23 research outputs found
Globular Cluster Population of Hickson Compact Group 22a and 90c
We present the first measurement of the globular cluster populations of
galaxies in Hickson compact groups, in order to investigate the effect of these
high density environments on the formation and evolution of globular cluster
systems. Based on V and R band images that we obtained of HCG 22a and HCG 90c
with the ESO New Technology Telescope (NTT), we find a total globular cluster
population of for HCG 22a and for 90c. The specific
frequency for HCG 22a was found to be and for HCG 90c. A power-law fit to the globular cluster radial profile of HCG
22a yields and for HCG 90c we found . A comparison of the globular cluster radial profiles with
the surface brightness of the parent galaxy shows that the globular cluster
systems are at least as extended as the halo light. The measured values for the
specific frequency are consistent with a scenario in which the host galaxies
were in a low density ``field-like'' environment when they formed their
globular cluster systems.Comment: 28 pages, 13 figures, 3 tables, accepted for publication in the
Astrophysical Journa
The Luminosity of SN 1999by in NGC 2841 and the Nature of `Peculiar' Type Ia Supernovae
We present UBVRIJHK photometry and optical spectroscopy of the so-called
'peculiar' Type Ia supernova 1999by in NGC 2841. The observations began one
week before visual maximum light which is well-defined by daily observations.
The light curves and spectra are similar to those of the prototypical
subluminous event SN 1991bg. We find that maximum light in B occurred on 1999
May 10.3 UT (JD 2,451,308.8 +/- 0.3) with B=13.66 +/- 0.02 mag and a color of
B_max-V_max=0.51 +/- 0.03 mag. The late-time color implies minimal dust
extinction from the host galaxy. Our photometry, when combined with the recent
Cepheid distance to NGC 2841 (Macri et al. 2001), gives a peak absolute
magnitude of M_B=-17.15 +/- 0.23 mag, making SN 1999by one of the least
luminous Type Ia events ever observed. We estimate a decline rate parameter of
dm15(B)=1.90 mag, versus 1.93 for SN 1991bg, where 1.10 is typical for
so-called 'normal' events. We compare SN 1999by with other subluminous events
and find that the B_max-V_max color correlates strongly with the decline rate
and may be a more sensitive indicator of luminosity than the fading rate for
these objects. We find a good correlation between luminosity and the depth of
the spectral feature at 580 nm, which had been attributed solely to Si II. We
show that in cooler photospheres the 580 nm feature is dominated by Ti II,
which provides a simple physical explanation for the correlation. Using only
subluminous Type Ia supernovae we derive a Hubble parameter of H_0=75 +12 -11
km/s Mpc, consistent with values found from brighter events.Comment: 36 preprint pages including 18 figures. Near-IR photometry of the SN
has been added to the paper. Scheduled to appear in ApJ vol. 613 (September
2004). High-resolution version available from
http://www.nd.edu/~pgarnavi/sn99by/sn99by.p
Far Ultraviolet and H-alpha Imaging of Nearby Spirals: The OB Stellar Population in the Diffuse Ionized Gas
(Abridged) We have compared H-alpha and far ultraviolet (FUV) images of 10
nearby spirals, with the goal of understanding the contribution of field OB
stars to the ionization of the diffuse ionized gas (DIG) in spiral galaxies.
The FUV images were obtained by the Ultraviolet Imaging Telescope (UIT) and the
H-alpha images were obtained using various ground-based telescopes. In all of
the galaxies, the F_H-alpha/F_UIT flux ratio is lower in the DIG than in the
HII regions. This is likely an indication that the mean spectral type for OB
stars in the field is later than that in HII regions. Comparison of the
N_Lyc/L_UIT ratio with models of evolving stellar populations shows that the
stellar population in the DIG is consistent with either an older single burst
population or a steady state model with constant star formation and an initial
mass function (IMF) slope steeper than alpha=2.35. We compared the
F_H\alpha/F_UIT ratio in the DIG of these galaxies with that in M33. If the
mean spectral types of stars in HII regions and in the DIG are the same as in
M33, and the difference in extinction between DIG and HII regions is constant
among galaxies, then the analysis suggests that field stars are important
sources of ionization in most galaxies, and may be the dominant source in some
galaxies.Comment: 29 pages, 8 figures, 6 tables. Accepted for publication in Ap