Scale Length Ratio Research Articles

Bars and boxy/peanut bulges in thin and thick discs

Boxy/peanut (b/p) bulges, the vertically extended inner parts of bars, are ubiquitous in barred galaxies in the local Universe, including our own Milky Way. At the same time, the majority of external galaxies and the Milky Way also possess a thick disc. However, the dynamical effect of thick discs in the b/p formation and evolution is not fully understood. Here, we investigate the effect of thick discs in the formation and evolution of b/ps by using a suite of N-body models of (kinematically cold) thin and (kinematically hot) thick discs. Within the suite of models, we systematically vary the mass fraction of the thick disc, and the thin-to-thick disc scale length ratio. The b/ps form in almost all our models via a vertical buckling instability, even in the presence of a massive thick disc. The thin disc b/p is much stronger than the thick disc b/p. With an increasing thick-disc mass fraction, the final b/p structure becomes progressively weaker in strength and larger in extent. Furthermore, the time interval between the bar formation and the onset of buckling instability becomes progressively shorter with an increasing thick-disc mass fraction. The breaking and restoration of the vertical symmetry (during and after the b/p formation) show a spatial variation – the inner bar region restores vertical symmetry rather quickly (after the buckling), while in the outer bar region the vertical asymmetry persists long after the buckling happens. Our findings also predict that at higher redshifts, when discs are thought to be thicker, b/ps would have a more “boxy” appearance than an “X-shaped” one. This remains to be tested in future observations at higher redshifts.

Bars and boxy/peanut bulges in thin and thick discs

The Milky Way and a majority of external galaxies possess a thick disc. However, the dynamical role of the (geometrically) thick disc in the bar formation and evolution is not fully understood. Here, we investigate the effect of thick discs in the formation and evolution of bars by means of a suite of N-body models of (kinematically cold) thin and (kinematically hot) thick discs. We systematically varied the mass fraction of the thick disc, the thin-to-thick disc scale length ratio, and the thick disc scale height to examine the bar formation under diverse dynamical scenarios. Bars form almost always in our models, even in the presence of a massive thick disc. The part of the bar that consists of the thick disc closely follows the overall growth and temporal evolution of the part of the bar that consists of the thin disc, but the part of the bar in the thick disc is weaker than the part of the bar in the thin disc. The formation of stronger bars is associated with a simultaneous greater loss of angular momentum and a more intense radial heating. In addition, we demonstrate a preferential loss of angular momentum and a preferential radial heating of disc stars in the azimuthal direction within the extent of the bar in both thin and thick disc stars. For purely thick-disc models (without any thin disc), the bar formation critically depends on the disc scale length and scale height. A larger scale length and/or a larger vertical scale height delays the bar formation time and/or suppresses the bar formation almost completely in thick-disc-only models. We find that the Ostriker-Peeble criterion predicts the bar instability scenarios in our models better than the Efstathiou-Lake-Negroponte criterion.

On the evolution of the size of Lyman alpha haloes across cosmic time: no change in the circumgalactic gas distribution when probed by line emission

ABSTRACT Lyman α (Lyα) is now routinely used as a tool for studying high-redshift galaxies, and its resonant nature means it can trace neutral hydrogen around star-forming galaxies. Integral field spectrograph measurements of high-redshift Lyα emitters indicate that significant extended Lyα halo emission is ubiquitous around such objects. We present a sample of redshift 0.23 to 0.31 galaxies observed with the Hubble Space Telescope selected to match the star formation properties of high-z samples while optimizing the observations for detection of low surface brightness Lyα emission. The Lyα escape fractions range between 0.7 and 37 per cent, and we detect extended Lyα emission around six out of seven targets. We find Lyα halo to UV scale length ratios around 6:1, which is marginally lower than high-redshift observations, and halo flux fractions between 60 and 85 per cent – consistent with high-redshift observations – when using comparable methods. However, our targets show additional extended stellar UV emission: we parametrize this with a new double exponential model. We find that this parametrization does not strongly affect the observed Lyα halo fractions. We find that deeper Hα data would be required to firmly determine the origin of Lyα halo emission; however, there are indications that Hα is more extended than the central FUV profile, potentially indicating conditions favourable for the escape of ionizing radiation. We discuss our results in the context of high-redshift galaxies, cosmological simulations, evolutionary studies of the circumgalactic medium in emission, and the emission of ionizing radiation.

