388 research outputs found
Near-infrared bulge-disc correlations of lenticular galaxies
We consider the luminosity and environmental dependence of structural
parameters of lenticular galaxies in the near-infrared K band. Using a
two-dimensional galaxy image decomposition technique, we extract bulge and disk
structural parameters for a sample of 36 lenticular galaxies observed by us in
the K band. By combining data from the literature for field and cluster
lenticulars with our data, we study correlations between parameters that
characterise the bulge and the disk as a function of luminosity and
environment. We find that scaling relations such as the Kormendy relation,
photometric plane and other correlations involving bulge and disk parameters
show a luminosity dependence. This dependence can be explained in terms of
galaxy formation models in which faint lenticulars (M_T > -24.5) formed via
secular formation processes that likely formed the pseudobulges of late-type
disk galaxies, while brighter lenticulars (M_T < -24.5) formed through a
different formation mechanism most likely involving major mergers. On probing
variations in lenticular properties as a function of environment, we find that
faint cluster lenticulars show systematic differences with respect to faint
field lenticulars. These differences support the idea that the bulge and disk
components fade after the galaxy falls into a cluster, while simultaneously
undergoing a transformation from spiral to lenticular morphologies.Comment: Published in MNRAS; added correct Bibliographic reference; 10 pages,
2 tables and 11 figure
Bar fraction in lenticular galaxies: dependence on luminosity and environment
We present a study of bars in lenticular galaxies based on a sample of 371
galaxies from the SDSS-DR 7 and 2MASS in optical and near-infrared bands,
respectively. We found a bar in 15% of the lenticular galaxies in our sample,
which is consistent with recent studies. The barred galaxy fraction shows a
luminosity dependence, with faint lenticular galaxies (MK > -24.5, total
absolute magnitude in K band) having a larger fraction of bars than bright
lenticular galaxies (MK < -24.5). A similar trend is seen when Mr = -21.5, the
total absolute magnitude in SDSS r band is used to divide the sample into faint
and bright lenticular galaxies. We find that faint galaxies in clusters show a
higher bar fraction than their counterparts in the field. This suggests that
the formation of bars in lenticular galaxies not only depends on the total
luminosity of galaxy but also on the environment of the host galaxy.Comment: Accepted for publication in MNRAS: Letters, 5 pages, 4 figure
Tracing the vertical composition of disc galaxies through colour gradients
(Abbreviated) Optical observations of a statistically complete sample of
edge-on disc galaxies are used to study the intrinsic vertical colour gradients
in the galactic discs, to constrain the effects of population gradients,
residual dust extinction and gradients in the galaxies' metal abundance. It
appears that the intrinsic vertical colour gradients are either non-existent,
or small and relatively constant as a function of position along the galaxies'
major axes. Our results are consistent with the absence of any vertical colour
gradient in the discs of the early-type sample galaxies. In most galaxies
small-scale variations in the magnitude and even the direction of the vertical
gradient are observed: at larger galactocentric distances they generally
display redder colours with increasing z height, whereas the opposite is often
observed in and near the galactic centres. For a significant fraction of our
sample galaxies another mechanism in addition to the effects of stellar
population gradients is required to explain the magnitude of the observed
gradients. The non-zero colour gradients in a significant fraction of our
sample galaxies are likely (at least) partially due to residual dust extinction
at these z heights, as is also evidenced from the sometimes significant
differences between the vertical colour gradients measured on either side of
the galactic planes. We suggest that initial vertical metallicity gradients, if
any, have likely not been accentuated by accretion or merging events over the
lifetimes of our sample galaxies. On the other hand, they may have weakened any
existing vertical metallicity gradients, although they also may have left the
existing correlations unchanged.Comment: 17 pages LaTeX, incl. 5 embedded postscript figures, resubmitted to
MNRAS (referee's comments taken into account
2MASS photometry of edge-on spiral galaxies. I. Sample and general results
A sample of edge-on spiral galaxies aimed at a study of the main structural
and photometric parameters of edge-on galaxies both of early and late types is
presented. The data were taken from the 2MASS in the J, H and K_s filters. The
sample consists of 175 galaxies in the K_s-filter, 169 galaxies in the H-filter
and 165 galaxies in the J-filter. We present bulge and disc decompositions of
each galaxy image. All galaxies have been modelled with a Sersic bulge and
exponential disc with the BUDDA v2.1 package.
