Outskirts of spiral galaxies: result of a secular evolution process?
J. Bakos, I. Trujillo, R. Azzollini, J. E. Beckman, M. Pohlen
备注：
6 pages, 4 figures. To appear in "Tumbling, twisting, and winding galaxies: Pattern speeds along the Hubble sequence", E. M. Corsini and V. P. Debattista (eds.), Memorie della Societa` Astronomica Italiana
We present our recent results on the properties of the outskirts of disk galaxies. In particular, we focus on spiral galaxies with stellar disk truncations in their radial surface brightness profiles. Using SDSS, UDF and GOODS data we show how the position of the break (i.e., a direct estimator of the size of the stellar disk) evolves with time since z~1. Our findings agree with an evolution on the radial position of the break by a factor of 1.3+/-0.1 in the last 8 Gyr for galaxies with similar stellar masses. We also present radial color gradients and how they evolve with time. At all redshift we find a radial inside-out bluing reaching a minimum at the position of the break radius, this minimum is followed by a reddening outwards. Our results constrain several galaxy disk formation models and favour a scenario where stars are formed inside the break radius and are relocated in the outskirts of galaxies through secular processes.
Based on CO(2-1) public data, we study the monoxide oxygen gas excitation conditions and the magnetic field strength of NGC 2841, NGC 3077, NGC 3184, NGC 3351 spiral galaxies. For their galaxy outskirts, we found kinetic temperatures in the range of $35 - 38 ~K$, CO column densities $10^{15} - 10^{16} ~cm^{-2}$ and H2 masses of $4 ~10^6 - 6 ~ 10^8 M_\odot$. An H2 density 1$0^3 cm^{-3}$ is suitable to explain the 2 sigma upper limits of the CO(2-1) line intensity. We constrain the magnetic field strength for our sample of spiral galaxies and their outskirts, evaluating a simplified expression o…
Stellar bars are important internal drivers of secular evolution in disk galaxies. Using a sample of nearby spiral galaxies with weak and strong bars, we explore the relationships between the star formation feature and stellar bars in galaxies. We find that galaxies with weak bars tend to be coincide with low concentrical star formation activity, while those with strong bars show a large scatter in the distribution of star formation activity. We find enhanced star formation activity in bulges towards stronger bars, although not predominantly, consistent with previous studies. Our results sugge…
Based on CO(2-1) public data, we study the monoxide oxygen gas excitation conditions and the magnetic field strength of four spiral galaxies. For the galaxy outskirts, we found kinetic temperatures in the range of $\lesssim$35--38 K, CO column densities $\lesssim 10^{15}$--$10^{16}$ cm$^{-2}$, and H$_2$ masses $\lesssim 4\times 10^6$--$6\times10^8$ M$_\odot$. An H$_2$ density $\lesssim 10^3$ cm$^{-3}$ is suitable to explain the 2$σ$ upper limits of the CO(2-1) line intensity. We constrain the magnetic field strength for our sample of spiral galaxies and their outskirts by using their masses an…
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…
In the outskirts of nearby spiral galaxies, star formation is observed in extremely low gas surface densities. Star formation in these regions, where the interstellar medium is dominated by diffuse atomic hydrogen, is difficult to explain with classic star formation theories. In this letter, we introduce runaway stars as an explanation for this observation. Runaway stars, produced by collisional dynamics in young stellar clusters, can travel kiloparsecs during their main-sequence lifetime. Using galactic-scale hydrodynamic simulations including a treatment of individual stars, we demonstrate t…
The properties and star formation processes in the far-outer disks of nearby spiral and dwarf irregular galaxies are reviewed. The origin and structure of the generally exponential profiles in stellar disks is considered to result from cosmological infall combined with a non-linear star formation law and a history of stellar migration and scattering from spirals, bars, and random collisions with interstellar clouds. In both spirals and dwarfs, the far-outer disks tend to be older, redder and thicker than the inner disks, with the overall radial profiles suggesting inside-out star formation plu…
