Superthin Galaxies Research Articles

Formation of Superthin Galaxies in IllustrisTNG

Superthin galaxies are observed to have stellar disks with extremely small minor-to-major axis ratios. In this work, we investigate the formation of superthin galaxies in the TNG100 simulation. We trace the merger history and investigate the evolution of galaxy properties of a selected sample of superthin galaxies and a control sample of galaxies that share the same joint probability distribution in the stellar-mass and color diagram. Through making comparisons between the two galaxy samples, we find that present-day superthin galaxies had similar morphologies as the control sample counterparts at higher redshifts, but have developed extended flat “superthin” morphologies since z ∼ 1. During this latter evolution stage, superthin galaxies undergo an overwhelmingly higher frequency of prograde mergers (with orbit-spin angle θ orb ≤ 40°). Accordingly the spins of their dark matter halos have grown significantly and become noticeably higher than those of their normal disk counterparts. This further results in the buildup of their stellar disks at larger distances much beyond the regimes of normal disk galaxies. We also discuss the formation scenario of those superthin galaxies that live in larger dark matter halos as satellite galaxies therein.

Are superthin galaxies low-surface-brightness galaxies seen edge-on? The star formation probe

ABSTRACT Superthin galaxies (STs) are edge-on disc galaxies with strikingly high planar-to-vertical axes ratios of ∼10–20 with no bulge component, and central surface brightness in the B band > 23 mag arcsec−2 comparable to low-surface-brightness galaxies (LSBs). Although STs and LSBs have similar dynamical, stellar, and atomic hydrogen (H i) masses on an average, it is tricky to conclude if they constitute the same galaxy population, given the edge-on and face-on orientations of the STs and the LSBs, respectively. We systematically study star formation rate (SFR) in a sample of STs and LSBs using SED fitting of photometric data in 10 bands including GALEX: FUV, NUV, SDSS: u,g,r,i,z and 2MASS: J, H, Ks using stellar population synthesis models employing the publicly available software magphys (Multi-Wavelength Analysis of Galaxy Physical Properties). The estimated median SFRs for LSBs and STs are $0.4^{+2.2}_{-0.3}$$ and $0.2^{+0.9}_{-0.2}\, {\rm M}_{\odot }{\rm yr}^{-1}$, respectively. Our calculations indicate that this deficit in the SFR of an ST can be attributed to inclination and opacity effects. Therefore, we conclude that STs and LSBs have equal intrinsic SFR over and above other physical properties, which possibly implies that STs are just LSBs seen in edge-on.

H i 21 cm observation and mass models of the extremely thin galaxy FGC 1440

ABSTRACT We present observations and models of the kinematics and distribution of neutral hydrogen (H i) in the superthin galaxy FGC 1440 with an optical axial ratio a/b = 20.4. Using the Giant Meterwave Radio telescope (GMRT), we imaged the galaxy with a spectral resolution of 1.7 km s−1 and a spatial resolution of 15${_{.}^{\prime\prime}}$9 × 13${_{.}^{\prime\prime}}$5. We find that FGC 1440 has an asymptotic rotational velocity of 141.8 km s−1. The structure of the H i disc in FGC 1440 is that of a typical thin disc warped along the line of sight, but we cannot rule out the presence of a central thick H i disc. We find that the dark matter halo in FGC 1440 could be modelled by a pseudo-isothermal (PIS) profile with $\\ R_{\rm c}/ R_{\rm d} \lt 2$, where Rc is the core radius of the PIS halo and Rd the exponential stellar disc scale length. We note that in spite of the unusually large axial ratio of FGC 1440, the ratio of the rotational velocity to stellar vertical velocity dispersion, $\frac{V_{\rm Rot}}{\sigma _{z}} \sim 5 - 8$, which is comparable to other superthins. Interestingly, unlike previously studied superthin galaxies which are outliers in the log10(j*) − log10(M*) relation for ordinary bulgeless disc galaxies, FGC 1440 is found to comply with the same. The values of j for the stars, gas, and the baryons in FGC 1440 are consistent with those of normal spiral galaxies with similar mass.

