532 research outputs found
The significant contribution of minor mergers to the cosmic star formation budget
We estimate an empirical lower limit for the fraction of cosmic star formation that is triggered by minor mergers in the local Universe. Splitting the star formation budget by galaxy morphology, we find that early-type galaxies (ETGs) host ~14 per cent of the budget, while Sb/Sc galaxies host the bulk (~53 per cent) of the local star formation activity. Recent work indicates that star formation in nearby ETGs is driven by minor mergers, implying that at least ~14 per cent of local star formation is triggered by this process. A more accurate estimate can be derived by noting that an infalling satellite likely induces a larger starburst in a galaxy of 'later' morphological type, both due to higher availability of gas in the accreting galaxy and also because a bigger bulge better stabilizes the disc against star formation. This enables us to use the star formation in ETGs to estimate a lower limit for the fraction of star formation in late-type galaxies (LTGs) that is minor-merger-driven. Using a subsample of ETGs that is mass-and environment-matched to the LTGs (implying a similar infalling satellite population), we estimate this limit to be ~24 per cent. Thus, a lower limit for the fraction of cosmic star formation that is induced by minor mergers is ~35 per cent [14 per cent (ETGs) + 0.24 × 86 per cent (LTGs)]. The observed positive correlation between black hole and galaxy mass further implies that a similar fraction of black hole accretion may also be triggered by minor mergers. Detailed studies of minor-merger remnants are therefore essential, to quantify the role of this important process in driving stellar mass and black hole growth in the local Universe.Peer reviewedSubmitted Versio
Magnetic Field and Displacement sensor based on Giant Magneto-impedance effect
A two-core transducer assembly using a Fe73.5Nb3Cu1Si13.5B9 ribbon to detect
a change of magnetic field is proposed and tested for displacement (linear and
angular) and current sensor. Two identical inductors, with the ribbon as core,
are a part of two series resonance network, and are in high impedance state
when excited by a small a.c field of 1MHz in absence of d.c biasing field
(Hdc). When the magnetic state of one inductor is altered by biasing field,
produced by a bar magnet or current carrying coil, an ac signal proportional to
Hdc is generated by transducer. The results for the sensitivity and linearity
with displacement (linear and angular) of a magnet and with field from the
current carrying coil are presented for two particular configurations of the
transducer. High sensitivities of voltage response as much as
12micro-volt/micro-meter and 3mV/degree have been obtained for the transducer
as a linear and angular displacement sensor respectively in the transverse
configuration of exciting a.c and biasing d.c fields.Comment: 16 pages,7 figure
The star formation histories of early-type galaxies: insights from the rest-frame ultra-violet
Our current understanding of the star formation histories of early-type
galaxies is reviewed, in the context of recent observational studies of their
ultra-violet (UV) properties. Combination of UV and optical spectro-photometric
data indicates that the bulk of the stellar mass in the early-type population
forms at high redshift (z > 2), typically over short timescales (< 1 Gyr).
Nevertheless, early-types of all luminosities form stars over the lifetime of
the Universe, with most luminous (-23 < M(V) < -21) systems forming 10-15% of
their stellar mass after z = 1 (with a scatter to higher value), while their
less luminous (M(V) > -21) counterparts form 30-60% of their mass in the same
redshift range. The large scatter in the (rest-frame) UV colours in the
redshift range 0 < z < 0.7 indicates widespread low-level star formation in the
early-type population over the last 8 billion years. The mass fraction of young
(< 1 Gyr old) stars in luminous early-type galaxies varies between 1% and 6% at
z~0 and is in the range 5-13% at z~0.7. The intensity of recent star formation
and the bulk of the UV colour distribution is consistent with what might be
expected from minor mergers (mass ratios < 1:6) in an LCDM cosmology.Comment: Brief Review, Mod. Phys. Lett.