We present observations of pseudobulges in S0 and spiral galaxies using imaging data taken with the Spitzer Infra-Red Array Camera. We have used 2-d bulge-disk-bar decomposition to determine structural parameters of 185 S0 galaxies and 31 nearby spiral galaxies. Using the Sersic index and the position on the Kormendy diagram to classify their bulges as either classical or pseudo, we find that 25 S0s (14%) and 24 spirals (77%) host pseudoblges. The fraction of pseudobulges we find in spiral galaxies is consistent with previous results obtained with optical data and show that the evolution of a …
We present a study of the magnetic field properties of NGC 4038/9 (the `Antennae' galaxies), the closest example of a late stage merger of two spiral galaxies. Wideband polarimetric observations were performed using the Karl G. Jansky Very Large Array between 2 and 4 GHz. Rotation measure synthesis and Faraday depolarization analysis was performed to probe the magnetic field strength and structure at spatial resolution of $\sim1$ kpc. Highly polarized emission from the southern tidal tail is detected with intrinsic fractional polarization close to the theoretical maximum ($0.62\pm0.18$), estim…
The manifold theory of barred-spiral structure provides a dynamical mechanism explaining how spiral arms beyond the ends of galactic bars can be supported by chaotic flows extending beyond the bar's co-rotation zone. We discuss its applicability to N-body simulations of secularly evolving barred galaxies. In these simulations, we observe consecutive `incidents' of spiral activity, leading to a time-varying disc morphology. Besides disc self-excitations, we provide evidence of a newly noted excitation mechanism related to the `off-centering' effect: particles ejected in elongated orbits at majo…
We present gravitational N-body simulations of the secular morphological evolution of disk galaxies induced by density wave modes. In particular, we address the demands collective effects place on the choice of simulation parameters, and show that the common practice of the use of a large gravity softening parameter was responsible for the failure of past simulations to correctly model the secular evolution process in galaxies, even for those simulations where the choice of basic state allows an unstable mode to emerge, a prerequisite for obtaining the coordinated radial mass flow pattern need…
The outskirts of galaxies offer extreme environments where we can test our understanding of the formation, evolution, and destruction of molecules and their relationship with star formation and galaxy evolution. We review the basic equations that are used in normal environments to estimate physical parameters like the molecular gas mass from CO line emission and dust continuum emission. Then we discuss how those estimates may be affected when applied to the outskirts, where the average gas density, metallicity, stellar radiation field, and temperature may be lower. We focus on observations of …
Using the potential-density phase shift approach developed by the present authors in earlier publications, we estimate the magnitude of radial mass accretion/excretion rates across the disks of six nearby spiral galaxies having a range of Hubble types. Our goal is to examine these rates in the context of bulge building and secular morphological evolution along the Hubble sequence. Stellar surface density maps of the sample galaxies are derived from SINGS 3.6um and SDSS i-band images. Corresponding molecular and atomic gas surface densities are derived from published CO(1-0) and HI interferomet…
Secular and environmental effects play a significant role in regulating the star formation rate and hence the evolution of the galaxies. Since UV flux is a direct tracer of the star formation in galaxies, the UltraViolet Imaging Telescope (UVIT) onboard ASTROSAT enables us to characterize the star forming regions in a galaxy with its remarkable spatial resolution. In this study, we focus on the secular evolution of NGC 628, a spiral galaxy in the local universe. We exploit the resolution of UVIT to resolve up to $\sim$ 63 pc in NGC 628 for identification and characterization of the star formin…
The outskirts of galaxies - especially the very extended HI disks of galaxies - are strongly affected by their local environment. I highlight the giant 2X-HI disks of nearby galaxies (M 83, NGC 3621, and NGC 1512), studied as part of the Local Volume HI Survey (LVHIS), their kinematics and relation to XUV disks, signatures of tidal interactions and accretion events, the MHI - DHI relation as well as the formation of tidal dwarf galaxies. - Using multi-wavelength data, I create 3D visualisations of the gas and stars in galaxies, with the shape of their warped disks obtained through kinematic mo…
