We present simulations of a 500 pc$^2$ region, containing gas of mass 4 $\times$ 10$^6$ M$_\odot$, extracted from an entire spiral galaxy simulation, scaled up in resolution, including photoionising feedback from stars of mass > 18 M$_\odot$. Our region is evolved for 10 Myr and shows clustered star formation along the arm generating $\approx$ 5000 cluster sink particles $\approx$ 5% of which contain at least one of the $\approx$ 4000 stars of mass > 18 M$_\odot$. Photoionisation has a noticeable effect on the gas in the region, producing ionised cavities and leading to dense features at the edge of the HII regions. Compared to the no-feedback case, photoionisation produces a larger total mass of clouds and clumps, with around twice as many such objects, which are individually smaller and more broken up. After this we see a rapid decrease in the total mass in clouds and the number of clouds. Unlike studies of isolated clouds, our simulations follow the long range effects of ionisation, with some already-dense gas becoming compressed from multiple sides by neighbouring HII regions. This causes star formation that is both accelerated and partially displaced throughout the spiral arm with up to 30% of our cluster sink particle mass forming at distances > 5 pc from sites of sink formation in the absence of feedback. At later times, the star formation rate decreases to below that of the no-feedback case.
To pinpoint the peak location of the synchrotron total intensity emission in a spiral arm, we use a map of the spiralarm locations (from the observed arm tangent). Thus In a typical spiral arm in Galactic Quadrant I, we find the peak of the synchrotron radiation to be located about 220 +/-40 pc away from the inner arm edge (hot dust lane) inside the spiral arm. While most of the galactic disk has a clockwise largescale magnetic field, we make a statistical analysis to delimitate more precisely the smaller reverse annulus wiith a counterclockwise galactic magnetic field. We find an annulus widt…
The start of the Sagittarius spiral arm (Sagittarius origin) and the start of the Norma spiral arm (Norma origin) - model-computed and observed arm tangents at galactic longitudes -20 degrees < l < +23 degrees
We use classifications provided by citizen scientists though Galaxy Zoo to investigate the correlation between bulge size and arm winding in spiral galaxies. Whilst the traditional spiral sequence is based on a combination of both measures, and is supposed to favour arm winding where disagreement exists, we demonstrate that, in modern usage, the spiral classifications Sa-Sd are predominantly based on bulge size, with no reference to spiral arms. Furthermore, in a volume limited sample of galaxies with both automated and visual measures of bulge prominence and spiral arm tightness, there is at …
Understanding the effect spiral structure has on star formation properties of galaxies is important to completing our picture of spiral structure evolution. Previous studies have investigated connections between spiral arm properties with star formation, but the effect that the number of spiral arms has on this process is unclear. Here we use the Galaxy and Mass Assembly (GAMA) survey paired with the citizen science visual classifications from the Galaxy Zoo project to explore galaxies' spiral arm number and how it connects to the star formation process. We use the votes from the GAMA-KiDS Gal…
Generally, identifying the spiral arms of a spiral galaxy is not a hard task. However, defining the main characteristics, width and length of those structure is not a common task. Previous studies have used different tracers: Star clusters, Massers, H$α$. It was until recently that individual stars were used as tracers of spiral structures. The basic method of measuring the width of spiral arms assumes a Gaussian distribution around the mean concentration, either of gas or other tracer. In this work we use NGC 5236's stars as tracers. We estimated the surface stellar density of arms and inter-…
We compare the observed radial velocity of different arm tracers, taken near the tangent to a spiral arm. A slight difference is predicted by the density wave theory, given the shock predicted at the entrance to the inner spiral arm. In many of these spiral arms, the observed velocity offset confirms the prediction of the density wave theory (with a separation between the maser velocity and the CO gas peak velocity, of about 20 km/s) - when the observed offset is bigger than the error estimates. The time to reach the next spiral arm is given.
