819 research outputs found
The V_c-sigma_c relation in high and low surface brightness galaxies
We investigate the relation between the asymptotic circular velocity, V_c,
and the central stellar velocity dispersion, sigma_c, in galaxies. We consider
a new sample of high surface brightness spiral galaxies (HSB), low surface
brightness spiral galaxies (LSB), and elliptical galaxies with HI-based V_c
measurements. We find that: 1) elliptical galaxies with HI measurements fit
well within the relation; 2) a linear law can reproduce the data as well as a
power law (used in previous works) even for galaxies with sigma_c < 70 km/s; 3)
LSB galaxies, considered for the first time with this respect, seem to behave
differently, showing either larger V_c values or smaller sigma_c values.Comment: 2 pages, 2 figures, to appear in Proc. IAU Symp. 222, "The Interplay
among Black Holes, Stars and ISM in Galactic Nuclei" eds. Th. Storchi
Bergmann, L.C. Ho & H.R. Schmitt (Cambridge University Press
Star formation in the central regions of galaxies
Massive star formation in the central regions of spiral galaxies plays an
important role in the dynamical and secular evolution of their hosts. Here, we
summarise a number of recent investigations of the star formation history and
the physical conditions of the gas in circumnuclear regions, to illustrate not
only the detailed results one can achieve, but also the potential of using
state-of-the-art spectroscopic and analysis techniques in researching the
central regions of galaxies in general. We review how the star formation
history of nuclear rings confirms that they are long-lived and stable
configurations. Gas flows in from the disk, through the bar, and into the ring,
where successive episodes of massive star formation occur. Analysing the ring
in NGC 7742 in particular, we determine the physical conditions of the line
emitting gas using a combination of ionisation and stellar population
modelling, concluding that the origin of the nuclear ring in this non-barred
galaxy lies in a recent minor merger with a small gas-rich galaxy.Comment: Invited contribution, to appear in "Mapping the Galaxy and other
galaxies", Eds. K. Wada and F. Combes, Springer, in pres
Diffuse Gas in Retired Galaxies: Nebular Emission Templates and Constraints on the Sources of Ionization
We present emission line templates for passively evolving ("retired")
galaxies, useful for investigation of the evolution of the ISM in these
galaxies, and characterization of their high-temperature source populations.
The templates are based on high signal-to-noise () co-added spectra
(\AA) of gas-rich Sloan Digital Sky Survey galaxies
devoid of star-formation and active galactic nuclei. Stacked spectra are
provided for the entire sample and sub-samples binned by mean stellar age. In
Johansson~et al (2014), these spectra provided the first measurements of the He
II 4686\AA\ line in passively-evolving galaxies, and the observed He
II/H ratio constrained the contribution of accreting white dwarfs (the
"single-degenerate" scenario) to the type Ia supernova rate. In this paper, the
full range of unambiguously detected emission lines are presented. Comparison
of the observed [O I] 6300\AA/H ratio with photoionization models
further constrains any high-temperature single-degenerate scenario for type Ia
supernovae (with 1.5 T/ 10) to 3-6\%
of the observed rate in the youngest age bin (i.e. highest SN Ia rate). Hence,
for the same temperatures, in the presence of an ambient population of post-AGB
stars, we exclude additional high-temperature sources with a combined ionizing
luminosity of for stellar
populations with mean ages of 1 - 4 Gyrs. Furthermore, we investigate the
extinction affecting both the stellar and nebular continuum. The latter shows
about five times higher values. This contradicts isotropically distributed dust
and gas that renders similar extinction values for both cases.Comment: Accepted for publication in MNRAS, 16 pages, 12 figure
A Stellar Dynamical Mass Measurement of the Black Hole in NGC 3998 from Keck Adaptive Optics Observations
We present a new stellar dynamical mass measurement of the black hole in the
nearby, S0 galaxy NGC 3998. By combining laser guide star adaptive optics
observations obtained with the OH-Suppressing Infrared Imaging Spectrograph on
the Keck II telescope with long-slit spectroscopy from the Hubble Space
Telescope and the Keck I telescope, we map out the stellar kinematics on both
small spatial scales, well within the black hole sphere of influence, and on
large scales. We find that the galaxy is rapidly rotating and exhibits a sharp
central peak in the velocity dispersion. Using the kinematics and the stellar
luminosity density derived from imaging observations, we construct
three-integral, orbit-based, triaxial stellar dynamical models. We find the
black hole has a mass of M_BH = (8.1_{-1.9}^{+2.0}) x 10^8 M_sun, with an
I-band stellar mass-to-light ratio of M/L = 5.0_{-0.4}^{+0.3} M_sun/L_sun
(3-sigma uncertainties), and that the intrinsic shape of the galaxy is very
round, but oblate. With the work presented here, NGC 3998 is now one of a very
small number of galaxies for which both stellar and gas dynamical modeling have
been used to measure the mass of the black hole. The stellar dynamical mass is
nearly a factor of four larger than the previous gas dynamical black hole mass
measurement. Given that this cross-check has so far only been attempted on a
few galaxies with mixed results, carrying out similar studies in other objects
is essential for quantifying the magnitude and distribution of the cosmic
