160 research outputs found
A Morphological-type dependence in the mu_0-log(h) plane of Spiral galaxy disks
We present observational evidence for a galaxy `Type' dependence to the
location of a spiral galaxy's disk parameters in the mu_0-log(h) (central disk
surface-brightness - disk scale-length) plane. With a sample of ~40 Low Surface
Brightness galaxies (both bulge- and disk-dominated) and ~80 High Surface
Brightness galaxies, the early-type disk galaxies (<=Sc) tend to define a
bright envelope in the mu_0-log(h) plane, while the late-type (>=Scd) spiral
galaxies have, in general, smaller and fainter disks. Below the defining
surface brightness threshold for a Low Surface Brightness galaxy (i.e. more
than 1 mag fainter than the 21.65 B-mag arcsec^(-2) Freeman value), the
early-type spiral galaxies have scale-lengths greater than 8-9 kpc, while the
late-type spiral galaxies have smaller scale-lengths. All galaxies have been
modelled with a seeing-convolved Sersic r^(1/n) bulge and exponential disk
model. We show that the trend of decreasing bulge shape parameter (n) with
increasing Hubble type and decreasing bulge-to-disk luminosity ratio, which has
been observed amongst the High Surface Brightness galaxies, extends to the Low
Surface Brightness galaxies, revealing a continuous range of structural
parameters.Comment: To be published in ApJ. Inc. three two-part figure
The Millennium Galaxy Catalogue: morphological classification and bimodality in the colour-concentration plane
Using 10 095 galaxies (B < 20 mag) from the Millennium Galaxy Catalogue, we
derive B-band luminosity distributions and selected bivariate brightness
distributions for the galaxy population. All subdivisions extract highly
correlated sub-sets of the galaxy population which consistently point towards
two overlapping distributions. A clear bimodality in the observed distribution
is seen in both the rest-(u-r) colour and log(n) distributions. The rest-(u-r)
colour bimodality becomes more pronounced when using the core colour as opposed
to global colour. The two populations are extremely well separated in the
colour-log(n) plane. Using our sample of 3 314 (B < 19 mag) eyeball classified
galaxies, we show that the bulge-dominated, early-type galaxies populate one
peak and the bulge-less, late-type galaxies occupy the second. The early- and
mid-type spirals sprawl across and between the peaks. This constitutes
extremely strong evidence that the fundamental way to divide the luminous
galaxy population is into bulges and discs and that the galaxy bimodality
reflects the two component nature of galaxies and not two distinct galaxy
classes. We argue that these two-components require two independent formation
mechanisms/processes and advocate early bulge formation through initial
collapse and ongoing disc formation through splashback, infall and
merging/accretion. We calculate the B-band luminosity-densities and
stellar-mass densities within each subdivision and estimate that the z ~ 0
stellar mass content in spheroids, bulges and discs is 35 +/- 2 per cent, 18
+/- 7 and 47 +/- 7 per cent respectively. [Abridged]Comment: Accepted for publication in MNRAS, 23 pages, 17 figures. Comments
welcome. MGC website is at: http://www.eso.org/~jliske/mgc
Recommended from our members
The Role of Animal Models for Research on Severe Malaria
In light of the recent controversies over the role of animal models for research into the development of new treatments for severe malaria, particularly cerebral disease, a group of scientists came together to discuss the relative merits of a range of animal models and their overlap with the complex clinical syndromes of human disease. While it was not possible to fully resolve differences over the utility of the Plasmodium berghei ANKA model of experimental cerebral malaria, the meeting did bring the two research communities closer together to identify further work to provide information needed to validate the model and revitalise the development of other animal models displaying features of human pathology. The driving force behind this was the desire to ensure better translation of experimental findings into effective treatments for severe malaria
Evidence for an outer disk in the Prototype `Compact Elliptical' Galaxy M32
M32 is the prototype for the relatively rare class of galaxies referred to as
`compact ellipticals'. It has been suggested that M32 may be a tidally
disturbed r^(1/4) elliptical galaxy, or the remnant bulge of a disk-stripped
early-type spiral galaxy. This paper reveals that the surface brightness
profile, the velocity dispersion measurements, and the estimated supermassive
black hole mass in M32 are inconsistent with the galaxy having, and probably
ever having had, an r^(1/4) light profile. Instead, the radial surface
brightness distribution of M32 resembles an almost perfect (bulge + exponential
