312 research outputs found
Internal Kinematics of Distant Field Galaxies: I. Emission Line Widths for a Complete Sample of Faint Blue Galaxies at <z>=0.25
We present measurements of the OII(3727) emission line width for a complete
sample of 24 blue field galaxies (21.25=0.25, obtained
with the AUTOFIB fibre spectrograph on the Anglo-Australian Telescope. Most
emission lines are spectrally resolved, yet all have dispersions sigma<100km/s.
Five of the 24 sample members have OII doublet line flux ratios which imply gas
densities in excess of 100 cm^-3. The line emission in these galaxies may be
dominated by an active nucleus and the galaxies have been eliminated from the
subsequent analysis. The remaining 19 linewidths are too large by a factor of
two (7sigma significance) to be attributed to turbulent motions within an
individual star forming region, and therefore most likely reflect the orbital
motion of ionized gas in the galaxy. We use Fabry--Perot observations of nearby
galaxies to construct simulated datasets that mimic our observational setup at
z=0.25; these allow us to compute the expected distribution of (observable)
linewidths sigma_v for a galaxy of a given ``true'' (optical) rotation speed
v_c. These simulations include the effects of random viewing angles, clumpy
line emission, finite fibre aperture, and internal dust extinction on the
emission line profile. We assume a linewidth--luminosity--colour relation: ln[
v_c(M_B,B-R) ] = ln[v_c(-19,1)] - eta*(M_B+10) + zeta*[(B-R)-1] and determine
the range of parameters consistent with our data. We find a mean rotation speed
of v_c(-19,1)=66+-8km/s (68% confidence limits) for the distant galaxies with
M_B=-19 and B-R=1, with a magnitude dependence for v_c of eta=0.07+-0.08, and a
colour dependence of zeta =0.28+-0.25. Through comparison with several local
samples we show that this value of v_c(-19,1) is significantly lower than the
optical rotation speed of present-day galaxies with the same absolute magnitudeComment: TeX Text and Tables, no Figures. Compressed and uuencoded PS file of
the complete paper (43 pages including 9 figures) available at
http://zwicky.as.arizona.edu/~rix/; submitted to MNRA
Intergalactic HI in the NGC5018 group
The cold interstellar and intergalactic medium is in the small group of galaxies whose brightest member is the elliptical galaxy NGC5018. Researchers' attention was first drawn to this galaxy as possibly containing cold interstellar gas by the detection by the Infrared Astronomy Satellite (IRAS) of emission at lambda 60 microns and lambda 100 microns at an intensity of about 1 Jy (Knapp et al. 1989), which is relatively strong for an elliptical (Jura et al. 1987). These data showed that the temperature of the infrared emission is less than 30K and that its likely source is therefore interstellar dust. A preliminary search for neutral hydrogen (HI) emission from this galaxy using the Very Large Array (VLA) showed that there appears to be HI flowing between NGC5018 and the nearby Sc galaxy NGC5022 (Kim et al. 1988). Since NGC5018 has a well-developed system of optical shells (cf. Malin and Carter 1983; Schweizer 1987) this observation suggests that NGC5018 may be in the process of forming its shell system by the merger of a cold stellar system with the elliptical, as suggested by Quinn (1984). Researchers describe follow-up HI observations of improved sensitivity and spatial resolution, and confirm that HI is flowing between NCG5022 and NGC5018, and around NGC5018. The data show, however, that the HI bridge actually connects NGC5022 and another spiral in the group, MCG03-34-013, both spatially and in radial velocity, and that in doing so it flows through and around NGC5018, which lies between the spiral galaxies. This is shown by the total HI map, with the optical positions of the above three galaxies labelled
Resolving the Controversy Over the Core Radius of 47 Tucanae (NGC 104)
This paper investigates the discrepancy between recent measurements of the
density profile of the globular cluster 47 Tuc that have used HST data sets.
Guhathakurta et al. (1992) used pre-refurbishment WFPC1 V-band images to derive
r_c = 23" +/- 2". Calzetti et al. (1993) suggested that the density profile is
a superposition of two King profiles (r_c = 8" and r_c = 25") based on U-band
FOC images. De Marchi et al. (1996) used deep WFPC1 U-band images to derive r_c
= 12" +/- 2". Differences in the adopted cluster centers are not the cause of
the discrepancy. Our independent analysis of the data used by De Marchi et al.
