Stellar Scale Heights Research Articles

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.

Transport of gas from disk to halo in starforming galaxies

Abstract Using 3-D gas dynamic simulations, we study the supernova (SNe) driven transport of gas from the galactic disk. We assume that SNe are distributed randomly and uniformly in the galactic plane and we consider sufficiently high volume SNe rates that are typical for starforming galaxies: νSN = (0.3 − 3) × 10−11 pc−3 yr−1. We found that under such conditions, a major part of gas locked initially in the galactic disk is transported up to ∼ 1 − 5 stellar scale heights within several millions years. As expected gas transport is more efficient in the case of a thinner stellar disk. An decrease/increase of SN rate in the galactic disk with the same stellar scale height leads to an enlarging/shortening of time scale for gas transport. Independent of SN rate, the major fraction of the swept up gas is in the cold phase (T < 103 K), though its volume filling factor is rather small, ∼1-3%. Hot gas with T > 106 K is elevated to larger heights than cold gas.

THE RELATIONSHIP BETWEEN MONO-ABUNDANCE AND MONO-AGE STELLAR POPULATIONS IN THE MILKY WAY DISK

ABSTRACT Studying the Milky Way disk structure using stars in narrow bins of [Fe/H] and [α/Fe] has recently been proposed as a powerful method to understand the Galactic thick and thin disk formation. It has been assumed so far that these mono-abundance populations (MAPs) are also coeval, or mono-age, populations. Here we study this relationship for a Milky Way chemodynamical model and show that equivalence between MAPs and mono-age populations exists only for the high-[α/Fe] tail, where the chemical evolution curves of different Galactic radii are far apart. At lower [α/Fe]-values an MAP is composed of stars with a range in ages, even for small observational uncertainties and a small MAP bin size. Due to the disk inside-out formation, for these MAPs younger stars are typically located at larger radii, which results in negative radial age gradients that can be as large as 2 Gyr kpc−1. Positive radial age gradients can result for MAPs at the lowest [α/Fe] and highest [Fe/H] end. Such variations with age prevent the simple interpretation of observations for which accurate ages are not available. Studying the variation with radius of the stellar surface density and scale height in our model, we find good agreement to recent analyses of the APOGEE red-clump (RC) sample when 1–4 Gyr old stars dominate (as expected for the RC). Our results suggest that the APOGEE data are consistent with a Milky Way model for which mono-age populations flare for all ages. We propose observational tests for the validity of our predictions and argue that using accurate age measurements, such as from asteroseismology, is crucial for putting constraints on Galactic formation and evolution.

A constant intrinsic thickness for dwarf irregular galaxies?

Abstract We assess the intrinsic thickness parameter, qo, for a sample of dwarf irregular galaxies and compare to larger, more massive spiral galaxy systems. We use optical photometry to determine b/a (minor-to-major axial ratio), and we use the H i kinematic inclination angle to derive qo. We find that qo ranges from 0.1 to 0.8 for the dwarfs. We find a trend in qo with luminosity, indicating that fainter dwarfs are thicker than brighter ones, similar to previous studies. However, we also find a trend in qo with H i kinematic inclination, which indicates that either the radius at which we measure b/a may be too small, or, that the stellar and gas discs have different inclinations. Because we selected only those objects that have nearly identical morphological and kinematic position angles, we find the latter reason unlikely. We find a weak trend in qo with hz/RD (ratio of stellar scaleheight to scalelength), which points to b/a not fully representing the stellar disc distribution. We conclude that a constant qo may not be appropriate for dwarf irregulars.