On the Sun’s distance from the center and the shape of the inner halo in the Galaxy: Gaia EDR3, HST and literature globular clusters

A statistical method is used to derive both the Sun’s distance r0 from the Galactic Center (GC) and the 3D geometry of the inner (< 25 kpc) halo. The spatial distribution of the 138 Gaia EDR3 globular clusters (GCs) with distances established on a combination of HST and literature data of Baumgardt and Vasiliev (2021) is explored. An estimate by using these ancient objects of the pressure-supported subsystem of the Galaxy with newly derived distances leads to the mean r0=7.81±0.14 kpc. The distribution of GCs within 25 kpc is almost spherically symmetric, and has the shape of an ellipsoid with a major axis of its symmetry slightly elongated toward the Sun and two minor axes of almost the same length. The obtained scale-length ratio of the major axis to the minor axis in the plane and to the vertical axis of the ellipsoid is ≈ 1:0.8:0.7. Based on the papers of a series, for practical use we argue to employ the following Sun’s distances from the GC and the plane: r0=8.15±0.15 kpc and z0=15±5 pc.

Radial distribution of dust, stars, gas, and star-formation rate in DustPedia face-on galaxies

The purpose of this work is the characterization of the radial distribution of dust, stars, gas, and star-formation rate (SFR) in a sub-sample of 18 face-on spiral galaxies extracted from the DustPedia sample. This study is performed by exploiting the multi-wavelength, from UV to sub-mm bands, DustPedia database, in addition to molecular (12CO) and atomic (HI) gas maps and metallicity abundance information available in the literature. We fitted the surface brightness profiles of the tracers of dust and stars, the mass surface density profiles of dust, stars, molecular gas, and total gas, and the SFR surface density profiles with an exponential curve and derived their scale-lengths. We also developed a method to solve for the CO-to-H2 conversion factor (alpha_CO) per galaxy by using dust and gas mass profiles. Although each galaxy has its own peculiar behaviour, we identified a common trend of the exponential scale-lengths vs. wavelength. On average, the scale-lengths normalized to the B-band 25 mag/arcsec^2 radius decrease from UV to 70 micron, from 0.4 to 0.2, and then increase back up to 0.3 at 500 microns. The main result is that, on average, the dust mass surface density scale-length is about 1.8 times the stellar one derived from IRAC data and the 3.6 micron surface brightness, and close to that in the UV. We found a mild dependence of the scale-lengths on the Hubble stage T: the scale-lengths of the Herschel bands and the 3.6 micron scale-length tend to increase from earlier to later types, the scale-length at 70 micron tends to be smaller than that at longer sub-mm wavelength with ratios between longer sub-mm wavelengths and 70 micron that decrease with increasing T. The scale-length ratio of SFR and stars shows a weak increasing trend towards later types.

DECIPHERING THE 3D STRUCTURE OF THE OLD GALACTIC BULGE FROM THE OGLE RR LYRAE STARS

We have analyzed a sample of 27,258 fundamental-mode RR Lyrae variable stars (type RRab) detected recently toward the Galactic bulge by the Optical Gravitational Lensing Experiment (OGLE) survey. The data support our earlier claim that these metal-poor stars trace closely the barred structure formed of intermediate-age red clump giants. The distance to the Galactic center (GC) inferred from the bulge RR Lyrae stars is R_0=8.27+/-0.01(stat)+/-0.40(sys) kpc. We show that their spatial distribution has the shape of a triaxial ellipsoid with an major axis located in the Galactic plane and inclined at an angle of i=20+/-3 deg to the Sun-GC line of sight. The obtained scale-length ratio of the major axis to the minor axis in the Galactic plane and to the axis vertical to the plane is 1:0.49(2):0.39(2). We do not see the evidence for the bulge RR Lyrae stars forming an X-shaped structure. Based on the light curve parameters, we derive metallicities of the RRab variables and show that there is a very mild but statistically significant radial metallicity gradient. About 60% of the bulge RRab stars form two very close sequences on the period-amplitude (or Bailey) diagram, which we interpret as two major old bulge populations: A and B. Their metallicities likely differ. Population A is about four times less abundant than the slightly more metal-poor population B. Most of the remaining stars seem to represent other, even more metal-poor populations of the bulge. The presence of multiple old populations indicates that the Milky Way bulge was initially formed through mergers.