We observe the density wave angular pattern speed OMEGA-p to be near 12 to 17 km / s / kpc, by the separation between a typical optical HII region (from the spiral arm dust lane) and using a HII evolution time model to yield its relative speed, and independently by the separation between a typical radio maser (from the spiral arm dust lane) with a maser model.
The origin of galactic spiral arms is one of fundamental problems in astrophysics. Based on the local analysis Toomre (1981) proposed the swing amplification mechanism in which the self-gravity forms spiral arms as leading waves of stars rotate to trailing ones due to galactic shear. The structure of spiral arms is characterized by their number and pitch angle. We perform global $N$-body simulations of spiral galaxies to investigate the dependence of the spiral structure on disk parameters and compare the simulation results with the swing amplification model. We find that the spiral structure …
We report measurements of parallax and proper motion for five 6.7-GHz methanol maser sources in the outer regions of the Perseus arm as part of the BeSSeL Survey of the Galaxy. By combining our results with previous astrometric results, we determine an average spiral arm pitch angle of $9.2\pm1.5$ deg and an arm width of 0.39 kpc for this spiral arm. For sources in the interior side of the Perseus arm, we find on average a radial inward motion in the Galaxy of $13.3\pm5.4$ km s$^{-1}$ and counter to Galactic rotation of $6.2\pm3.2$ km s$^{-1}$. These characteristics are consistent with models …
An updated catalog of 205 observed tangents to the spiral arms (in Galactic longitudes) since 1980 is presented. This represents an addition of 80 arm tangents in 6 years (since 2016). Most arm tangents are observed at telescopes in the radio regime. In this study, the separation of each arm tracer from the dust lane is analyzed to obtain the relative speed away from the dust lane (an age gradient). Each arm tracer is observed to be separated from the dust lane, showing an age gradient of about 11.3 +/- 2 Myr/kpc across the spiral arm; this gives a relative speed away from the dust lane of abo…
The nature of galactic spiral arms in disc galaxies remains elusive. Regardless of the spiral model, arms are expected to play a role in sculpting the star-forming interstellar medium. As such, different arm models may result in differences in the structure of the interstellar medium and molecular cloud properties. In this study we present simulations of galactic discs subject to spiral arm perturbations of different natures. We find very little difference in how the cloud population or gas kinematics vary between the different grand-design spirals, indicting that the interstellar medium on cl…
We investigate how the properties of spiral arms relate to other fundamental galaxy properties. To this end, we use previously published measurements of those properties, and our own measurements of arm-interarm luminosity contrasts for a large sample of galaxies, using 3.6$μ$m images from the Spitzer Survey of Stellar Structure in Galaxies. Flocculent galaxies are clearly distinguished from other spiral arm classes, especially by their lower stellar mass and surface density. Multi-armed and grand-design galaxies are similar in most of their fundamental parameters, excluding some bar propertie…
We perform local $N$-body simulations of disk galaxies and investigate the evolution of spiral arms. We calculate the time autocorrelation of the surface density of spiral arms and find that the typical evolution timescale is described by the epicycle period. We investigate the distribution of the orbital elements of stars and find that in spiral arms the epicycle motions of stars are in phase while the spatial distribution of the guiding center is nearly uniform. These facts clearly show that the phase synchronization of the epicycle motion takes place, which is theoretically predicted by the…
In this work we determine the parameters of spiral structure for a sample of face-on spiral galaxies. In practice, the solution of this problem is a hard task because of the diversity of the observed characteristics of spiral structure, such as the arm number, their shape, arm contrast etc. In this work we study spiral structure in galaxies based on an analysis of photometric cuts perpendicular to the arm direction. The method is based on an approximation of these slices with an analytical function and derivation of the parameters of spiral structure (arm width, asymmetry, pitch angle) using t…
A number of giant cometary HII regions (GCH) sheathed inside molecular bow shocks (MBS) are found along spiral arms of the barred galaxy M83. The open cone structure is explained by a model of expanded HII front in a gaseous arm with steep density gradient, and the bow shock is shown to be formed by encounter of an HII region with the supersonic gas flow across the arm. It is suggested that dual-side compression of molecular gas at the bow head between the MBS and GCH enhances star formation along the spiral arms.
