VLT Enables Most Accurate Distance Measurement to Spiral Galaxy NGC300 (Forwarded)

ESO Education and Public Relations Dept.

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Contact:
Wolfgang Gieren
Astronomy Group, Physics Department
Universidad de Concepcion, Chile
Phone: +56 41 203103

For immediate release: 1 August 2005

ESO Press Release 20/05

Moving Closer to the Grand Spiral

VLT Enables Most Accurate Distance Measurement to Spiral
Galaxy NGC 300

Cepheids and the Distance Scale

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.

Cepheid Variables in NGC 300

To tackle this problem, Wolfgang Gieren (University of
Concepcion, Chile) and his team [1] 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.

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 ESO Press Photos 18a-h in 2002. Last year, the
team presented the distance of NGC 300 as derived from these
optical images in V- and I-bands.

Moving to the Near-Infrared

ESO PR Photo 23a/05
Observed Fields in NGC 300

Caption of ESO PR Photo 23a/05: 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.

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", says 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.

An Improved Distance Determination to NGC 300

ESO PR Photo 23b/05
The Cepheid Period-Luminosity in the Near-Infrared

Caption: ESO PR Photo 23b/05 shows the Cepheid period-
luminosity relations in the J- and K-bands, as obtained
from the current VLT data on NGC 300. The mean magnitudes
for each Cepheid were determined from two independent
measurements of Cepheid brightness at different pulsation
phases. The slopes of the best-fitting relations were
adopted from the Large Magellanic Cloud Cepheids.

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", says 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", emphasizes 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 [2].

More information

The information contained in this press release is based on
a research article published in the August 1, 2005 issue of
The Astrophysical Journal (The Araucaria Project: Near-
Infrared Photometry of Cepheid Variables in the Sculptor
Galaxy NGC 300). The Araucaria Project is a key activity
of the Chilean FONDAP Center for Astrophysics.

Notes

[1]: The team is composed of Wolfgang Gieren (Principal
Investigator of the Araucaria Project), Grzegorz Pietrzynski
and Igor Soszynski (Universidad de Concepcion, Chile), Rolf-
Peter Kudritzki and Fabio Bresolin (Institute for Astronomy,
Hawaii, USA), Dante Minniti (Pontificia Universidad Catolica,
Chile), and Jesper Storm (Astrophysikalisches Institut
Potsdam, Germany).

[2]: This distance determination is tied to an assumed
distance of 163,000 light-years to the Large Magellanic
Cloud (LMC). The accurate measurement of the LMC distance
itself to which the extragalactic distance scale is
currently tied will still require years of hard work.

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