最近的观测表明，在固定质量和红移的演化星系的属性显着的相似性，促使我们考虑的可能性，大多数星系可能会演变的共同历史，包括恒星形成，类星体吸积，并最终静止。我们通过定义一个“同步时标”的星系作为质量和红移的函数来量化这一点，它表征了不同星系的共同质量在不同的宇宙时代以相同的方式演变的程度。我们测量这个同步时间尺度使用9个不同的恒星形成星系的观测文献和SDSS类星体观测跨越$0 < z \lesssim 6$。令人惊讶的是，这个同步时间尺度是一个常数，对于所有质量和时间的组合，大约为1.5 Gyr。我们还发现，关闭恒星形成的星系的恒星质量与关闭类星体的黑洞质量之比约为30：1，远低于低红移静止星系的500：1。因此，我们提出了一个模型，其中恒星形成的“主序列”，类似的类星体的行为，和其他观测形成一个星系演化的“主序列”，其中恒星形成发生最早，其次是超大质量黑洞吸积，和两者之间的反馈是由确定性的，而不是随机过程占主导地位。
英文摘要：
Recent observations indicate a remarkable similarity in the properties of evolving galaxies at fixed mass and redshift, prompting us to consider the possibility that most galaxies may evolve with a common history encompassing star formation, quasar accretion, and eventual quiescence. We quantify this by defining a "synchronization timescale" for galaxies as a function of mass and redshift that characterizes the extent to which different galaxies of a common mass are evolving in the same manner at various cosmic epochs. We measure this synchronization timescale using 9 different star-forming galaxy observations from the literature and SDSS quasar observations spanning $0 < z \lesssim 6$. Surprisingly, this synchronization timescale is a constant, approximately 1.5 Gyr for all combinations of mass and time. We also find that the ratio between the stellar mass of galaxies turning off star formation and black hole mass of turnoff quasars is approximately 30:1, much lower than the 500:1 for quiescent galaxies at low redshift. As a result, we propose a model in which the star-forming "main sequence", analogous quasar behavior, and other observations form a galactic evolution "main sequence", in which star formation occurs earliest, followed by supermassive black hole accretion, and feedback between the two are dominated by deterministic rather than stochastic processes.
The complex history of NGC 1427A revealed by its star clusters and star formation history
University of Vienna（维也纳大学）；European Southern Observatory（欧洲南方天文台）；Leibniz-Institut für Astrophysik Potsdam (AIP)（波茨坦莱布尼茨天体物理研究所）；Pontificia Universidad Católica de Chile（智利天主教 Pontificia 大学）
The primary method for inferring the stellar mass ($M_*$) of a galaxy is through spectral energy distribution (SED) modeling. However, the technique rests on assumptions such as the galaxy star formation history and dust attenuation law that can severely impact the accuracy of derived physical properties from SED modeling. Here, we examine the effect that the assumed star formation history (SFH) has on the stellar properties inferred from SED fitting by ground truthing them against mock observations of high-resolution cosmological hydrodynamic galaxy formation simulations. Classically, SFHs ar…
We study the star formation history (SFH) of the ultra-diffuse galaxy (UDG) Dragonfly 44 (DF44) based on the simultaneous fit to near-ultraviolet to near-infrared photometry and high signal-to-noise optical spectroscopy. In fitting the observations we adopt an advanced physical model with a flexible SFH, and we discuss the results in the context of the degeneracies between stellar population parameters. Through reconstructing the mass-assembly history with a prior for extended star formation (akin to methods in the literature) we find that DF44 formed 90 per cent of its stellar mass by $z\sim …
We present the star formation history (SFH) of the Sextans dwarf spheroidal galaxy based on deep archive $B$,$I$ photometry taken with Suprime-Cam at Subaru telescope focusing our analysis on the inner region of the galaxy, fully located within the core radius. Within the errors of our SFH we have not detected any metallicity gradient along the considered radial distance interval. As a main result of this work we can state that the Sextans dwarf spheroidal stopped forming stars less than $\sim1.3$ Gyr after Big Bang in correspondance to the end of the reionization epoch. We have been able to c…
The Local Group (LG) is an environment accessible to detailed studies of galaxy formation, providing a complement to the early universe. In particular, spectroscopy of resolved stellar populations in the LG provides kinematical and chemical information for individual stars that can be used to infer the history of $L_\star$ galaxies like the Milky Way (MW) and Andromeda (M31). The Gaia revolution in the MW has enabled comprehensive observational studies of the MW's formation history. The exquisite detail in which the MW has been studied is currently not achievable in any other $L_\star$ galaxy.…
