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Post Info TOPIC: NGC300 Cepheids


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NGC 300 black hole

Astronomers using ESOs Very Large Telescope have detected a stellar mass black hole much farther away than any other previously known. The newly discovered black hole is in the spiral galaxy NGC 300, about six million light years away from the Sun. With a mass of about twenty times that of the Sun, it is also the second most massive stellar mass black hole ever found and it is entwined with a star that will soon become a black hole itself. The team of scientists will publish their findings about this intriguing system in the journal Monthly Notices of the Royal Astronomical Society.
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Cepheids in the Sculptor group

For the first time, astronomers have detected Cepheids in the Sculptor group galaxy NGC 55 and used these cosmic yardsticks to measure its distance. That distance matches the distance of the spiral galaxy NGC 300, suggesting the two galaxies may orbit each other.

The Sculptor group is the nearest galaxy group to the Local Group and contains five major galaxies: NGC 55, NGC 247, NGC 253, NGC 300, and NGC 7793. Except for NGC 247, all of these galaxies reside in the constellation Sculptor, hence the group's name.
The Cepheids reveal that the distance to NGC 55 is 6.2 million light-years, nearly the same as the distance to the spiral NGC 300.

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RE: NGC-300

The milky way may be a lot bigger than previously thought.
Australian and US Astronomers have discovered whole new "suburbs" of stars in the galaxy NGC 300.

The astronomers used the 8 metre Gemini South telescope in Chile to reveal the faint ancient outer parts of the galaxy NGC 300, showing that that galaxy is at least twice as big as previously thought.
The finding implies that our own Galaxy too is probably much larger than previously thought.

The research was published on the 10th August 2005 in the Astrophysical Journal.
NGC300 is some 6.1 million light years from earth. It resembles our own Galaxy, the Milky Way, with most of its stars lying in a thin disk like a pancake.
Using the Gemini Multi-Object Spectrograph instrument on the Gemini South telescope in Chile, and using extremely sensitive measurements, going more than ten times fainter than any previous images of the galaxy, the observers were able to see stars in the disk up to 47,000 light-years from the galaxy's centre - double the previously known radius of the disk.
The outskirts of NGC 300 are filled by faint, old stars; but a few billion years ago were brightly lit with large stars.
The discovery has implications for our own Galaxy. Most current estimates put the size of the Milky Way at around 100,000 light-years across, about the same as the new estimate for NGC 300.

"However, our galaxy is much more massive and brighter than NGC 300. So on this basis, our Galaxy is also probably much larger than we previously thought--perhaps as much as 200,000 light-years across" - Professor Joss Bland-Hawthorn, Anglo-Australian Observatory, lead author.

The observers found no evidence that the outer part of NGC 300 was falling abruptly in brightness, or truncating, as happens in many galaxies.

"We now realize that there are distinctly different types of galaxy disks. Probably most truncate--the density of stars in the disk drops off sharply. But NGC 300 just seems to go on forever. The density of stars in the disk falls off very smoothly and gradually" - Professor Ken Freeman, Research School of Astronomy and Astrophysics at the Australian National University, team member.

The observers traced NGC 300's disk out to the point where the surface density of stars was equivalent to a one-thousandth of a Sun per square light-year.

"This is the most extended and diffuse population of stars ever seen" - Bland-Hawthorn.

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NGC300 Cepheids

VLT Enables Most Accurate Distance Measurement to Spiral Galaxy NGC 300

Cepheid pulsating stars have been used as distance indicators since the early discovery of Henrietta Leavitt almost a hundred years ago. From her photographic data regarding one of the Milky Way's neighbour galaxies, the Small Magellanic Cloud, she found that the brightness of these stars closely correlate with their pulsation periods.
This period-luminosity relation, once calibrated, allows a precise distance determination of a galaxy once Cepheids have been discovered in it, and their periods and mean magnitudes have been measured.
While the Cepheid method doesn't reach out far enough in the Universe to directly determine cosmological parameters like the Hubble constant, Cepheid distances to relatively nearby resolved galaxies have laid the foundation for such work in the past, as in the Hubble Space Telescope Key Project on the Extragalactic Distance Scale.
Cepheids indeed constitute one of the first steps in the cosmic distance ladder.

