Regions Of Disk Galaxies Research Articles

SDSS-IV MaNGA: constraints on the conditions for star formation in galaxy discs

Regions of disc galaxies with widespread star formation tend to be both gravitationally unstable and self-shielded against ionizing radiation, whereas extended outer discs with little or no star formation tend to be stable and unshielded on average. We explore what drives the transition between these two regimes, specifically whether discs first meet the conditions for self-shielding (parameterized by dust optical depth, $\tau$) or gravitational instability (parameterized by a modified version of Toomre's instability parameters, $Q_{\rm thermal}$, which quantifies the stability of a gas disc that is thermally supported at $T=10^4$ K). We first introduce a new metric formed by the product of these quantities, $Q_{\rm thermal}\tau$, which indicates whether the conditions for disk instability or self-shielding are easier to meet in a given region of a galaxy, and we discuss how $Q_{\rm thermal}\tau$ can be constrained even in the absence of direct gas information. We then analyse a sample of 13 galaxies with resolved gas measurements and find that on average galaxies will reach the threshold for disk instabilities ($Q_{\rm thermal}<1$) before reaching the threshold for self-shielding ($\tau>1$). Using integral field spectroscopic observations of a sample of 236 galaxies from the MaNGA survey, we find that the value of $Q_{\rm thermal}\tau$ in star-forming discs is consistent with similar behavior. These results support a scenario where disc fragmentation and collapse occurs before self-shielding, suggesting that gravitational instabilities are the primary condition for widespread star formation in galaxy discs. Our results support similar conclusions based on recent galaxy simulations.

Bars and boxy/peanut bulges in thin and thick discs

We explore trends in the morphology and line-of-sight (los) velocity of stellar populations in the inner regions of disc galaxies using N-body simulations with a thin (kinematically cold) and a thick (kinematically hot) disc which form a bar and a boxy/peanut (b/p) bulge. The bar in the thin disc component is ~50% stronger than the thick disc bar and is more elongated, with an axis ratio almost half that of the thick disc bar. The thin disc b/p bulge has a pronounced X-shape, while the thick disc b/p is weaker with a rather boxy shape. This leads to the signature of the b/p bulge in the thick disc being weaker and further away from the plane than in the thin disc. Regarding the kinematics, we find that the los velocity of thick disc stars in the outer parts of the b/p bulge can be higher than that of thin disc stars, by up to 40% and 20% for side-on and Milky Way-like orientations of the bar, respectively. This is due to the different orbits followed by thin and thick disc stars in the bar-b/p region, which are affected by two factors. First, thin disc stars are trapped more efficiently in the bar-b/p instability and thus lose more angular momentum than their thick disc counterparts and second, thick disc stars have large radial excursions and therefore stars from large radii with high angular momenta can be found in the bar region. We also find that the difference between the los velocities of the thin and thick disc in the b/p bulge (Δvlos) correlates with the initial difference between the radial velocity dispersions of the two discs (Δσ). We therefore conclude that stars in the bar-b/p bulge will have considerably different morphologies and kinematics depending on the kinematic properties of the disc population they originate from.

Kinematic Decoupled Core Paradigm: Counterrotation or Just Warping

Several mechanisms have been proposed in recent years to explain kinematic decoupled cores (KDCs) in early type galaxies as well as the large differences in angular momentum between KDCs and host galaxy. Most of the proposed scenarios involve large fractions of merging events, high speed interactions with dwarf spheroidal galaxies, cusp effect of the dark matter density profiles, etc. We here argue that counterrotation as well as fast and slow rotation of disks or spheroids at the center of galaxies can also be explained by a misalignment of the central spheroid equatorial plane with regard to that defined by the observed external stellar rotation. Contrary to what happens at the outer region of disk galaxies, once instability has led to the inner warped core, the perturbed orbits can maintain a common orientation due to the rigid body like rotation at the central region of the galaxy. The spatial configuration that furnishes the smallest angular momentum difference between the KDC and the host galaxy is completely defined by observed parameters in the plane of the sky, namely, the inclination of the inner and outer disks and the angle between the two lines of nodes. As an example we modeled the paradigmatic and extreme case of the 2D radial velocity field of NGC 4382 nucleus. Tilt angles of the KDC not larger than 30 degrees also allow explaining fast and low rotators of the called “Sauron paradigm” in a unified scenario. The maximum for the three parameters, namely, velocity of the inner rotator, difference of position angle and difference with the outer rotation velocity of the whole Sauron sample, are consistently correlated in agreement with the proposed scenario. These quantities do not correlate with the galaxies magnitude, mass (since large and dwarf spheroidals show apparent counterrotation as well) or environment, also suggesting that an internal phenomenon like the central spheroid warping, that we are here proposing, may be at work.