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 t…
Many spiral galaxies have extended outer H~I disks and display low levels of star formation, inferred from the far-ultraviolet emission detected by {\it GALEX}, well beyond the optical radius. Here, we investigate the supernova (SN) rate in the outskirts of galaxies, using the largest and most homogeneous set of nearby supernovae (SNe) from the Lick Observatory Supernova Search (LOSS). While SN rates have been measured with respect to various galaxy properties, such as stellar mass and metallicity, their relative frequency in the outskirts versus the inner regions has not yet been studied. Und…
Through vertical resonances, bars can produce pseudo-bulges, within secular evolution. Bulges and pseudo-bulges have doubled their mass since z=1. The frequency of bulge-less galaxies at z=0 is difficult to explain, especially since clumpy galaxies at high z should create classical bulges in all galaxies. This issue is solved in modified gravity models. Bars and spirals in a galaxy disk, produce gravity torques that drive the gas to the center and fuel central star formation and nuclear activity. At 0.1-1kpc scale, observations of gravity torques show that only about one third of Seyfert galax…
Context: Planetary nebulae (PNe) are excellent tracers of stellar populations with low surface brightness, and therefore provide a powerful method to detect and explore the rich system of substructures discovered around the main spiral galaxies of the Local Group. Aims: We searched the outskirts of the Local Group spiral galaxy M33 (the Triangulum) for PNe to gain new insights into the extended stellar substructure on the northern side of the disc and to study the existence of a faint classical halo. Methods: The search is based on wide field imaging covering a 4.5 square degree area out to a …
Current observational evidence suggests that the star formation rate (SFR) efficiency of neutral atomic hydrogen gas measured in Damped Ly-alpha Systems (DLAs) at z~3 is more than 10 times lower than predicted by the Kennicutt-Schmidt (KS) relation. To understand the origin of this deficit, and to investigate possible evolution with redshift and galaxy properties, we measure the SFR efficiency of atomic gas at z~1, z~2, and z~3 around star-forming galaxies. We use new robust photometric redshifts in the Hubble Ultra Deep Field to create galaxy stacks in these three redshift bins, and measure t…
Stellar migration, whether due to trapping by transient spirals (churning), or to scattering by non-axisymmetric perturbations, has been proposed to explain the presence of stars in outer disks. After a review of the basic theory, we present compelling, but not yet conclusive, evidence that churning has been important in the outer disks of galaxies with type II (down-bending) profiles, while scattering has produced the outer disks of type III (up-bending) galaxies. In contrast, field galaxies with type I (pure exponential) profiles appear to not have experienced substantial migration. We concl…
The two-phase formation history of spiral galaxies traced by the cosmic evolution of the bar fraction
我们研究了星系棒的演化和与磁盘和球体形成的放大宇宙学模拟的样本中的链接。我们的模拟样本集中在现今恒星质量在10^10-10^11兆太阳范围内的星系，在场和松散的群环境中，具有各种各样的质量增长历史。在我们的模型中，在z>1.5时，现在螺旋的祖先几乎不存在条形，它们仍然很罕见，而且通常太弱而无法观察到z~1。在这一特征时期之后，可观测到的强条的比例迅速上升，80%的旋涡星系中存在条，其中三分之二在z<0.5时很容易观测到。这与观测到的棒分数的红移演化是定量一致的。我们的模型预测，随着红移的增加，酒吧分数的减少应该继续与可观察到的酒吧1的分数<10-15% in disk galaxies at z>。我们的模型还预测了较低质量星系中稍后的酒吧形成，与现有数据一致。我们发现，酒吧形成的特征时代，即红移z~0.8-1，对应的时代，今天的螺旋获得其磁盘为主的形态。在更高的红移，盘倾向于被合并和引力不稳定性迅速破坏，很少发展出明显的棒。棒形成时期对应于螺旋星系形成的早期“剧烈”阶段（z>1）和晚期“长期”阶段（z<0.8）之间的过渡。在长期阶段，酒吧的存在大大有助于增长的隆起，但隆起的质量预算仍然占主导地位的合并，相互作用和磁盘的不稳定性在高红移的贡献。z>1时的早期棒通常是短暂的，而z<1时形成的大多数棒一直持续到z=0，晚期宇宙学气体的注入是维持其中一些棒的必要条…
Spiral patterns in some disc galaxies have two arms in the centre, and three or more arms on the periphery. The same result is also obtained in numerical simulations of stellar and gaseous discs. We argue that such patterns may occur due to fast cooling of the gas, resulting in formation of giant molecular clouds. The timescale of this process is 50 Myr, the factor of 10 shorter than of ordinary secular instability. The giant molecular clouds give rise to multiarm spirals through the mechanism of swing amplification.
