340 research outputs found
The Circumnuclear Ring of Ionized Gas in NGC3593
We present the results of narrow-band Halpha+NII imaging of the early-type
spiral NGC3593 in combination with a study of the flux radial profiles of the
NII (lambda: 654.80, 658.34 nm), Halpha, and SII (lambda: 671.65, 673.08 nm)
emission lines along its major axis. The galaxy is known to contain two
counterrotating stellar discs of different size and luminosity. We find that
the Halpha emission mainly derives from a small central region of 57 arcsec x
25 arcsec. It consists of a filamentary pattern with a central ring. This has a
diameter of about 17 arcsec (~ 0.6/h kpc) and it contributes about half of the
total Halpha flux. The ring is interpreted as the result of the interaction
between the acquired retrograde gas which later formed the smaller
counterrotating stellar disc and the pre-existing prograde gas of the galaxy.Comment: Accepted for pubblication in Astronomy and Astrophysics; one latex
file (corsini.tex), and 2 encapsulated postscript figures
(corsini_fig1.ps,corsini_fig2.ps). To be compiled with aa.cls latex2e macro
style (pslatex option): 6 pages after latex compilatio
Mixed Early and Late-Type Properties in the Bar of NGC 6221: Evidence for Evolution along the Hubble Sequence?
Rotation curves and velocity dispersion profiles are presented for both the
stellar and gaseous components along five different position angles (P.A.=5,
50, 95, 125 and 155 degrees) of the nearby barred spiral NGC 6221. The observed
kinematics extends out to about 80" from the nucleus. Narrow and broad-band
imaging is also presented. The radial profiles of the fluxes ratio [NII]/Halpha
reveal the presence of a ring-like structure of ionized gas, with a radius of
about 9" and a deprojected circular velocity of about 280 km/s. The analysis of
the dynamics of the bar indicates this ring is related to the presence of an
inner Lindblad resonance (ILR) at 1.3 kpc. NGC6221 is found to exhibit
intermediate properties between those of the early-type barred galaxies: the
presence of a gaseous ring at an ILR, the bar edge located between the ILR's
and the corotation radius beyond the steep rising portion of the rotation
curve, the dust-lane pattern, and those of the late-type galaxies: an almost
exponential surface brightness profile, the presence of Halpha regions along
all the bar, the spiral-arm pattern. It is consistent with scenarios of
bar-induced evolution from later to earlier-type galaxies.Comment: 1 File ds7406.tar.gz which contains: one latex file (ds7406.tex), and
10 encsulated postscript figures (ds7406f**.eps). To be compiled with aa-l
latex2e macro style. To be published in A&A Sup. Serie
The Star Formation Demographics of Galaxies in the Local Volume
We examine the connections between the current global star formation
activity, luminosity, dynamical mass and morphology of galaxies in the Local
Volume, using H-alpha data from the 11 Mpc H-alpha and Ultraviolet Galaxy
Survey (11HUGS). Taking the equivalent width (EW) of the H-alpha emission line
as a tracer of the specific star formation rate, we analyze the distribution of
galaxies in the M_B-EW and rotational velocity (V_{max})-EW planes.
Star-forming galaxies show two characteristic transitions in these planes. A
narrowing of the galaxy locus occurs at M_B~-15 and V_{max}~50 km/s, where the
scatter in the logarithmic EWs drops by a factor of two as the
luminosities/masses increase, and galaxy morphologies shift from predominately
irregular to late-type spiral. Another transition occurs at M_B~-19 and
V_{max}~120 km/s, above which the sequence turns off toward lower EWs and
becomes mostly populated by intermediate and early-type bulge-prominent
spirals. Between these two transitions, the mean logarithmic EW appears to
remain constant at 30 A. We comment on how these features reflect established
empirical relationships, and provide clues for identifying the large-scale
physical processes that both drive and regulate star formation, with emphasis
on the low-mass galaxies that dominate our approximately volume-limited sample.Comment: 5 pages, accepted for publication in ApJL pending editing for lengt
Modeling gas and stellar kinematics in disc galaxies
We present V-band surface photometry and major-axis kinematics of stars and
ionized gas of three early-type spiral galaxies, namely NGC 772, NGC 3898 and
NGC 7782. For each galaxy we built a self-consistent Jeans model for the
stellar kinematics, adopting the light distribution of bulge and disc derived
by means of a two-dimensional parametric photometric decomposition. This
allowed us to investigate the presence of non-circular gas motions, and derive
the mass distribution of luminous and dark matter in these objects. We found
that the observed gas rotation corresponds to the circular velocity except for
the innermost region (|r|<8") of NGC 3898. This behaviour is quite common,
although not ubiquitous, in the few bulge-dominated galaxies, for which
dynamical modeling allows the comparison between the gas velocity and the
circular speed.Comment: 1 single page, 1 encapsulated figure. Poster contribution at the
Euroconference "The Evolution of Galaxies. I- Observational clues.", held in
Granada (Spain), May 23-27 2000. To be published as a special issue of
Astrophysics and Space Scienc
Star Formation in Satellite Galaxies
We present narrow-band observations of the H emission in a sample of
31 satellite orbiting isolated giant spiral galaxies. The sample studied spans
the range mag. The H emission was detected in all the
spiral and irregular objects with fluxes in the range erg cm s. The average and maximum values for the
current star formation rates are 0.68 and 3.66 M_\sun yr respectively.
Maps of the spatial distribution of ionized gas are presented. The star-forming
regions show a rich structure in which frequently discrete complexes are
imposed over more diffuse structures. In general, the current star formation
rates are smaller that the mean values in the past obtained from the current
stellar content; this probably indicates a declining rhythm with time in the
generation of new stars. However, the reserve of gas is enough to continue
fueling the current levels of star formation activity for at least another
Hubble time. Four of the objects (NGC 2718b, NGC 4541e, NGC 5965a and NGC
5965a) with higher current star formation rates show clear signs of
interaction with close companions of comparable brightness at projected
distances of 25, 20 and 2 kpc respectively. The only two galaxies in our sample
that do not show star formation activity are members of these interacting
systems, and it is unclear if this is a consequence of intrinsic properties
(both are Hubble early types) or if it is related with possible disruption of
the external parts due to the interaction. In the case of the pair NGC 2718a-b
there are indications of gas transport between both galaxies.Comment: 25 pages, 11 figures. Accepted for publication in A
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