Fourcade-Figueroa galaxy: A clearly disrupted superthin edge-on galaxy

Context. Studies of the stellar and the H I gas kinematics in dwarf and low surface brightness (LSB) galaxies are essential for deriving constraints on their dark matter distribution. Moreover, a key component to unveil in the evolution of LSBs is to determine why some of them can be classified as superthin. Aims. We aim to investigate the nature of the proto-typical superthin galaxy Fourcade-Figueroa (FF) to understand the role played by the dark matter halo in forming its superthin shape and to investigate the mechanism that explains the observed disruption in the approaching side of the galaxy. Methods. Combining new H I 21 cm observations obtained with the Giant Metrewave Radio Telescope with archival data from the Australia Telescope Compact Array we were able to obtain sensitive H I observations of the FF galaxy. These data were modelled with a 3D tilted ring model in order to derive the rotation curve and surface brightness density of the neutral hydrogen. We subsequently used this model, combined with a stellar profile from the literature, to derive the radial distribution of the dark matter in the FF galaxy. Additionally, we used a more direct measurement of the vertical H I gas distribution as a function of the galactocentric radius to determine the flaring of the gas disk. Results. For the FF galaxy, the Navarro-Frenk-White dark matter distribution provides the best fit to the observed rotation curve. However, the differences with a pseudo-isothermal halo are small. Both models indicate that the core of the dark matter halo is compact. Even though the FF galaxy classifies as superthin, the gas thickness about the galactic centre exhibits a steep flaring of the gas that agrees with the edge of the stellar disk. In addition, FF is clearly disrupted towards its north-west side, clearly observed at optical and H I wavelengths. As suggested previously in the literature, the compact dark matter halo might be the main cause for the superthin structure of the stellar disk in FF. This idea is strengthened through the detection of the disruption; the fact that the galaxy is disturbed also appears to support the idea that it is not isolation that causes its superthin structure.

Spectral Observations of Superthin Galaxies

Abstract We conduct spectral observations of 138 superthin galaxies (STGs) with high radial-to-vertical stellar disk scale ratios with the Dual Imaging Spectrograph on the 3.5 m telescope at the Apache Point Observatory (APO) to obtain the ionized gas rotation curves with R ∼5000 resolution. We also performed near-infrared (NIR) H and Ks photometry for 18 galaxies with the NICFPS camera on the 3.5 m telescope. The spectra, the NIR photometry, and published optical and NIR photometry are used for modeling that utilizes the thickness of the stellar disk and rotation curves simultaneously. The projection and dust extinction effects are taken into account. We evaluate eight models that differ in their free parameters and constraints. As a result, we estimated the masses and scale lengths of the galactic dark halos. We find systematic differences between the properties of our red and blue STGs. The blue STGs have a large fraction of dynamically underevolved galaxies whose vertical velocity dispersion is low in both gas and stellar disks. The dark halo-to-disk scale ratio is shorter in the red STGs than in the blue ones, but in a majority of all STGs, this ratio is under 2. The optical color (r − i) of the STGs correlates with their rotation curve maximum, vertical velocity dispersion in stellar disks, and mass of the dark halo. We conclude that there is a threshold central surface density of 50 M ⊙ pc−2 below which we do not observe very thin, rotationally supported galactic disks.

A Spectroscopic Survey of Superthin Galaxies

Abstract We present spectroscopic observations of superthin galaxies. Superthin galaxies have the thinnest stellar disks among disk galaxies. A sample of 138 superthins was observed in visible light with the 3.5 m telescope at Apache Point Observatory in New Mexico to obtain the rotation curves of the ionized gas in the galaxies. The sample represents the largest survey of superthin galaxies so far and provides a database to investigate the kinematic and dynamic properties of this special type of extragalactic objects. Here we present the rotation curves of our sample objects.