By including the most recent observations of HI 21-cm absorption through near-by galactic disks, we confirm our previous assertion that there is an anti-correlation between the abundance of cool neutral atomic gas and impact parameter. Comparing the measured neutral hydrogen column densities of the sample with the absorption strength, we find a peak in the mean spin temperature of ~2310 K at an impact parameter of ~ 14 kpc, with > 1000 K in the remainder of the disk. This is significantly different to the spin temperature distribution in the Milky Way, which exhibits a constant ~ 250 - 400 K o…
We summarize the results obtained from our suite of chemical evolution models for spiral disks, computed for different total masses and star formation efficiencies. Once the gas, stars and star formation radial distributions are reproduced, we analyze the Oxygen abundances radial profiles for gas and stars, in addition to stellar averaged ages and global metallicity. We examine scenarios for the potential origin of the apparent flattening of abundance gradients in the outskirts of disk galaxies, in particular the role of molecular gas formation prescriptions.
We analyze radial and azimuthal variations of the phase balance between the molecular and atomic ISM in the Milky Way. In particular, the azimuthal variations -- between spiral arm and interarm regions -- are analyzed without any explicit definition of spiral arm locations. We show that the molecular gas mass fraction, i.e., fmol=H2/ (HI+H2) in mass, varies predominantly in the radial direction: starting from ~100% at the center, remaining ~>50% (~>60%) to R~6kpc, and decreasing to ~10-20% (~50%) at R=8.5 kpc when averaged over the whole disk thickness (in the mid plane). Azimuthal, arm-intera…
We present a quantification of the properties of bars in two N-body+SPH cosmological simulations of spiral galaxies, named GA and AqC. The initial conditions were obtained using the zoom-in technique and represent two dark matter (DM) halos of $2-3\times10^{12}\ {\rm M}_\odot$, available at two different resolutions. The resulting galaxies are presented in the companion paper of Murante et al. (2014). We find that the GA galaxy has a bar of length $8.8$ kpc, present at the two resolution levels even though with a slightly different strength. Classical bar signatures (e.g. pattern of streaming …
The analysis of the chemical composition of galaxies provides fundamental insights into their evolution. This holds true also in the case of the outer regions of spiral galaxies. This Chapter presents the observational data, accumulated in the past few years mostly from the analysis of HII region spectra, concerning the metallicity of the outer disks of spirals that are characterized by extended HI envelopes and low star formation rates. I present evidence from the literature that the metal radial distribution flattens at large galactocentric distances, with levels of enrichment that exceed th…
M31 and M33 serve as ideal places to study distributions of dust in the outskirts of spiral galaxies. In this letter, using about 0.2 million stars selected from the LAMOST data and combining precise photometry and parallaxes from the Gaia DR2, we have constructed a two-dimensional foreground dust reddening map towards the M31 and M33 region ($111.2° {\le} gl {\le} 136.2°, -36.5° {\le} gb {\le} -16.5°$). The map has a typical spatial resolution of about 12 arc-minute and precision of 0.01 mag. The complex structure of dust clouds towards the M31 is revealed. By carefully removing the foregroun…
Recent observations show that the Milky Way's metallicity distribution function (MDF) changes its shape as a function of radius. This new evidence of radial migration within the stellar disc sets additional constraints on Galactic models. By performing controlled test particle simulations in a very detailed, observationally motivated model of the Milky Way, we demonstrate that, in the inner region of the disc, the MDF is shaped by the joint action of the bar and spiral arms, while at outer radii the MDF is mainly shaped by the spiral arms. We show that the spiral arms are able to imprint their…
The main orbital signatures of the secular evolution of an isolated self-gravitating stellar Mestel disc are recovered using a dressed Fokker-Planck formalism in angle-action variables. The shot-noise-driven formation of narrow ridges of resonant orbits is recovered in the WKB limit of tightly wound transient spirals, for a tepid Toomre-stable tapered disc. The relative effect of the bulge, the halo, the disc temperature and the spectral properties of the shot noise are investigated in turn. For such galactic discs all elements seem to impact the locus and direction of the ridge. For instance,…