Energy Density Driven Galactic Formation and Evolution

© La Fortune This article is distributed under the terms of the Creative Commons Attribution 4.0 International License, which permits unrestricted use, distribution, and redistribution in any medium, provided that the original author and source are credited. This letter is inspired by the findings of M. Rabinowitz in his recent publication, “Why Observable Space Is Solely Three Dimensional” (Rabinowitz 2014). His paper provides coherent reasoning why our physical universe can only have one time and three spatial dimensions. He also states that “flux,” i.e., electrostatics and by analogy, gravitation, must remain consistent with threedimensional space with forces obeying the inverse-square law.

The Inner Regions of Disk Galaxies: A Constant Baryonic Fraction?

For disk galaxies (spirals and irregulars), the inner circular-velocity gradient dRV0 (inner steepness of the rotation curve) correlates with the central surface brightness ∑*,0 with a slope of ~0.5. This implies that the central dynamical mass density scales almost linearly with the central baryonic density. Here I show that this empirical relation is consistent with a simple model where the central baryonic fraction ƒbar,0 is fixed to 1 (no dark matter) and the observed scatter is due to differences in the baryonic mass-to-light ratio Mbar / LR (ranging from 1 to 3 in the R-band) and in the characteristic thickness of the central stellar component Δz (ranging from 100 to 500 pc). Models with lower baryonic fractions are possible, although they require some fine-tuning in the values of Mbar/LR and Δz. Regardless of the actual value of ƒbar,0, the fact that different types of galaxies do not show strong variations in ƒbar,0 is surprising, and may represent a challenge for models of galaxy formation in a Λ Cold Dark Matter (ΛCDM) cosmology.

THE DISKMASS SURVEY. I. OVERVIEW

We present a survey of the mass surface-density of spiral disks, motivated by outstanding uncertainties in rotation-curve decompositions. Our method exploits integral-field spectroscopy to measure stellar and gas kinematics in nearly face-on galaxies sampled at 515, 660, and 860 nm, using the custom-built SparsePak and PPak instruments. A two-tiered sample, selected from the UGC, includes 146 nearly face-on galaxies, with B<14.7 and disk scale-lengths between 10 and 20 arcsec, for which we have obtained H-alpha velocity-fields; and a representative 46-galaxy subset for which we have obtained stellar velocities and velocity dispersions. Based on re-calibration of extant photometric and spectroscopic data, we show these galaxies span factors of 100 in L(K) (0.03 < L/L(K)* < 3), 8 in L(B)/L(K), 10 in R-band disk central surface-brightness, with distances between 15 and 200 Mpc. The survey is augmented by 4-70 micron Spitzer IRAC and MIPS photometry, ground-based UBVRIJHK photometry, and HI aperture-synthesis imaging. We outline the spectroscopic analysis protocol for deriving precise and accurate line-of-sight stellar velocity dispersions. Our key measurement is the dynamical disk-mass surface-density. Star-formation rates and kinematic and photometric regularity of galaxy disks are also central products of the study. The survey is designed to yield random and systematic errors small enough (i) to confirm or disprove the maximum-disk hypothesis for intermediate-type disk galaxies, (ii) to provide an absolute calibration of the stellar mass-to-light ratio well below uncertainties in present-day stellar-population synthesis models, and (iii) to make significant progress in defining the shape of dark halos in the inner regions of disk galaxies.

The search for inner polar disks with integral field spectroscopy: the case of NGC 2855 and NGC 7049

The presence of non-circular and off-plane gas motions is frequently observed in the inner regions of disk galaxies. We have measured with integral-field spectroscopy the surface-brightness distribution and kinematics of the ionized gas in NGC 2855 and NGC 7049. These two early-type spiral galaxies were selected to possibly host a kinematically-decoupled gaseous component in orthogonal rotation with respect to the galaxy disk. We have modeled the ionized-gas kinematics and distribution of both galaxies assuming that the gaseous component is distributed either on two orthogonally-rotating disks or in a single and strongly warped disk. In both galaxies the velocity field and distribution of the inner gas are consistent with the presence of an inner polar disk. In NGC 2855 it correponds to the innermost and strongly warped portion of the main disk. In NGC 7049 it is a central and geometrically-decoupled disk, which is nested in the main disk.