The environmental dependence of the structure of galactic discs in STAGES S0 galaxies: implications for S0 formation

We present an analysis of V-band radial surface brightness {\mu}(r) profiles for S0s in different environments using HST/ACS imaging and data from the Space Telescope A901/2 Galaxy Evolution Survey (STAGES). Using a sample of ~280 field and cluster S0s, we find that in both environments, ~25 per cent have a pure exponential disc (Type I) and ~50 per cent exhibit an up-bending disc break (antitruncation, Type III). However, we find hardly any (< 5 per cent) down-bending disc breaks (truncations, Type II) in our S0s and many cases (~20 per cent) where no exponential component was observed. We also find no evidence for an environmental dependence on the disc scalelength or break strength (outer-to-inner scalelength ratio), implying that the galaxy environment does not affect the stellar distribution in S0 stellar discs. Comparing disc structure between these S0s and the spirals from our previous studies, we find: i) no evidence for the Type I scalelength being dependent on morphology; and ii) some evidence suggesting the Type II/III break strength is smaller (weaker) in S0s compared to spirals. Taken together, these results suggest that the stellar distribution in S0s is not drastically affected by the galaxy environment. However, some process inherent to the morphological transformation of spirals into S0s does affect the stellar disc causing a weakening of {\mu}(r) breaks and may even eliminate truncations from S0s. In further tests, we perform analytical bulge-disc decompositions on our S0s and compare the results to those for spirals from our previous studies. For Type III galaxies, we find that bulge light can account for the excess light at large radii in up to ~50 per cent of S0s but in only ~15 per cent of spirals. We propose that this result is consistent with a fading stellar disc (evolving bulge-to-disc ratio) being an inherent process in the transformation of spirals into S0s.

Gyrokinetic Theory of Turbulent Acceleration of Parallel Rotation in Tokamak Plasmas

A mechanism for turbulent acceleration of parallel rotation is discovered using gyrokinetic theory. This new turbulent acceleration term cannot be written as a divergence of parallel Reynolds stress. Therefore, turbulent acceleration acts as a local source or sink of parallel rotation. The physics of turbulent acceleration is intrinsically different from the Reynolds stress. For symmetry breaking by positive intensity gradient, a positive turbulent acceleration, i.e., cocurrent rotation, is predicted. The turbulent acceleration is independent of mean rotation and mean rotation gradient, and so constitutes a new candidate for the origin of spontaneous rotation. A quasilinear estimate for ion temperature gradient turbulence shows that the turbulent acceleration of parallel rotation is explicitly linked to the ion temperature gradient scale length and temperature ratio Ti0/Te0. Methods for testing the effects of turbulent parallel acceleration by gyrokinetic simulation and experiment are proposed.

The environmental dependence of the structure of outer galactic discs in STAGES spiral galaxies