We present the star formation history (SFH) of the Sculptor dwarf spheroidal galaxy based on deep g,r photometry taken with DECam at the Blanco telescope, focusing our analysis on the central region of the galaxy extended up to $\sim 3$ core radii. We have investigated how the SFH changes radially, subdividing the sampled area into four regions, and have detected a clear trend of star formation. All the SFHs show a single episode of star formation, with the innermost region presenting a longer period of star formation of $\sim 1.5$ Gyr and for the outermost region the main period of star forma…
We apply the spectral energy distribution (SED) fitting code ProSpect to multiwavelength imaging for $\sim$7,000 galaxies from the GAMA survey at $z<0.06$, in order to extract their star formation histories. We combine a parametric description of the star formation history with a closed-box evolution of metallicity where the present-day gas-phase metallicity of the galaxy is a free parameter. We show with this approach that we are able to recover the observationally determined cosmic star formation history (CSFH), an indication that stars are being formed in the correct epoch of the Universe, …
We present deep imaging of the ultra-diffuse Andromeda XIX dwarf galaxy from the Advance Camera for Surveys on the Hubble Space Telescope which resolves its stellar populations to below the oldest main sequence turn-off. We derive a full star formation history for the galaxy using MATCH, and find no evidence of star formation in the past 8 Gyr. We calculate a quenching time of $τ_{90}=9.7\pm0.2$~Gyr, suggesting Andromeda~XIX ceased forming stars very early on. This early quenching, combined with its extremely large half-light radius, low density dark matter halo and lower than expected metalli…
We present a combination of the Schwarzschild orbit-superposition dynamical modelling technique with the spatially-resolved mean stellar age and metallicity maps to uncover the formation history of galaxies. We apply this new approach to a remarkable 5-pointing mosaic of VLT/MUSE observations obtained by Guérou et al. (2016) extending to a maximum galactocentric distance of $120''$ ($\sim 5.6\ {\rm kpc}$) along the major axis, corresponding to $\sim 2.5\ {\rm R}_e$. Our method first identifies `families' of orbits from the dynamical model that represent dynamically-distinct structures of the g…
Both observations and cosmological simulations have recently shown that there is a large scatter in the number of satellites of Milky Way (MW)-like galaxies. In this study, we investigate the relation between the satellite number and galaxy group assembly history, using the $r-$band magnitude gap ($Δm_{12}$) between the first and the second brightest galaxy as an indicator. From 20 deg$^2$ of Hyper Suprime-Cam Subaru Strategic Program Wide layer, we identify 17 dwarf satellite candidates around NGC 4437, a spiral galaxy with about one-fourth of the MW stellar mass. We estimate their distances …
We present a photometric study of the Andromeda XVIII dwarf spheroidal galaxy associated with M31, and situated well outside of the virial radius of the M31 halo. The galaxy was resolved into stars with Hubble Space Telescope/Advanced Camera for Surveys revealing the old red giant branch and red clump. With the new observational data we determined the Andromeda XVIII distance to be D = 1.33+-0.08 Mpc using the tip of red giant branch method. Thus, the dwarf is situated at the distance of 579 kpc from M31. We model the star formation history of Andromeda XVIII from the stellar photometry and Pa…
We investigate the dynamical evolution of galaxies in groups with different formation epochs. Galaxy groups have been selected to be in different dynamical states, namely dynamically old and dynamically young, which reflect their early and late formation times, respectively, based on their halo mass assembly. Brightest galaxies in dynamically young groups have suffered their last major galaxy merger typically $\sim 2$ Gyr more recently than their counterparts in dynamically old groups. Furthermore, we study the evolution of velocity dispersion in these two classes and compare them with the ana…
Galacto-forensic of LMC's orbital history as a probe for the dark matter potential in the outskirt of the Galaxy
观测到的大麦哲伦云和小麦哲伦云的三维速度为探测星系晕外围的星系势提供了机会。Besla et al.基于银河系势的正则NFW模型，（2007）重建了LMC和SMC的轨道，并认为它们目前正在第一次围绕银河系的近银河系轨道上。出于最近几次修订的太阳的运动围绕银河系中心，我们重新检查LMC的轨道历史，并表明它敏感地依赖于暗物质的质量分布超出其目前的银河系距离。我们利用数值模拟的结果，考虑一系列可能的结构和演化模型的银河系的潜力。我们发现，在理论和观测的不确定性，它是可能的大麦哲伦星云有多个近银河系通道的哈勃时间尺度上，特别是如果银河系的圆速度在太阳的位置大于$\sim 228$公里秒$^{-1}$。基于这些模型，更准确地确定大麦哲伦星系的运动可以用来确定星系晕外围的暗物质分布。
