2,111 research outputs found
Spatially resolved optical and near infrared spectroscopy of I Zw 18
For a long time, I Zw 18 has been recognized as the lowest abundance extra-galactic HII region (with the possible recent exception of SBS 0335-052; Izotov 1989). As such, it is important for many studies, including the determination of the primordial helium abundance. Recent imaging studies of I Zw 18 have revealed a more complex structure to I Zw 18 than the simple two-component model previously assumed. This has given rise to concern about the reliability of chemical abundance measurements derived for I Zw 18. Researchers have obtained long-slit spectra covering the wavelength range lambda 3650 to lambda 10,000, which allow us to measure physical parameters and chemical abundances as a function of position. With these new data we can investigate the SE component, which has not been studied previously, and we can address some of the concerns about abundance uncertainties
Measuring the Hubble Constant with the Hubble Space Telescope
Ten years ago our team completed the Hubble Space Telescope Key Project on
the extragalactic distance scale. Cepheids were detected in some 25 galaxies
and used to calibrate four secondary distance indicators that reach out into
the expansion field beyond the noise of galaxy peculiar velocities. The result
was H_0 = 72 +/- 8 km/sec/Mpc and put an end to galaxy distances uncertain by a
factor of two. This work has been awarded the Gruber Prize in Cosmology for
2009.Comment: Gruber Prize Lecture to be published in Transactions of the IA
Malmquist Bias and the Distance to the Virgo Cluster
This paper investigates the impact of Malmquist bias on the distance to the
Virgo cluster determined by the H_0 Key Project using M100, and consequently on
the derived value of H_0. Malmquist bias is a volume-induced statistical effect
which causes the most probable distance to be different from the raw distance
measured. Consideration of the bias in the distance to the Virgo cluster raises
this distance and lowers the calculated value of H_0. Monte Carlo simulations
of the cluster have been run for several possible distributions of spirals
within the cluster and of clusters in the local universe. Simulations
consistent with known information regarding the cluster and the errors of
measurement result in a bias of about 6.5%-8.5%. This corresponds to an
unbiased distance of 17.2-17.4 Mpc and a value of H_0 in the range 80-82
km/s/Mpc.
The problem of determining the bias to Virgo illustrates several key points
regarding Malmquist bias. Essentially all conventional astronomical distance
measurements are subject to this bias. In addition, the bias accumulates when
an attempt is made to construct "distance ladders" from measurements which are
individually biased. As will be shown in the case of Virgo, the magnitude and
direction of the bias are sensitive to the spatial distribution of the parent
poputation from which the observed object is drawn - a distribution which is
often poorly known. This leads to uncertainty in the magnitude of the bias, and
adds to the importance of minimizing the number of steps in "distance ladders".Comment: 19 pages, 3 figures, Latex, To appear in Ap
High chemical abundances in stripped Virgo spiral galaxies
Based on a comparison of the oxygen abundances in H 2 regions in field and Virgo cluster late type spiral galaxies, Shields, Skillman, & Kennicutt (1991) suggested that the highly stripped spiral galaxies in the Virgo cluster have systematically higher abundances than comparable field galaxies. In April 1991 and May 1992 we used the blue channel spectrograph on the MMT to obtain new observations of 30 H 2 regions in Virgo spiral galaxies. These spectra cover the wavelength range from (O II) lambda 3727 to (S II) lambda 6731. We now have observed at least 4 H II regions in 9 spiral galaxies in the Virgo cluster. Combining (O II) and (O III) line strengths, we calculate the H II region oxygen abundances based on the empirical calibration of Edmunds & Pagel (1984). These observations show: (1) The stripped, low luminosity Virgo spirals (N4689, N4571) truly have abundances characteristic of much more luminous field spirals; (2) Virgo spirals which show no evidence of stripping (N4651, N4713) have abundances comparable to field galaxies; and (3) Evidence for transition galaxies (e.g., N4254, N4321), with marginally stripped disks and marginal abundance enhancements. The new observations presented here confirm the validity of the oxygen over-abundances in the stripped Virgo spirals. Shields et al. (1991) discussed two different mechanisms for producing the higher abundances in the disks of stripped galaxies in Virgo. The first is the supression of infall of near-primordial material, the second is the suppression of radial inflow of metal-poor gas. Distinguishing between the two cases will require more observations of the Virgo cluster spirals and a better understanding of which parameters determine the variation of abundance with radius in field spirals (cf., Garnett & Shields 1987)
Molecular hydrogen deficiency in HI-poor galaxies and its implications for star formation
We use a sample of 47 homogeneous and high sensitivity CO images taken from
the Nobeyama and BIMA surveys to demonstrate that, contrary to common belief, a
significant number (~40%) of HI-deficient nearby spiral galaxies are also
depleted in molecular hydrogen. While HI-deficiency by itself is not a
sufficient condition for molecular gas depletion, we find that H2 reduction is
associated with the removal of HI inside the galaxy optical disk. Those
HI-deficient galaxies with normal H2 content have lost HI mainly from outside
their optical disks, where the H2 content is low in all galaxies. This finding
is consistent with theoretical models in which the molecular fraction in a
galaxy is determined primarily by its gas column density. Our result is
supported by indirect evidence that molecular deficient galaxies form stars at
a lower rate or have dimmer far infrared fluxes than gas rich galaxies, as
expected if the star formation rate is determined by the molecular hydrogen
content. Our result is consistent with a scenario in which, when the atomic gas
column density is lowered inside the optical disk below the critical value
required to form molecular hydrogen and stars, spirals become quiescent and
passive evolving systems. We speculate that this process would act on the
time-scale set by the gas depletion rate and might be a first step for the
transition between the blue and red sequence observed in the color-magnitude
diagram.Comment: 12 pages, 9 figures, accepted for publication in Ap
PPAK Wide-field Integral Field Spectroscopy of NGC 628: II. Emission line abundance analysis
In this second paper of the series, we present the 2-dimensional (2D)
emission line abundance analysis of NGC 628, the largest object within the PPAK
Integral Field Spectroscopy (IFS) Nearby Galaxies Survey: PINGS. We introduce
the methodology applied to the 2D IFS data in order to extract and deal with
large spectral samples, from which a 2D abundance analysis can be later
performed. We obtain the most complete and reliable abundance gradient of the
galaxy up-to-date, by using the largest number of spectroscopic points sampled
in the galaxy, and by comparing the statistical significance of different
strong-line metallicity indicators. We find features not previously reported
for this galaxy that imply a multi-modality of the abundance gradient
consistent with a nearly flat-distribution in the innermost regions of the
galaxy, a steep negative gradient along the disc and a shallow gradient or
nearly-constant metallicity beyond the optical edge of the galaxy. The N/O
ratio seems to follow the same radial behaviour. We demonstrate that the
observed dispersion in metallicity shows no systematic dependence with the
spatial position, signal-to-noise or ionization conditions, implying that the
scatter in abundance for a given radius is reflecting a true spatial physical
variation of the oxygen content. Furthermore, by exploiting the 2D IFS data, we
were able to construct the 2D metallicity structure of the galaxy, detecting
regions of metal enhancement, and showing that they vary depending on the
choice of the metallicity estimator. The analysis of axisymmetric variations in
the disc of NGC 628 suggest that the physical conditions and the star formation
history of different-symmetric regions of the galaxy have evolved in a
different manner.Comment: Accepted for publication in MNRAS, 40 pages, 22 figures, online data:
http://www.ast.cam.ac.uk/ioa/research/ping
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