Bulgeless Galaxies Research Articles

Galactic bulges: overview

AbstractThis overview of galactic bulges begins with a discussion of the various kinds of bulges (classical, boxy/peanut-shaped, pseudo) and their likely formation mechanisms. Other specific topics include the Galactic bar/bulge and its chemical evolution, the bulge of M31, the relation between bulges and metal-poor halos (often lumped together as spheroids), the morphology-density relation and the formation of S0 galaxies, the color-structure bimodality, and scaling laws for bulges. Finally I will briefly discuss the current difficulty of forming bulgeless disk galaxies in ΛCDM.

Virgo Cluster Early-Type Dwarf Galaxies with the Sloan Digital Sky Survey. I. On the Possible Disk Nature of Bright Early-Type Dwarfs

We present a systematic search for disks in 476 Virgo cluster early-type dwarf (dE) galaxies. Disk features (spiral arms, edge-on disks, or bars) were identified by applying unsharp masks to a combined image from three bands (g, r, i), as well as by subtracting the axisymmetric light distribution of each galaxy from that image. 14 objects are unambiguous identifications of disks, 10 objects show 'probable disk' features, and 17 objects show 'possible disk' features. The number fraction of these galaxies, for which we introduce the term dEdi, reaches more than 50% at the bright end of the dE population, and decreases to less than 5% for magnitudes B>16. The luminosity function of our full dE sample can be explained by a superposition of dEdis and ordinary dEs, strongly suggesting that dEdis are a distinct type of galaxy. This is supported by the projected spatial distribution: dEdis show basically no clustering and roughly follow the spatial distribution of spirals and irregulars, whereas ordinary dEs are distributed similarly to the strongly clustered E/S0 galaxies. While the flattening distribution of ordinary dEs is typical for spheroidal objects, the distribution of dEdis is significantly different and agrees with their being flat oblate objects. We therefore conclude that the dEdis are not spheroidal galaxies that just have an embedded disk component, but are instead a population of genuine disk galaxies. Several dEdis display well-defined spiral arms with grand design features that clearly differ from the flocculent, open arms typical for late-type spirals that have frequently been proposed as progenitors of early-type dwarfs. This raises the question of what process is able to create such spiral arms - with pitch angles like those of Sab/Sb galaxies - in bulgeless dwarf galaxies. (Abridged)

Stellar Populations in the Nuclei of Late‐Type Spiral Galaxies

(Abridged) As part of an ongoing effort to study the stellar nuclei of very late-type, bulge-less spirals, we present results from a high-resolution spectroscopic survey of nine such nuclear star clusters, undertaken with VLT/UVES. We fit the spectra with population synthesis models and measure Lick-type indices to determine mean luminosity-weighted ages, which range from 4.1*10^7 to 1.1*10^10 years and are insensitive to assumed metallicity or internal extinction. The average metallicity of nuclear clusters in late-type spirals is slightly sub-solar (<Z> = 0.015) but shows significant scatter. The fits also show that the nuclear cluster spectra are best described by a mix of several generations of stars. This is supported by the fact that only models with composite stellar populations yield mass-to-light ratios that match those obtained from dynamical measurements. The last star formation episode was on average 34 Myr ago, while all clusters experienced some star formation in the last 100 Myr. We thus conclude that the nuclear clusters undergo repeated episodes of star formation. The robustness with respect to possible contamination from the underlying galaxy disk is demonstrated by comparison to spectra obtained with HST/STIS. Combining these results with those from Walcher et al. (2005), we have thus shown that the stellar nuclei of these bulge-less galaxies are massive and dense star clusters that form stars recurrently until the present day. This unique set of properties is likely due to the central location of these clusters in their host galaxies.

Morphologies in a Cluster of Extremely Red Galaxies with Old Stellar Populations atz= 1.34

We have identified a clustering of red galaxies from deep optical/IR images obtained as part of the Institute for Astronomy Deep Survey. Photometric spectral-energy distributions indicate that most of these galaxies comprise nearly pure old stellar populations at a redshift near 1.4, and Keck spectroscopy of the three brightest red galaxies confirm this interpretation and give redshifts ranging from 1.335 to 1.338. Four of the galaxies are close together on the sky and less than 30 from an R=13.5 star, and we have obtained deep adaptive-optics imaging of this group. Detailed analysis and modeling of the surface-brightness profiles of these galaxies shows that two are normal ellipticals, one is an S0, and one appears to be an essentially pure disk of old stars, with no significant bulge. All four are highly relaxed, symmetric systems. While the old, bulgeless disk galaxy represents a type that is rare at the present epoch, the other three galaxies have structural parameters that are essentially indistinguishable from those of present-day galaxies and differ only in the age of their stellar populations.