disk) profile, which is accompanied by a marked increase in the ellipticity
profile and an associated change in the position angle profile where the `disk'
starts to dominate. Compelling evidence that this bulge/disk interpretation is
accurate comes from the best-fitting r^(1/n) bulge model which has a Sersic
index n=1.5, in agreement with the recently discovered relation between a
bulge's Sersic index and the mass of its supermassive black hole. An index n>4
would also be inconsistent with the stellar velocity dispersion of M32. The
bulge-to-disk size ratio r_e /h equals 0.20, and the logarithm of the
bulge-to-disk luminosity ratio log(B/D) equals 0.22, typical of lenticular
galaxies. The effective radius of the bulge is 27" (~100 pc), while the
scale-length of the disk is less well determined: due to possible tidal-
stripping of the outer profile beyond 220-250", the scale-length may be as
large as 1.3 kpc. M32 is a relatively face-on, nucleated, dwarf galaxy with a
low surface brightness disk and a high surface brightness bulge. This finding
brings into question the existence of the compact elliptical class of galaxies.Comment: Accepted for publication in ApJ Letter
When Is a Bulge Not a Bulge? Inner Disks Masquerading as Bulges in NGC 2787 and NGC 3945
We present a detailed morphological, photometric, and kinematic analysis of
two barred S0 galaxies with large, luminous inner disks inside their bars. We
show that these structures, in addition to being geometrically disk-like, have
exponential profiles (scale lengths 300--500 pc) distinct from the
central, non-exponential bulges. We also find them to be kinematically
disk-like. The inner disk in NGC 2787 has a luminosity roughly twice that of
the bulge; but in NGC 3945, the inner disk is almost ten times more luminous
than the bulge, which itself is extremely small (half-light radius
100 pc, in a galaxy with an outer ring of radius 14 kpc) and only
5% of the total luminosity -- a bulge/total ratio much more typical of
an Sc galaxy. We estimate that at least 20% of (barred) S0 galaxies may have
similar structures, which means that their bulge/disk ratios may be
significantly overestimated. These inner disks dominate the central light of
their galaxies; they are at least an order of magnitude larger than typical
``nuclear disks'' found in ellipticals and early-type spirals. Consequently,
they must affect the dynamics of the bars in which they reside.Comment: LaTeX, 37 pages, 14 EPS figures. To appear in The Astrophysical
Journal (November 10, 2003 issue). Version with full-resolution figures
available at http://www.iac.es/galeria/erwin/research
A New Empirical Model for the Structural Analysis of Early-type Galaxies and a Critical Review of the Nuker Model
The Nuker law was designed to match the inner few (~3-10) arcseconds of
predominantly nearby (< 30 Mpc) early-type galaxy light-profiles; it was never
intended to describe an entire profile. The Sersic model, on the other hand,
was developed to fit the entire profile; however, due to the presence of
partially depleted galaxy cores, the Sersic model cannot always describe the
very inner region. We have therefore developed a new empirical model consisting
of an inner power-law, a transition region, and an outer Sersic model to
connect the inner and outer structure of elliptical galaxies. Moreover, the
stability of the Nuker model parameters are investigated. Surprisingly, none
are found to be stable quantities; all are shown to vary systematically with a
profile's fitted radial extent, and often by more than 100%. Considering
elliptical galaxies spanning a range of 7.5 magnitudes, the central stellar
density of the underlying host galaxy is observed to increase with galaxy
luminosity until the onset of core formation, detected only in the brightest
elliptical galaxies. We suggest that the so-called ``power-law'' galaxies may
actually be described by the Sersic model over their entire radial range
Fundamental Planes, and the "barless" M-sigma relation, for supermassive black holes
The residuals about the standard M-sigma relation correlate with the
effective radius, absolute magnitude, and Sersic index of the host bulge -
although, it is noted here that the elliptical galaxies do not partake in such
correlations. Moreover, it is revealed that barred galaxies (with their
relatively small, faint, and low stellar concentration bulges) can deviate from
the M-sigma relation by delta(log M) = -0.5 to -1.0 dex (in the sense that
their sigma values are too large) and generate much of the aforementioned
correlations. Removal of the seven barred galaxies from the Tremaine et al. set
of 31 galaxies results in a ``barless M-sigma'' relation with an intrinsic
scatter of 0.17 dex (cf. 0.27 dex for the 31 galaxies) and a total scatter of
0.25 dex (cf. 0.34 dex for the 31 galaxies). The introduction of third
parameters does not reduce the scatter of the barless M-sigma relation.