Impact of Dark Matter Subhalos on Extended H i Disks of Galaxies: Possible Formation of H i Fine Structures and Stars

Recent observations have discovered star formation activities in the extreme outer regions of disk galaxies. However, it remains unclear what physical mechanisms are responsible for triggering star formation in such low-density gaseous environments of galaxies. In order to understand the origin of these outer star-forming regions, we numerically investigate how the impact of dark matter subhalos orbiting a gas-rich disk galaxy embedded in a massive dark matter halo influences the dynamical evolution of the outer H I gas disk of the galaxy. We find that if the masses of the subhalos (Msb) in a galaxy with an extended H I gas disk are as large as 10-3Mh, where Mh is the total mass of the galaxy's dark halo, local fine structures can be formed in the extended H I disk. We also find that the gas densities of some apparently filamentary structures can exceed a threshold gas density for star formation and thus be likely to be converted into new stars in the outer part of the H I disk in some models with larger Msb. These results thus imply that the impact of dark matter subhalos (dark impact) can be important for better understanding the origin of recent star formation discovered in the extreme outer regions of disk galaxies. We also suggest that characteristic morphologies of local gaseous structures formed by the dark impact can indirectly prove the existence of dark matter subhalos in galaxies. We discuss the origin of giant H I holes observed in some gas-rich galaxies (e.g., NGC 6822) in the context of the dark impact.

Peculiarities in the Formation of Molecular Clouds in the Central Regions of Spiral Galaxies

The limitations imposed by the shear instability on the formation of gigantic molecular clouds in the central regions of spiral galaxies are examined. The criteria obtained here are illustrated using the example of six galaxies for which the detailed rotation curves are known. The different mechanisms for formation of molecular clouds which apply in the central and edge regions of disk galaxies are evaluated.

Minor-axis velocity gradients in disk galaxies$^{\bf,}$

We present the ionized-gas kinematics and photometry of a sample of 4 spiral galaxies which are characterized by a zero-velocity plateau along the major axis and a velocity gradient along the minor axis, respectively. By combining these new kinematical data with those available in the literature for the ionized-gas component of the S0s and spirals listed in the Revised Shapley-Ames Catalog of Bright Galaxies we realized that about 50% of unbarred galaxies show a remarkable gas velocity gradient along the optical minor axis. This fraction rises to about 60% if we include unbarred galaxies with an irregular velocity profile along the minor axis. This phenomenon is observed all along the Hubble sequence of disk galaxies, and it is particularly frequent in early-type spirals. Since minor-axis velocity gradients are unexpected if the gas is moving onto circular orbits in a disk coplanar to the stellar one, we conclude that non-circular and off-plane gas motions are not rare in the inner regions of disk galaxies.

Kinematics of the Ionized Gas in the Inner Regions of Disk Galaxies

We have studied the kinematics of the ionized gas in the inner regions of disk galaxies in order to investigate the presence of circumnuclear Keplerian disks (CNKDs) of ionized gas and kinematically decoupled components in a sample of 25 disk galaxies. We obtained long-slit spectra at intermediate or high spectral resolution along the major axis of all the sample galaxies, whereas minor-axis spectra, broadband, and narrowband imaging have been obtained for only a subsample of them. We have measured the ionized-gas kinematics in the inner regions of 23 galaxies, whose morphological types range from S0 to Sc. The velocity gradients, velocity dispersions, and integrated fluxes of the emission lines have been measured at and . We can distinguish three classes of ′′ ′′ r 1 r 4 galaxy spectra taking into account the two-dimensional shape of the emission lines, the comparison of the velocity gradients, and the ratio of the fluxes at the different radii where we measured them. We interpret the variety of position-velocity diagrams by applying a modeling technique which assumes that the gas resides in an infinitesimally thin disk whose mean motion is characterized by circular orbits in the plane of the galaxy. Under these assumptions, two parameters play an important role in giving rise to the observed shape of the emission line: the value of the central mass concentration and the cuspiness of the intrinsic gaseous disk surface brightness. This modeling technique takes into account the instrumental effects (slit width and pixel size of the detector) and the seeing. This kind of analysis allows the identification of spiral galaxies that are good candidates to harbor central supermassive black holes and to be followed up with higher spatial resolution. We find that 13% of sample galaxies exhibit features in the position-velocity diagrams that are consistent with the presence of a CNKD (J. G. Funes et al. 2001, in preparation). We also demonstrate the possibility of inferring the presence of gaseous CNKDs using optical ground-based telescopes properly equipped. The peculiar bidimensional shape of the emission lines of five early-type disk galaxies have been modeled as due to the motion of a gaseous disk rotating in the combined potential of a central pointlike mass and of an extended stellar disk. An upper limit of about 10 M, has been given for the central mass concentration of NGC 2179, NGC 4343, NGC 4435, and NGC 4459 (F. Bertola et al. 1998, ApJ, 509, L93). The possibility of the detection of low-mass black holes is also discussed. The study of kinematically decoupled components in disk galaxies focused on three Sa galaxies, namely, NGC 3595, NGC 4698, and NGC 4672. The galaxy NGC 3593 is characterized by the presence of two counterrotating stellar disks of different size and luminosity. We present the results of narrowband Ha and near-infrared [N ii] imaging of the early-type spiral NGC 3593 in combination with a study of the flux radial profiles of the (ll6548, 6583), [N ii] Ha, and (ll6717, 6731) emission lines along its major [S ii] axis. We found that the Ha emission mainly derives from a small central region of . It consists of a filamentary pattern ′′ ′′ 57 # 25 with a central ring. The ring, which hosts an ongoing process of star formation, is interpreted as the result of the interaction between the acquired retrograde gas which later formed the smaller counterrotating stellar disk and the preexisting prograde gas of the galaxy (E. M. Corsini et al. 1998, AA this is also true for the outer parts of the bulge if a parametric photometric decomposition is adopted (F. Bertola et al. 1999, ApJ, 519, L127). The morphological features of NGC 4672, as well as its velocity curves and velocity dispersion profiles of stars and ionized gas along both its major and minor axes, are similar to those we observed in NGC 4698. We conclude that also NGC 4672 has a bulge elongated perpendicularly to the disk and a stellar core rotating perpendicularly to the disk. The presence of the isolated core suggests that the disk component is the end result of the acquisition of external material in polar orbits around a preexisting oblate spheroid as in the case of the ring component of AM 2020 504, the prototype of polar ring elliptical galaxies (M. Sarzi et al. 2000, A&A, 360, 439).