Star formation in galaxies: the role of spiral arms
研究旋臂中的恒星形成不仅可以告诉我们恒星形成的演化和分子云，还可以告诉我们星系中螺旋结构的本质。我将使用最近的模拟和观察结果来解决这两个问题。银河系尺度的模拟开始详细研究GMC的演变，因为它们形成螺旋臂，然后通过恒星反馈或剪切分散。这一过程的总时间尺度似乎与GMC的穿越时间相当，$10^5$ M $_{\odot}$云的穿越时间为几百万年，更大质量的GMC的穿越时间为2000万年左右。模拟和观测都表明，在螺旋臂中发现了大量的云，可能是云与云碰撞的结果。包括恒星在内的模拟也应该告诉我们GMC中的恒星年龄分布，以及螺旋臂上的恒星年龄分布。更普遍的是，最近对螺旋星系的研究表明，螺旋臂中气体流动的动力学在长寿命和短暂的螺旋臂中是不同的，导致恒星的年龄模式不同。这样的结果可以用来帮助建立银河系螺旋结构的主要驱动力（图姆雷不稳定性，酒吧，或附近的同伴星系）与未来的调查。
Context. The physical processes driving the formation of Galactic spiral arms are still under debate. Studies using open clusters favour the description of the Milky Way spiral arms as long-lived structures following the classical density wave theory. Current studies comparing the Gaia DR2 field stars kinematic information of the Solar neighbourhood to simulations, find a better agreement with short-lived arms with a transient behaviour. Aims. Our aim is to provide an observational, data-driven view of the Milky Way spiral structure and its dynamics using open clusters as the main tracers, and…
This study extends to the structure of the Galaxy. Our main goal is to focus on the first spiral arm beyond the Perseus arm, often called the Cygnus arm or the Outer Norma arm, by appraising the distributions of the masers near the Cygnus arm. The method is to employ masers whose trigonometric distances were measured with accuracy. The maser data come from published literature, having been obtained via the existing networks (US VLBA, the Japanese VERA, the European VLBI, and the Australian LBA). The new results for Cygnus are split in two groups: those located near a recent CO fitted global mo…
For a sample of 4378 nearby spiral and S0 galaxies, Yu and Ho (2020) used Fourier analysis of Sloan Digital Sky Survey images to show that the strengths of the spiral arms and the pitch angles of the arms are inversely correlated with central concentration. In the current study, we search for trends in the Yu and Ho (2020) spiral arm parameters with environment and specific star formation rate (sSFR). When comparing galaxies with similar concentrations, we do not find a significant difference in the arm strengths or pitch angles of spiral galaxies in clusters compared to field galaxies. When d…
Different spiral generation mechanisms are expected to produce different morphological and kinematic features. In this first paper in a series we carefully study the parameters of spiral structure in 155 face-on spiral galaxies, selected from the Sloan Digital Sky Survey, in the three $gri$ bands. We use a method for deriving a set of parameters of spiral structure, such as the width of the spiral arms, their fraction to the total galaxy luminosity and their colour, which have not been properly studied before. Our method is based on an analysis of a set of photometric cuts perpendicular to the…
Spiral-driven instabilities may drive gas inflow to enhance central star formation in disk galaxies. We investigate this hypothesis using the Sloan Digital Sky Survey (SDSS) in a sample of 2779 nearby unbarred star-forming main-sequence spiral galaxies. The strength of spiral arms is quantified by their average Fourier amplitude relative to the axisymmetric disk. The star formation properties in the central 1--3\,kpc region were derived from the SDSS spectra. We show that galaxies with stronger spiral arms not only tend to have more intense central specific star formation rate (sSFR), larger B…
One of the main theories for explaining the formation of spiral arms in galaxies is the stationary density wave theory. This theory predicts the existence of an age gradient across the arms. We use the stellar cluster catalogues of the galaxies NGC 1566, M51a, and NGC 628 from the Legacy Extragalactic UV Survey (LEGUS) program. In order to test for the possible existence of an age sequence across the spiral arms, we quantified the azimuthal offset between star clusters of different ages in our target galaxies. We found that NGC 1566, a grand-design spiral galaxy with bisymmetric arms and a str…
