1,070 research outputs found
Metallicity inhomogeneities in local star-forming galaxies as sign of recent metal-poor gas accretion
We measure the oxygen metallicity of the ionized gas along the major axis of
seven dwarf star-forming galaxies. Two of them, SDSSJ1647+21 and SDSSJ2238+14,
show 0.5 dex metallicity decrements in inner regions with enhanced
star-formation activity. This behavior is similar to the metallicity drop
observed in a number of local tadpole galaxies by Sanchez Almeida et al. (2013)
and interpreted as showing early stages of assembling in disk galaxies, with
the star formation sustained by external metal-poor gas accretion. The
agreement with tadpoles has several implications: (1) it proves that galaxies
other than the local tadpoles present the same unusual metallicity pattern. (2)
Our metallicity inhomogeneities were inferred using the direct method, thus
discarding systematic errors usually attributed to other methods. (3) Taken
together with the tadpole data, our findings suggest a threshold around one
tenth the solar value for the metallicity drops to show up. Although galaxies
with clear metallicity drops are rare, the physical mechanism responsible for
them may sustain a significant part of the star-formation activity in the local
Universe. We argue that the star-formation dependence of the mass-metallicity
relationship, as well as other general properties followed by most local disk
galaxies, are naturally interpreted as side effects of pristine gas infall.
Alternatives to the metal poor gas accretion are examined too.Comment: Accepted for publication in ApJ. 10 pages. 5 Fig
A Constant Bar Fraction out to Redshift z~1 in the Advanced Camera for Surveys Field of the Tadpole Galaxy
Bar-like structures were investigated in a sample of 186 disk galaxies larger
than 0.5 arcsec that are in the I-band image of the Tadpole galaxy taken with
the HST ACS. We found 22 clear cases of barred galaxies, 21 galaxies with small
bars that appear primarily as isophotal twists in a contour plot, and 11 cases
of peculiar bars in clump-cluster galaxies, which are face-on versions of chain
galaxies. The latter bars are probably young, as the galaxies contain only weak
interclump emission. Four of the clearly barred galaxies at z~0.8-1.2 have
grand design spirals. The bar fraction was determined as a function of galaxy
inclination and compared with the analogous distribution in the local Universe.
The bar fraction was also determined as a function of galaxy angular size.
These distributions suggest that inclination and resolution effects obscure
nearly half of the bars in our sample. The bar fraction was also determined as
a function of redshift. We found a nearly constant bar fraction of 0.23+-0.03
from z~0 to z=1.1. When corrected for inclination and size effects, this
fraction is comparable to the bar fraction in the local Universe, ~0.4, as
tabulated for all bar and Hubble types in the Third Reference Catalogue of
Galaxies. The average major axis of a barred galaxy in our sample is ~10 kpc
after correcting for redshift with a LambdaCDM cosmology. Galaxy bars were
present in normal abundance at least ~8 Gy ago (z~1); bar dissolution cannot be
common during a Hubble time unless the bar formation rate is comparable to the
dissolution rate.Comment: to appear in ApJ, Sept 1, 2004, Vol 612, 18 pg, 12 figure
Hierarchical Structure Formation and Modes of Star Formation in Hickson Compact Group 31
The handful of low-mass, late-type galaxies that comprise Hickson Compact
Group 31 is in the midst of complex, ongoing gravitational interactions,
evocative of the process of hierarchical structure formation at higher
redshifts. With sensitive, multicolor Hubble Space Telescope imaging, we
characterize the large population of <10 Myr old star clusters that suffuse the
system. From the colors and luminosities of the young star clusters, we find
that the galaxies in HCG 31 follow the same universal scaling relations as
actively star-forming galaxies in the local Universe despite the unusual
compact group environment. Furthermore, the specific frequency of the globular
cluster system is consistent with the low end of galaxies of comparable masses
locally. This, combined with the large mass of neutral hydrogen and tight
constraints on the amount of intragroup light, indicate that the group is
