arXiv subjects

Alister W. Graham

Publications and source records attributed to Alister W. Graham.

176 records · Page 10Linked to original sources

arXiv2002

The Correlation Between Supermassive Black Hole Mass and the Structure of Ellipticals and Bulges

We demonstrate a strong correlation between supermassive black hole (SMBH) mass and the global structure of ellipticals and bulges: more centrally concentrated bulges and ellipticals (higher Sersic index $n$) host higher-mass black holes. This correlation is as good as that previously found between SMBH mass and central velocity dispersion, with comparable scatter. In addition, by carefully modeling the bulges of disk galaxies so that bars, inner disks, and the like do not accidentally contribute to bulge light, we find that the correlation between SMBH mass and bulge or elliptical luminosity is similarly close.

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arXiv2002

Constraints on Jupiters from observations of Galactic Bulge microlensing events during 2000

We present observations of 8 Galactic Bulge microlensing events taken with the 1.0m JKT on La Palma during 2000 June and July. The JKT observing schedule was optimized using a prioritizing algorithm to automatically update the target list. For most of these events we have sampled the lightcurves at times where no information was available from the OGLE alert team. We assume a point-source point-lens (PSPL) model and perform a maximum likelihood fit to both our data and the OGLE data to constrain the event parameters of the fit. We then refit the data assuming a binary lens and proceed to calculate the probability of detecting planets with mass ratio $q=10^{-3}$. We have seen no clear signatures of planetary deviations on any of the 8 events and we quantify constraints on the presence of planetary companions to the lensing stars. For two well observed events, 2000BUL31 and 2000BUL33, our detection probabilities peak at $\sim$30% and $\sim$20% respectively for $q=10^{-3}$ and $a \sim R_{E}$ for a $Δχ^2$ threshold value of 60.

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arXiv2002

The Photometric Plane of Elliptical Galaxies

The Sersic r^(1/n) index n of an elliptical galaxy (or bulge) has recently been shown to correlate strongly (r=0.8) with a galaxy's central velocity dispersion. This index could therefore prove extremely useful and cost-effective (in terms of both telescope time and data reduction) for many fields of extragalactic research. It is a purely photometric quantity which apparently not only traces the mass of a bulge but has additionally been shown to reflect the degree of bulge concentration. This paper explores the affect of replacing the central velocity dispersion term in the Fundamental Plane with the Sersic index n. Using a sample of early-type galaxies from the Virgo and Fornax clusters, various (B-band) `Photometric Planes' were constructed and found to have a scatter of 0.14-0.17 dex in log(r_e), or a distance error of 38-48% per galaxy (the higher values arising from the inclusion of the S0 galaxies). The corresponding Fundamental Plane yielded a 33-37% error in distance for the same galaxy (sub-)samples (i.e. ~15-30% less scatter). The gains in using a hyperplane (i.e. adding the Sersic index to the Fundamental Plane as a fourth parameter) were small, giving a 27-33% error in distance, depending on the galaxy sample used. The Photometric Plane has been used here to estimate the Virgo-Fornax distance modulus; giving a value of Δμ=0.62(+/-0.30) mag (cf. 0.51(+/-0.21), HST Key Project on the Extragalactic distance Scale). The prospects for using the Photometric Plane at higher redshift appears promising. Using published data on the intermediate redshift cluster Cl 1358+62 (z=0.33) gave a Photometric Plane distance error of 35-41% per galaxy.