We present an analysis of V-band radial surface brightness profiles for spiral galaxies from the field and cluster environments using Hubble Space Telescope/Advanced Camera for Surveys imaging and data from the Space Telescope A901/2 Galaxy Evolution Survey (STAGES). We use a large sample of ~330 face-on to intermediately inclined spiral galaxies and assess the effect of the galaxy environment on the azimuthally averaged radial surface brightness mu profiles for each galaxy in the outer stellar disc (24 < mu < 26.5 mag per sq arcsec). For galaxies with a purely exponential outer disc (~50 per cent), we determine the significance of an environmental dependence on the outer disc scalelength h_out. For galaxies with a broken exponential in their outer disc, either down-bending (truncation, ~10 per cent) or up-bending (anti-truncation, ~40 per cent), we measure the strength T (outer-to-inner scalelength ratio, log_10(h_out/h_in) of the mu breaks and determine the significance of an environmental dependence on break strength T. Surprisingly, we find no evidence to suggest any such environmental dependence on either outer disc scalelength h_out or break strength T, implying that the galaxy environment is not affecting the stellar distribution in the outer stellar disc. We also find that for galaxies with small effective radii (r_e < 3 kpc) there is a lack of outer disc truncations in both the field and cluster environments. Our results suggest that the stellar distribution in the outer disc of spiral galaxies is not significantly affected by the galaxy environment.

Probing the stellar populations in the outskirts of spiral galaxies

We present our results on the stellar population 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 data we show color gradients. We find that the color radial profile exhibits a "U'-shape", showing a minimum at the position of the break of the surface brightness profile. We obtain stellar surface mass density profiles of truncated galaxies as well, these show a peculiar behaviour: they follow very closely an exponential decrement. This suggests the idea that the observed properties of truncated galaxies are not caused by a drop in the mass distribution but by a different stellar population in the outer regions of the disks. Confronting this with current theoretical scenarios we find that this is likely to be a result of secular evolution, in which scenario stars are being formed inside the break and then being motioned outwards which would result in an inverted age gradient corresponding to the observed color profiles. Having maintained this idea, using multiwave-length data from GALEX, SDSS, UKIDSS and SPITZER we showed a dependence of the inner-outer scale-length ratio of truncated disks. Our results suggest that there is an existing general trend of the scale-length ratio: from bluer to redder bands the scale-length ratio decreases, which is in accordance with the stellar disk being dominated by an older stellar population in the outer disk.

Optimum Condition of Sharp-Density-Transition Scheme for the Generation of a High-Quality Electron Bunch in the Laser Wakefield Acceleration

In this paper, the effects of scale length and density transition ratio on the generation of a high-quality electron bunch with a sharp density transition in the laser wakefield accelerator were analyzed using 2-D particle-in-cell simulations. The simulation results show the effect of the density transition on the quality of the beam, i.e., the total charge of the electron bunch depends on the scale length and the density transition ratio and the size of the electron bunch only depends on the density transition ratio. The simulations also show that the energy spread of the bunch in the first period can be reduced with a sharp scale length. For this paper, the optimum condition for the generation of a high-quality electron bunch was found in the parameter space of both the scale length and the density transition ratio.

Multi-wavelength characterization of the outskirts of spiral galaxies

AbstractWe have explored late-type spiral galaxies with truncated radial surface brightness profiles. Based on the study of optical color profiles, Bakos et al. (2008) suggested that truncated (TYPE II) galaxies have a generally older stellar population in their outer regions: one observable consequence of this would be a strong dependence of the structural parameters of the outer disk on the observing wavelength. To corroborate this result, we obtained surface brightness profiles using data from GALEX(UV), SDSS(optical), UKIDSS(NIR) and SPITZER-IRAC(IR). We have characterized the behaviour of the outer disk by obtaining the ratio of the inner and outer scale-lengths. Here we show the example of NGC0450. Our results suggest that there is an existing general trend of the scale-length ratio: from bluer to redder bands the scale-length ratio decreases, which is in accordance with the idea of the old smooth stellar disk in the outer disk.

Effects of Scale Length and Transition Density Ratio on the Electron Trapping and Acceleration with a Sharp Density Transition in Laser Wakefield Accelerator

In this paper, the effects of scale length and transition density ratio on the electron trapping and acceleration with a sharp density transition in laser wakefield accelerator (LWFA) have been studied using one-dimensional particle-in-cell (PIC) simulations. Simulation results show that the number of accelerated electrons depends on the plasma density in the high density region and the scale length of the density transition region. For the detailed investigation of the dynamical features of the electron trapping and acceleration, spatiotemporal patterns of the laser wake wave are also used. Throughout this work, the optimum condition for the electron acceleration could be found in the relevant parameter space of the scale length and the transition density ratio.

