分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： NGC 2915 is a unique nearby galaxy that is classified as an isolated blue compact dwarf based on its optical appearance but has an extremely extended H i gas disk with prominent Sd-type spiral arms. To unveil the starburst-triggering mystery of NGC 2915, we performed a comprehensive analysis of deep VLT/MUSE integral field spectroscopic observations that cover the star-forming region in the central kiloparsec of the galaxy. We find that episodes of bursty star formation have recurred in different locations throughout the central region, and the most recent one peaked around 50 Myr ago. The bursty star formation has significantly disturbed the kinematics of the ionized gas but not the neutral atomic gas, which implies that the two gas phases are largely spatially decoupled along the line of sight. No evidence for an active galactic nucleus is found based on the classical line-ratio diagnostic diagrams. The ionized gas metallicities have a positive radial gradient, which confirms the previous study based on several individual H ii regions and may be attributed to both the stellar feedback-driven outflows and metal-poor gas inflow. Evidence for metal-poor gas infall or inflow includes discoveries of high-speed collisions between gas clouds of different metallicities, localized gas metallicity drops and unusually small metallicity differences between gas and stars. The central stellar disk appears to be counter-rotating with respect to the extended H i disk, implying that the recent episodes of bursty star formation have been sustained by externally accreted gas.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： NGC 2915 is a unique nearby galaxy that is classified as an isolated blue compact dwarf based on its optical appearance but has an extremely extended H i gas disk with prominent Sd-type spiral arms. To unveil the starburst-triggering mystery of NGC 2915, we performed a comprehensive analysis of deep VLT/MUSE integral field spectroscopic observations that cover the star-forming region in the central kiloparsec of the galaxy. We find that episodes of bursty star formation have recurred in different locations throughout the central region, and the most recent one peaked around 50 Myr ago. The bursty star formation has significantly disturbed the kinematics of the ionized gas but not the neutral atomic gas, which implies that the two gas phases are largely spatially decoupled along the line of sight. No evidence for an active galactic nucleus is found based on the classical line-ratio diagnostic diagrams. The ionized gas metallicities have a positive radial gradient, which confirms the previous study based on several individual H ii regions and may be attributed to both the stellar feedback-driven outflows and metal-poor gas inflow. Evidence for metal-poor gas infall or inflow includes discoveries of high-speed collisions between gas clouds of different metallicities, localized gas metallicity drops and unusually small metallicity differences between gas and stars. The central stellar disk appears to be counter-rotating with respect to the extended H i disk, implying that the recent episodes of bursty star formation have been sustained by externally accreted gas.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Yao Yao
Haiyang Liu
Xu Kong
Yulong Gao
Guangwen Chen
Xinkai Chen
Zhixiong Liang
Zesen Lin
Yimeng Tang
Hongxin Zhang
摘要： The scaling relationship is a fundamental probe of the evolution of galaxies. Using the integral field spectroscopic data from the Mapping Nearby Galaxies at Apache Point Observatory survey, we select 1698 spaxels with significant detection of the auroral emission line \oiii$\lambda$4363 from 52 galaxies to investigate the scaling relationships at the low-metallicity end. We find that our sample's star formation rate is higher and its metallicity is lower in the scaling relationship than the star-forming sequence after removing the contribution of the Fundamental Metallicity Relation.We also find that the stellar ages of our sample are younger ($<$ 1 Gyr) and the stellar metallicities are also lower. Morphological parameters from Deep Learning catalog indicate that our galaxies are more likely to be merger. These results suggest that their low metallicity regions may be related to interaction, the inflow of metal-poor gas may dilute the interstellar medium and form new metal-poor stars in these galaxies during interaction.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Aaron J. Romanowsky
Søren S. Larsen
Alexa Villaume
Jeffrey L. Carlin
Joachim Janz
David J. Sand
Jay Strader
Jean P. Brodie
Sukanya Chakrabarti
Chloe M. Cheng
Denija Crnojević
Duncan A. Forbes
Christopher T. Garling
Jonathan R. Hargis
Ananthan Karunakaran
Ignacio Martín-Navarro
Knut A. G. Olsen
Nicole Rider
Bitha Salimkumar
Vakini Santhanakrishnan
摘要： The classical globular clusters found in all galaxy types have half-light radii of $r_{\rm h} \sim$ 2-4 pc, which have been tied to formation in the dense cores of giant molecular clouds. Some old star clusters have larger sizes, and it is unclear if these represent a fundamentally different mode of low-density star cluster formation. We report the discovery of a rare, young "faint fuzzy" star cluster, NGC 247-SC1, on the outskirts of the low-mass spiral galaxy NGC 247 in the nearby Sculptor group, and measure its radial velocity using Keck spectroscopy. We use Hubble Space Telescope imaging to measure the cluster half-light radius of $r_{\rm h} \simeq 12$ pc and a luminosity of $L_V \simeq 4\times10^5 \mathrm{L}_\odot$. We produce a colour-magnitude diagram of cluster stars and compare to theoretical isochrones, finding an age of $\simeq$ 300 Myr, a metallicity of [$Z$/H] $\sim -0.6$ and an inferred mass of $M_\star \simeq 9\times10^4 \mathrm{M}_\odot$. The narrow width of blue-loop star magnitudes implies an age spread of $\lesssim$ 50 Myr, while no old red-giant branch stars are found, so SC1 is consistent with hosting a single stellar population, modulo several unexplained bright "red straggler" stars. SC1 appears to be surrounded by tidal debris, at the end of a $\sim$ 2 kpc long stellar filament that also hosts two low-mass, low-density clusters of a similar age. We explore a link between the formation of these unusual clusters and an external perturbation of their host galaxy, illuminating a possible channel by which some clusters are born with large sizes.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Low-mass compact stellar systems (CSSs; $M_{\star}$ $<$ 10$^{10}$ M$_{\odot}$) are thought to be a mixed bag of objects with various formation mechanisms. Previous surveys of CSSs were biased to relatively high-density environments and cannot provide a complete view of the environmental dependence of the formation of CSSs. We conduct the first-ever unbiased flux-limited census of nearby quiescent CSSs over a total sky area of $\sim$ 200 deg$^{2}$ observed by the GAMA spectroscopic survey. The complete sample includes 82 quiescent CSSs, of which 85\% fall within the stellar mass range of classical compact ellipticals (cEs).\ By quantifying the local environment with the normalized projected distance $D/R_{\rm vir}$ to the nearest luminous neighboring galaxy, we find that these CSSs have a bimodal $D/R_{\rm vir}$ distribution, with one group peaking near $\sim$ 0.1$\times$$R_{\rm vir}$ (satellite) and the other peaking near $\sim$ 10$\times$$R_{\rm vir}$ (field). In contrast to the CSSs, ordinary quiescent galaxies of similar masses have unimodal $D/R_{\rm vir}$ distribution.\ Satellite CSSs are older and more metal-rich than field CSSs on average. The bimodal $D/R_{\rm vir}$ distribution of quiescent CSSs reinforces the existence of two distinct formation channels (tidal stripping and born-to-be) for cEs and may be understood in two mutually inclusive perspectives, i.e., substantial tidal stripping happens only when satellite galaxies travel sufficiently close to their massive hosts, and there exists an excess of high-density cE-bearing subhalos close to massive halos.
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