1,085 research outputs found
Low surface brightness galaxies and tidally triggered star formation
Counts of companions to low surface brightness (LSB) galaxies are presented and these are compared to counts of companions to normal galaxies obtained with the same techniques and criteria. Our results are consistent with LSB's having no clustered companions and support the hypothesis that LSB galaxies have low star-formation rates because they lack external tidal triggering
Are Some Milky Way Globular Clusters Hosted by Undiscovered Galaxies?
The confirmation of a globular cluster (GC) in the recently discovered
ultrafaint galaxy Eridanus II (Eri II) motivated us to examine the question
posed in the title. After estimating the halo mass of Eri II using a published
stellar mass - halo mass relation, the one GC in this galaxy supports extending
the relationship between the number of GCs hosted by a galaxy and the galaxy's
total mass about two orders of magnitude in stellar mass below the previous
limit. For this empirically determined specific frequency of between 0.06 and
0.39 globular clusters per 10 of total mass, the surviving Milky
Way (MW) subhalos with masses smaller than could host as many
as 5 to 31 GCs, broadly consistent with the actual population of outer halo MW
GCs, although matching the radial distribution in detail remains a challenge.
Using a subhalo mass function from published high resolution numerical
simulations and a Poissonian model for populating those halos with the
aforementioned empirically constrained frequency, we find that about 90 of
these GCs lie in lower-mass subhalos than that of Eri II. From what we know
about the stellar mass-halo mass function, the subhalo mass function, and the
mass-normalized GC specific frequency, we conclude that some of the MW's outer
halo GCs are likely to be hosted by undetected subhalos with extremely modest
stellar populations.Comment: 5 pages, 4 figures; ApJL in pres
A study of HII regions in spiral galaxies using multiobject spectroscopy
Multiobject spectroscopy results of HII regions in nearby late-type spiral galaxies are given. Results include excitation measurements, log ((O III)/H beta), for 81 regions in M 101, 30 regions in NGC 2403, and 13 regions in M 51. Researchers conclude that late-type spirals can be classified into two distinct populations, examine possible causes of this division, and derive metallicity gradients for these galaxies. M 51 appears to have an anomalously shallow abundance gradient
The Case of the Off-Center, Levitating Bar in the Large Magellanic Cloud
I explore the hypothesis that many of the unusual aspects of the apparent
stellar bar of the Large Magellanic Cloud are the result of viewing a triaxial
stellar bulge that is embedded in a highly obscuring thin disk. Specifically,
this hypothesis explains the observed off-center position of the bar within the
disk, the differing apparent distances of the bar and disk, the near alignment
of the bar's major axis position angle and the disk's line-of-nodes, and the
asymmetric appearance of the bar itself. Indirectly, it may also play a role in
explaining the microlensing rate toward the LMC and the recently observed large
velocity dispersion of RR Lyrae stars.Comment: 5 emulateapj style pages, accepted for publication in ApJ
Giant disk galaxies : Where environment trumps mass in galaxy evolution
We identify some of the most HI massive and fastest rotating disk galaxies in
the local universe with the aim of probing the processes that drive the
formation of these extreme disk galaxies. By combining data from the Cosmic
Flows project, which has consistently reanalyzed archival galaxy HI profiles,
and 3.6m photometry obtained with the Spitzer Space Telescope, with which
we can measure stellar mass, we use the baryonic Tully-Fisher (BTF)
relationship to explore whether these massive galaxies are distinct. We discuss
several results, but the most striking is the systematic offset of the
HI-massive sample above the BTF. These galaxies have both more gas and more
stars in their disks than the typical disk galaxy of similar rotational
velocity. The "condensed" baryon fraction, , the fraction of the baryons
in a dark matter halo that settle either as cold gas or stars into the disk, is
twice as high in the HI-massive sample than typical, and almost reaches the
universal baryon fraction in some cases, suggesting that the most extreme of
these galaxies have little in the way of a hot baryonic component or cold
baryons distributed well outside the disk. In contrast, the star formation
efficiency, measured as the ratio of the mass in stars to that in both stars
and gas, shows no difference between the HI-massive sample and the typical disk
galaxies. We conclude that the star formation efficiency is driven by an
internal, self-regulating process, while is affected by external factors.
We also found that the most massive HI detected galaxies are located
preferentially in filaments. We present the first evidence of an environmental
effect on galaxy evolution using a dynamical definition of a filament.Comment: 14 pages, in press MNRA
Deep imaging of Eridanus II and its lone star cluster
We present deep imaging of the most distant dwarf discovered by the Dark
Energy Survey, Eridanus II (Eri II). Our Magellan/Megacam stellar photometry
reaches mag deeper than previous work, and allows us to confirm the
presence of a stellar cluster whose position is consistent with Eri II's
center. This makes Eri II, at , the least luminous galaxy known to
host a (possibly central) cluster. The cluster is partially resolved, and at
it accounts for of Eri II's luminosity. We derive
updated structural parameters for Eri II, which has a half-light radius of
pc and is elongated (), at a measured
distance of kpc. The color-magnitude diagram displays a blue,
extended horizontal branch, as well as a less populated red horizontal branch.
A central concentration of stars brighter than the old main sequence turnoff
hints at a possible intermediate-age ( Gyr) population; alternatively,
these sources could be blue straggler stars. A deep Green Bank Telescope
observation of Eri II reveals no associated atomic gas.Comment: 7 pages, 4 figures; ApJL accepte
Distant Galaxy Clusters Identified From Optical Background Fluctuations
We present the first high redshift (0.3 < z < 1.1) galaxy clusters found by
systematically identifying optical low surface brightness fluctuations in the
background sky. Using spectra obtained with the Keck telescope and I-band
images from the Palomar 1.5m telescope, we conclude that at least eight of the
ten candidates examined are high redshift galaxy clusters. The identification
of such clusters from low surface brightness fluctuations provides a
complementary alternative to classic selection methods based on overdensities
of resolved galaxies, and enables us to search efficiently for rich high
redshift clusters over large areas of the sky. The detections described here
are the first in a survey that covers a total of nearly 140 sq. degrees of the
sky and should yield, if these preliminary results are representative, over 300
such clusters.Comment: Submitted to ApJ
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