MONTE-CARLO RADIATIVE TRANSFER MODEL OF THE DIFFUSE GALACTIC LIGHT

Monte-Carlo radiative models of the diffuse Galactic light (DGL) in our Galaxy are calculated using the dust radiative transfer code MoCafe, which is three-dimensional and takes full account of multiple scattering. The code is recently updated to use a fast voxel traversal algorithm, which has dramatically increased the computing speed. The radiative transfer models are calculated with the generally accepted dust scale-height of 0.1 kpc. The stellar scale-heights are assumed to be 0.1 or 0.35 kpc, appropriate for far-ultraviolet (FUV) and optical wavelengths, respectively. The face-on optical depth, measured perpendicular to the Galactic plane, is also varied from 0.2 to 0.6, suitable to the optical to FUV wavelengths, respectively. We find that the DGL at high Galactic latitudes is mostly due to backward or large-angle scattering of starlight originating from the local stars within a radial distance of r < 0.5 kpc from the Earth. On the other hand, the DGL measured in the Galactic plane is mostly due to stars at a distance range that corresponds to an optical depth of <TEX>$${\sim_\sim}$$</TEX> 1 measured from the Earth. Therefore, the low-latitude DGL at the FUV wavelength band would be mostly caused by the stars located at a distance of <TEX>$r{\leq}0.5$</TEX> kpc and the optical DGL near the Galactic plane mainly originates from stars within a distance range of <TEX>$1{\leq}r{\leq}2kpc$</TEX>. We also calculate the radiative transfer models in a clumpy two-phase medium. The clumpy two-phase models provide lower intensities at high Galactic latitudes compared to the uniform density models, because of the lower effective optical depth in clumpy media. However, no significant difference in the intensity at the Galactic plane is found.

A QUMOND galactic N-body code - I. Poisson solver and rotation curve fitting

Here we present a new particle-mesh galactic N-body code that uses the full multigrid algorithm for solving the modified Poisson equation of the Quasi Linear formulation of Modified Newtonian Dynamics (QUMOND). A novel approach for handling the boundary conditions using a refinement strategy is implemented and the accuracy of the code is compared with analytical solutions of Kuzmin disks. We then employ the code to compute the predicted rotation curves for a sample of five spiral galaxies from the THINGS sample. We generated static N-body realisations of the galaxies according to their stellar and gaseous surface densities and allowed their distances, mass-to-light ratios (M/L) and both the stellar and gas scale-heights to vary in order to estimate the best fit parameters. We found that NGC 3621, NGC 3521 and DDO 154 are well fit by MOND using expected values of the distance and M/L. NGC 2403 required a moderately larger $M/L$ than expected and NGC 2903 required a substantially larger value. The surprising result was that the scale-height of the dominant baryonic component was well constrained by the rotation curves: the gas scale-height for DDO 154 and the stellar scale-height for the others. In fact, if the suggested stellar scale-height (one-fifth the stellar scale-length) was used in the case of NGC 3621 and NGC 3521 it would not be possible to produce a good fit to the inner rotation curve. For each of the four stellar dominated galaxies, we calculated the vertical velocity dispersions which we found to be, on the whole, quite typical compared with observed stellar vertical velocity dispersions of face on spirals. We conclude that modelling the gas scale-heights of the gas rich dwarf spiral galaxies will be vital in order to make precise conclusions about MOND.

THE STABILITY OF LOW SURFACE BRIGHTNESS DISKS BASED ON MULTI-WAVELENGTH MODELING

To investigate the structure and composition of the dusty interstellar medium (ISM) of low surface brightness (LSB) disk galaxies, we have used multiwavelength photometry to construct spectral energy distributions for three low-mass, edge-on LSB galaxies. We use Monte Carlo radiation transfer codes that include the effects of transiently heated small grains and polycyclic aromatic hydrocarbon molecules to model and interpret the data. We find that unlike the high surface brightness galaxies previously modeled, the dust disks appear to have scale heights equal to or exceeding their stellar scale heights. This result supports the findings of previous studies that low mass disk galaxies have dust scale heights comparable to their stellar scale heights and suggests that the cold ISM of low mass, LSB disk galaxies may be stable against fragmentation and gravitational collapse. This may help to explain the lack of observed dust lanes in edge-on LSB galaxies and their low current star formation rates.