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arXiv2002

Mass-to-light ratios from the fundamental plane of spiral galaxy disks

The best-fitting 2-dimensional plane within the 3-dimensional space of spiral galaxy disk observables (rotational velocity v_{rot}, central disk surface brightness μ_0 = -2.5log I_0, and disk scale-length h) has been constructed. Applying a three-dimensional bisector method of regression analysis to a sample of ~100 spiral galaxy disks that span more than four mag arcsec^{-2} in central disk surface brightness yields v_{rot} ~ I_0^{0.50+/-0.05}h^{0.77+/-0.07} (B-band) and v_{rot} ~ I_0^{0.43+/-0.04}h^{0.69+/-0.07} (R-band). Contrary to popular belief, these results suggest that in the B-band, the dynamical mass-to- light ratio (within 4 disk scale-lengths) is largely independent of surface brightness, varying as I_0^{0.00+/-0.10}h^{0.54+/-0.14}. Consistent results were obtained when the expanse of the analysis was truncated by excluding the low surface brightness galaxies. Previous claims that M/L_B varies with I_0^{-1/2} are shown to be misleading and/or due to galaxy selection effects. Not all low-surface-brightness disk galaxies are dark matter dominated. The situation is however different in the near-infrared where L_{K'} \~ v^4 and M/L_{K'} is shown to vary as I_0^{-1/2}. Theoretical studies of spiral galaxy disks should not assume a constant M/L ratio within any given passband. The B-band dynamical mass-to-light ratio (within 4 disk scale-lengths) has no obvious correlation with (B-R) disk colour, while in the K'-band it varies as -1.25+/-0.28(B-R). Combining the present observational data with recent galaxy model predictions implies that the logarithm of the stellar-to-dynamical mass ratio is not a constant value, but increases as disks become redder, varying as 1 .70+/-0.28(B-R).

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arXiv2002

Evidence for an outer disk in the Prototype `Compact Elliptical' Galaxy M32

M32 is the prototype for the relatively rare class of galaxies referred to as `compact ellipticals'. It has been suggested that M32 may be a tidally disturbed r^(1/4) elliptical galaxy, or the remnant bulge of a disk-stripped early-type spiral galaxy. This paper reveals that the surface brightness profile, the velocity dispersion measurements, and the estimated supermassive black hole mass in M32 are inconsistent with the galaxy having, and probably ever having had, an r^(1/4) light profile. Instead, the radial surface brightness distribution of M32 resembles an almost perfect (bulge + exponential disk) profile, which is accompanied by a marked increase in the ellipticity profile and an associated change in the position angle profile where the `disk' starts to dominate. Compelling evidence that this bulge/disk interpretation is accurate comes from the best-fitting r^(1/n) bulge model which has a Sersic index n=1.5, in agreement with the recently discovered relation between a bulge's Sersic index and the mass of its supermassive black hole. An index n>4 would also be inconsistent with the stellar velocity dispersion of M32. The bulge-to-disk size ratio r_e /h equals 0.20, and the logarithm of the bulge-to-disk luminosity ratio log(B/D) equals 0.22, typical of lenticular galaxies. The effective radius of the bulge is 27" (~100 pc), while the scale-length of the disk is less well determined: due to possible tidal- stripping of the outer profile beyond 220-250", the scale-length may be as large as 1.3 kpc. M32 is a relatively face-on, nucleated, dwarf galaxy with a low surface brightness disk and a high surface brightness bulge. This finding brings into question the existence of the compact elliptical class of galaxies.

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arXiv2002

Triaxial stellar systems following the r^{1/n} luminosity law: an analytical mass-density expression, gravitational torques and the bulge/disc interplay

We have investigated the structural and dynamical properties of triaxial stellar systems whose surface brightness profiles follow the r^{1/n} luminosity law - extending the analysis of Ciotti (1991) who explored the properties of spherical r^{1/n} systems. A new `analytical' expression that accurately reproduces the spatial (i.e. deprojected) luminosity density profiles (error < 0.1%) is presented for detailed modelling of the Sersic family of luminosity profiles. We evaluate both the symmetric and the non--axisymmetric components of the gravitational potential and force, and compute the torques as a function of position. For a given triaxiality, stellar systems with smaller values of n have a greater non-axisymmetric gravitational field component. We also explore the strength of the non-axisymmetric forces produced by bulges with differing n and triaxiality on systems having a range of bulge-to-disc ratios. The increasing disc-to-bulge ratio with increasing galaxy type (decreasing n) is found to heavily reduce the amplitude of the non-axisymmetric terms, and therefore reduce the possibility that triaxial bulges in late-type systems may be the mechanism or perturbation for nonsymmetric structures in the disc. Using seeing-convolved r^{1/n}-bulge plus exponential-disc fits to the K-band data from a sample of 80 nearby disc galaxies, we probe the relations between galaxy type, Sersic index n and the bulge-to-disc luminosity ratio. These relations are shown to be primarily a consequence of the relation between n and the total bulge luminosity. In the K-band, the trend of decreasing bulge-to-disc luminosity ratio along the spiral Hubble sequence is predominantly, although not entirely, a consequence of the change in the total bulge luminosity; the trend between the total disc luminosity and Hubble type is much weaker.