The main conclusions of our general statistical analysis of the sample are:
(1) The distribution of the apparent bulge axis ratio q_b for the subsample
with n < 2 can be attributed to triaxial, nearly prolate bulges that are seen
from different projections, while n > 2 bulges seem to be oblate spheroids with
moderate flattening.
(2) For the sample galaxies, the effective radius of the bulge r_{e,b}, the
disc scalelength h and the disc scaleheight z_0 are well correlated. However,
there is a clear trend for the ratio r_{e,b}/h to increase with n.
(3) There is a hint that the fundamental planes of discs, which links only
disc parameters and the maximum rotational velocity of gas, are different for
galaxies with different bulges.
(4) The investigation of the Photometric Plane of sample bulges shows that
the plane is not flat and has a prominent curvature towards small values of n.
For bulges this fact was not noticed earlier.
(5) The clear relation between the flattening of stellar discs h/z_0 and the
relative mass of a spherical component, including a dark halo, is confirmed not
for bulgeless galaxies but for galaxies with massive bulges. (Abridged)Comment: 20 pages, 13 figures, accepted for publication in MNRA
The supermassive black hole mass - S\'ersic index relations for bulges and elliptical galaxies
Scaling relations between supermassive black hole mass, M_BH, and host galaxy
properties are a powerful instrument for studying their coevolution. A complete
picture involving all of the black hole scaling relations, in which each
relation is consistent with the others, is necessary to fully understand the
black hole-galaxy connection. The relation between M_BH and the central light
concentration of the surrounding bulge, quantified by the S\'ersic index n, may
be one of the simplest and strongest such relations, requiring only
uncalibrated galaxy images. We have conducted a census of literature S\'ersic
index measurements for a sample of 54 local galaxies with directly measured
M_BH values. We find a clear M_BH - n relation, despite an appreciable level of
scatter due to the heterogeneity of the data. Given the current M_BH - L_sph
and the L_sph - n relations, we have additionally derived the expected M_BH - n
relations, which are marginally consistent at the 2 sigma level with the
observed relations. Elliptical galaxies and the bulges of disc galaxies are
each expected to follow two distinct bent M_BH - n relations due to the
S\'ersic/core-S\'ersic divide. For the same central light concentration, we
predict that M_BH in the S\'ersic bulges of disc galaxies are an order
magnitude higher than in S\'ersic elliptical galaxies if they follow the same
M_BH - L_sph relation.Comment: 12 pages, 6 figures, 5 tables, accepted for publication in MNRA
The central black hole mass of the high-sigma but low-bulge-luminosity lenticular galaxy NGC 1332
The masses of the most massive supermassive black holes (SMBHs) predicted by
the M_BH-sigma and M_BH-luminosity relations appear to be in conflict. Which of
the two relations is the more fundamental one remains an open question. NGC
1332 is an excellent example that represents the regime of conflict. It is a
massive lenticular galaxy which has a bulge with a high velocity dispersion
sigma of ~320 km/s; bulge--disc decomposition suggests that only 44% of the
total light comes from the bulge. The M_BH-sigma and the M_BH-luminosity
predictions for the central black hole mass of NGC 1332 differ by almost an
order of magnitude. We present a stellar dynamical measurement of the SMBH mass
using an axisymmetric orbit superposition method. Our SINFONI integral-field
unit (IFU) observations of NGC 1332 resolve the SMBH's sphere of influence
which has a diameter of ~0.76 arcsec. The sigma inside 0.2 arcsec reaches ~400
km/s. The IFU data allow us to increase the statistical significance of our
results by modelling each of the four quadrants separately. We measure a SMBH
mass of (1.45 \pm 0.20) x 10^9 M_sun with a bulge mass-to-light ratio of 7.08
\pm 0.39 in the R-band. With this mass, the SMBH of NGC 1332 is offset from the
M_BH-luminosity relation by a full order of magnitude but is consistent with
the M_BH-sigma relation.Comment: 15 pages, 12 figures, accepted for publication in MNRA
Does the stellar disc flattening depend on the galaxy type?