We present COOL J1323+0343, an early-type galaxy at $z = 1.0153 \pm 0.0006$, strongly lensed by a cluster of galaxies at z = $z = 0.353 \pm 0.001$. This object was originally imaged by DECaLS and noted as a gravitational lens by COOL-LAMPS, a collaboration initiated to find strong-lensing systems in recent public optical imaging data, and confirmed with follow-up data. With ground-based grzH imaging and optical spectroscopy from the Las Campanas Observatory and the Nordic Optical Telescope, we derive a stellar mass, metallicity, and star-formation history from stellar-population synthesis mode…
The observed present-day stellar mass function (PDMF) of the solar neighborhood is a mixture of stellar populations born in star-forming events that occurred over the life-time of the thin disk of the Galaxy. Assuming stars form in embedded clusters which have stellar initial mass functions (IMFs) which depend on the metallicity and density of the star-forming gas clumps, the integrated galaxy-wide IMF (IGIMF) can be calculated. The shape of the IGIMF thus depends on the SFR and metallicity. Here, the shape of the PDMF for stars more massive than $1\,M_\odot$ in combination with the mass densi…
Studying the evolution of dwarf galaxies can provide insights into the characteristics of systems that can act as building blocks of massive galaxies. This paper discusses the history of star formation and gas flows (inflow and outflow) of a dwarf irregular galaxy in the Local Group, NGC 6822, from the viewpoint of gas-phase and stellar chemical abundance. Gas-phase oxygen abundance, stellar metallicity distribution and gas fraction data are compared to chemical evolution models in which continuous star formation and gas flows are assumed. If the galaxy is assumed to be a closed or an accretio…
Recent studies have found higher galaxy metallicities in richer environments. It is not yet clear, however, whether metallicity-environment dependencies are merely an indirect consequence of environmentally dependent formation histories, or of environment related processes directly affecting metallicity. Here, we present a first detailed study of metallicity-environment correlations in a cosmological hydrodynamical simulation, in particular the Illustris simulation. Illustris galaxies display similar relations to those observed. Utilizing our knowledge of simulated formation histories, and lev…
We present the star formation history of the extremely metal-poor dwarf galaxy DDO 68, based on our photometry with the Advanced Camera for Surveys. With a metallicity of only $12+\log(O/H)=7.15$ and a very isolated location, DDO 68 is one of the most metal-poor galaxies known. It has been argued that DDO 68 is a young system that started forming stars only $\sim 0.15$ Gyr ago. Our data provide a deep and uncontaminated optical color-magnitude diagram that allows us to disprove this hypothesis, since we find a population of at least $\sim 1$ Gyr old stars. The star formation activity has been …
We introduce a practical methodology for investigating the star formation and chemical evolution history of a galaxy: age-divided mean stellar populations (ADPs) from full spectrum fitting. In this method, the mass-weighted mean stellar populations and mass fractions (f_mass) of young and old stellar components in a galaxy are separately estimated, which are divided with an age cut (selected to be 10^9.5 yr ~ 3.2 Gyr in this paper). To examine the statistical reliability of ADPs, we generate 10,000 artificial galaxy spectra, each of which consists of five random simple stellar population compo…
NGC7252, which is one of the nearest major-merger galaxy remnants, is an ideal laboratory to study the processes inherent to the transformation of disc galaxies to ellipticals as observed about ~1Gyr after the collision. We obtained wide-field IFU spectroscopy with the VLT-VIMOS integral-field spectrograph covering the central 50"x50" of NGC7252 to map the stellar and ionised gas kinematics, and the distribution and conditions of the ionised gas, revealing the extent of ongoing star formation and recent star formation history. We find that the inner gas disc is not counter-rotating with respec…
The existence of massive galaxies lacking a classical bulge has often been proposed as a challenge to $Λ$CDM. However, recent simulations propose that a fraction of massive disc galaxies might have had very quiescent merger histories, and also that mergers do not necessarily build classical bulges. We test these ideas with deep MUSE observations of NGC 5746, a massive ($\sim 10^{11}$ M$_\odot$) edge-on disc galaxy with no classical bulge. We analyse its stellar kinematics and stellar populations, and infer that a massive and extended disc formed very early: 80% of the galaxy's stellar mass for…
