分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Aiming at discriminating different gravitational potential models of the Milky Way, we perform tests based on the kinematic data powered by the Gaia DR2 astrometry, over a large range of $(R,z)$ locations. Invoking the complete form of Jeans equations that admit three integrals of motion, we use the independent $R$- and $z$-directional equations as two discriminators ($T_R$ and $T_z$). We apply the formula for spatial distributions of radial and vertical velocity dispersions proposed by Binney et al., and successfully extend it to azimuthal components, $\sigma_\theta(R,z)$ and $V_\theta(R,z)$; the analytic form avoids the numerical artifacts caused by numerical differentiation in Jeans-equations calculation given the limited spatial resolutions of observations, and more importantly reduces the impact of kinematic substructures in the Galactic disk. It turns out that whereas the current kinematic data are able to reject Moffat's Modified Gravity (let alone the Newtonian baryon-only model), Milgrom's MOND is still not rejected. In fact, both the carefully calibrated fiducial model invoking a spherical dark matter (DM) halo and MOND are equally consistent with the data at almost all spatial locations (except that probably both have respective problems at low-$|z|$ locations), no matter which a tracer population or which a meaningful density profile is used. Because there is no free parameter at all in the quasi-linear MOND model we use, and the baryonic parameters are actually fine-tuned in the DM context, such an effective equivalence is surprising, and might be calling forth a transcending synthesis of the two paradigms.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present a new investigation of the intergalactic medium (IGM) near reionization using dark gaps in the Lyman-$\beta$ (Ly$\beta$) forest. With its lower optical depth, Ly$\beta$ offers a potentially more sensitive probe to any remaining neutral gas compared to commonly used Ly$\alpha$ line. We identify dark gaps in the Ly$\beta$ forest using spectra of 42 QSOs at $z_{\rm em}>5.5$, including new data from the XQR-30 VLT Large Programme. Approximately $40\%$ of these QSO spectra exhibit dark gaps longer than $10h^{-1}{\rm Mpc}$ at $z\simeq5.8$. By comparing the results to predictions from simulations, we find that the data are broadly consistent both with models where fluctuations in the Ly$\alpha$ forest are caused solely by ionizing ultraviolet background (UVB) fluctuations and with models that include large neutral hydrogen patches at $z<6$ due to a late end to reionization. Of particular interest is a very long ($L=28h^{-1}{\rm Mpc}$) and dark ($\tau_{\rm eff} \gtrsim 6$) gap persisting down to $z\simeq 5.5$ in the Ly$\beta$ forest of the $z_{\rm}=5.85$ QSO PSO J025$-$11. This gap may support late reionization models with a volume-weighted average neutral hydrogen fraction of $ \langle x_{\rm HI}\rangle \gtrsim 5\%$ by $z=5.6$. Finally, we infer constraints on $\langle x_{\rm HI}\rangle$ over $5.5 \lesssim z \lesssim 6.0$ based on the observed Ly$\beta$ dark gap length distribution and a conservative relationship between gap length and neutral fraction derived from simulations. We find $\langle x_{\rm HI}\rangle \leq 0.05$, 0.17, and 0.29 at $z\simeq 5.55$, 5.75, and 5.95, respectively. These constraints are consistent with models where reionization ends significantly later than $z = 6$.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present a new investigation of the intergalactic medium (IGM) near the end of reionization using "dark gaps" in the Lyman-alpha (Ly$\alpha$) forest. Using spectra of 55 QSOs at $z_{\rm em}>5.5$, including new data from the XQR-30 VLT Large Programme, we identify gaps in the Ly$\alpha$ forest where the transmission averaged over 1 comoving $h^{-1}\,{\rm Mpc}$ bins falls below 5%. Nine ultra-long ($L > 80~h^{-1}\,{\rm Mpc}$) dark gaps are identified at $z<6$. In addition, we quantify the fraction of QSO spectra exhibiting gaps longer than $30~h^{-1}\,{\rm Mpc}$, $F_{30}$, as a function of redshift. We measure $F_{30} \simeq 0.9$, 0.6, and 0.15 at $z = 6.0$, 5.8, and 5.6, respectively, with the last of these long dark gaps persisting down to $z \simeq 5.3$. Comparing our results with predictions from hydrodynamical simulations, we find that the data are consistent with models wherein reionization extends significantly below redshift six. Models wherein the IGM is essentially fully reionized that retain large-scale fluctuations in the ionizing UV background at $z \lesssim 6$ are also potentially consistent with the data. Overall, our results suggest that signature of reionization in the form of islands of neutral hydrogen and/or large-scale fluctuations in the ionizing background remain present in the IGM until at least $z \simeq 5.3$.