reaches the following conclusions: (1) De Marchi et al.'s r_c ~ 12" value is
spuriously low, a result of radially-varying bias in the star counts in a
magnitude limited sample -- photometric errors and a steeply rising stellar
luminosity function cause more stars to scatter across the limiting magnitude
into the sample than out of it, especially near the cluster center where
crowding effects are most severe. (2) Changing the limiting magnitude to the
main sequence turnoff, away from the steep part of the luminosity function,
partially alleviates the problem and results in r_c = 18". (3) Combining such a
limiting magnitude with accurate photometry derived from PSF fitting, instead
of the less accurate aperture photometry employed by De Marchi et al., results
in a reliable measurement of the density profile which is well fit by r_c = 22"
+/- 2". Archival WFPC2 data are used to derive a star list with a higher degree
of completeness, greater photometric accuracy, and wider areal coverage than
the WFPC1 and FOC data sets; the WFPC2-based density profile supports the above
conclusions, yielding r_c = 24" +/- 1.9".Comment: 22 pages, 5 figures, 1 table; accepted for publication in PASP; see
http://www.ucolick.org/~raja/hgg.tar.gz for full-resolution figure
An Extremely Lithium-Rich Bright Red Giant in the Globular Cluster M3
We have serendipitously discovered an extremely lithium-rich star on the red
giant branch of the globular cluster M3 (NGC 5272). An echelle spectrum
obtained with the Keck I HIRES reveals a Li I 6707 Angstrom resonance doublet
of 520 milli-Angstrom equivalent width, and our analysis places the star among
the most Li-rich giants known: log[epsilon(Li)] ~= +3.0. We determine the
elemental abundances of this star, IV-101, and three other cluster members of
similar luminosity and color, and conclude that IV-101 has abundance ratios
typical of giants in M3 and M13 that have undergone significant mixing. We
discuss mechanisms by which a low-mass star may be so enriched in Li, focusing
on the mixing of material processed by the hydrogen-burning shell just below
the convective envelope. While such enrichment could conceivably only happen
rarely, it may in fact regularly occur during giant-branch evolution but be
rarely detected because of rapid subsequent Li depletion.Comment: 7-page LaTeX file, including 2 encapsulated ps figures + 1 table;
accepted for publication in the Astrophysical Journal Letter
Internal Dynamics, Structure and Formation of Dwarf Elliptical Galaxies: II. Rotating Versus Non-Rotating Dwarfs
We present spatially-resolved internal kinematics and stellar chemical
abundances for a sample of dwarf elliptical (dE) galaxies in the Virgo Cluster
observed with Keck/ESI. We find that 4 out of 17 dEs have major axis rotation
velocities consistent with rotational flattening, while the remaining dEs have
no detectable major axis rotation. Despite this difference in internal
kinematics, rotating and non-rotating dEs are remarkably similar in terms of
their position in the Fundamental Plane, morphological structure, stellar
populations, and local environment. We present evidence for faint underlying
disks and/or weak substructure in a fraction of both rotating and non-rotating
dEs, but a comparable number of counter-examples exist for both types which
show no evidence of such structure. Absorption-line strengths were determined
based on the Lick/IDS system (Hbeta, Mgb, Fe5270, Fe5335) for the central
region of each galaxy. We find no difference in the line-strength indices, and
hence stellar populations, between rotating and non-rotating dE galaxies. The
best-fitting mean age and metallicity for our 17 dE sample are 5 Gyr and Fe/H =
-0.3 dex, respectively, with rms spreads of 3 Gyr and 0.1 dex. The majority of
dEs are consistent with solar alpha/Fe abundance ratios. By contrast, the
stellar populations of classical elliptical galaxies are, on average, older,
more metal rich, and alpha-enhanced relative to our dE sample. The local
environments of both dEs types appear to be diverse in terms of their proximity
to larger galaxies in real or velocity space within the Virgo Cluster. Thus,
rotating and non-rotating dEs are remarkably similar in terms of their
structure, stellar content, and local environments, presenting a significant
challenge to theoretical models of their formation. (abridged)Comment: 33 pages, 12 figures. To appear in the October 2003 Astronomical
Journal. See http://www.ucolick.org/~mgeha/geha_dE.ps.gz for version with
high resolution figure
NGC 770: A Counter-Rotating Core in a Low-Luminosity Elliptical Galaxy
We present evidence for a counter-rotating core in the low-luminosity (M_B =
-18.2) elliptical galaxy NGC 770 based on internal stellar kinematic data. This
counter-rotating core is unusual as NGC 770 is not the primary galaxy in the
region and it lies in an environment with evidence of on-going tidal
interactions. We discovered the counter-rotating core via single-slit Keck/ESI
echelle spectroscopy; subsequent integral field spectroscopy was obtained with
the Gemini/GMOS IFU. The counter-rotating region has a peak rotation velocity
of 21 km/s as compared to the main galaxy's rotation speed of greater than 45
km/s in the opposite direction. The counter-rotating region extends to a radius
of 4'' (0.6 kpc), slightly smaller than the half-light radius of the galaxy
which is 5.3'' (0.8 kpc) and is confined to a disk whose scale height is less
than 0.8'' (0.1 kpc). We compute an age and metallicity of the inner
counter-rotating region of 3 +/- 0.5 Gyr and [Fe/H] = 0.2 +/- 0.2 dex, based on
Lick absorption-line indices. The lack of other large galaxies in this region
