7,673 research outputs found
Nebular Emission from Star-Forming Galaxies
We present a new model for computing consistently the line and continuum
emission from galaxies, based on a combination of recent population synthesis
and photoionization codes. We use effective parameters to describe the HII
regions and the diffuse gas ionized by single stellar generations in a galaxy
[...]. We calibrate the nebular properties of our model using the observed
[OIII]/Hbeta, [OII]/[OIII], [SII]/Halpha, and [NII]/[SII] ratios of a
representative sample of nearby spiral and irregular, starburst, and HII
galaxies. To compute whole (line plus continuum) spectral energy distributions,
we include the absorption by dust in the neutral interstellar medium (ISM)
using a recent simple prescription, which is consistent with observations of
nearby starburst galaxies. Our model enables us to interpret quantitatively the
observed optical spectra of galaxies in terms of stars, gas, and dust
parameters. We find that the range of ionized-gas properties spanned by nearby
galaxies implies factors of 3.5 and 14 variations in the Halpha and [OII]
luminosities produced per unit star formation rate (SFR). When accounting for
stellar Halpha absorption and absorption by dust in the neutral ISM, the actual
uncertainties in SFR estimates based on the emergent Halpha and [OII]
luminosities are as high as several decades. We derive new estimators of the
SFR, the gas-phase oxygen abundance, and the effective absorption optical depth
of the dust in galaxies. We show that, with the help of other lines such as
[OII], Hbeta, [OIII], [NII], or [SII], the uncertainties in SFR estimates based
on Halpha can be reduced to a factor of only 2-3, even if the Halpha line is
blended with the adjacent [NII] lines. Without Halpha, however, the SFR is
difficult to estimate from the [OII], Hbeta, and [OIII] lines. (abridged)Comment: To appear in MNRAS; 17 pages with 10 embedded PS figures (mn.sty
Assessing the impact of local taxation on property prices: a spatial matching contribution
This article provides empirical evidence on the impact of local taxation on property prices, controlling for the local public spending, using data on property taxation and real estate transactions, over the period 1994â2004. Our empirical methodology pairs transactions in the same spatial environments. Spatial differencing and Instrumental Variables (IV) methodology allow us to compare sales across municipality boundaries and to control for the potential endogeneity of local taxation and public spending. Our results suggest that the local Property Tax (PT) rate has no impact on property prices, while the amount of taxes paid appears to have a negative effect on property price.FISCAL CAPITALIZATION;LOCAL TAXATION;PROPERTY PRICES;BORDERS
The ages and metallicities of galaxies in the local universe
We derive stellar metallicities, light-weighted ages and stellar masses for a
magnitude-limited sample of 175,128 galaxies drawn from the Sloan Digital Sky
Survey Data Release Two (SDSS DR2). We compute median-likelihood estimates of
these parameters using a large library of model spectra at medium-high
resolution, covering a comprehensive range of star formation histories. The
constraints we derive are set by the simultaneous fit of five spectral
absorption features, which are well reproduced by our population synthesis
models. By design, these constraints depend only weakly on the alpha/Fe element
abundance ratio. Our sample includes galaxies of all types spanning the full
range in star formation activity, from dormant early-type to actively
star-forming galaxies. We show that, in the mean, galaxies follow a sequence of
increasing stellar metallicity, age and stellar mass at increasing 4000AA-break
strength (D4000). For galaxies of intermediate mass, stronger Balmer absorption
at fixed D4000 is associated with higher metallicity and younger age. We
investigate how stellar metallicity and age depend on total galaxy stellar
mass. Low-mass galaxies are typically young and metal-poor, massive galaxies
old and metal-rich, with a rapid transition between these regimes over the
stellar mass range 3x10^9<M/Msun<3x10^10. Both high- and low-concentration
galaxies follow these relations, but there is a large dispersion in stellar
metallicity at fixed stellar mass, especially for low-concentration galaxies of
intermediate mass. Despite the large scatter, the relation between stellar
metallicity and stellar mass is similar to the correlation between gas-phase
oxygen abundance and stellar mass for star-forming galaxies. [abriged]Comment: 22 pages, 14 figures, accepted for publication on MNRAS, data
available at http://www.mpa-garching.mpg.de/SDSS
Star Formation, Metallicity and Dust Properties Derived from the SAPM Galaxy Survey Spectra
We have derived star formation rates (SFRs), gas-phase oxygen abundances and