A method which we have developed for determining corotation radii, has allowed us to map in detail the radial resonant structures of barred spiral galaxies. Here we have combined this information with new determinations of the bar strength and the pitch angle of the innermost segment of the spiral arms to find relationships between these parameters of relevance to the dynamical evolution of the galaxies. We show how (1) the bar mass fraction, (2) the scaled bar angular momentum, (3) the pitch angle, and (4) the shear parameter vary along the Hubble sequence, and we also plot along the Hubble s…
Spiral arms across stellar populations in simulations via the local dimension method
Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona（宇宙科学研究所，巴塞罗那大学）；Institut d’Estudis Espacials de Catalunya（加泰罗尼亚空间研究研究所）；Jeremiah Horrocks Institute, University of Central Lancashire（杰里米·霍罗克斯研究所，中央兰开夏大学）
The Milky Way spiral arms are well established from star counts as well as from the locus of molecular clouds and other young objects, however, they have only recently started to be observed from a kinematics point of view. Using the kinematics of thin disc stars in Gaia EDR3 around the extended solar neighbourhood, we create x-y projections coloured by the radial, residual rotational, and vertical Galactocentric velocities ($U,ΔV,W$). The maps are rich in substructures and reveal the perturbed state of the Galactic disc. We find that local differences between rotational velocity and the azimu…
Large-scale galactic shocks, predicted by density wave theory, trigger star formation (SF-arms) downstream from the potential of the oldest stars (P-arms), resulting in a color jump from red to blue across spiral arms in the direction of rotation, while aging of these newly formed young stars induces the opposite but coexisting classic age gradient further downstream from the SF-arms. As the techniques for measuring pitch angle are intensity-weighted, they trace both the SF-arms and P-arms and are not sensitive to the classic age gradient. Consequently, the measured pitch angle of spiral arms …
The solar neighborhood contains disc stars that have recently crossed spiral arms in the Galaxy. We propose that boundaries in local velocity distributions separate stars that have recently crossed or been perturbed by a particular arm from those that haven't. Ridges in the stellar velocity distributions constructed from the second Gaia data release trace orbits that could have touched nearby spiral arms at apocentre or pericentre. The multiple ridges and arcs seen in local velocity distributions are consistent with the presence of multiple spiral features and different pattern speeds and impl…
From the Sun, a look at the edge of each spiral arm in our Milky Way (seen tangentially, along the line of sight) can yield numerous insights. Using different arm tracers (dust, masers, synchrotron emission, CO gas, open star clusters), we observe here for the first time an age gradient (about 12 +/-2 Myrs/kpc), much as predicted by the density wave theory. This implies that the arm tracers are leaving the dust lane at a relative speed of about 81 +/-10 km/s. We then compare with recent optical data obtained from the Gaia satellite, pertaining to the spiral arms.
Clumps in spiral galaxies at $z \lesssim 3$: Disentangling two spatial modes of star formation
Pulkovo Astronomical Observatory, Russian Academy of Sciences（俄罗斯科学院普尔科沃天文台）；Sternberg Astronomical Institute, Lomonosov Moscow State University（莫斯科国立大学斯滕伯格天文研究所）；Saint Petersburg State University（圣彼得堡国立大学）
Context: The Faraday rotation measure (RM) is often used to study the magnetic field strength and orientation within the ionized medium of the Milky Way. Observations indicate a RM in the spiral arms that exceeds the commonly assumed range. This raises the question of under what conditions spiral arms create such strong RM. Aims: We investigate the effect of spiral arms on Galactic RMs through shock compression of the interstellar medium (ISM). It has recently been suggested that the Sagittarius spiral arm creates a strong peak in RM where the line of sight (LOS) is tangent to the arm, and tha…