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arXiv2001

A Correlation between Galaxy Light Concentration and Supermassive Black Hole Mass

We present evidence for a strong correlation between the concentration of bulges and the mass of their central supermassive black hole (M_bh) -- more concentrated bulges have more massive black holes. Using C_{r_e}(1/3) from Trujillo, Graham & Caon (2001b) as a measure of bulge concentration, we find that log (M_bh/M_sun) = 6.81(+/-0.95)C_{r_e}(1/3) + 5.03(+/-0.41). This correlation is shown to be marginally stronger (Spearman's r_s=0.91) than the relationship between the logarithm of the stellar velocity dispersion and log M_bh (Spearman's r_s=0.86), and has comparable, or less, scatter (0.31 dex in log M_bh), which decreases to 0.19 dex when we use only those galaxies whose supermassive black hole's radius of influence is resolved and remove one well understood outlying data point).

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arXiv2001

Galaxy Light Concentration. I. Index stability and the connection with galaxy structure, dynamics, and supermassive black holes

We explore the stability of different galaxy light concentration indices as a function of the outermost observed galaxy radius. With a series of analytical light-profile models, we show mathematically how varying the radial extent to which one measures a galaxy's light can strongly affect the derived galaxy concentration. The "mean concentration index", often used for parameterizing high-redshift galaxies, is shown to be horribly unstable, even when modeling one-component systems such as elliptical, dwarf elliptical and pure exponential disk galaxies. The C_31 concentration index performs considerably better but is also heavily dependent on the radial extent, and hence exposure depth, of any given galaxy. We show that the recently defined central concentration index is remarkably stable against changes to the outer radius, providing a meaningful and reliable estimate of galaxy concentration. The index n from the r^(1/n) models is shown to be monotonically related with the central concentration of light, giving the index n a second and perhaps more tangible meaning. With a sample of elliptical and dwarf elliptical galaxies, we present correlations between the central light concentration and the global parameters: luminosity (Pearson's r = -0.82), effective radius (r = 0.67), central surface brightness (r = -0.88), and velocity dispersion (r = 0.80). The more massive elliptical galaxies are shown to be more centrally concentrated. We speculate that the physical mechanism behind the recently observed correlation between the central velocity dispersion (mass) of a galaxy and the mass of its central supermassive black hole may be connected with the central galaxy concentration. That is, we hypothesize that it may not simply be the amount of mass in a galaxy but rather how that mass is distributed that controls the mass of the central black hole.

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arXiv2001

A K-band central disk surface-brightness correlation with scale-length for early-type disk galaxies, and the inclination correction

The K-band light-profiles from two statistically complete, diameter-limited samples of disk galaxies have been simultaneously modelled with a seeing-convolved Sersic r^{1/n} bulge and a seeing-convolved exponential disk. This has enabled an accurate separation of the bulge and disk light, and hence an estimate of the central disk surface brightness mu_{0,K} and the disk scale-length h. There exists a bright envelope of galaxy disks in the mu_{0,K}-log h diagram; for the early-type ( 50 deg) galaxies having morphological types ranging from S0 to Sc, the need for an inclination correction to the K-band disk surface brightness is demonstrated. Measured central disk surface brightnesses are found to be significantly (>5-sigma) brighter for the high-inclination disk galaxies than for the low-inclination disk galaxies. With no surface brightness inclination correction or allowance for the trend between mu_{0,K} and log h, the standard deviation to the distribution of mu_{0,K} values is ~1 mag arcsec^{-2}, while the standard deviation about the mean mu_{0,K}-log h relation decreases from 0.69 mag arcsec^{-2}, when no inclination correction is applied, to 0.47 mag arcsec^{-2} when the inclination correction is applied.