The nature of the red disc-like galaxies at high redshift: dust attenuation and intrinsically red stellar populations

We investigate which conditions of dust attenuation and stellar populations allow models of dusty, continuously star-forming, bulge-less disc galaxies at 0.8 ≲ z ≲ 3.2 to meet the different colour selection criteria of high-z ‘red’ galaxies (e.g. RC−K > 5.3, IC−K > 4, J−K > 2.3). As a main novelty, we use stellar population models that include the thermally pulsating asymptotic giant branch (TP-AGB) phase of stellar evolution. The star formation rate of the models declines exponentially as a function of time, the e-folding time being longer than 3 Gyr. In addition, we use calculations of radiative transfer of the stellar and scattered radiation through different dusty interstellar media in order to explore the wide parameter space of dust attenuation. We find that synthetic discs can exhibit red optical/near-infrared colours because of reddening by dust, but only if they have been forming stars for at least ∼1 Gyr. Extremely few models barely exhibit RC−K > 5.3, if the inclination i= 90° and if the opacity 2 ×τV≳ 6. Hence, RC−K-selected galaxies at 1 ≲ z ≲ 2 most probably are either systems with an old, passively evolving bulge or starbursts. Synthetic discs at 1 ≲ z ≲ 2 exhibit 4 < IC−K < 4.8, if they are seen edge on (i.e. at i∼ 90°) and if 2 ×τV≳ 0.5. This explains the large fraction of observed, edge-on disc-like galaxies with Ks < 19.5 and F814W-Ks≳ 4. Finally, models with 2 ≲ z ≲ 3.2 exhibit 2.3 < J−K < 3, with no bias towards i∼ 90° and for a large range in opacity (e.g. 2 ×τV > 1 for i∼ 70°). In conclusion, red disc-like galaxies at 0.8 ≲ z ≲ 3.2 may not necessarily be dustier than nearby disc galaxies (with 0.5 ≲ 2 ×τV ≲ 2) and/or much older than ∼1 Gyr. This result is due both to a realistic description of dust attenuation and to the emission contribution by TP-AGB stars, with ages of 0.2 to 1–2 Gyr and intrinsically red colours.

An Equilibrium Dark Matter Halo with the Burkert Profile

Abstract In this paper a prescription for generating an equilibrium spherical system depicted with cuspy density profiles is extended to a core density profile, the Burkert profile, which is observationally more suitable to dwarfs. By using a time-saving Monte Carlo method instead of $N$-body simulations, we show that the Burkert halo, which is initialized with a distribution function that depends only on energy, is in a practically stable equilibrium. The one generated with the local Maxwellian approximation is unstable, where the flat core density structure tends to steepen. This is fundamentally different from a previous study on a halo with a cuspy density profile. The deviation of the unstable “Burkert” from the initial Burkert profile is found to be closely related to taking a Gaussian as the velocity distribution at any given point. The significance of not using the local Maxwellian approximation is further demonstrated by exploring the dynamical evolution of compact super star clusters (SSCs) in the Burkert halo. In particular, the local Maxwellian approximation results in underestimating the dynamical friction and overestimating sinking time scale. Accordingly, this leads to a lower probability of forming massive bulges and young nuclear star clusters in bulgeless galaxies.

Elliptical galaxies from mergers of discs

We analyse N-body galaxy merger experiments involving disc galaxies. Mergers of disc-bulge-halo models are compared to those of bulge-less, disc-halo models to quantify the effects of the central bulge on the merger dynamics and on the structure of the remnant. Our models explore galaxy mass ratios 1:1 through 3:1, and use higher bulge mass fraction than previous studies. A full comparison of structural and dynamical properties to observations is carried out. The presence of central bulges results in longer tidal tails, oblate final intrinsic shapes, surface brightness profiles with higher Sersic index, steeper rotation curves, and oblate-rotator internal dynamics. Mergers of bulge-less galaxies do not generate long-lasting tidal tails, and their strong triaxiality seems inconsistent with observations; these remnants show shells, which we do not find in models including central bulges. Giant ellipticals with boxy isophotes and anisotropic dynamics cannot be produced by the mergers modeled here; they could be the result of mergers between lower luminosity ellipticals, themselves plausibly formed in disc-disc mergers.