Furthermore, removal of the barred galaxies, or all the disk galaxies, from an
expanded and updated set of 40 galaxies with reliable black hole mass
measurements gives a consistent result, such that log(M_bh/M_sun) =
(8.25+/-0.05) + (3.68+/-0.25)log [sigma/200].
The "barless" sigma-L relation for galaxies with black hole mass measurements
is found to be consistent with that from the SDSS sample of early-type
galaxies. In addition, the barless M-sigma relation, the M-L relation, and the
M-n relation are all shown to yield SMBH masses less than 2-4 x 10^9 M_sun.Comment: 16 pages. Originally submitted to ApJ on September 10, 200
Galaxy and mass assembly (GAMA) : The wavelength-dependent sizes and profiles of galaxies revealed by MegaMorph
We investigate the relationship between colour and structure within galaxies using a large, volume-limited sample of bright, low-redshift galaxies with optical-near-infrared imaging from the Galaxy AndMass Assembly survey.We fit single-component,wavelength-dependent, elliptical Sérsic models to all passbands simultaneously, using software developed by the MegaMorph project. Dividing our sample by n and colour, the recovered wavelength variations in effective radius (Re) and Sérsic index (n) reveal the internal structure, and hence formation history, of different types of galaxies. All these trends depend on n; some have an additional dependence on galaxy colour. Late-type galaxies (nr 2.5), even though they maintain constant n with wavelength, revealing that ellipticals are a superimposition of different stellar populations associated with multiple collapse and merging events. Processes leading to structures with larger Re must be associated with lower metallicity or younger stellar populations. This appears to rule out the formation of young cores through dissipative gas accretion as an important mechanism in the recent lives of luminous elliptical galaxies.Peer reviewe
Galactic bulges from Hubble Space Telescope NICMOS observations: the lack of r^{1/4} bulges
We use HST near-infrared imaging to explore the shapes of the surface
brightness profiles of bulges of S0-Sbc galaxies at high resolution. Modeling
extends to the outer bulge via bulge-disk decompositions of combined HST -
ground based profiles. Compact, central unresolved components similar to those
reported by others are found in ~84% of the sample. We also detect a moderate
frequency (~34%) of nuclear components with exponential profiles which may be
disks or bars. Adopting the S\'ersic r^{1/n} functional form for the bulge,
none of the bulges have an r^{1/4} behaviour; derived S\'ersic shape-indices
are = 1.7 \pm 0.7. For the same sample, fits to NIR ground-based profiles
yield S\'ersic indices up to n = 4-6. The high- of ground-based profiles are
a result of nuclear point sources blending with the bulge extended light due to
seeing. The low S\'ersic indices are not expected from merger violent
relaxation, and argue against significant merger growth for most bulges.Comment: 4 pages, 3 figures, accepted for the Astrophysical Journal Letter
Galaxy And Mass Assembly (GAMA) : stellar mass functions by Hubble type
This work was supported by the Austrian Science Foundation FWF under grant P23946. AWG was supported under the Australian Research Council's funding scheme FT110100263.We present an estimate of the galaxy stellar mass function and its division by morphological type in the local (0.025 < z < 0.06) Universe. Adopting robust morphological classifications as previously presented (Kelvin et al.) for a sample of 3727 galaxies taken from the Galaxy And Mass Assembly survey, we define a local volume and stellar mass limited sub-sample of 2711 galaxies to a lower stellar mass limit of M = 109.0 MΘ. We confirm that the galaxy stellar mass function is well described by a double-Schechter function given by Μ* = 1010.64 MΘ, α1 = 0.43, φ1* = 4.18 dex-1 Mpc-3, α2 = −1.50 and φ2* = 0.74 dex-1 Mpc-3. The constituent morphological-type stellar mass functions are well sampled above our lower stellar mass limit, excepting the faint little blue spheroid population of galaxies. We find approximately 71-4+3 per cent of the stellar mass in the local Universe is found within spheroid-dominated galaxies; ellipticals and S0-Sas. The remaining 29-3+4 per cent falls predominantly within late-type disc-dominated systems, Sab-Scds and Sd-Irrs. Adopting reasonable bulge-to-total ratios implies that approximately half the stellar mass today resides in spheroidal structures, and half in disc structures. Within this local sample, we find approximate stellar mass proportions for E : S0-Sa : Sab-Scd : Sd-Irr of 34 : 37 : 24 :5.Publisher PDFPeer reviewe
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