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arXiv2001

A Morphological-type dependence in the mu_0-log(h) plane of Spiral galaxy disks

We present observational evidence for a galaxy `Type' dependence to the location of a spiral galaxy's disk parameters in the mu_0-log(h) (central disk surface-brightness - disk scale-length) plane. With a sample of ~40 Low Surface Brightness galaxies (both bulge- and disk-dominated) and ~80 High Surface Brightness galaxies, the early-type disk galaxies (<=Sc) tend to define a bright envelope in the mu_0-log(h) plane, while the late-type (>=Scd) spiral galaxies have, in general, smaller and fainter disks. Below the defining surface brightness threshold for a Low Surface Brightness galaxy (i.e. more than 1 mag fainter than the 21.65 B-mag arcsec^(-2) Freeman value), the early-type spiral galaxies have scale-lengths greater than 8-9 kpc, while the late-type spiral galaxies have smaller scale-lengths. All galaxies have been modelled with a seeing-convolved Sersic r^(1/n) bulge and exponential disk model. We show that the trend of decreasing bulge shape parameter (n) with increasing Hubble type and decreasing bulge-to-disk luminosity ratio, which has been observed amongst the High Surface Brightness galaxies, extends to the Low Surface Brightness galaxies, revealing a continuous range of structural parameters.

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arXiv2001

On the estimation of galaxy structural parameters: the Sersic Model

This paper addresses some questions which have arisen from the use of the Sérsic r^{1/n} law in modelling the luminosity profiles of early type galaxies. The first issue deals with the trend between the half-light radius and the structural parameter n. We show that the correlation between these two parameters is not only real, but is a natural consequence from the previous relations found to exist between the model-independent parameters: total luminosity, effective radius and effective surface brightness. We also define a new galaxy concentration index which is largely independent of the image exposure depth, and monotonically related with n. The second question concerns the curious coincidence between the form of the Fundamental Plane and the coupling between _e and r_e when modelling a light profile. We explain, through a mathematical analysis of the Sérsic law, why the quantity r_e _e^{0.7} appears almost constant for an individual galaxy, regardless of the value of n (over a large range) adopted in the fit to the light profile. Consequently, Fundamental Planes of the form r_e _e^{0.7} propto sigma_0^x (for any x, and where sigma_0 is the central galaxy velocity dispersion) are insensitive to galaxy structure. Finally, we address the problematic issue of the use of model-dependent galaxy light profile parameters versus model-independent quantities for the half-light radii, mean surface brightness and total galaxy magnitude. The former implicitly assume that the light profile model can be extrapolated to infinity, while the latter quantities, in general, are derived from a signal-to-noise truncated profile. We quantify (mathematically) how these parameters change as one reduces the outer radius of an r^{1/n} profile, and reveal how these can vary substantially when n>4.