The current main problem with the Cepheid method is that its dependence on a galaxy's metallicity, that is, its content in elements more heavy than hydrogen and helium, has never been measured accurately so far. Another intriguing difficulty with the method is the fact that the total absorption of the Cepheid's light on its way to Earth, and in particular the amount of absorption within the Cepheid's host galaxy, must be precisely established to avoid significant errors in the distance determination.

Location of the three fields in the Sculptor spiral galaxy NGC 300 for which deep imaging in the near-infrared J and K filters was obtained with VLT/ISAAC. The fields contain together 16 Cepheids with periods from 6 to 83 days.

To tackle this problem, Wolfgang Gieren (University of Concepcion, Chile) and his team devised a Large Programme at ESO: the Araucaria Project. Its aim is to obtain distances to relatively nearby galaxies with a precision better than 5 percent.
The team was able to measure the distance to NGC 300 with the unprecedented total uncertainty of only about 3 percent. The astronomers found that NGC 300 is located 6.13 million light-years away.

One of the key galaxies of the team's Araucaria Project is the beautiful, near face-on galaxy NGC 300 in the Sculptor Group. In a wide-field imaging survey carried out at the ESO/MPG 2.2-m telescope on La Silla in 1999-2000, the team had discovered more than a hundred Cepheid variables spanning a broad range in pulsation period. Pictures of the galaxy, and some of its Cepheids from these data were released in in 2002. Last year, the team presented the distance of NGC 300 as derived from these optical images in V- and I-bands.

The team complemented this unique dataset with new data taken with the ISAAC near-infrared camera and spectrometer on ESO's 8.2-m VLT Antu telescope.

"There are three substantial advantages in the Cepheid distance work when images obtained through near-infrared passbands are used instead of optical data" - Wolfgang Gieren.

The most important gain is the fact that the absorption of starlight in the near-infrared, and particularly in the K-band, is dramatically reduced as compared to the effect interstellar matter has at visible wavelengths.
A second advantage is that Cepheid light curves in the infrared have smaller amplitudes and are much more symmetrical than their optical counterparts, making it possible to measure a Cepheid's mean K-band brightness just from a very few, and in principle from just one observation at known pulsation phase. In contrast, optical work requires the observation of full light curves to determine accurate mean magnitudes.
The third basic advantage in the infrared is a reduced sensitivity of the period-luminosity relation to metallicity, and to blending with other stars in the crowded fields of a distant galaxy.
Taking this into account, one of the main purposes of the team's Large Programme has been to conduct near-infrared follow-up observations of Cepheids in their project's target galaxies which have previously been discovered in optical wide-field surveys.
Deep images in the J and K bands of three fields in NGC 300 containing 16 Cepheids were taken with VLT/ISAAC in 2003.

"The high quality of the data allowed a very accurate measurement of the mean J- and K- magnitudes of the Cepheids from just 2 observations of each star obtained at different times"- Grzegorz Pietrzynski, another member of the team, also from Concepcion.

Using these remarkable data the period-luminosity relations were constructed.

"They are the most accurate infrared PL relations ever obtained for a Cepheid sample in a galaxy beyond the Magellanic Clouds" - Wolfgang Gieren.

The total absorption of light ("reddening") of the Cepheids in NGC 300 was obtained by combining the values for the distance of the galaxy obtained in the various optical and near-infrared bands in which NGC 300 was observed.
This led to the discovery that there is a very significant contribution to the total reddening from absorption intrinsic to NGC 300. This intrinsic absorption has an important effect on the determination of the distance but had not been taken into account previously.
The team was able to measure the distance to NGC 300 with the unprecedented total uncertainty of only about 3 percent. The astronomers found that NGC 300 is located 6.13 million light-years away…
NGC 300 is a Spiral galaxy (Sd-type) in the constellation Sculptor.


-- Edited by Blobrana at 19:53, 2005-08-01

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