effective dust absorption optical depths for a sample of galaxies drawn from
the Stromlo-APM redshift survey using the new Charlot and Longhetti (2001;
CL01) models, which provide a physically consistent description of the effects
of stars, gas and dust on the integrated spectra of galaxies. Our sample
consists of 705 galaxies with measurements of the fluxes and equivalent widths
of Halpha, [OII], and one or both of [NII] and [SII]. For a subset of the
galaxies, 60 and 100 micron IRAS fluxes are available. We compare the star
formation rates derived using the models with those derived using standard
estimators based on the Halpha, the [OII] and the far-infrared luminosities of
the galaxies. The CL01 SFR estimates agree well with those derived from the
IRAS fluxes, but are typically a factor of ~3 higher than those derived from
the Halpha or the [OII] fluxes, even after the usual mean attenuation
correction of A_Halpha=1 mag is applied to the data. We show that the reason
for this discrepancy is that the standard Halpha estimator neglects the
absorption of ionizing photons by dust in HII regions and the contamination of
Halpha emission by stellar absorption. We also use our sample to study
variations in star formation and metallicity as a function of galaxy absolute
bJ magnitude. For this sample, the star formation rate per unit bJ luminosity
is independent of magnitude. The gas-phase oxygen abundance does increase with
bJ luminosity, although the scatter in metallicity at fixed magnitude is large.Comment: 17 pages, 8 figures, accepted for publication in MNRA
The host galaxies of radio-loud AGN: mass dependencies, gas cooling and AGN feedback
The properties of the host galaxies of a well-defined sample of 2215
radio-loud AGN with redshifts 0.03 < z < 0.3, defined from the SDSS, are
investigated. These are predominantly low radio luminosity sources, with 1.4GHz
luminosities of 10^23 to 10^25 W/Hz. The fraction of galaxies that host
radio-loud AGN with L(1.4GHz) > 10^23 W/Hz is a strong function of stellar
mass, rising from nearly zero below a stellar mass of 10^10 Msun to more than
30% at 5x10^11 Msun. The integral radio luminosity function is derived in six
ranges of stellar and black hole mass. Its shape is very similar in all of
these ranges and can be well fitted by a broken power-law. Its normalisation
varies strongly with mass, as M_*^2.5 or M_BH^1.6; this scaling only begins to
break down when the predicted radio-loud fraction exceeds 20-30%. There is no
correlation between radio and emission line luminosities for the radio-loud AGN
in the sample and the probability that a galaxy of given mass is radio-loud is
independent of whether it is optically classified as an AGN. The host galaxies
of the radio-loud AGN have properties similar to those of ordinary galaxies of
the same mass.
All of these findings support the conclusion that the optical AGN and low
radio luminosity AGN phenomena are independent and are triggered by different
physical mechanisms. Intriguingly, the dependence on black hole mass of the
radio-loud AGN fraction mirrors that of the rate at which gas cools from the
hot atmospheres of elliptical galaxies. It is speculated that gas cooling
provides a natural explanation for the origin of the radio-loud AGN activity,
and it is argued that AGN heating could plausibly balance the cooling of the
gas over time. [Abridged]Comment: Accepted for publication in MNRAS. LaTeX, 16 pages. Figure 10 is in
colou
Near-Infrared Microlensing of Stars by the Super-Massive Black Hole in the Galactic Center
We investigate microlensing amplification of faint stars in the dense stellar
cluster in the Galactic Center (GC) by the super-massive black hole (BH). Such
events would appear very close to the position of the radio source SgrA*, which
is thought to coincide with the BH, and could be observed during the monitoring
of stellar motions in the GC. We use the observed K-band (2.2 um) luminosity
function (KLF) in the GC and in Baade's Window, as well as stellar population
synthesis computations, to construct KLF models for the inner 300 pc of the
Galaxy. These, and the observed dynamical properties of this region, are used
to compute the rates of microlensing events, which amplify stars above
specified detection thresholds. We present computations of the lensing rates
and amplifications as functions of the event durations (weeks to years), for a
range of detection thresholds. We find that short events dominate the total
rate and that long events tend to have large amplifications. For the current
detection limit of K=17 mag, the total microlensing rate is 0.003 1/yr, and the
rate of events with durations >1 yr is 0.001 1/yr. Recent GC proper motion
studies have revealed the possible presence of one or two variable K-band
sources very close to SgrA* (Genzel et al 97; Ghez et al 98). These sources may
have attained peak brightnesses of K~15 mag, about 1.5-2 mag above the