The G-dwarf problem in the Galaxy

This paper has two parts: one about observational constraints, and the other about chemical evolution models. In the first part, the empirical differential metallicity distribution (EDMD) is deduced from three different samples involving (i) local thick disk stars derived from Gliese and scaled in situ samples within the range, - 1.20 ⩽ [ Fe / H ] ⩽ - 0.20 [Wyse, R.F.G., Gilmore, G., 1995. AJ 110, 2771]; (ii) 46 likely metal-weak thick disk stars within the range, - 2.20 ⩽ [ Fe / H ] ⩽ - 1.00 [Chiba, M., Beers, T.C., 2000. AJ 119, 2843]; (iii) 287 chemically selected G dwarfs within 25 pc from the Sun, with the corrections performed in order to take into account the stellar scale height [Rocha-Pinto, H.J., Maciel, W.J., 1996. MNRAS 279, 447]; in addition to previous results [Caimmi, R., 2001b. AN 322, 241; Caimmi, R., 2007. NewA 12, 289] related to (iv) 372 solar neighbourhood halo subdwarfs [Ryan, S.G., Norris, J.E., 1991. AJ 101, 1865]; and (v) 268 K-giant bulge stars [Sadler, E.M., Rich, R.M., Terndrup, D.M., 1996. AJ 112, 171]. The metal-poor and metal-rich EDMD related to the thick disk shows similarities with their halo and bulge counterparts, respectively. Then the thick disk is conceived as made of two distinct regions: the halo-like and the bulge-like thick disk, and the related EDMD is deduced. Under the assumption that each distribution is typical for the corresponding subsystem, the EDMD of the thick disk, the thick + thin disk, and the Galaxy, is determined by weighting the mass. In the second part, models of chemical evolution for the halo-like thick disk, the bulge-like thick disk, and the thin disk, are computed assuming the instantaneous recycling approximation. The EDMD data are fitted, to an acceptable extent, by simple models of chemical evolution implying both homogeneous and inhomogeneous star formation, provided that star formation is inhibited during thick disk formation, with respect to the thin disk. The initial mass function (IMF) is assumed to be a universal power law, which implies the same value of the true yield in different subsystems. The theoretical differential metallicity distribution (TDMD) is first determined for the halo-like thick disk, the bulge-like thick disk, and the thin disk separately, and then for the Galaxy by weighting the mass. An indicative comparison is performed between the EDMD deduced for the disk both in presence and in absence of [O/Fe] plateau, and its counterpart computed for (vi) N = 523 nearby stars within the range, - 1.5 < [ Fe / H ] < 0.5 , for which the oxygen abundance has been determined both in presence and in absence of the local thermodynamical equilibrium (LTE) approximation [Ramirez, I., Allende Prieto, C., Lambert, D.L., 2007. A&A 465, 271]. Both distributions are found to exhibit a similar trend, although systematic differences exist. In addition, the related empirical age-metallicity relation (EAMR) cannot be fitted by the theoretical age-metallicity relation (TAMR) predicted by the model, and the reasons for this discrepancy are explained.

Chain Galaxies in the Tadpole Advanced Camera for Surveys Field

Colors and magnitudes were determined for 69 chain galaxies, 58 other linear structures, 32 normal edge-on galaxies, and all of their large star formation clumps in the Hubble Space Telescope Advanced Camera for Surveys field of the Tadpole galaxy. Redshifts of 0.5-2 are inferred from comparisons with published color-evolution models. The linear galaxies have no red nuclear bulges like the normal disk galaxies in our field, but the star formation clumps in each have about the same colors and magnitudes. Light profiles along the linear galaxies tend to be flat, unlike the exponential profiles of normal galaxies. Although the most extreme of the linear objects look like beaded filaments, they are all probably edge-on disks that will evolve to late Hubble type galaxies. The lack of an exponential profile is the result of either a dust scale height that is comparable to the stellar scale height or an intrinsically irregular structure. Examples of galaxies that could be face-on versions of linear galaxies are shown. They have an irregular clumpy structure with no central bulge and with clump colors and magnitudes that are comparable to those in the linear galaxies. Radiative transfer solutions to the magnitudes and surface brightnesses of inclined dusty galaxies suggest that edge-on disks should become more prominent near the detection limit for surface brightness. The surface brightness distribution of the edge-on galaxies in this field confirm this selection effect. The star formation regions are much more massive than in modern galaxies, averaging up to ~109 M☉ for kiloparsec scales.

Origin of radially increasing stellar scaleheight in a galactic disk

For the past twenty years, it has been accepted that the vertical scaleheight of the stellar disk in spiral galaxies is constant with radius. However, there is no clear physical explanation for this in the literature. Here we calculate the vertical stellar scaleheight for a self-gravitating stellar disk including the additional gravitational force of the HI and H 2 gas and the dark matter halo. We apply our model to two edge-on galaxies, NGC 891 and NGC 4565, and find that the resulting scaleheight shows a linear increase of nearly a factor of two within the optical disk for both these galaxies. Interestingly, we show that the observed data when looked at closely, do not imply a constant scaleheight but actually support this moderate flaring in scaleheight.