scatter in the black hole mass - host galaxy relations.Comment: 19 pages, 15 figures, accepted for publication in Ap
Upper limits on the mass of supermassive black holes from HST/STIS archival data
The growth of supermassive black holes (SBHs) appears to be closely linked
with the formation of spheroids. There is a pressing need to acquire better
statistics on SBH masses, since the existing samples are preferentially
weighted toward early-type galaxies with very massive SBHs. With this
motivation we started a project aimed at measuring upper limits on the mass of
the SBHs in the center of all the nearby galaxies (D<100 Mpc) for which
STIS/G750M spectra are available in the HST archive. These upper limits will be
derived by modeling the central emission-line widths observed in the Halpha
region over an aperture of ~0.1''. Here we present our results for a subsample
of 20 S0-Sb galaxies within 20 Mpc.Comment: 2 pages, 1 figure. To appear in the proceedings of "Black Holes: from
Stars to Galaxies", IAU Symp. No. 238, V. Karas & G. Matt (eds.), Cambridge
University Pres
The bulge-disk orthogonal decoupling in galaxies: NGC 4698 and NGC 4672
We report the case of the geometrical and kinematical decoupling between the
bulge and the disk of the Sa galaxy NGC 4698. The R-band isophotal map of this
spiral shows that the bulge structure is elongated perpendicularly to the major
axis of the disk. At the same time a central stellar velocity gradient is found
along the major axis of the bulge. We also present the Sa NGC 4672 as good
candidate of a spiral hosting a bulge and a disk orthogonally decoupled with
respect to one other. This decoupling of the two fundamental components of a
visible galaxy suggests that the disk could represent a second event in the
history of early-type spirals.Comment: 4 pages, 3 figures (LaTeX, cupconf.sty). To appear in "The Formation
of Bulges" C. M. Carollo, H. C. Ferguson, R. F. G. Wyse (eds.), Cambridge
University Pres
The pseudobulge of NGC 1292
The photometric and kinematic properties of Sb NGC 1292 suggest it hosts a
pseudobulge. The properties of the stellar population of such a pseudobulge are
consistent with a slow buildup within a scenario of secular evolution.Comment: 2 pages, 1 figure to appear in the proceedings of "Formation and
Evolution of Galaxy Disks", Rome, October 2007, Eds. J. Funes and E. M.
Corsin
Diffuse gas in galaxies sheds new light on the origin of Type Ia supernovae
Date of Acceptance: 05/05/2014We measure the strength of He II λ4686 nebular emission in passively evolving ('retired') galaxies, aiming to constrain their populations of hot accreting white dwarfs (WDs) in the context of the single-degenerate (SD) scenario of Type Ia supernovae (SNe Ia). In the SD scenario, as a WD burns hydrogen-rich material accreted from a companion star, it becomes a powerful source of ionizing ultraviolet emission. If significant populations of such sources exist in galaxies, strong emission in the recombination lines of He II should be expected from the interstellar medium. To explore this conjecture, we select from the Sloan Digital Sky Survey ~11 500 emission-line galaxies with stellar ages >1 Gyr showing no signs of active galactic nuclei activity and co-add their spectra in bins of stellar population age. For the first time, we detect He II λ4686 nebular emission in retired galaxies and find it to be significantly weaker than that expected in the SD scenario, especially in the youngest age bin (1-4 Gyr) where the SN Ia rate is the highest. Instead, the strength of the observed He II λ4686 nebular emission is consistent with post-asymptotic giant branch stars being the sole ionizing source in all age bins. These results limit populations of accretingWDs with photospheric temperatures (Teff) in the range ~(1.5-6) Ă 105 K to the level at which they can account for no more than ~5-10 per cent of the observed SN Ia rate. Conversely, should all WD progenitors of SN Ia go through the phase of steady nuclear burning with Teff ~ (1.5-6) Ă 105 K, they do not increase their mass by more than ~0.03Mâ in this regime.Peer reviewedFinal Accepted Versio
Polar bulges and polar nuclear discs: the case of NGC 4698
The early-type spiral NGC 4698 is known to host a nuclear disc of gas and
stars which is rotating perpendicularly with respect to the galaxy main disc.
In addition, the bulge and main disc are characterised by a remarkable
geometrical decoupling. Indeed they appear elongated orthogonally to each
other. In this work the complex structure of the galaxy is investigated by a
detailed photometric decomposition of optical and near-infrared images. The
intrinsic shape of the bulge was constrained from its apparent ellipticity, its
twist angle with respect to the major axis of the main disc, and the
inclination of the main disc. The bulge is actually elongated perpendicular to
the main disc and it is equally likely to be triaxial or axisymmetric. The
central surface brightness, scalelength, inclination, and position angle of the
nuclear disc were derived by assuming it is infinitesimally thin and
exponential. Its size, orientation, and location do not depend on the observed
passband. These findings support a scenario in which the nuclear disc is the
end result of the acquisition of external gas by the pre-existing triaxial
bulge on the principal plane perpendicular to its shortest axis and
perpendicular to the galaxy main disc. The subsequent star formation either
occurred homogeneously all over the extension of the nuclear disc or through an
inside-out process that ended more than 5 Gyr ago.Comment: 6 pages, 3 figures. Accepted for publication in MNRAS Letter
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