Radial Gas Flows in Colliding Galaxies: Connecting Simulations and Observations

We investigate the detailed response of gas to the formation of transient and long-lived dynamical structures induced in the early stages of a disk-disk collision, and identify observational signatures of radial gas inflow through a detailed examination of the collision simulation of an equal mass bulge dominated galaxy. Our analysis and discussion mainly focuses around the evolution of the diffuse and dense gas in the early stages of the collision, when the two disks are interacting but have not yet merged. Stars respond to the tidal interaction by forming both transient arms and long lived m = 2 bars, but the gas response is more transient, flowing directly toward the central regions within about 10 years after the initial collision. The rate of inflow declines when more than half of the total gas supply reaches the inner few kpc, where the gas forms a dense nuclear ring inside the stellar bar. The average gas inflow rate to the central 1.8 kpc is ∼ 7M⊙ yr −1 with a peak rate of 17M⊙ yr . Gas with high volume density is found in the inner parts of the post-collision disks at size scales close to the spatial resolution of the simulations, and this may be a direct result of shocks traced by the discontinuity in the gas velocity field. The evolution of gas in a bulgeless progenitor galaxy is also discussed, and a possible link to the “chain galaxy” population observed at high redshifts is inferred. The evolution of the structural parameters such as asymmetry and concentration of both stars and gas are studied in detail. Further, a new structure parameter (the compactness parameter K) that traces the evolution of the size scale of the gas relative to the stellar disk is introduced, and this may be a useful tracer to determine the merger chronology of colliding systems. Non-circular gas kinematics driven by the perturbation of the non-axisymmetric structure can produce distinct emission Department of Astronomy, University of Massachusetts, Amherst, MA 01002 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 Department of Astronomy, Case Western Reserve University, Cleveland, OH 44106 Research Corporation Cottrell Scholar and NSF CAREER Fellow

Bulgeless Galaxies and Their Angular Momentum Problem

The specific angular momentum of cold dark matter (CDM) halos in a ΛCDM universe is investigated. Their dimensionless specific angular momentum λ' = j/(VvirRvir) with Vvir and Rvir the virial velocity and virial radius, respectively, depends strongly on their merging histories. We investigate a set of ΛCDM simulations and explore the specific angular momentum content of halos formed through various merging histories. Halos with a quiet merging history, dominated by minor mergers and accretion until the present epoch, acquire by tidal torques on average only 2%-3% of the angular momentum required for their rotational support (λ' = 0.02). This is in conflict with observational data for a sample of late-type bulgeless galaxies that indicates that those galaxies reside in dark halos with exceptionally high values of λ' ≈ 0.06-0.07. Minor mergers and accretion preserve or slowly increase the specific angular momentum of dark halos with time. This mechanism, however, is not efficient enough in order to explain the observed spin values for late-type dwarf galaxies. Energetic feedback processes have been invoked to solve the problem that gas loses a large fraction of its specific angular momentum during infall. Under the assumption that dark halos hosting bulgeless galaxies acquire their mass via quiescent accretion, our results indicate yet another serious problem: the specific angular momentum gained during the formation of these objects is not large enough to explain their observed rotational properties, even if no angular momentum would be lost during gas infall.

On the Origin of Nuclear Star Clusters in Late-Type Spiral Galaxies

A large fraction of bulgeless disk galaxies contain young compact stellar systems at their centers, in spite of the local gravitational stability of these disks. We evaluate two contrasting hypotheses for the origin of the nuclear star clusters in late-type disk galaxies. The clusters could not have migrated from distant eccentric locations in the disk. Instead they must have formed in situ, requiring radial transport of gas toward the center of the disk. This transport could be a consequence of the development of the magnetorotational instability in the differentially rotating warm neutral medium. We evaluate the rate of gas transport into the disk center and find that it is sufficient to support continuous star formation in that location. Enhanced stellar surface brightness in the inner few hundred parsecs and the formation of a compact stellar system in the central few parsecs are unavoidable in dark matter halos with divergent density profiles. We illustrate our conclusions on a model of the nearest late-type disk galaxy M33.

