Structure, mass and stability of galactic disks

In this review I concentrate on three areas related to structure of disks in spiral galaxies. First I will review the work on structure, kinematics and dynamics of stellar disks. Next I will review the progress in the area of flaring of HI layers. These subjects are relevant for the presence of dark matter and lead to the conclusion that disk are in general not `maximal', have lower M/L ratios than previously suspected and are locally stable w.r.t. Toomre's Q criterion for local stability. I will end with a few words on `truncations' in stellar disks.Comment: Invited review at "Galaxies and their Masks" for Ken Freeman's 70-th birthday, Sossusvlei, Namibia, April 2010. A version with high-res. figures is available at http://www.astro.rug.nl/~vdkruit/jea3/homepage/Namibiachapter.pd

Star formation thresholds and galaxy edges: why and where

We study global star formation thresholds in the outer parts of galaxies by investigating the stability of disk galaxies embedded in dark halos. The disks are self-gravitating, contain metals and dust, and are exposed to UV radiation. We find that the critical surface density for the existence of a cold interstellar phase depends only weakly on the parameters of the model and coincides with the empirically derived surface density threshold for star formation. Furthermore, it is shown that the drop in the thermal velocity dispersion associated with the transition from the warm to the cold gas phase triggers gravitational instability on a wide range of scales. The presence of strong turbulence does not undermine this conclusion if the disk is self-gravitating. Models based on the hypothesis that the onset of thermal instability determines the star formation threshold in the outer parts of galaxies can reproduce many observations, including the threshold radii, column densities, and the sizes of stellar disks as a function of disk scale length and mass. Finally, prescriptions are given for implementing star formation thresholds in (semi-)analytic models and three-dimensional hydrodynamical simulations of galaxy formation.Comment: 16 pages, 6 figures, accepted for publication in the Astrophysical Journal. Version 2: text significantly revised (major improvements), physics unchanged. Version 3: minor correction

The Opacity of Spiral Galaxy Disks IV: Radial Extinction Profiles from Counts of Distant Galaxies seen through Foreground Disks

Dust extinction can be determined from the number of distant field galaxies seen through a spiral disk. To calibrate this number for the crowding and confusion introduced by the foreground image, Gonzalez et al.(1998) and Holwerda et al. (2005) developed the ``Synthetic Field Method'' (SFM), which analyses synthetic fields constructed by adding various deep exposures of unobstructed background fields to the candidate foreground galaxy field. The advantage of the SFM is that it gives the average opacity for area of galaxy disk without assumptions about either the distribution of absorbers or of the disk starlight. However it is limited by low statistics of the surviving field galaxies, hence the need to combine a larger sample of fields. This paper presents the first results for a sample of 32 deep HST/WFPC2 archival fields of 29 spirals. The radial profiles of average dust extinction in spiral galaxies based on calibrated counts of distant field galaxies is presented here, both for individual galaxies as well as for composites from our sample. The effects of inclination, spiral arms and Hubble type on the radial extinction profile are discussed. (Abbreviated)Comment: 43 pages, 16 figures, 3 tables, accepted for publication in the Astronomical Journal, (typos, table update, updates abstract

van der Kruit to Spitzer: A New Look at the FIR-Radio Correlation

We present an initial look at the far infrared-radio correlation within the star-forming disks of four nearby, nearly face-on galaxies (NGC~2403, NGC~3031, NGC~5194, and NGC~6946). Using {\it Spitzer} MIPS imaging and WSRT radio continuum data, we are able to probe variations in the logarithmic 70~$\mu$m/22~cm ($q_{70}$) flux density ratios across each disk at sub-kpc scales. We find general trends of decreasing $q_{70}$ with declining surface brightness and with increasing radius. We also find that the dispersion in $q_{70}$ within galaxies is comparable to what is measured {\it globally} among galaxies at around 0.2 dex. We have also performed preliminary phenomenological modeling of cosmic ray electron (CR$e^{-}$) diffusion using an image-smearing technique, and find that smoothing the infrared maps improves their correlation with the radio maps. The best fit smoothing kernels for the two less active star-forming galaxies (NGC~2403 and NGC~3031) have much larger scale-lengths than that of the more active star-forming galaxies (NGC~5194 and NGC~6946). This difference may be due to the relative deficit of recent CR$e^{-}$ injection into the interstellar medium (ISM) for the galaxies having largely quiescent disks.Comment: 6 pages, 3 figures, To appear in the proceedings of the "Island Universes: Structure and Evolution of Disk Galaxies" conference held in Terschelling, Netherlands, July 2005, ed. R. de Jong (Springer: Dordrecht

Structural Parameters of Thin and Thick Disks in Edge-On Disk Galaxies

We analyze the global structure of 34 late-type, edge-on, undisturbed, disk galaxies spanning a wide range of mass. We measure structural parameters for the galaxies using two-dimensional least-squares fitting to our $R$-band photometry. The fits require both a thick and a thin disk to adequately fit the data. The thick disks have larger scale heights and longer scale lengths than the embedded thin disks, by factors of ~2 and ~1.25, respectively. The observed structural parameters agree well with the properties of thick and thin disks derived from star counts in the Milky Way and from resolved stellar populations in nearby galaxies. We find that massive galaxies' luminosities are dominated by the thin disk. However, in low mass galaxies (Vc < 120 km/s), thick disk stars contribute nearly half of the luminosity and dominate the stellar mass. Thus, although low mass dwarf galaxies appear blue, the majority of their stars are probably quite old. Our data are most easily explained by a formation scenario where the thick disk is assembled through direct accretion of stellar material from merging satellites while the thin disk is formed from accreted gas. The baryonic fraction in the thin disk therefore constrains the gas-richness of the merging pre-galactic fragments. If we include the mass in HI as part of the thin disk, the thick disk contains <10% of the baryons in high mass galaxies, and ~25-30% of the baryons in low-mass galaxies. We discuss how our trends can be explained by supernova-driven outflow at early times as well as the possibilities for predicting abundance trends in thick disks, and for removing discrepancies between semi-analytic galaxy formation models and the observed colors of low mass galaxies. (abstract abridged)Comment: 25 pages, 24 figures, accepted for publication in A

Stellar Disk Truncations: Where do we stand ?

