Most of my work these days deals with high resolution studies of the core properties of early type galaxies. HST/ACS images and ground based spectra (from KPNO, Keck and Gemini) are combined to provide the most accurate view to date of the cores of 100 early-type galaxies in the Virgo clusters, and 43 early-type galaxies in the Fornax clusters. Please see the ACS Virgo Cluster Survey webpage for a detailed description of these projects.

When I was a second year graduate student at the Johns Hopkins University in Baltimore, my advisor, Holland Ford, and his collaborators announced what was deemed to be the first secure detection of a supermassive black hole in a galaxy. The galaxy was M87, the central galaxy in the Virgo cluster. The black hole at its center was measured to be over a billion times more massive than our Sun. A year later, Holland Ford, Walter Jaffe, and myself published a second detection, in the galaxy NGC 4261. Since then, thanks to the efforts of many teams, the detections have multiplied, so much that we now believe that supermassive black holes might be present in the center of every galaxy, including our own.

I am planning to add to this page a tutorial about black hole detections and demographics. In the meantime, these links will have to do!

The expansion rate of the Universe is a fundamental parameter in modern astrophysics. In a standard Big Bang cosmology, the Universe expands uniformly according to the Hubble law, v = H0 d, where v is the recession velocity of a galaxy at a distance d, and H0 is the Hubble constant, the expansion rate at the current epoch. If an estimate of the total energy density of the Universe is available, the Hubble constant immediately defines the age and size of the Universe, as well as its radius of curvature. The critical density of the Universe, which tells us about the growth of cosmic structure; the proportion of primordial light elements synthesis in the Big Bang; and practically all physical properties of galaxies and active galactic nuclei (AGNs), such as their mass and intrinsic luminosity, all hinge on the value of the Hubble constant.

The discovery, more than seven decades ago, that local galaxies obey the Hubble law, provided indisputable evidence that the Universe is indeed expanding. However, measuring the Hubble constant has proven to be a challenging task. The figure to the left shows determinations of the Hubble constant from Edwin Hubble's time to the present. Contrary to popular belief, Hubble was not the first nor the second astronomer to measure the constant that carries his name: based on Hubble's data, G. Lemaitre and H. Robinson both published estimates before Hubble's first attempt. The plot is made with data compiled by Prof. John Huchra.

 

The HST Key Project on the Extragalactic Distance Scale

Back in 1984, when the four flagship research programs for the yet to be launched Hubble Space Telescope were announced, the "HST Key Project on the Extra Galactic Distance Scale" was one of the selected few. The goal of the project was to measure the Hubble constant with 10% accuracy. The plan of attack was to measure accurate distances to 18 galaxies, from very nearby systems like M81, to large spirals in the Virgo and Fornax clusters of galaxies. The most accurate of the primary "standard candles", Cepheid variable stars, were to be employed for these distance measurements. Unfortunately, even at the distance of the Virgo cluster, 15 million parsecs (45 million light years) the motion of galaxies is still heavily influenced by local perturbations, which can mask the underlying uniform flow due to the expansion of the Universe. To push beyond the local supercluster, and well into the unperturbed Hubble flow, the Key Project planned to use the 18 Cepheid distances as stepping stones to calibrate secondary distance indicators, such as the Tully-Fisher relation, the Surface Brightness Fluctuation method, and the Fundamental Plane for Early Type galaxies. These distance indicators may not be as accurate and/or well understood as Cepheids, but they can target galaxies at distances up to 10 times the distance of the Virgo cluster, where not even HST can ever hope to discover Cepheids.

Cepheid variables are stars in a late evolutionary stage. Because of radial pulsations, their luminosity varies in a very regular fashion with periods between 1 and 80 days. Cepheids are good distance indicators because their mean luminosity correlates well with their variation period. A nice description of how Cepheids can be used to measure distances can be found in this Key Project press release. The figure to the right shows a composite I-band Period-Luminosity Relation for all Cepheids discovered in the 24 galaxies observed for this purpose with the Hubble Space Telescope. The count is just shy of 800 objects. Most of them were discovered by the Key Project, but several are part of an independent, and equally ambitious project led by Allan Sandage, of the Carnegie Observatory, and Abhijit Saha, now at NOAO. References for the data can be found here.