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arXiv2000

An investigation into the prominence of spiral galaxy bulges

From a diameter-limited sample of 86 `face-on' spiral galaxies, the bulge-to-disk size and luminosity ratios, and other quantitative measurements for the prominence of the bulge are derived. The bulge and disk parameters have been estimated using a seeing convolved Sersic r^(1/n) bulge and a seeing convolved exponential disk. In general, early-type spiral galaxy bulges have Sersic values of n>1, and late-type spiral galaxy bulges have values of n<1. In the B-band, only 8 galaxies have a bulge shape parameter n consistent with the exponential value of 1, and only 5 galaxies do in the K-band. Application of the r^(1/n) bulge models results in a larger mean r_e/h ratio for the early-type spiral galaxies than the late-type spiral galaxies. Although, this result is shown not to be statistically significant. The mean B/D luminosity ratio is, however, significantly larger (> 3-sigma) for the early-type spirals than the late-type spirals. This apparent contradiction with the r_e/h values can be explained with an iceberg-like scenario, in which the bulges in late-type spiral galaxies are relatively submerged in their disk. This can be achieved by varying the relative bulge/disk stellar density while maintaining the same effective bulge-to-disk size ratio. The absolute bulge magnitude - log(n) diagram is used as a diagnostic tool for comparative studies with dwarf elliptical and ordinary elliptical galaxies. At least in the B-band, these objects occupy distinctly different regions of this parameter space. While the dwarf ellipticals appear to be the faint extension to the brighter elliptical galaxies, the bulges of spiral galaxies are not.

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arXiv1999

The influence of bulge profile shapes on claims for a scale-free Hubble sequence for spirals

We investigate recent claims that the Hubble sequence of spiral galaxies is scale-free. Fundamental to this investigation is the fact that within the photometric data of 86 spiral galaxies from de Jong & van der Kruit - from which these claims were made - a trend exists between morphological type and bulge profile shape. While late-type spiral bulges are described by an exponential luminosity profile, the early-type spiral bulges are better described by an r^{1/2} or r^{1/4} law. Taking the scale-lengths from the best-fitting surface brightness profile models (i.e. either using exponential, r^{1/2}, or r^{1/4} law profile parameters), we show that in all six passbands used (BVRIHK) the early-type spirals have a larger r_e/h ratio than late-type spiral galaxies. In contrast to this, fitting an exponential profile to the bulges of all spirals results in the mean r_e/h ratio for the early-type spiral galaxies actually being smaller than the mean r_e/h ratio for the late-type spiral galaxies (at the 3-sigma significance level using K-band data).

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arXiv1998

Some effects of galaxy structure and dynamics on the Fundamental Plane. II. A Virgo-Fornax distance modulus

The influence of broken structural homology, and the implied broken dynamical homology, is examined for the Fundamental Plane (FP). A bisector method of linear regression was applied, in 3-dimensions, to derive the best FP and a bootstrapping procedure has been used to estimate the associated uncertainties. For 25 E and S0 Virgo galaxies, the `standard' FP, constructed using R^{1/4} model parameters for the effective radii (R_{e,4}) and the mean surface brightness within this radius (Sigma_{e,4}) and using central velocity dispersion measurements (sigma_0), gave a relation described by R_{e,4}~sigma_{0}^{1.10+/-0.14}Sigma_{e,4}^{-0.55+/-0.09}. Using Sersic R^{1/n} light profile model parameters and the computed velocity dispersion within an aperture of infinite radius (sigma_{tot,n}) gave an `improved' FP described by the relation R_{e,n}~sigma_{tot,n}^{1.37+/-0.16}Sigma_{e,n}^{-0.76+/-0.05}. Upon removal of the known S0 galaxies from the sample, the above planes were observed to change to R_{e,4}~sigma_{0}^{1.19+/-0.21}Sigma_{e,4}^{-0.60+/-0.11} and R_{e,n}~sigma_{tot,n}^{1.72+/-0.24}Sigma_{e,n}^{-0.74+/-0.09}. The perpendicular rms residuals about these planes are 0.084 and 0.050 dex, respectively. A similar analysis of the Fornax cluster produced a FP given by the relation R_{e,n}~sigma_{tot,n}^{2.03+/-0.78}Sigma_{e,n}^{-1.07+/-0.30}, in tantalizing agreement with the plane expected from the virial theorem, but with discouragingly large errors due to the small sample size of only 7. Similarly to Virgo, the perpendicular rms residuals about this plane is 0.050 dex. A Virgo-Fornax distance modulus was computed using Working-Hotelling confidence bands. The `standard' FP parameters gave a value of 0.45+/-0.16 mag, where as the `improved' FP parameters gave a value of 0.25+/-0.12 mag.

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