The Masses of the Progenitors of Planetary Nebulae

ABSTRACTBipolar, elliptical, and circular planetary nebulae appear to be possessed of differing Galactic scale heights, a feature which is usually assumed to arise from the differing distributions and masses of their progenitor stars. It would appear that more massive progenitors are responsible for bipolar sources, whilst the least massive progenitors give rise to circular sources. This, clearly, is an important result, since it implies that shell formation mechanisms must be closely intertwined with progenitor type. We point out, however, that previous estimates of nebular scale height are at best somewhat sketchy and may even be wrong. At least two prior analyses, for instance, appear to compare reddened values of nebular scale height with extinction‐corrected stellar scale heights. We have therefore attempted, in the following, to undertake a dereddening of nebular scale heights and perform a fresh comparison of these results with those for Galactic stars of differing spectral types. We find that circular and elliptical sources are likely to originate from early F‐type stars and bipolar sources from stars of spectral type A7 or earlier. The results appear to be insensitive to uncertainties in the distance scale.

A Pressure Anomaly for HiiRegions in Irregular Galaxies

The pressures of giant H II regions in six dwarf irregular galaxies are found to be a factor of ~10 larger than the average pressures of the corresponding galaxy disks, obtained from the stellar and gaseous column densities. This is unlike the situation for spiral galaxies, where these two pressures are approximately equal. Either the H II regions in these dwarfs are all so young that they are still expanding, or there is an unexpected source of disk self-gravity that increases the background pressure. We consider first whether any additional self-gravity might come from disk dark matter that either is cold H2 gas in diffuse or self-gravitating clouds with weak CO emission, or is the same material as the halo dark matter inferred from rotation curves. The H2 solution is possible because cold molecular clouds would be virtually invisible in existing surveys if they were also CO-weak from the low metal abundances in these galaxies. Cosmological dark matter might be possible too because of the relatively large volume fraction occupied by the disk within the overall galaxy potential. There is a problem with both of these solutions, however: the vertical scale heights inferred for irregular galaxies are consistent with the luminous matter alone. The amount of disk dark matter that is required to explain the high H II region pressures would give gas and stellar scale heights that are too small. The anomalous pressures in star-forming regions are more likely the result of local peaks in the gravitational field that come from large gas concentrations. These peaks also explain the anomalously low average column density thresholds for star formation that were found earlier for irregular galaxies, and they permit the existence of a cool H I phase as the first step toward dense molecular cores. The evidence for concentrations of H I in regions of star formation is summarized; the peak column densities are shown to be consistent with local pressure equilibrium for the H II regions. Strongly self-gravitating star-forming regions should also limit the dispersal of metals into the intergalactic medium. The third possibility is that all of the visible H II regions in these dwarf galaxies are strongly overpressured and still expanding. The mean time to pressure equilibrium is ~15 times their current age, which implies that the observed population is only 7% of the total if they live that long; the rest are presumably too faint to see. The expansion model also implies that the volume-filling factor can reach ~100 times the current factor, in which case faint and aging H II regions should merge and occupy nearly the entire dwarf galaxy volume. This would explain the origin of the giant H I shells seen in these galaxies as the result of old, expanded H II regions that were formerly driven by OB associations. The exciting clusters would now be so old and dispersed that they would not be recognized easily. The shells are still round because of a lack of shear.

The origin of galactic discs with exponential z-profiles

A new solution is presented for the puzzling, observed universality of the exponential luminosity profiles, perpendicular to the disk plane of spiral and lenticular galaxies. It is shown that such exponential z-profiles result naturally from gaseous protodisks which settle into isothermal equilibrium prior to star formation. Subsequent cooling leads to a gravitational contraction of the gas towards the equatorial plane and to a stellar exponential z-profile if the star formation rate is assumed to be comparable to the cooling rate of the gas. The final stellar scale height depends only on the initial gas temperature and local surface density. This model therefore provides a new method to investigate the early energetic state of galactic protodisks with measured scale heights and surface densities along the disk plane.