How ‘cold’ are the stellar discs of superthin galaxies?

ABSTRACT Superthin galaxies are a class of bulgeless, low surface brightness galaxies with strikingly high values of planar-to-vertical axes ratio $\rm (b/a\gt 10\!-\!20)$, possibly indicating the presence of an ultracold stellar disc. Using the multicomponent galactic disc model of gravitationally coupled stars and gas in the force field of the dark matter halo as well as the stellar dynamical code AGAMA (Action-based Galaxy Modelling Architecture), we determine the vertical velocity dispersion of stars and gas as a function of galactocentric radius for five superthin galaxies (UGC 7321, IC 5249, FGC 1540, IC2233, and UGC00711) using observed stellar and atomic hydrogen (H i) scale heights as constraints, using a Markov chain Monte Carlo Method. We find that the central vertical velocity dispersion for the stellar disc in the optical band varies between σ0s ∼ 10.2−18.4 $\rm {kms}\,^{-1}$ and falls off with an exponential scale length of 2.6–3.2 Rd where Rd is the exponential stellar disc scale length. Interestingly, in the 3.6 μm, the same, averaged over the two components of the stellar disc, varies between 5.9 and 11.8 $\rm {kms}\,^{-1}$, both of which confirm the presence of ‘ultracold’ stellar discs in superthin galaxies. Interestingly, the global median of the multicomponent disc dynamical stability parameter QN of our sample superthins is found to be 5 ± 1.5, which higher than the global median value of 2.2 ± 0.6 for a sample of spiral galaxies.

Dynamical modelling of disc vertical structure in superthin galaxy ‘UGC 7321’ in braneworld gravity: an MCMC study

ABSTRACT Low surface brightness (LSBs) superthins constitute classic examples of very late-type galaxies, with their disc dynamics strongly regulated by their dark matter haloes. In this work, we consider a gravitational origin of dark matter in the braneworld scenario, where the higher dimensional Weyl stress term projected on to the three-brane acts as the source of dark matter. In the context of the braneworld model, this dark matter is referred to as the ‘dark mass’. This model has been successful in reproducing the rotation curves of several LSB and high surface brightness galaxies. Therefore, it is interesting to study the prospect of this model in explaining the vertical structure of galaxies which has not been explored in the literature so far. Using our two-component model of gravitationally coupled stars and gas in the external force field of this dark mass, we fit the observed scale heights of stellar and atomic hydrogen (H i) gas of superthin galaxy ‘UGC7321’ using the Markov Chain Monte Carlo approach. We find that the observed scale heights of ‘UGC7321’ can be successfully modelled in the context of the braneworld scenario. In addition, the model predicted rotation curve also matches the observed one. The implications on the model parameters are discussed.

H i scale height in spiral galaxies

ABSTRACT We model the galactic discs of seven nearby large spiral galaxies as three-component systems consist of stars, molecular gas, and atomic gas in vertical hydrostatic equilibrium. We set up the corresponding joint Poisson–Boltzmann equation and solve it numerically to estimate the 3D distribution of H i in these galaxies. While solving the Poisson–Boltzmann equation, we do not consider a constant H i velocity dispersion (σHI); rather, we develop an iterative method to self-consistently estimate the σHI profile in a galaxy by using the observed second-moment profile of the H i spectral cube. Using the density solutions, we determine the H i vertical scale height in our galaxies. We find that the H i discs flare in a linear fashion as a function of radius. H i scale height in our galaxies is found to vary between a few hundred parsecs at the centre to ∼1–2 kpc at the outskirts. We estimate the axial ratio of the H i discs in our sample galaxies and find a median ratio of 0.1, which is much lower than what is found for dwarf galaxies, indicating much thinner H i discs in spiral galaxies. Very low axial ratios in three of our sample galaxies (NGC 5055, NGC 6946, and NGC 7331) suggest them to be potential superthin galaxies. Using the H i distribution and the H i hole sizes in NGC 6946, we find that most of the H i holes in this galaxy are broken out into the circumgalactic medium and this breaking out is more effective in the inner radii as compared to the outer radii.

