Civil Rights
Movements, leaders, victories and the continuing fight for equality.
q2K BHP
Black History Portal
THE BHP WIRE —
HIDDEN TRUTHS
What's New!
Explore the people, places, events, achievements, struggles and stories that shaped our journey.
Movements, leaders, victories and the continuing fight for equality.
Innovation, patents, science, technology and world-changing contributions.
Pioneers, champions, Negro Leagues, records, activism and excellence.
Meet the people whose lives, choices and achievements shaped the journey.
Black towns, communities, institutions and places where history happened.
Moments that changed communities, movements, institutions and the nation.
In August 1908, a white mob attacked Springfield, Illinois’s Black community, destroying homes and businesses and lynching two Black men. National outrage over the violence helped spur the movement that created the NAACP the following year.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
A cooling flow occurs when the intracluster medium (ICM) in the centres of galaxy clusters should be rapidly cooling at the rate of tens to thousands of solar masses per year.[1] This should happen as the ICM (a plasma) is quickly losing its energy by the emission of X-rays. The X-ray brightness of the ICM is proportional to the square of its density, which rises steeply towards the centres of many clusters. Also the temperature falls to typically a third or a half of the temperature in the outskirts of the cluster. The typical [predicted] timescale for the ICM to cool is relatively short, less than a billion years. As material in the centre of the cluster cools out, the pressure of the overlying ICM should cause more material to flow inwards (the cooling flow).
In a steady state, the rate of mass deposition, i.e. the rate at which the plasma cools, is given by
where L is the bolometric (i.e. over the entire spectrum) luminosity of the cooling region, T is its temperature, k is the Boltzmann constant and μm is the mean molecular mass.
It is currently thought that the very large amounts of expected cooling are in reality much smaller, as there is little evidence for cool X-ray emitting gas in many of these systems.[2] This is the cooling flow problem. Theories for why there is little evidence of cooling include[3]
Heating by AGN is the most popular explanation, as they emit a lot of energy over their lifetimes, and some of the alternatives listed have theoretical problems.
Source: Wikipedia. Article content is retrieved live through the MediaWiki API.
A cooling flow occurs when the intracluster medium (ICM) in the centres of galaxy clusters should be rapidly cooling at the rate of tens to thousands of solar masses per year. This should happen as the ICM (a plasma) is quickly losing its energy by the emission of X-rays. The X-ray brightness of the ICM is proportional to the square of its density, which rises steeply towards the centres of many clusters. Also the temperature falls to typically a third or a half of the temperature in the outskirts of the cluster. The typical [predicted] timescale for the ICM to cool is relatively short, less than a billion years. As material in the centre of the cluster cools out, the pressure of the overlying ICM should cause more material to flow inwards (the cooling flow). In a steady state, the rate of mass deposition, i.e. the rate at which the plasma cools, is given by M ˙ = 2 5 L μ m k T , {\displaystyle {\dot {M}}={\frac {2}{5}}{\frac {L\mu m}{kT}},} where L is the bolometric (i.e. over the entire spectrum) luminosity of the cooling region, T is its temperature, k is the Boltzmann constant and μm is the mean molecular mass.
A brightest cluster galaxy (BCG) is defined as the brightest galaxy in a cluster of galaxies. BCGs include the most massive galaxies in the universe. They are generally elliptical galaxies which lie close to the geometric and kinematical center of their host galaxy cluster, hence at the bottom of the cluster potential well. They are also generally coincident with the peak of the cluster X-ray emission. Formation scenarios for BCGs include: Cooling flow—star formation from the central cooling flow in high density cooling centers of X-ray cluster halos. The cooling flow begins due to the entropy in the galaxy falling below a key value. The study of accretion populations in BCGs has cast doubt over this theory and astronomers have seen no evidence of cooling flows in radiative cooling clusters. The two remaining theories exhibit healthier prospects. Galactic cannibalism—galaxies sink to the center of the cluster due to dynamical friction and tidal stripping. Galactic merger—rapid galactic mergers between several galaxies take place during cluster collapse. It is possible to differentiate the cannibalism model from the merging model by considering the formation period of the BCGs. In the cannibalism model, there are numerous small galaxies present in the evolved cluster, whereas in the merging model, a hierarchical cosmological model is expected due to the collapse of clusters. It has been shown that the orbit decay of cluster galaxies is not effective enough to account for the growth of BCGs. The merging model is now generally accepted as the most likely one, but recent observations are at odds with some of its predictions. For example, it has been found that the stellar mass of BCGs was assembled much earlier than the merging model predicts. This is due to the lack of significant stellar mass evolution in size, luminosity, or structure over a range of redshifts, suggesting that most BCGs were assembled by redshift z~1.5−2.0, which refutes any late hierarchical assembly and emphases passive evolution. Growth primarily comes from minor mergers and gas accretion after assembly. Discrepancies between observations and hierarchical simulations highlight the need for model revisions to better account for early mass assembly. BCGs are divided into various classes of galaxies: giant ellipticals (gE), D galaxies and cD galaxies. cD and D galaxies both exhibit an extended diffuse envelope surrounding an elliptical-like nucleus akin to regular elliptical galaxies. The light profiles of BCGs are often described by a Sersic surface brightness law, a double Sersic profile or a de Vaucouleurs law. The different parametrizations of the light profile of BCG's, as well as the faintness of the diffuse envelope lead to discrepancies in the reported values of the sizes of these objects. An examination of x-ray observational results for high red-shift cluster galaxies (0.81< z < 1.17) found that nine of the ten high red-shift cluster galaxies in the sample are dynamically disturbed. This was inferred from offsets of the BCG mass center from the X-ray centroid and X-ray maximum value. Based on a comparison of earlier work cited on lower red-shift galaxies (0.14< z
Air cooling is a method of dissipating heat. It works by expanding the surface area or increasing the flow of air over the object to be cooled, or both. An example of the former is to add cooling fins to the surface of the object, either by making them integral or by attaching them tightly to the object's surface (to ensure efficient heat transfer). In the case of the latter, it is done by using a fan blowing air into or onto the object one wants to cool. The addition of fins to a heat sink increases its total surface area, resulting in greater cooling effectiveness. There are two types of cooling pads that can be used for air cooling: one is the honeycomb design and another one is excelsior. In all cases, the air has to be cooler than the object or surface from which it is expected to remove heat. This is due to the second law of thermodynamics, which states that heat will only move spontaneously from a hot reservoir (the heat sink) to a cold reservoir (the air).
Variable refrigerant flow (VRF), is an HVAC technology invented by Daikin Industries, Ltd. in 1982. Daikin Industries, Ltd. named this "VRV" and holds the registered trademark for it. Similar to ductless mini-split systems, VRFs use refrigerant as the primary cooling and heating medium, and are usually less complex than conventional chiller-based systems. This refrigerant is conditioned by one or more condensing units (which may be outdoors or indoors, water or air cooled), and is circulated within the building to multiple indoor units. VRF systems, unlike conventional chiller-based systems, allow for varying degrees of cooling in more specific areas (because there are no large air handlers, only smaller indoor units), may supply hot water in a heat recovery configuration without affecting efficiency, and switch to heating mode (heat pump) during winter without additional equipment, all of which may allow for reduced energy consumption. Also, air handlers and large ducts are not used which can reduce the height above a dropped ceiling, increase space in the dropped ceiling space, as well as structural impact as VRF uses smaller penetrations for refrigerant pipes instead of ducts. With Liquid VRF, the space otherwise required for air handlers is also significantly reduced.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →Shirley Chisholm, elected in 1968.