The metallicity distribution of G dwarfs in the solar neighbourhood

We derive a new metallicity distribution of G dwarfs in the solar neighbourhood, using uvby photometry and up-to-date parallaxes. Our distribution comprises 287 G dwarfs within 25 pc from the Sun, and differs considerably from the classic solar neighbourhood distribution of Pagel & Patchett and Pagel by having a prominent single peak around [Fe/H] = -0.20 dex. The raw data are corrected for observational errors and cosmic scatter assuming a deviation of 0.1. In order to obtain the true abundance distribution, we use the correction factors given by Sommer-Larsen, which take into account the stellar scale heights. The distribution confirms the G dwarf problem, that is, the paucity of metal-poor stars relative to the predictions of the simple model of chemical evolution. Another feature of this distribution, which was already apparent in previous ones, is the small number of metal-rich stars again in comparison with the simple model. Our results indicate that it is very difficult to fit the simple model to this distribution, even with the definition of an `effective yield'. A comparison with several models from the literature is made. We find that models with infall are the most appropriate to explain the new metallicity distribution. We also show that the metallicity distribution is compatible with a major era of star formation occurring 5 to 8 Gyr ago, similar to results found by several authors.

The gravitational potential of the bar in NGC 4314

We present near-infrared images of NGC 4314 in the J, H, and K bands. Colors in these bands are constant across the bar of NGC 4314, indicating that the mass-to-light ratio is constant over much of the galaxy. We have developed an efficient procedure for mapping the gravitational potential of a nonaxisymmetric galaxy based on its appearance in the near-infrared using a Fourier transform method. We account for the thickness of the galaxy's stellar disk by convolving its K image with a function that depends upon the vertical stellar scale height. We apply this procedure to NGC 4314 and find that prograde orbits integrated in the potential are consistent with the shape of the bar for a disk vertical scale height of h = 350 +/- 100 pc and a corotation radius of r = 3.5 +/- 0.5 kpc. We find that the inner m = 4 Lindblad resonance is approximately at the location of knoblike features seen near the end of the bar. The shape of nearly closed orbits at this radius are lozenge or diamond shaped and have low speeds along the major axis of the bar. The low speeds imply an increase in density along the major axis of the bar. The angle of the major axis of the m = 4 component of the surface density begins to twist at the location of its resonance, and the angle of the m = 2 component begins to twist at a larger radius near the corotation radius.

Near-infrared surface photometry of three early-type, edge-on spiral galaxies - NGC 4594, NGC 7123, and NGC 7814

view Abstract Citations (36) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Near-Infrared Surface Photometry of Three Early-Type, Edge-on Spiral Galaxies: NGC 4594, NGC 7123, and NGC 7814 Wainscoat, R. J. ; Hyland, A. R. ; Freeman, K. C. Abstract We present near-infrared surface photometry of three early-type, edge-on spiral galaxies. Two of these galaxies--NGC 7814 and NGC 7123--are almost exactly edge-on and have dust lanes which at optical wavelengths hide much of the disk light. At K (2.2 microns), the disk is seen as emission in each of these galaxies. The bulge light distribution in each of these galaxies is well-fitted by the de Vaucouleurs R^1/4^ law. Bulge/disk decompositions have been performed for each galaxy, and disk parameters determined. We find that the disks of NGC 7814 and NGC 7123 are a substantially more important component of these galaxies than might be expected from examining optical photographs or optical surface photometry, and that the scale height of the dust in the disk must be very similar to the stellar scale height in order to so effectively eliminate the stellar disk emission at optical wavelengths. We show that the disk of NGC 4594 contains a stellar ring; the dust in NGC 4594 also has a ringlike structure. Publication: The Astrophysical Journal Pub Date: January 1990 DOI: 10.1086/168216 Bibcode: 1990ApJ...348...85W Keywords: Galactic Structure; Infrared Photometry; Interstellar Matter; Near Infrared Radiation; Spiral Galaxies; Early Stars; Infrared Astronomy Satellite; Infrared Imagery; Light Curve; Astrophysics; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 4594; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 7123; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 7814; GALAXIES: INTERSTELLAR MATTER; GALAXIES: PHOTOMETRY; GALAXIES: STRUCTURE full text sources ADS | data products SIMBAD (3) NED (3)