limits possible scenarios for the formation of the counter-rotating core. We
discuss several scenarios and favor one in which NGC 770 accreted a small
gas-rich dwarf galaxy during a very minor merging event. If this scenario is
correct, it represents one of the few known examples of merging between two
dwarf-sized galaxies.Comment: 26 pages, 9 figures. Accepted to AJ. See this
http://www.ociw.edu/~mgeha/geha.ps.gz for version with high resolution
figure
Tracing the Dynamical History of the Globular Cluster 47 Tucanae
We use two stellar populations in the globular cluster 47 Tucanae to trace
its dynamical history: blue stragglers and low mass main sequence stars. We
assumed that the blue stragglers were formed through stellar collisions in all
regions of the cluster. We find that in the core of the cluster, models of
collisional blue stragglers agree well with the observations as long as blue
stragglers are still continuing to form and the mass function in the cluster is
extremely biased towards massive stars (x=-8 where a Salpeter mass function has
x=+1.35). We show that such an extreme mass function is supported by direct
measurements of the luminosity function of main sequence stars in the centre of
the cluster. In the middle region of our dataset (25'' to 130'' from the
cluster centre), blue straggler formation seems to have stopped about half a
Gyr ago. In the outskirts of the cluster, our models are least successful at
reproducing the blue straggler data. Taken at face value, they indicate that
blue straggler formation has been insignificant over the past billion years,
and that a Salpeter mass function applies. However, it is more likely that the
dominant formation mechanism in this part of the cluster is not the collisional
one, and that our models are not appropriate for this region of the cluster. We
conclude that blue stragglers can be used as tracers of dynamics in globular
clusters, despite our incomplete understanding of how and where they were
formed.Comment: Accepted by Ap
Stellar Kinematics and Structural Properties of Virgo Cluster Dwarf Early-Type Galaxies from the SMAKCED Project. I. Kinematically Decoupled Cores and Implications for Infallen Groups in Clusters
We present evidence for kinematically decoupled cores (KDCs) in two dwarf
early-type (dE) galaxies in the Virgo cluster, VCC 1183 and VCC 1453, studied
as part of the SMAKCED stellar absorption-line spectroscopy and imaging survey.
These KDCs have radii of 1.8'' (0.14 kpc) and 4.2'' (0.33 kpc), respectively.
Each of these KDCs is distinct from the main body of its host galaxy in two
ways: (1) inverted sense of rotation; and (2) younger (and possibly more
metal-rich) stellar population. The observed stellar population differences are
probably associated with the KDC, although we cannot rule out the possibility
of intrinsic radial gradients in the host galaxy. We describe a statistical
analysis method to detect, quantify the significance of, and characterize KDCs
in long-slit rotation curve data. We apply this method to the two dE galaxies
presented in this paper and to five other dEs for which KDCs have been reported
in the literature. Among these seven dEs, there are four significant KDC
detections, two marginal KDC detections, and one dE with an unusual central
kinematic anomaly that may be an asymmetric KDC.The frequency of occurence of
KDCs and their properties provide important constraints on the formation
history of their host galaxies. We discuss different formation scenarios for
these KDCs in cluster environments and find that dwarf-dwarf wet mergers or gas
accretion can explain the properties of these KDCs. Both of these mechanisms
require that the progenitor had a close companion with a low relative velocity.
This suggests that KDCs were formed in galaxy pairs residing in a poor group
environment or in isolation whose subsequent infall into the cluster quenched
star formation.Comment: 14 pages, accepted for publication in Ap
Stellar Stream and Halo Structure in the Andromeda Galaxy From a Subaru/Hyper Suprime-Cam Survey
We present wide and deep photometry of the northwest part of the halo of the
Andromeda galaxy (M31) using Hyper Suprime-Cam on the Subaru Telescope. The
survey covers 9.2 deg field in the , , and bands and shows
a clear red giant branch (RGB) of M31's halo stars and a pronounced red clump
(RC) feature. The spatial distribution of RC stars shows a prominent stream
feature, the North Western (NW) Stream, and a diffuse substructure in the south
part of our survey field. We estimate the distances based on the RC method and
obtain = 24.63(random)(systematic) and 24.29(random)(systematic) mag for the NW stream and diffuse
substructure, respectively, implying that the NW Stream is located behind M31,
whereas the diffuse substructure is located in front. We also estimate
line-of-sight distances along the NW Stream and find that the south part of the
stream is 20 kpc closer to us relative to the north part. The distance to
the NW Stream inferred from the isochrone fitting to the color-magnitude
diagram favors the RC-based distance, but the TRGB-based distance estimated for
-selected RGB stars does not agree with it. The surface number density
distribution of RC stars across the NW Stream is found to be approximately
Gaussian with a FWHM of 25 arcmin (5.7 kpc), with a slight skew to the
south-west side. That along the NW Stream shows a complicated structure
including variations in number density and a significant gap in the stream.Comment: 24 pages, 18 figures, accepted for publication in Ap
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