We investigate the isolated, quiescent ultra-diffuse galaxy (UDG) DGSAT I and its globular cluster (GC) system using two orbits of Hubble Space Telescope Advanced Camera for Surveys imaging in the F606W and F814W filters. This is the first study of GCs around a UDG in a low-density environment. DGSAT I was previously found to host an irregular blue low surface brightness clump, that we confirm as very likely belonging to the galaxy rather than being a chance projection, and represents a recent episode of star formation (${\sim}500~\mathrm{Myr}$) that challenges some UDG formation scenarios. We…
Discovery of a Barred-Spiral Galaxy at $z_{spec}$ = 3.16 II. The Star Formation History
University of Washington（华盛顿大学）；University of Massachusetts Amherst（马萨诸塞大学阿默斯特分校）；University of Pittsburgh（匹兹堡大学）；Centro de Astrobiología (CAB), CSIC-INTA（天体生物学中心（CAB），西班牙国家科学研究委员会-国家航空航天技术研究所）；University of Wisconsin, Madison（威斯康星大学麦迪逊分校）；The University of Texas at Austin（德克萨斯大学奥斯汀分校）；MIT Kavli Institute for Astrophysics and Space Research（麻省理工学院卡弗里天体物理学与空间研究学院）
We use deep Hubble Space Telescope imaging to derive a distance to the Virgo Cluster ultradiffuse galaxy (UDG) VCC 615 using the tip of the red giant branch (TRGB) distance estimator. We detect 5,023 stars within the galaxy, down to a 50% completeness limit of F814W = 28.0, using counts in the surrounding field to correct for contamination due to background sources and Virgo intracluster stars. We derive an extinction-corrected F814W tip magnitude of $m_{\rm tip,0} = 27.19^{+0.07}_{-0.05}$, yielding a distance of $d=17.7^{+0.6}_{-0.4}$ Mpc. This places VCC 615 on the far side of the Virgo Clus…
We examine the influence of dark matter halo assembly on the evolution of a simulated $\sim L^\star$ galaxy. Starting from a zoom-in simulation of a star-forming galaxy evolved with the EAGLE galaxy formation model, we use the genetic modification technique to create a pair of complementary assembly histories: one in which the halo assembles later than in the unmodified case, and one in which it assembles earlier. Delayed assembly leads to the galaxy exhibiting a greater present-day star formation rate than its unmodified counterpart, whilst in the accelerated case the galaxy quenches at $z\si…
Deriving a simple, analytic galaxy star formation history (SFH) using observational data is a complex task without the proper tool to hand. We therefore present SNITCH, an open source code written in Python, developed to quickly (~2 minutes) infer the parameters describing an analytic SFH model from the emission and absorption features of a galaxy spectrum dominated by star formation gas ionisation. SNITCH uses the Flexible Stellar Population Synthesis models of Conroy et al. (2009), the MaNGA Data Analysis Pipeline and a Markov Chain Monte Carlo method in order to infer three parameters (time…
Thin halo star streams originate from the evaporation of globular clusters and therefore provide information about the early epoch globular cluster population. The observed tidal tails from halo globular clusters in the Milky Way are much more shorter than expected from a star cluster orbiting for 10~Gyr. The discrepancy is likely the result of the assumption that the clusters have been orbiting in a non-evolving galactic halo for a Hubble time. As a first step towards more realistic stream histories, a toy model that combines an idealized merger model with a simplified model of the internal c…
We use the Keck Cosmic Web Imager integral-field unit spectrograph to: 1) measure the global stellar population parameters for the ultra-diffuse galaxy (UDG) Dragonfly 44 (DF44) to much higher precision than previously possible for any UDG, and 2) for the first time measure spatially-resolved stellar population parameters of a UDG. We find that DF44 falls below the mass--metallicity relation established by canonical dwarf galaxies both in and beyond the Local Group. We measure a flat radial age gradient ($m_{\rm age} \sim +0.01_{-0.08}^{+0.07}$ log Gyr kpc$^{-1}$) and a flat-to-positive metall…
We explore the radial variation of star formation histories in dwarf galaxies simulated with Feedback In Realistic Environments (FIRE) physics. The sample contains 9 low-mass field dwarfs with M_ star = 10^5 - 10^7 M_sun from previous FIRE results, and a new suite of 17 higher mass field dwarfs with M_star = 10^7 - 10^9 M_sun introduced here. We find that age gradients are common in our dwarfs, with older stars dominant at large radii. The strength of the gradient correlates with overall galaxy age such that earlier star formation produces a more pronounced gradient. The relation between forma…
