~10x lower than the value expected from their observed star formation history. This problem is evident in current theoretical treatments of chemical evolution and galaxy formation; it may indicate a serious flaw in our understanding of the interplay between star formation and metal production.
قيم البحث
اقرأ أيضاً
We present a sample of 38 intervening Damped Lyman $alpha$ (DLA) systems identified towards 100 $z>3.5$ quasars, observed during the XQ-100 survey. The XQ-100 DLA sample is combined with major DLA surveys in the literature. The final combined sample
consists of 742 DLAs over a redshift range approximately $1.6 < z_{rm abs} < 5.0$. We develop a novel technique for computing $Omega_{rm HI}^{rm DLA}$ as a continuous function of redshift, and we thoroughly assess and quantify the sources of error therein, including fitting errors and incomplete sampling of the high column density end of the column density distribution function. There is a statistically significant redshift evolution in $Omega_{rm HI}^{rm DLA}$ ($geq 3 sigma$) from $z sim 2$ to $z sim$ 5. In order to make a complete assessment of the redshift evolution of $Omega_{rm HI}$, we combine our high redshift DLA sample with absorption surveys at intermediate redshift and 21cm emission line surveys of the local universe. Although $Omega_{rm HI}^{rm DLA}$, and hence its redshift evolution, remains uncertain in the intermediate redshift regime ($0.1 < z_{rm abs} < 1.6$), we find that the combination of high redshift data with 21cm surveys of the local universe all yield a statistically significant evolution in $Omega_{rm HI}$ from $z sim 0$ to $z sim 5$ ($geq 3 sigma$). Despite its statistical significance, the magnitude of the evolution is small: a linear regression fit between $Omega_{rm HI}$ and $z$ yields a typical slope of $sim$0.17$times 10^{-3}$, corresponding to a factor of $sim$ 4 decrease in $Omega_{rm HI}$ between $z=5$ and $z=0$.
Omega Centauri is a peculiar Globular Cluster formed by a complex stellar population. To shed light on this, we studied 172 stars belonging to the 5 SGBs that we can identify in our photometry, in order to measure their [Fe/H] content as well as esti
mate their age dispersion and the age-metallicity relation. The first important result is that all of these SGBs has a distribution in metallicity with a spread that exceeds the observational errors and typically displays several peaks that indicate the presence of several sub-populations. We were able to identified at least 6 of them based on their mean [Fe/H] content. These metallicity-based sub-populations are seen to varying extents in each of the 5 SGBs. Taking advantage of the age-sensitivity of the SGB we showed that, first of all, at least half of the sub-populations have an age spread of at least 2 Gyrs. Then we obtained an age-metallicity relation that is the most complete up to date for this cluster. The interpretation of the age-metallicity relation is not straightforward, but it is possible that the cluster (or what we can call its progenitor) was initially composed of two populations having different metallicities. Because of their age, it is very unlikely that the most metal-rich derives from the most metal-poor by some kind of chemical evolution process, so they must be assumed as two independent primordial objects or perhaps two separate parts of a single larger object, that merged in the past to form the present-day cluster.
We investigate the heat source of the neutral gas comprising DLAs. Unlike the Lya forest, where the extragalactic background radiation field ionizes and heats the gas, we find that grain photoelectric heating by the FUV background is not sufficient t
o balance the C II 158um cooling rate inferred from DLAs. In these systems, a local energy source is required. We show that in the case of the z=1.919 DLA toward Q2206-19, the local source is FUV emission from the associated galaxy found by Moller et al (2002): the mean intensity inferred from photometry is in good agreement with the intensity Jnu required to explain the cooling rate. The FUV mean intensity predicted for a cold neutral medium (CNM) model, Jnu=(1.7+2.7-1.0)x10^(-18) cgs (95% c.l.), is the largest expected from our CII* study of 45 DLAs. This may explain why this is the only confirmed DLA yet detected in emission at z>1.9. We argue that in most DLAs with detected CII* absorption, Jnu is between 10^{-19} and 10^{-18} and heats the gas which is a CNM. By contrast, in most DLAs with upper limits on CII* absorption the gas is a warm neutral medium (WNM). Surprisingly, the upper limits are compatible with the same range of Jnu values suggesting the majority of DLAs are heated by radiation fields generated by a limited range of star formation rates per unit H I area, between 10^{-3} and 10^{-2} Msol/kpc^2. We also show that CII* absorption is unlikely to arise in gas that is ionized.
We report evidence for an anti-correlation between spin temperature $T_s$ and metallicity [Z/H], detected at $3.6 sigma$ significance in a sample of 26 damped Lyman-$alpha$ absorbers (DLAs) at redshifts $0.09 < z < 3.45$. The anti-correlation is dete
cted at $3 sigma$ significance in a sub-sample of 20 DLAs with measured covering factors, implying that it does not stem from low covering factors. We obtain $T_s = (-0.68 pm 0.17) times {rm [Z/H]} + (2.13 pm 0.21)$ from a linear regression analysis. Our results indicate that the high $T_s$ values found in DLAs do not arise from differences between the optical and radio sightlines, but are likely to reflect the underlying gas temperature distribution. The trend between $T_s$ and [Z/H] can be explained by the larger number of radiation pathways for gas cooling in galaxies with high metal abundances, resulting in a high cold gas fraction, and hence, a low spin temperature. Conversely, low-metallicity galaxies have fewer cooling routes, yielding a larger warm gas fraction and a high $T_s$. Most DLAs at $z>1.7$ have low metallicities, [Z/H] $< -1$, implying that the HI in high-$z$ DLAs is predominantly warm. The anti-correlation between $T_s$ and [Z/H] is consistent with the presence of a mass-metallicity relation in DLAs, suggested by the tight correlation between DLA metallicity and the kinematic widths of metal lines. Most high-$z$ DLAs are likely to arise in galaxies with low masses ($M_{rm vir} < 10^{10.5} M_odot$), low metallicities ([Z/H]$< -1$, and low cold gas fractions.
The age-metallicity relation for F and G dwarf stars in the solar neighborhood, based on the stellar metallicity data of Edvardsson et al. (1993), shows an apparent scatter that is larger than expected considering the uncertainties in metallicities a
nd ages. A number of theoretical models have been put forward to explain the large scatter. However, we present evidence, based on Edvardsson et al. (1993) data, along with Hipparcos parallaxes and new age estimates, that the scatter in the age-metallicity relation depends on the distance to the stars in the sample, such that stars within 30 pc of the Sun show significantly less scatter in [Fe/H]. Stars of intermediate age from the Edvardsson et al. sample at distances 30-80 pc from the Sun are systematically more metal-poor than those more nearby. We also find that the slope of the apparent age-metallicity relation is different for stars within 30 pc than for those stars more distant. These results are most likely an artifact of selection biases in the Edvardsson et al. star sample. We conclude that the intrinsic dispersion in metallicity at fixed age is < 0.15 dex, consistent with the < 0.1 dex scatter for Galactic open star clusters and the interstellar medium.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