In the light of several recent developments we revisit the phenomenon of galactic stellar disk truncations. Even 25 years since the first paper on outer breaks in the radial light profiles of spiral galaxies, their origin is still unclear. The two most promising explanations are that these 'outer edges' either trace the maximum angular momentum during the galaxy formation epoch, or are associated with global star formation thresholds. Depending on their true physical nature, these outer edges may represent an improved size characteristic (e.g., as compared to D_25) and might contain fossil evidence imprinted by the galaxy formation and evolutionary history. We will address several observational aspects of disk truncations: their existence, not only in normal HSB galaxies, but also in LSB and even dwarf galaxies; their detailed shape, not sharp cut-offs as thought before, but in fact demarcating the start of a region with a steeper exponential distribution of starlight; their possible association with bars; as well as problems related to the line-of-sight integration for edge-on galaxies (the main targets for truncation searches so far). Taken together, these observations currently favour the star-formation threshold model, but more work is necessary to implement the truncations as adequate parameters characterising galactic disks.Comment: LaTeX, 10 pages, 6 figures, presented at the "Penetrating Bars through Masks of Cosmic Dust" conference in South Africa, proceedings published by Kluwer, and edited by Block, D.L., Freeman, K.C., Puerari, I., & Groess, R; v3 to match published versio

Radial Profiles of Star Formation in the Far Outer Regions of Galaxy Disks

Star formation in galaxies is triggered by a combination of processes, including gravitational instabilities, spiral wave shocks, stellar compression, and turbulence compression. Some of these persist in the far outer regions where the column density is far below the threshold for instabilities, making the outer disk cutoff somewhat gradual. We show that in a galaxy with a single exponential gas profile the star formation rate can have a double exponential with a shallow one in the inner part and a steep one in the outer part. Such double exponentials have been observed recently in the broad-band intensity profiles of spiral and dwarf Irregular galaxies. The break radius in our model occurs slightly outside the threshold for instabilities provided the Mach number for compressive motions remains of order unity to large radii. The ratio of the break radius to the inner exponential scale length increases for higher surface brightness disks because the unstable part extends further out. This is also in agreement with observations. Galaxies with extended outer gas disks that fall more slowly than a single exponential, such as 1/R, can have their star formation rate scale approximately as a single exponential with radius, even out to 10 disk scale lengths. Halpha profiles should drop much faster than the star formation rate as a result of the rapidly decreasing ambient density.Comment: To appear in ApJ. Available from ftp.lowell.edu/pub/dah/papers/sfouterdisks

The Distribution of Dark Matter in a Ringed Galaxy

Outer rings are located at the greatest distance from the galaxy center of any feature resonant with a bar. Because of their large scale, their morphology is sensitive to the distribution of the dark matter in the galaxy. We introduce here how study of these rings can constrain the mass-to-light ratio of the bar, and so the percentage of dark matter in the center of these galaxies. We compare periodic orbits integrated in the ringed galaxy NGC 6782 near the outer Lindblad resonance to the shape of the outer ring. The non-axisymmetric component of the potential resulting from the bar is derived from a near-infrared image of the galaxy. The axisymmetric component is derived assuming a flat rotation curve. We find that the pinched non-self-intersecting periodic orbits are more elongated for higher bar mass-to-light ratios and faster bars. The inferred mass-to-light ratio of the bar depends on the assumed inclination of the galaxy. With an assumed galaxy inclination of i=41 degrees, for the orbits to be consistent with the observed ring morphology the mass-to-light ratio of the bar must be high, greater than 70% of a maximal disk value. For i=45 degrees, the mass-to-light ratio of the bar is $75\pm 15$ of the maximal disk value. Since the velocity field of these rings can be used to constrain the galaxy inclination as well as which periodic orbit is represented in the ring, further study will yield tighter constraints on the mass-to-light ratio of the bar. If a near maximal disk value for the bar is required, then either there would be little dark matter within the bar, or the dark matter contained in the disk of the galaxy would be non-axisymmetric and would rotate with the bar.Comment: AAS Latex + jpg Figures, Accepted for publication in Ap

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.Comment: 8 pages, 4 .EPS figures, Astron. & Astrophys Letters, In press (Vol 390, L35 - L38

Deep CCD Surface Photometry of the Edge-On Spiral NGC 4244

We have obtained deep surface photometry of the edge-on spiral galaxy NGC 4244. Our data reliably reach 27.5 R magnitude arcsec^{-2}, a significant improvement on our earlier deep CCD surface photometry of other galaxies. NGC 4244 is a nearby Scd galaxy whose total luminosity is approximately one magnitude fainter than the peak of the Sc luminosity function. We find that it has a simple structure: a single exponential disk, with a scale height h_Z = 246 +/- 2 pc, a scale length h_R = 1.84 +/- 0.02 kpc and a disk cutoff at a radius R(max) = 10.0 kpc (5.4 scale lengths). We confirm a strong cutoff in the stellar disk at R(max), which happens over only 1 kpc. We do not see any statistically significant evidence for disk flaring with radius. Unlike the more luminous Sc galaxies NGC 5907 and M 33, NGC 4244 does not show any evidence for a second component, such as a thick disk or halo, at mu(R) < 27.5 magnitude arcsec^{-2}.Comment: 36 pages, including 12 figures; accepted for publication in Sept 99 A