Modelling the Galactic bar using OGLE-II red clump giant stars

Red clump giant stars can be used as distance indicators to trace the mass distribution of the Galactic bar. We use RCG stars from 44 bulge fields from the OGLE-II microlensing collaboration database to constrain analytic tri-axial models for the Galactic bar. We find the bar major axis is oriented at an angle of 24 - 27 degrees to the Sun-Galactic centre line-of-sight. The ratio of semi-major and semi-minor bar axis scale lengths in the Galactic plane x_0, y_0, and vertical bar scale length z_0, is x_0 : y_0 : z_0 = 10 : 3.5 : 2.6, suggesting a slightly more prolate bar structure than the working model of Gerhard (2002) which gives the scale length ratios as x_0 : y_0 : z_0 = 10 : 4 : 3 .

A Comparison of Hα and Stellar Scale Lengths in Virgo and Field Spirals

The scale lengths of the old stars and ionized gas distributions are compared for similar samples of Virgo Cluster members and field spiral galaxies via Hα and broad R-band surface photometry. While the R-band and Hα scale lengths are, on average, comparable for the combined sample, we find significant differences between the field and cluster samples. While the Hα scale lengths of the field galaxies are a factor of 1.14 ± 0.07 longer, on average, than their R-band scale lengths, the Hα scale lengths of Virgo Cluster members are, on average, 20% smaller than their R-band scale lengths. Furthermore, in Virgo, the scale length ratios are correlated with the size of the star-forming disk: galaxies with smaller overall Hα extents also show steeper radial falloff of star formation activity. At the same time, we find no strong trends in scale length ratio as a function of other galaxy properties, including galaxy luminosity, inclination, morphological type, central R-band light concentration, or bar type. Our results for Hα emission are similar to other results for dust emission, suggesting that Hα and dust have similar distributions. The environmental dependence of the Hα scale length places additional constraints on the evolutionary process(es) that cause gas depletion and a suppression of the star formation rate in clusters of galaxies.

I Zw 1: Decomposition of the nearby QSO host

The paper presents an analysis of the nearby QSO host I Zw 1 based on new J-band imaging data, obtained with ISAAC at the Very Large Telescope (VLT) of the European Southern Observatory (ESO). As one of the best-studied QSOs with exceptional properties lying between those of QSOs, narrow-line Seyfert 1s, and ultraluminous infrared galaxies , I Zw 1 is a prime candidate for a detailed case study of a QSO host. With its high angular resolution and sensitivity, the new J-ba nd image provides strong evidence for an interaction between I Zw 1 and the western companion galaxy. We present a procedure for subtracting the QSO nucleus from the I Zw 1 image to obtain an undisturbed view on the underlying host galaxy. Based on the derived surface brightness profile and the gas rotatio n curve, the host is classified as a high-surface-brightness Freeman type I galaxy with a central disk surface brightness signific antly larger than that of normal spirals. The surface-brigh tness profile is decomposed into a Plummer bulge and a Kuzmin disk wi th similar luminosities and a bulge-to-disk scale length ra tio of 0.29, comparable to that of nearby spiral hosts with active galactic nuclei (AGN). Different models for the decomposition of the gas rotation curve are discussed. The resulting J-band mass-to-light ratio (M/L) of (0.7± 0.4) M⊙/L⊙ for the disk component suggests a mean solar-type stellar population with a tendency towards young stars. The bulge exhibits a lower M/L of (0.4± 0.2) M⊙/L⊙, which supports previous findings of enhanced starburst act ivity in the nuclear and circumnuclear region.

Orthogonal and conformal surface grid generation

Surface grid-generation algorithms are based on a projection strategy, consisting in generating a grid on the surface parametric plane, and then to project it on the surface. However during the projection step the grid characteristics, such as point distribution and orthogonality, are often not correctly transported. To overcome these difficulties, a new approach is proposed which can be divided into two main steps: (i) the parameterization of the surface by isothermal coordinates, (ii) the generation of a two-dimensional grid on the conformal parametric plane and its projection on the surface. In this way the projection is based on a conformal map, preserving angles and scale-length ratios.