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Intervening CIV absorbers are key tracers of metal-enriched gas in galaxy halos over cosmic time. Previous studies suggest that the CIV cosmic mass density ($\Omega_{\rm CIV}$) decreases slowly over 1.5 $\lesssim z\lesssim$ 5 before declining rapidly at $z\gtrsim$ 5, but the cause of this downturn is poorly understood. We characterize the $\Omega_{\rm CIV}$ evolution over 4.3 $\lesssim z\lesssim$ 6.3 using 260 absorbers found in 42 XSHOOTER spectra of $z\sim$ 6 quasars, of which 30 come from the ESO Large Program XQR-30. The large sample enables us to robustly constrain the rate and timing of the downturn. We find that $\Omega_{\rm CIV}$ decreases by a factor of 4.8 $\pm$ 2.0 over the ~300 Myr interval between $z\sim$ 4.7 and $z\sim$ 5.8. The slope of the column density (log N) distribution function does not change, suggesting that CIV absorption is suppressed approximately uniformly across 13.2 $\leq$ log N/cm$^{-2}$ < 15.0. Assuming that the carbon content of galaxy halos evolves as the integral of the cosmic star formation rate density (with some delay due to stellar lifetimes and outflow travel times), we show that chemical evolution alone could plausibly explain the fast decline in $\Omega_{\rm CIV}$ over 4.3 $\lesssim z\lesssim$ 6.3. However, the CIV/CII ratio decreases at the highest redshifts, so the accelerated decline in $\Omega_{\rm CIV}$ at $z\gtrsim$ 5 may be more naturally explained by rapid changes in the gas ionization state driven by evolution of the UV background towards the end of hydrogen reionization.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Luminous quasars are powerful targets to investigate the role of feedback from supermassive black-holes (BHs) in regulating the growth phases of BHs themselves and of their host galaxies, up to the highest redshifts. Here we investigate the cosmic evolution of the occurrence and kinematics of BH-driven outflows, as traced by broad absorption line (BAL) features, due to the C IV ionic transition. We exploit a sample of 1935 quasars quasars at $z=2.1-6.6$ with bolometric luminosity log($L_{\rm bol}/$erg s$^{-1})\gtrsim46.8$, drawn from the Sloan Digital Sky Survey and from the X-shooter legacy survey of Quasars at Reionisation (XQR-30). We consider rest-frame optical bright quasars to minimise observational biases due to quasar selection criteria and we apply a homogeneous BAL identifiation analysis, based on employing composite template spectra to estimate the quasar intrinsic emission. We find a BAL quasar fraction close to 20\% at $z\sim2-4$, while it increases to almost 50\% at $z\sim6$. The velocity and width of the BAL features also increase at $z\gtrsim4.5$. We exclude that the redshift evolution of the BAL properties is due to differences in terms of quasar luminosity and accretion rate. These results suggest significant BH feedback occurring in the 1 Gyr old Universe, likely affecting the growth of BHs and, possibly, of their host galaxies, as supported by models of early BH and galaxy evolution.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Intervening metal absorption lines in the spectra of z > 6 quasars are fundamental probes of the ionization state and chemical composition of circumgalactic and intergalactic gas near the end of the reionization epoch. Large absorber samples are required to robustly measure typical absorber properties and to refine models of the synthesis, transport, and ionization of metals in the early Universe. The "Ultimate XSHOOTER legacy survey of quasars at z~5.8-6.6" (XQR-30) has obtained high signal-to-noise spectra of 30 luminous quasars, nearly quadrupling the existing sample of 12 high quality z~6 quasar spectra. We use this unprecedented sample to construct a catalog of 778 systems showing absorption in one or more of MgII (360 systems), FeII (184), CII (46), CIV (479), SiIV (127), and NV (13) which span 2 5 absorbers, especially for CIV and SiIV which are important probes of the ionizing photon background at high redshift. The sample is 50% (90%) complete for rest-frame equivalent widths W > 0.03AA (0.09AA). We publicly release the absorber catalog along with completeness statistics and a Python script to compute the absorption search path for different ions and redshift ranges. This dataset is a key legacy resource for studies of enriched gas from the era of galaxy assembly to cosmic noon, and paves the way for even higher redshift studies with the James Webb Space Telescope and 30m-class telescopes.