Dark matter distribution in superthin galaxies

ABSTRACT This paper presents a new method of calculating dark matter density profiles for superthin axial symmetric galaxies without a bulge. This method is based on a simple physical model, which includes an infinitely thin galactic disc immersed in a spherically symmetric halo of dark matter. To obtain the desired distribution density, it suffices to know a distribution of visible matter surface density in a galaxy and a dependence of angular velocity on the radius. As a byproduct, the well-known expression, which reproduces surface density of a superthin galaxy expressed through a rotation law, was obtained.

Near‐infrared photometry of superthin edge‐on galaxies

AbstractWe perform near‐infrared photometry of a large sample of 49 superthin edge‐on galaxies. These galaxies are selected based on optical photometry because of high radial‐to‐vertical scale ratio in their stellar disks. The near infrared (NIR) H and K observations were conducted with the cryogenic‐cooled camera ASTRONIRCAM on the 2.5 m telescope at the Caucasus Mountain Observatory of Lomonosov Moscow State University. A majority of galaxies in our sample show comparable or better photometric depth than the Sloan Digital Sky Survey (SDSS) optical images. We estimate the structural parameters of stellar disks in the galaxies and find that the NIR scale height of stellar disks is comparable to that estimated from the optical, SDSS g, r and i, whereas the H and K scale length of the stellar disks is significantly shorter than in the g, r, and i. We investigate if a realistic distribution of dust alone can explain the difference in the scale length and find that in the majority of the galaxies the radial variation of the stellar population is actually responsible for the color distribution. The latter suggests a younger age of the disks periphery, and the inside out building up of stellar disks in the superthin galaxies.

The specific angular momenta of superthin galaxies: Cue to their origin?

ABSTRACT Superthin galaxies are low surface brightness (LSB) bulgeless disc galaxies having stellar discs with unusually high planar-to-vertical axes ratio b/a > 10–20, the formation and evolution of which is not well understood. We calculate the specific angular momenta of a sample of six superthins and nine other bulgeless LSBs using stellar photometry, atomic hydrogen (HI) surface density, and high-resolution HI rotation curves available in the literature. We find that the stellar specific angular mometum js, and hence the stellar disc size given by the exponential stellar disc scale length RD, of three superthins and seven LSBs lie above the 95.4 ${{\ \rm per\ cent}}$ confidence band of the js − Vrot regression line for ordinary bulgeless disc galaxies, Vrot being the asymptotic rotational velocity. Further, we find that superthins and LSBs have higher js and RD values for a given value of stellar mass Ms at high values of statistical significance, compared to ordinary disc galaxies. Therefore, we conclude, a superthin is may be distinguished by a characteristically larger disc size which could possibly explain the origin of its large planar-to-vertical axes ratio. Interestingly, we find that the median spin parameter $\lambda = \frac{ j_{\rm {stars}} }{ {\sqrt{(}2) V_{\rm {vir}} R_{\rm {vir}} } }$, Vvir and Rvir being the virial velocity and virial radius of the galaxy respectively, is 0.13 ± 0.01 for superthin galaxies which is an order of magnitude higher than those of LSBs and ordinary disc galaxies, which may have important implications for the existence of superthin stellar discs in these LSB galaxies.