undergoing its first epoch of interaction-induced star formation. In both the
main galaxies and the tidal-dwarf candidate, F, stellar complexes, which are
sensitive to the magnitude of disk turbulence, have both sizes and masses more
characteristic of z=1-2 galaxies. After subtracting the light from compact
sources, we find no evidence for an underlying old stellar population in F --
it appears to be a truly new structure. The low velocity dispersion of the
system components, available reservoir of HI, and current star formation rate
of ~10 solar masses per year, indicate that HCG31 is likely to both exhaust its
cold gas supply and merge within ~1 Gyr. We conclude that the end product will
be an isolated, X-ray-faint, low-mass elliptical.Comment: 24 pages, 14 figures (including low resolution versions of color
images), latex file prepared with emulateapj. Accepted for publication by the
Astronomical Journa
A Turbulent Origin for Flocculent Spiral Structure in Galaxies: II. Observations and Models of M33
Fourier transform power spectra of azimuthal scans of the optical structure
of M33 are evaluated for B, V, and R passbands and fit to fractal models of
continuum emission with superposed star formation. Power spectra are also
determined for Halpha. The best models have intrinsic power spectra with 1D
slopes of around -0.7pm0.7, significantly shallower than the Kolmogorov
spectrum (slope =-1.7) but steeper than pure noise (slope=0). A fit to the
power spectrum of the flocculent galaxy NGC 5055 gives a steeper slope of
around -1.5pm0.2, which could be from turbulence. Both cases model the optical
light as a superposition of continuous and point-like stellar sources that
follow an underlying fractal pattern. Foreground bright stars are clipped in
the images, but they are so prominent in M33 that even their residual affects
the power spectrum, making it shallower than what is intrinsic to the galaxy. A
model consisting of random foreground stars added to the best model of NGC 5055
fits the observed power spectrum of M33 as well as the shallower intrinsic
power spectrum that was made without foreground stars. Thus the optical
structure in M33 could result from turbulence too.Comment: accepted by ApJ, 13 pages, 10 figure
Constraints on the assembly and dynamics of galaxies. II. Properties of kiloparsec-scale clumps in rest-frame optical emission of z ~ 2 star-forming galaxies
We study the properties of luminous stellar clumps identified in deep, high
resolution HST/NIC2 F160W imaging at 1.6um of six z~2 star-forming galaxies
with existing near-IR integral field spectroscopy from SINFONI at the VLT.
Individual clumps contribute ~0.5%-15% of the galaxy-integrated rest-frame
~5000A emission, with median of about 2%; the total contribution of clump light
ranges from 10%-25%. The median intrinsic clump size and stellar mass are ~1kpc
and log(Mstar[Msun])~9, in the ranges for clumps identified in rest-UV or line
emission in other studies. The clump sizes and masses in the subset of disks
are broadly consistent with expectations for clump formation via gravitational
instabilities in gas-rich, turbulent disks given the host galaxies' global
properties. By combining the NIC2 data with ACS/F814W imaging available for one
source, and AO-assisted SINFONI Halpha data for another, we infer modest color,
M/L, and stellar age variations within each galaxy. In these two objects, sets
of clumps identified at different wavelengths do not fully overlap;
NIC2-identified clumps tend to be redder/older than ACS- or Halpha-identified
clumps without rest-frame optical counterparts. There is evidence for a
systematic trend of older ages at smaller galactocentric radii among the
clumps, consistent with scenarios where inward migration of clumps transports
material towards the central regions. From constraints on a bulge-like
component at radii <1-3kpc, none of the five disks in our sample appears to
contain a compact massive stellar core, and we do not discern a trend of bulge
stellar mass fraction with stellar age of the galaxy. Further observations are
necessary to probe the build-up of stellar bulges and the role of clumps in
this process.Comment: 29 pages, 11 figures. Revised version accepted for publication in the
Astrophysical Journa
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