The secular evolution of an infinitely thin tepid isolated galactic disc made of a finite number of particles is described using the inhomogeneous Balescu-Lenard equation. Assuming that only tightly wound transient spirals are present in the disc, a WKB approximation provides a simple and tractable quadrature for the corresponding drift and diffusion coefficients. It provides insight into the physical processes at work during the secular diffusion of a self-gravitating discrete disc and makes quantitative predictions on the initial variations of the distribution function in action space. When …
The distribution of galaxies in the stellar specific angular momentum versus stellar mass plane ($j_{\star}$-$M_{\star}$) provides key insights into their formation mechanisms. In this paper, we determine the location in this plane of a sample of ten field/group unbarred lenticular (S0) galaxies from the CALIFA survey. We performed a bulge-disc decomposition both photometrically and kinematically to study the stellar specific angular momentum of the disc components alone and understand the evolutionary links between S0s and other Hubble types. We found that eight of our S0 discs have a distrib…
We determine radial velocities and mass flow rates in a sample of 54 local spiral galaxies by modelling high-resolution and high-sensitivity data of the atomic hydrogen emission line. We found that, although radial inflow motions seem to be slightly preferred over outflow motions, their magnitude is generally small. Most galaxies show radial flows of only a few km/s throughout their HI disks, either inwards or outwards, without any clear increase in magnitude in the outermost regions, as we would expect for continuous radial accretion. Gas mass flow rates for most galaxies are less than 1 M$_\…
We investigate the contribution of outer HI disks to the observable population of merging black hole binaries. Like dwarf galaxies, the outer HI disks of spirals have low star formation rates and lower metallicities than the inner disks of spirals. Since low-metallicity star formation can produce more detectable compact binaries than typical star formation, the environments in the outskirts of spiral galaxies may be conducive to producing a rich population of massive binary black holes. We consider here both detailed controlled simulations of spirals and cosmological simulations, as well as th…
Understanding the evolutionary history of the Magellanic Clouds requires an in-depth exploration and characterization of the stellar content in their outer regions, which ultimately are key to tracing the epochs and nature of past interactions. We present new deep images of a shell-like over-density of stars in the outskirts of the Small Magellanic Cloud (SMC). The shell, also detected in photographic plates dating back to the fifties, is located at ~1.9 degr from the center of the SMC in the north-east direction.The structure and stellar content of this feature were studied with multi-band, o…
Pure disk galaxies without any bulge component, i.e., neither classical nor pseudo, seem to have escaped the affects of merger activity inherent to hierarchical galaxy formation models as well as strong internal secular evolution. We discover that a significant fraction (15 - 18 %) of disk galaxies in the Hubble Deep Field (0.4 < z < 1.0) as well as in the local Universe (0.02 < z < 0.05) are such pure disk systems (hereafter, PDS). The spatial distribution of light in these PDS is well described by a single exponential function from the outskirts to the centre and appears to have remained int…
We examine the dependence of the fraction of galaxies containing pseudo bulges on environment for a flux limited sample of $\sim$5000 SDSS galaxies. We have separated bulges into classical and pseudo bulge categories based on their position on the Kormendy diagram. Pseudo bulges are thought to be formed by internal processes and are a result of secular evolution in galaxies. We attempt to understand the dependence of secular evolution on environment and morphology. Dividing our sample of disc+bulge galaxies based on group membership into three categories: central and satellite galaxies in grou…