Ring Structure in Disk Galaxies: Erratum

view Abstract Citations References (1) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Ring Structure in Disk Galaxies: Erratum Zaspel, C. E. Abstract In the paper "Ring Structure in Disk Galaxies" by C. E. Zaspel (ApJ, 385, 83 [1992]) the assumption that the effect of the halo on the dynamics of a disk can be neglected was attributed to K. G. Begeman, A. H. Broeils, & R. H. Sanders (MNRAS, 249,523 [1991]). However, this assumption was made earlier by T. S. Van Albada & R. Sancisi (Phil. Trans. R. Soc. Lond. A, 320,447 [1986]), and it was shown to be true for the inner regions of disk galaxies by M. Persic & P. Salucci (ApJ, 355,44 [1990]). The author thanks M. Persic for bringing this to his attention. Publication: The Astrophysical Journal Pub Date: October 1992 DOI: 10.1086/171829 Bibcode: 1992ApJ...397..736Z Keywords: ERRATA; ADDENDA full text sources ADS | Related Materials (1) Original Paper: 1992ApJ...385...83Z

Rotating disk galaxies - Yet another case for dark matter

A method to probe the systematic variations of the dark matter mass fraction (DM) in the optical region of disk galaxies is proposed, based on studying the correlation between galaxy luminosity and the normalized radius where the rotation frequency exceeds a given multiple of the rotation frequency as measured at the optical disk radius. The results strongly suggest the ubiquitous existence of DM throughout the luminosity sequence of galaxies, thereby challenging the possibility that luminous matter alone can be responsible for the observed disk dynamics. By modeling the distribution of dark matter by means of a pseudoisothermal halo, it is shown that the disk-to-halo mass ratio at the optical edge varies with luminosity. 42 refs.

The luminosity-velocity diagram - A new distance estimator for spiral galaxies

view Abstract Citations (6) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Luminosity-Velocity Diagram: A New Distance Estimator for Spiral Galaxies? Madore, Barry F. ; Woods, David Abstract Evidence is presented for a newly identified feature of the inner regions of disk galaxies which appears to show promise as a new distance indicator. For a sample of 46 galaxies, found moving out from their centers, Delta M(r)/Delta log V(r) = about -3.30 (+ or - 0.40), where M(r) is the cumulative interior magnitude and V(r) is the local rotation rate. The stability of this slope (+ or - 12 percent) in turn suggets that such a feature might also be worth investigating further for stability in zero point, since the combination of velocity predicting luminosity is a classic means of distance determination. Publication: The Astrophysical Journal Pub Date: December 1987 DOI: 10.1086/185050 Bibcode: 1987ApJ...323L..25M Keywords: Astronomical Photometry; Galactic Rotation; Spiral Galaxies; Stellar Luminosity; Error Analysis; Galactic Structure; Astrophysics; GALAXIES: INTERNAL MOTIONS; GALAXIES: PHOTOMETRY; GALAXIES: STRUCTURE full text sources ADS | data products SIMBAD (2) NED (2)

Numerical investigation into the generation of spiral density waves by bars in rigidly rotating disks

A numerical study of the large-scale structure of a spiral galaxy is presented. The study involves the examination of the nonstationary response in a rigidly rotating gas disk to the potential of a bar. When the angular velocity of the bar exceeds the angular velocity of the disk, a quasi-stationary two-arm trailing spiral pattern, which rotates with the angular velocity of the bar, is generated. When the angular velocity of the disk exceeds that of the bar, there is no spiral pattern. Calculations confirm that the bars existing in the central regions of disk galaxies are capable of generating sustaining two-arm spiral patterns.