Mass modelling of a superthin galaxy, FGC 1540

We present high resolution H{\sc i} 21cm Giant Meterwave Radio Telescope (GMRT) observations of the superthin galaxy FGC1540 with a spatial resolution of 10$''$ $\times$ 8$''$ and a spectral resolution of 1.73 kms$^{-1}$ and an rms noise of 0.9 mJy per beam. We obtain its rotation curve as well as deprojected radial H{\sc i} surface density profile by fitting a 3-dimensional tilted ring model directly to the H{\sc i} data cubes by using the publicly-available software, Fully Automated Tirrific (FAT). We also present the rotation curve of FGC1540 derived from its optical spectroscopy study using the 6-m BTA telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. We use the rotation curve, the H{\sc i} surface density profile together with Spitzer 3.6 $\mu$m and the SDSS $i$--band data to construct the mass models for FGC1540. We find that both the Pseudo-isothermal (PIS), as well as Navarro-Frenk-White (NFW) dark matter (DM) halos, fit the observed rotation curve equally well. The PIS model indicates a compact dark matter halo ($R_{\rm C}/R_{\rm D}$ < 2), with the best-fitting core radius ($R_{\rm C}$) approximately half the exponential stellar disc scale length ($R_{\rm D}$), which is in agreement with the mass models of superthin galaxies studied earlier in the literature. Since the vertical thickness of the galactic stellar disc is determined by a balance between the net gravitational field and the velocity dispersion in the vertical direction, the compact dark matter halo may be primarily responsible in regulating the superthin vertical structure of the stellar disc in FGC1540 as was found in case of the superthin galaxy UGC7321.

Mass modelling of superthin galaxies: IC5249, UGC7321 and IC2233

Superthin galaxies are low surface brightness disc galaxies, characterised by optical discs with strikingly high values of planar-to-vertical axes ratios ($>$ 10), the physical origin and evolution of which continue to be a puzzle. We present mass models for three superthin galaxies: IC5249, UGC7321 and IC2233. We use high resolution rotation curves and gas surface density distributions obtained from HI 21 cm radio-synthesis observations, in combination with their two-dimensional structural surface brightness decompositions at Spitzer 3.6 $\mu$m band, all of which were available in the literature. We find that while models with the pseudo-isothermal (PIS) and the Navarro-Frenk-White (NFW) dark matter density profiles fit the observed rotation curves of IC5249 and UGC7321 equally well, those with the NFW profile does not comply with the slowly-rising rotation curve of IC2233. Interestingly, for all of our sample galaxies, the best-fitting mass models with a PIS dark matter density profile indicate a {\it compact} dark matter halo i.e., $R_c/R_D$ $<$ 2 where $R_c$ is the core radius of the PIS dark matter halo, and $R_D$ is the radial scale-length of the exponential stellar disc. The compact dark matter halo may be fundamentally responsible for the superthin nature of the stellar disc, and therefore our results may have important implications for the formation and evolution models of superthin galaxies in the universe.

Extending the Hα Survey for the Local Volume Galaxies

Images in the Ha emission line are presented for 35 nearby objects observed with the 6-m BTA telescope. Three of them, NGC 3377, NGC 3384, and NGC 3390, are bright E and S0 galaxies, one is an edge-on Sd galaxy UGC 7321, two are remote globular clusters associated with M 31, and the rest are dwarf galaxies of morphological types dIr, dTr, dSph, BCD, and Sm. The measured Hα fluxes are used to estimate the integral (SFR) and specific (sSFR) star formation rates for these galaxies. The values of log[sSFR] for all these objects lie below a limit of -0.4(Gyr-1). We note that the emission disk for the nearest super-thin edge-on galaxy UGC 7321 has an extremely large axis ratio of a/b = 38.