Evidence for a large stellar bar in the Low Surface Brightness galaxy UGC 7321

Late-type spiral galaxies are thought to be the dynamically simplest type of disk galaxy and our understanding of their properties plays a key role in galaxy formation and evolution scenarios. The low surface brightness (LSB) galaxy UGC 7321, a nearby, isolated, “superthin” edge-on galaxy, is an ideal object to study these purely disk-dominated bulge-less galaxies. Although late type spirals are believed to exhibit the simplest possible structure, even prior observations showed deviations from a pure single component exponential disk in the case of UGC 7321. We present for the first time photometric evidence for peanut-shaped outer isophotes from a deep optical ( R -band) image of UGC 7321. Observations and dynamical modeling suggest that boxy/peanut-shaped (b/p) bulges in general form through the buckling instability in bars of the parent galaxy disks. Together with recent HI observations supporting the presence of a stellar bar in UGC 7321, this could be the earliest known case of the buckling process during the evolutionary life of a LSB galaxy, whereby material in the disk-bar has started to be pumped up above the disk, but a genuine bulge has not yet formed.

A Structural and Dynamical Study of Late-Type, Edge-on Galaxies. II. Vertical Color Gradients and the Detection of Ubiquitous Thick Disks

We present an analysis of optical (B-R) and optical-infrared (R-Ks) color maps for 47 extremely late-type edge-on unwarped, bulgeless disk galaxies spanning a wide range of mass. The color maps show that the thin disks of these galaxies are embedded in a low surface brightness red envelope. This component is substantially thicker than the thin disk (a : b ∼ 4 : 1 vs. >8 : 1), extends to at least 5 vertical disk scale heights above the galaxy midplane, and has a radial scale length that appears to be uncorrelated with that of the embedded thin disk. The color of the red envelope is similar from galaxy to galaxy, even when the thin disk is extremely blue, and is consistent with a relatively old (>6 Gyr) stellar population that is not particularly metal-poor. The color difference between the embedded thin disk and the red stellar envelope varies systematically with rotation speed, reflecting an increasing age difference between the thin and thick components in lower mass galaxies, driven primarily by changes in the age of the thin disk. The red stellar envelopes are similar to the thick disk of the Milky Way, having common surface brightnesses, spatial distributions, mean ages, and metallicities. We argue that the ubiquity of the red stellar envelopes implies that the formation of the thick disk is a nearly universal feature of disk formation and is not necessarily connected to the formation of a bulge. Our data suggest that the thick disk forms early (>6 Gyr ago), even within galaxies where the bulk of the stars formed very recently (<2 Gyr). We argue that several aspects of our data and the observed properties of the Milky Way thick disk argue in favor of a merger origin for the thick disk population. If so, then the age of the thick disk marks the end of the epoch of major merging, and the age difference between the younger thin disk and the older thick disk can become a strong constraint on cosmological constants and models of galaxy and/or structure formation.

Observational Constraints on the Self‐interacting Dark Matter Scenario and the Growth of Supermassive Black Holes

We consider the consequences of SIDM for a velocity dependent cross section per unit mass. Accretion of SIDM onto seed black holes can produce supermassive black holes that are too large for certain combinations of parameters,which is used to obtain a new constraint on the dark matter interaction. Constraints due to other considerations are presented and previous ones are generalized. The black hole constraint is extremely sensitive to the slope \alpha, of the inner density profile of dark halos. For the most probable value of \alpha=1.3, there exists a narrow range in parameter space, such that all constraints are satisfied. However, the adiabatic compression of the dark halo by baryons as they cool and contract in normal galaxies yields a steeper cusp, \alpha=1.7. This gives a tighter constraint, which would exclude SIDM as a possible solution to the purported problems with CDM in the absence of other dynamical processes. Nevertheless, SIDM with parameters consistent with this stronger constraint, can explain the ubiquity of supermassive black holes in the centers of galaxies. A ``best fit'' model is presented which reproduces the supermassive black hole masses and their observed correlations with the velocity dispersion of the host bulges. Specifically, the fourth power dependence of black hole mass on velocity dispersion is a direct consequence of the power spectrum having an index of n=-2. Although the dark matter collision rates for this model are too small to directly remedy problems with CDM, mergers between dark halos harboring supermassive black holes at high redshift could ameliorate the cuspy halo problem. This scenario also explains the lack of comparable supermassive black holes in bulgeless galaxies like M33.