Exploring the formation of spheroidal galaxies out to z ∼ 1.5 in GOODS
The formation of massive spheroidal galaxies is studied on a visually classified sample extracted from the Advanced Camera for Surveys/Hubble Space Telescope (ACS/HST) images of the Great Observatories Origins Deep Survey north and south fields, covering a total area of 360 arcmin . The sample size (910 galaxies brighter than i = 24) allows us to explore in detail the evolution over a wide range of redshifts (0.4 10 M galaxies by a factor of 2 between z = 1 and 0, in contrast with a factor of ∼50 for lower mass galaxies (10 <M / M <10 ). One-quarter of the whole sample of early types are photometrically classified as blue galaxies. On a volume-limited sample out to z <0.7, the average stellar mass of the blue ellipticals is 5 × 10 M compared to 4 × 10 M for red ellipticals. On a volume-limited subsample out to z = 1.4 probing the brightest galaxies (M <-21), we find the median redshift of blue and red early types: 1.10 and 0.85, respectively. Blue early types only amount to 4 per cent of this sample (compared to 26 per cent in the full sample). The intrinsic colour distribution correlates overall bluer colours with blue cores (positive radial gradients of colour), suggesting an inside-out process of formation. The redshift evolution of the observed colour gradients is incompatible with a significant variation in stellar age within each galaxy. The slope of the Kormendy relation in the subsample of massive galaxies does not change over 0.4 <z <1.4 and is compatible with z = 0 values. The 'zero-point' of the Kormendy relation (i.e. the surface brightness at a fixed half-light radius) is 1 mag fainter (in the B band) for the subsample of low-mass (M <3.5 × 10 M ) early types.Peer reviewe
Anisotropic Dependence of Giant Magneto-Impedance of Amorphous Ferromagnetic Ribbon on Biasing Field
The magneto-impedance (MI) in amorphous ribbon of nominal composition
Fe73.5Nb3Cu1Si13.5B9 has been measured at 1MHz and at room temperature for
different configurations of exciting a.c and biasing d.c. fields. A large drop
in both resistance and reactance is observed as a function of d.c magnetic
field. When the d.c and a.c fields are parallel but normal to the axis of
ribbon, smaller magnetic field is needed to reduce the impedance to its small
saturated value compared to the situation when fields are along the axis of
ribbon. Larger d.c. field is required to lower the impedance when the d.c field
acts perpendicular to the plane of the ribbon. Such anisotropy in
magneto-impedance is related to the anisotropic response of the magnetization
of ribbon. The large change of impedance is attributed to large variation of
a.c permeability on the direction and magnitude of the dc biasing field.Comment: 12 pages, 7 figures, to be published in "International Journal of
Modern Physics B
Better age estimations using UV-optical colours: breaking the age-metallicity degeneracy
We demonstrate that the combination of GALEX UV photometry in the FUV (~1530
angstroms) and NUV (~2310 angstroms) passbands with optical photometry in the
standard U,B,V,R,I filters can efficiently break the age-metallicity
degeneracy. We estimate well-constrained ages, metallicities and their
associated errors for 42 GCs in M31, and show that the full set of
FUV,NUV,U,B,V,R,I photometry produces age estimates that are ~90 percent more
constrained and metallicity estimates that are ~60 percent more constrained
than those produced by using optical filters alone. The quality of the age
constraints is comparable or marginally better than those achieved using a
large number of spectrscopic indices.Comment: Published in MNRAS (2007), 381, L74 (doi:
10.1111/j.1745-3933.2007.00370.x
Galaxy Zoo: Multimergers and the Millennium Simulation
We present a catalogue of 39 multiple mergers, found using the mergers catalogue of the Galaxy Zoo project for z <0.1, and compare them to corresponding semi-analytical galaxies from the Millennium Simulation. We estimate the (volume-limited) multimerger fraction of the local Universe using our sample and find it to be at least 2 orders of magnitude less than binary mergers - in good agreement with the simulations (especially the Munich group). We then investigate the properties of galaxies in binary mergers and multimergers (morphologies, colours, stellar masses and environment) and compare these results with those predicted by the semi-analytical galaxies. We find that multimergers favour galaxies with properties typical of elliptical morphologies and that this is in qualitative agreement with the models. Studies of multimergers thus provide an independent (and largely corroborating) test of the Millennium semi-analytical models.Peer reviewe
- …