Suppression of gravitational instabilities by dominant dark matter halo in low-surface-brightness galaxies

The low surface brightness galaxies are gas-rich and yet have a low star formation rate, this is a well-known puzzle. The spiral features in these galaxies are weak and difficult to trace, although this aspect has not been studied much. These galaxies are known to be dominated by the dark matter halo from the innermost regions. Here we do a stability analysis for the galactic disc of UGC 7321, a low surface brightness, superthin galaxy, for which the various observational input parameters are available. We show that the disc is stable against local, linear axisymmetric and non-axisymmetric perturbations. The Toomre Q parameter values are found to be large (>> 1) mainly due to the low disc surface density and the high rotation velocity resulting due to the dominant dark matter halo, which could explain the observed low star formation rate. For the stars-alone case, the disc shows finite swing amplification but the addition of dark matter halo suppresses that amplification almost completely. Even the inclusion of the low-dispersion gas which constitutes a high disc mass fraction does not help in causing swing amplification. This can explain why these galaxies do not show strong spiral features. Thus the dynamical effect of a halo that is dominant from inner regions can naturally explain why star formation and spiral features are largely suppressed in low surface brightness galaxies, making these different from the high surface brightness galaxies.

Why are some galaxy discs extremely thin?

Some low surface brightness galaxies are known to have extremely thin stellar disks with the vertical to planar axes ratio 0.1 or less, often referred to as superthin disks. Although their existence is known for over three decades, the physical origin for the thin distribution is not understood. We model the stellar thickness for a two-component (gravitationally coupled stars and gas) disk embedded in a dark matter halo, for a superthin galaxy UGC 7321 which has a dense, compact halo, and compare with a typical dwarf galaxy HoII which has a non-compact halo. We show that while the presence of gas does constrain the disk thickness, it is the compact dark matter halo which plays the decisive role in determining the superthin disk distribution in low-mass disks. Thus the compact dark matter halo significantly affects the disk structure and this could be important for the early evolution of galaxies.

On the nature of the peculiar superthin LSB galaxy UGC 12281

AbstractUGC 12281 has been classified as having a pure disk and being a low surface brightness galaxy (LSBG), thus being an obvious member of the so-called superthin galaxies. At the same time it represents an extremely untypical type of LSBG due to its remarkable amount of current star formation and evidence for extraplanar ionized gas. This makes it become a perfect tool to investigate the triggering of star formation in LSB galaxies, located in an alleged isolated area. By means of deep photometry and long-slit spectroscopy we analyse the Hα halo and verify the existence of a potential dwarf companion which we found on processed SDSS images.

The Edge-On Perspective of Bulgeless, Simple Disk Galaxies

ABSTRACTThis review focuses on flat and superthin galaxies. These are edge-on bulgeless galaxies, which are composed of a simple, stellar disk. The properties of these simple disks are at the end of a continuum that extends smoothly from bulge-dominated disk galaxies to the pure disks. On average, simple disks are low-mass galaxies with low surface brightnesses, blue colors, and slow rotational velocities. Widely-accepted cosmological models of galaxy formation and evolution were challenged by a relatively large observed fraction of pure disk galaxies, and only very recent models can explain the existence of simple disk galaxies. This makes simple disks an optimal galaxy type for the study of galaxy formation in a hierarchical universe. They enable us to analyze the environmental and internal influence on galaxy evolution, to study the stability of the disks, and to explain the nature and distribution of dark matter in galaxies. This review summarizes the current status of edge-on simple disk galaxies in the universe.

Dark matter dominance at all radii in the superthin galaxy UGC 7321

We model the shape and density profile of the dark matter halo of the low surface brightness, superthin galaxy UGC 7321, using the observed rotation curve and the Hi scale height data as simultaneous constraints. We treat the galaxy as a gravitationally coupled system of stars and gas, responding to the gravitational potential of the dark matter halo. An isothermal halo of spherical shape with a core density in the range of 0.039–0.057M⊙pc-3 and a core radius between 2.5 and 2.9kpc, gives the best fit to the observations for a range of realistic gas parameters assumed. We find that the best-fit core radius is only slightly higher than the stellar disc scale length (2.1kpc), unlike the case of the high surface brightness galaxies where the halo core radius is typically 3–4 times the disc scale length of the stars. Thus our model shows that the dark matter halo dominates the dynamics of the low surface brightness, superthin galaxy UGC 7321 at all radii, including the inner parts of the galaxy.