We analyze the dependence of the stellar disc flatness on the galaxy
morphological type using 2D decomposition of galaxies from the reliable
subsample of the Edge-on Galaxies in SDSS (EGIS) catalogue. Combining these
data with the retrieved models of the edge-on galaxies from the Two Micron All
Sky Survey (2MASS) and the Spitzer Survey of Stellar Structure in Galaxies
(SG) catalogue, we make the following conclusions:
(1) The disc relative thickness in the near- and mid-infrared
passbands correlates weakly with morphological type and does not correlate with
the bulge-to-total luminosity ratio in all studied bands.
(2) Applying an 1D photometric profile analysis overestimates the disc
thickness in galaxies with large bulges making an illusion of the relationship
between the disc flattening and the ratio .
(3) In our sample the early-type disc galaxies (S0/a) have both flat and
"puffed" discs. The early spirals and intermediate-type galaxies have a large
scatter of the disc flatness, which can be caused by the presence of a bar:
barred galaxies have thicker stellar discs, on average. On the other hand, the
late-type spirals are mostly thin galaxies, whereas irregular galaxies have
puffed stellar discs.Comment: 17 pages, 17 figures, accepted for publication in MNRA
Rejuvenation of spiral bulges
We seek to understand whether the stellar populations of galactic bulges show
fingerprints of secular evolution triggered by the presence of the disc. For
this purpose we re-analyse the sample of Proctor and Sansom, deriving stellar
population ages and element abundances from absorption line indices. We obtain
very consistent constraints on ages from the three Balmer indices Hbeta,
Hgamma, and Hdelta, in good agreement with those of Proctor and Sansom based on
a completely different method. Like other studies in the literature, we find
that bulges have relatively low luminosity weighted ages, the lowest age
derived for the smallest bulges being 1.3 Gyr. Hence bulges are not generally
old but actually rejuvenated systems. We discuss evidence that this might be
true also for the bulge of the Milky Way. We show that the smallest bulges,
being the youngest with the lowest alpha/Fe ratios, must have experienced star
formation events involving 10-30 per cent of their total mass in the past 1-2
Gyr. No significant correlations of the stellar population parameters with
Hubble Type are found instead. We show that the relationships with sigma
coincide perfectly with those of early-type galaxies. Hence, bulges are
typically seen younger, metal-poorer and less alpha/Fe enhanced than early-type
galaxies, only because of their smaller masses. At a given velocity dispersion,
bulges and elliptical galaxies are indistinguishable as far as their stellar
populations are concerned. These results favour an inside-out formation
scenario and indicate that the discs in spiral galaxies of Hubble types Sbc and
earlier cannot have a significant influence on the evolution of the stellar
populations in the bulge component. The phenomenon of pseudobulge formation
must be restricted to spirals of types later than Sbc.Comment: 12 pages, 6 figures, accepted by MNRA
A log-quadratic relation for predicting supermassive black hole masses from the host bulge Sersic index
We reinvestigate the correlation between black hole mass and bulge
concentration. With an increased galaxy sample, updated estimates of galaxy
distances, black hole masses, and Sersic indices `n' - a measure of
concentration - we perform a least-squares regression analysis to obtain a
relation suitable for the purpose of predicting black hole masses in other
galaxies. In addition to the linear relation, log(M_bh) = 7.81(+/-0.08) +
2.69(+/-0.28)[log(n/3)] with epsilon_(intrin)=0.31 dex, we investigated the
possibility of a higher order M_bh-n relation, finding the second order term in
the best-fitting quadratic relation to be inconsistent with a value of zero at
greater than the 99.99% confidence level. The optimal relation is given by
log(M_bh) = 7.98(+/-0.09) + 3.70(+/-0.46)[log(n/3)] -
3.10(+/-0.84)[log(n/3)]^2, with epsilon_(intrin)=0.18 dex and a total absolute
scatter of 0.31 dex. Extrapolating the quadratic relation, it predicts black
holes with masses of ~10^3 M_sun in n=0.5 dwarf elliptical galaxies, compared
to ~10^5 M_sun from the linear relation, and an upper bound on the largest
black hole masses in the local universe, equal to 1.2^{+2.6}_{-0.4}x10^9
M_sun}. In addition, we show that the nuclear star clusters at the centers of
low-luminosity elliptical galaxies follow an extrapolation of the same
quadratic relation. Moreover, we speculate that the merger of two such
nucleated galaxies, accompanied by the merger and runaway collision of their
central star clusters, may result in the late-time formation of some
supermassive black holes. Finally, we predict the existence of, and provide
equations for, a relation between M_bh and the central surface brightness of
the host bulge
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