On the global structure of distant galactic disks

Radial and vertical profiles are determined for a sample of 34 edge-on disk galaxies in the HDFs, selected for their apparent diameter larger than 1.3 '' and their unperturbed morphology. The thickness and flatness of their galactic disks are determined and discussed with regard to evolution with redshift. We find that sub-L ∗ spiral galaxies with z � 1 have a relative thickness or flatness (characterized by hz/h the scaleheight to scalelength ratio) globally similar to those in the local Universe. A slight trend is however apparent, with the hz/h flatness ratio larger by a factor of � 1.5 in distant galaxies if compared to local samples. In absolute value, the disks are smaller than in present-day galaxies. About half of the z � 1 spiral disks show a non-exponential surface brightness distribution.

I Zw 1: Decomposition of the Nearby QSO Host

The paper presents an analysis of the nearby QSO host I Zw 1 based on new J-band imaging data, obtained with ISAAC at the Very Large Telescope (VLT) of the European Southern Observatory (ESO). As one of the best-studied QSOs with exceptional properties lying between those of QSOs, narrow-line Seyfert 1s, and ultraluminous infrared galaxies, I Zw 1 is a prime candidate for a detailed case study of a QSO host. With its high angular resolution and sensitivity, the new J-band image provides strong evidence for an interaction between I Zw 1 and the western companion galaxy. We present a procedure for subtracting the QSO nucleus from the I Zw 1 image to obtain an undisturbed view on the underlying host galaxy. Based on the derived surface brightness profile and the gas rotation curve, the host is classified as a high-surface-brightness Freeman type I galaxy with a central disk surface brightness significantly larger than that of normal spirals. The surface-brightness profile is decomposed into a Plummer bulge and a Kuzmin disk with similar luminosities and a bulge-to-disk scale length ratio of 0.29, comparable to that of nearby spiral hosts with active galactic nuclei (AGN). Different models for the decomposition of the gas rotation curve are discussed. The resulting J-band mass-to-light ratio (M/L) of (0.7+/-0.4) M_sun/L_sun for the disk component suggests a mean solar-type stellar population with a tendency towards young stars. The bulge exhibits a lower M/L of 0.4+/-0.2 M_sun/L_sun, which supports previous findings of enhanced starburst activity in the nuclear and circumnuclear region.

Structure of Disk‐dominated Galaxies. I. Bulge/Disk Parameters, Simulations, and Secular Evolution

(Abridged) A robust analysis of galaxy structural parameters, based on the modeling of bulge and disk brightnesses in the BVRH bandpasses, is presented for 121 face-on and moderately inclined late-type spirals. Each surface brightness (SB) profile is decomposed into a sum of a generalized Sersic bulge and an exponential disk. The reliability and limitations of our bulge-to-disk (B/D) decompositions are tested with extensive simulations of galaxy brightness profiles (1D) and images (2D). Galaxy types are divided into 3 classes according to their SB profile shapes; Freeman Type-I and Type-II, and a third ``Transition'' class for galaxies whose profiles change from Type-II in the optical to Type-I in the infrared. We discuss possible interpretations of Freeman Type-II profiles. The Sersic bulge shape parameter for nearby Type-I late-type spirals shows a range between n=0.1-2 but, on average, the underlying surface density profile for the bulge and disk of these galaxies is adequately described by a double-exponential distribution. We confirm a coupling between the bulge and disk with a scale length ratio r_e/h=0.22+/-0.09, or h_bulge/h_disk=0.13+/-0.06 for late-type spirals, in agreement with recent N-body simulations of disk formation and models of secular evolution. This ratio increases from ~0.20 for late-type spirals to ~0.24 for earlier types. The similar scaling relations for early and late-type spirals suggest comparable formation and/or evolution scenarios for disk galaxies of all Hubble types.