Using new and archival data, we study the kinematic properties of the nearest field S0 galaxy, NGC 3115, out to $\sim6.5$ half-light radii ($R_\mathrm{e}$) from its stars (integrated starlight), globular clusters (GCs) and planetary nebulae (PNe). We find evidence of three kinematic regions with an inner transition at $\sim0.2\ R_\mathrm{e}$ from a dispersion-dominated bulge ($V_\mathrm{rot}/σ<1$) to a fast-rotating disk ($V_\mathrm{rot}/σ>1$), and then an additional transition from the disk to a slowly rotating spheroid at $\sim2-2.5\, R_\mathrm{e}$, as traced by the red GCs and PNe (and poss…
We present a new method that incorporates the horizontal branch morphology into synthetic colour-magnitude diagram based star formation history determinations. This method, we call MORGOTH, self-consistently takes into account all the stellar evolution phases up to the early asymptothic giant branch, flexibly modelling red giant branch mass loss. We test MORGOTH on a range of synthetic populations, and find that the inclusion of the horizontal branch significantly increases the precision of the resulting star formation histories. When the main sequence turn-off is detected, MORGOTH can fit the…
Approximately 20% of early-type galaxies host small nuclear stellar discs that are tens to a few hundred parsecs in size. Such discs are expected to be easily disrupted during major galactic encounters, hence their age serve to constrain their assembly history. We use VIMOS integral-field spectroscopic observations for the intermediate-mass E0 galaxy NGC4458 and age-date its nuclear disc via high-resolution fitting of various model spectra. We find that the nuclear disc is at least 6 Gyr old. A clue to gain narrow limits to the stellar age is our knowledge of the nuclear disc contribution to t…
The diffuse light in clusters of galaxies, or intracluster light, has attracted a lot of attention lately due to its potential in describing the assembly history of galaxy clusters and to explain the observed growth of the brightest cluster galaxy with time. The properties of this light (color, stellar populations, extent) give clues about its formation and, consequently, the processes that shape the cluster. Here, I will present a review on intracluster light, its history, properties and the particular observational problems and limitations associated with the study of this diffuse component …
Cosmological simulations of the same spiral galaxy: satellite properties, the role of baryonic physics and star formation history in shaping dark matter cores/cusps
The observed properties of galaxies are strongly dependent on both their total stellar mass and their morphology. Furthermore, the environment is known to play a strong role in shaping them. The galaxy population in the local universe that is located in virialized clusters is found to be red, poorly star-forming, and mostly composed of early morphological types. Towards a holistic understanding of the mechanisms that drive galaxy evolution, we exploit the spectrophotometric data from the WINGS and OmegaWINGS local galaxy cluster surveys, and study the role of both the local and the large-scale…
We present a new derivation of the star formation history (SFH) of the dSph galaxy Fornax in two central regions, characterised by unprecedented precision and age resolution. It reveals that star formation has proceeded in sharp bursts separated by periods of low-level or quiescent activity. The SFH was derived through colour-magnitude diagram (CMD) fitting of two extremely deep Hubble Space Telescope CMDs, sampling the centre and one core radius. The attained age resolution allowed us to single out a major star formation episode at early times, a second strong burst $4.6\pm0.4$ Gyr ago and re…
Determining the star formation history (SFH) is key to understand the formation and evolution of dwarf galaxies. Recovering the SFH in resolved galaxies is mostly based on deep colour--magnitude diagrams (CMDs), which trace the signatures of multiple evolutionary stages of their stellar populations. In distant and unresolved galaxies, the integrated light of the galaxy can be decomposed, albeit made difficult by an age--metallicity degeneracy. Another solution to determine the SFH of resolved galaxies is based on evolved stars; these luminous stars are the most accessible tracers of the underl…