observational detection limits, and appear to have varied on a timescale of ~1
yr. This behavior is consistent with long-duration microlensing of faint stars
by the BH. However, we estimate that the probability that such an event could
have been detected during the course of the recent proper motion studies is
\~0.5%. A ten-fold improvement in the detection limit and 10 yr of monthly
monitoring would increase the total detection probability to ~20%. (Abridged)Comment: 29 p. with 5 figs. To appear in ApJ. Changed to reflect published
version. Short discussions of solar metallicity luminosity function and
star-star microlensing adde
A Simple Model for the Absorption of Starlight by Dust in Galaxies
We present a new model to compute the effects of dust on the integrated
spectral properties of galaxies, based on an idealized prescription of the main
features of the interstellar medium (ISM). The model includes the ionization of
HII regions in the interiors of the dense clouds in which stars form and the
influence of the finite lifetime of these clouds on the absorption of
radiation. We compute the production of emission lines and the absorption of
continuum radiation in the HII regions and the subsequent transfer of line and
continuum radiation in the surrounding HI regions and the ambient ISM. This
enables us to interpret simultaneously all the observations of a homogeneous
sample of nearby UV-selected starburst galaxies, including the ratio of far-IR
to UV luminosities, the ratio of Halpha to Hbeta luminosities, the Halpha
equivalent width, and the UV spectral slope. We show that the finite lifetime
of stellar birth clouds is a key ingredient to resolve an apparent discrepancy
between the attenuation of line and continuum photons in starburst galaxies. In
addition, we find that an effective absorption curve proportional to
lambda^-0.7 reproduces the observed relation between the ratio of far-IR to UV
luminosities and the UV spectral slope. We interpret this relation most simply
as a sequence in the overall dust content of the galaxies. The shallow
wavelength dependence of the effective absorption curve is compatible with the
steepness of known extinction curves if the dust has a patchy distribution. In
particular, we find that a random distribution of discrete clouds with optical
depths similar to those in the Milky Way provides a consistent interpretation
of all the observations. Our model for absorption can be incorporated easily
into any population synthesis model. (abridged)Comment: To appear in the 2000 July 20 issue of the Astrophysical Journal; 19
pages with 13 embedded PS figures (emulateapj5.sty
Results from the Blazar Monitoring Campaign at the Whipple 10m Gamma-ray Telescope
In September 2005, the observing program of the Whipple 10 m gamma-ray
telescope was redefined to be dedicated almost exclusively to AGN monitoring.
Since then the five Northern Hemisphere blazars that had already been detected
at Whipple are monitored routinely each night that they are visible. Thanks to
the efforts of a large number of multiwavelength collaborators, the first year
of this program has been very successful. We report here on the analysis of
Markarian 421 observations taken from November, 2005 to May, 2006 in the
gamma-ray, X-ray, optical and radio bands.Comment: 4 pages; contribution to the 30th International Cosmic Ray
Conference, Merida, Mexico, July 200
A fundamental plane for field star-forming galaxies
Star formation rate (SFR), metallicity and stellar mass are within the
important parameters of star--forming galaxies that characterize their
formation and evolution. They are known to be related to each other at low and
high redshift in the mass--metallicity, mass--SFR, and metallicity--SFR
relations. In this work we demonstrate the existence of a plane in the 3D space
defined by the axes SFR [log(SFR)(M_sun yr^-1)], gas metallicity [12+log(O/H)],
and stellar mass [log(M_star/M_sun)] of star-forming galaxies. We used
star--forming galaxies from the "main galaxy sample" of the Sloan Digital Sky
Survey--Data Release 7 (SDSS-DR7) in the redshift range 0.04 < z < 0.1 and
r-magnitudes between 14.5 and 17.77. Metallicities, SFRs, and stellar masses
were taken from the Max-Planck-Institute for Astrophysics-John Hopkins
University (MPA-JHU) emission line analysis database. From a final sample of
44214 galaxies, we find for the first time a fundamental plane for field
galaxies relating the SFR, gas metallicity, and stellar mass for star--forming
galaxies in the local universe. One of the applications of this plane would be
estimating stellar masses from SFR and metallicity. High redshift data from the
literature at redshift ~2.2 and 3.5, do not show evidence for evolution in this
fundamental plane.Comment: Accepted for publication in A&A. 4 pages, 4 Figures, and 2 online
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