 

 

 

 

 

The figure to the left shows the an image of the Virgo cluster galaxy M100 (in gray) taken with the Wide Field and Planetary Camera 2 on HST, superimposed to a ground based image of the same galaxy. Over 100,000 stars can be detected in the HST field of view. Of these only a few tens are Cepheids, none of which was detected from the ground. The success of HST in finding Cepheids is due to the combination of two factors: superb spatial resolution, which allows one to separate Cepheids from the surrounding stars in crowded backgrounds; and the ability to schedule observations at precise time intervals, a necessary conditions when trying to characterize temporal variations in brightness.

In 2001, almost two decades after its inception, The Key Project on the Extragalactic Distance Scale reached its stated goal and announced a value of the Hubble constant of 72 km/s/Mpc, with a formal 10% accuracy. A list of all Key Project papers can be found by clicking here.

 

 

 

 

 

In an "Einstein-de Sitter Universe", in which the density of matter is 1 and there is no cosmological constant, a value of the Hubble constant of 72 km/s/Mpc implies an age of the Universe of only 9 billion years, significantly younger than the age estimated for the Universe oldest star clusters (12-13 billion years). The Key Project value of the Hubble constant, combined with the constraints on the age of the Universe, are consistent with either a low density open universe, or a flat Universe dominated by dark energy. This can be seen in the plot to the left, where we assumed a value for the Hubble constant and for the age of the Universe of 72 km/s/Mpc and 12.5 Gyr respectively, with ±10% uncertainties. The dashed line indicates the case of a flat universe where the sum of matter and dark density is equal to 1. The solid curve represents a Universe with a zero cosmological constant. The large open circle marks the parameters predicted by the Einstein-de Sitter model. The figure is taken from Freedman et al. (2001)

 

Additional links:

The Key Project, and all other distance scale projects that have been carried out in the past several years, have significantly refined our knowledge of the Hubble constant. However, several aspects of the problem need additional work. In particular, I am concentrating on two projects:

 

Constraining the Metallicity Dependence of the Cepheid PL Relation.

Cepheid distances to galaxies can be systematically miscalculated if this (presumably small) dependence is not taken into account. We have selected a sample of galaxies with Cepheids spanning a wide range in metallicity. For each we measure the distance using the "Tip of the Red Giant Branch Method". A systematic difference between TRGB and Cepheid distance, if it correlates with Cepheid metallicity, would indicate that the Cepheid PL relation is indeed sensitive to the metallicity of the variable stars. This could lead to a 5-10% decrease in the value of the Hubble constant. Our work on this subject, in collaboration with Rob Kennicutt, Shoko Sakai and Abi Saha, has been published, and a copy of the paper (Sakai et al. 2004) can be found here.

 

Refining the Calibration of Pop II Distance Indicators.

Cepheids are relatively young stars, and are only found in the arms of spiral galaxies. Pop II distance indicators (SBF, the Planetary Nebulae Luminosity Function, the Globular Cluster Luminosity Function and the Fundamental Plane for Early Type Galaxies) are only applicable to the old stellar populations found preferentially in Early Type (elliptical and lenticular) galaxies. It follows that Cepheids cannot be used to find distances to Early Type galaxies, and therefore provide a reliable calibration to Pop II distance indicators. In collaboration with John Tonry, John Blakeslee, Jeremy Mould and Peter Stetson, we are using HST to measure Cepheid distances to spiral galaxies which are interacting with (and therefore at the same distance as) elliptical galaxies. This will provide a secure calibration for Pop II distance indicators, and further refine the value of the Hubble constant. The project will also produce a state of the art, and much needed, photometric calibration for the Wide Field and Planetary Camera on HST. The first measurement of a Cepheid distance in an early-type galaxy has now been published. A copy of the paper can be found here. A second paper is in preparation