Dark haloes of spiral galaxies: ISO photometry

We exclude hydrogen-burning stars, of any mass above the hydrogen-burning limit and any metallicity, as significant contributors to the massive haloes deduced from rotation curves to dominate the outer parts of spiral galaxies. We present and analyse images of four nearly edge-on bulgeless spiral galaxies (UGC 711, NGC 2915, UGC 12426, UGC 1459) obtained with ISOCAM (The CAMera instrument on board the Infrared Space Observatory) at 14.5 and 6.75 μm. Our sensitivity limit for detection of any diffuse infrared emission associated with the dark haloes in these galaxies is a few tens of μJy per 6 × 6 arcsec2 pixel, with this limit currently set by remaining difficulties in modelling the non-linear behaviour of the detectors. All four galaxies show zero detected signal from extended non-disc emission, consistent with zero halo-like luminosity density distribution. The 95 per cent upper limit on any emission, for NGC 2915 in particular, allows us to exclude very low mass main-sequence stars (M > 0.08 M⊙) and young brown dwarfs (≲1 Gyr) as significant contributors to dark matter in galactic haloes. Combining our results with those of the Galactic microlensing surveys, which exclude objects with M < 0.01 M⊙, excludes almost the entire possible mass range of compact baryonic objects from contributing to Galactic dark matter.

Dark haloes of spiral galaxies: ISO photometry

We exclude hydrogen-burning stars, of any mass above the hydrogen-burning limit and any metallicity, as significant contributors to the massive haloes deduced from rotation curves to dominate the outer parts of spiral galaxies. We present and analyse images of four nearly edge-on bulgeless spiral galaxies (UGC 711, NGC 2915, UGC 12426, UGC 1459) obtained with ISOCAM (The CAMera instrument on board the Infrared Space Observatory) at 14.5 and 6.75 μm. Our sensitivity limit for detection of any diffuse infrared emission associated with the dark haloes in these galaxies is a few tens of μJy per 6 × 6 arcsec2 pixel, with this limit currently set by remaining difficulties in modelling the non-linear behaviour of the detectors. All four galaxies show zero detected signal from extended non-disc emission, consistent with zero halo-like luminosity density distribution. The 95 per cent upper limit on any emission, for NGC 2915 in particular, allows us to exclude very low mass main-sequence stars (M > 0.08 M⊙) and young brown dwarfs (≲1 Gyr) as significant contributors to dark matter in galactic haloes. Combining our results with those of the Galactic microlensing surveys, which exclude objects with M < 0.01 M⊙, excludes almost the entire possible mass range of compact baryonic objects from contributing to Galactic dark matter.

Gasdynamics and Starbursts in Major Mergers

view Abstract Citations (1435) References (183) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Gasdynamics and Starbursts in Major Mergers Mihos, J. Christopher ; Hernquist, Lars Abstract Using numerical simulation, we study the development of gaseous inflows and triggering of starburst activity in mergers of disk galaxies of comparable mass. Our models cover a range of orbits and internal structures for the merging galaxies. In all encounters studied, the galaxies experience strong gaseous inflows and, using a density-dependent Schmidt law to model star formation, moderate to intense starburst activity. We find that galaxy structure plays a dominant role in regulating activity. The gaseous inflows are strongest when galaxies with dense central bulges are in the final stages of merging, while inflows in bulgeless galaxies are weaker and occur earlier in the interaction. Orbital geometry plays only a relatively modest role in the onset of collisionally induced activity. Through an analysis of the torques acting on the gas, we show that these inflows are generally driven by gravitational torques from the host galaxy (rather than the companion) and that dense bulges act to stabilize galaxies against bar modes and inflow until the galaxies merge, at which point rapidly varying gravitational torques drive strong dissipation and inflow of gas in the merging pair. The strongest inflows (and associated starburst activity) develop in coplanar encounters, while the activity in inclined mergers is somewhat less intense and occurs slightly later during the merger. To the extent that a Schmidt law is a reasonable description of star formation in these systems, the starbursts that develop in mergers of galaxies with central bulges represent an increase in the star formation rate of two orders of magnitude over that in isolated galaxies. We find that the gaseous and stellar morphology and star-forming properties of these systems provide a good match to those of observed ultraluminous infrared galaxies. Our results imply that the internal structure of the merging galaxies, rather than orbital geometry, may be the key factor in producing ultraluminous infrared galaxies. Publication: The Astrophysical Journal Pub Date: June 1996 DOI: 10.1086/177353 arXiv: arXiv:astro-ph/9512099 Bibcode: 1996ApJ...464..641M Keywords: GALAXIES: ACTIVE; GALAXIES: INTERACTIONS; GALAXIES: STARBURST; GALAXIES: STRUCTURE; METHODS: NUMERICAL; Astrophysics E-Print: 40 pages of LaTeX using AASTeX full text sources arXiv | ADS | data products SIMBAD (9)