37 research outputs found
Comparison Between Simulated and Observational Results of Galaxy Formation for Large Scale Structures
The Millennium simulation is the largest numerical simulation of how minor fluctuations in the density of the universe’s dark matter distribution are amplified by gravity to develop into the large scale structures(LSS) and galaxy clusters seen today(Springel et al. 2005). Although the simulations have been compared with the astronomical observations of the local universe, the simulations have not been widely compared with high redshift, early universe observations. In our study we compare the simulation data(Wang et al. 2008; Guo et al. 2008(in preparation)) for the first time with observations from the COSMOS survey(Scoville et al. 2006). Three quantities are proposed to characterize the structures and the structures distribution, namely the percent area occupied by LSS at each redshift, the average area of LSS and the shapes as characterized by the square root of the area divided by the circumference. We calculate these quantities for both the observations and the simulations, and quantify discrepancies between the existing simulations and observations. In particular, the simulations exhibit earlier development of dense structures than is seen in the observational data
Herschel Survey of the Palomar-Green QSOs at Low Redshift
We investigate the global cold dust properties of 85 nearby (z < 0.5) QSOs,
chosen from the Palomar-Green sample of optically luminous quasars. We
determine their infrared spectral energy distributions and estimate their
rest-frame luminosities by combining Herschel data from 70 to 500 microns with
near-infrared and mid-infrared measurements from the Two Micron All Sky Survey
(2MASS) and the Wide-Field Infrared Survey Explorer (WISE). In most sources the
far-infrared (FIR) emission can be attributed to thermally heated dust. Single
temperature modified black body fits to the FIR photometry give an average dust
temperature for the sample of 33~K, with a standard deviation of 8~K, and an
average dust mass of 7E6 Solar Masses with a standard deviation of 9E6 Solar
Masses. Estimates of star-formation that are based on the FIR continuum
emission correlate with those based on the 11.3 microns PAH feature, however,
the star-formation rates estimated from the FIR continuum are higher than those
estimated from the 11.3 microns PAH emission. We attribute this result to a
variety of factors including the possible destruction of the PAHs and that, in
some sources, a fraction of the FIR originates from dust heated by the active
galactic nucleus and by old stars.Comment: accepted for publication in ApJ
Detection of Dense Molecular Gas in Inter-Arm Spurs in M51
Spiral arm spurs are prominent features that have been observed in extinction
and 8m emission in nearby galaxies. In order to understand their molecular
gas properties, we used the Owens Valley Radio Observatory to map the
CO(J=1--0) emission in three spurs emanating from the inner northwestern spiral
arm of M51. We report CO detections from all three spurs. The molecular gas
mass and surface density are M M_{\sun} and
50 M_{\sun} pc. Thus, relative to the spiral arms,
the spurs are extremely weak features. However, since the spurs are extended
perpendicular to the spiral arms for 500 pc and contain adequate fuel for
star formation, they may be the birthplace for observed inter-arm HII regions.
This reduces the requirement for the significant time delay that would be
otherwise needed if the inter-arm star formation was initiated in the spiral
arms. Larger maps of galaxies at similar depth are required to further
understand the formation and evolution of these spurs and their role in star
formation - such data should be forthcoming with the new CARMA and future ALMA
telescopes and can be compared to several recent numerical simulations that
have been examining the evolution of spiral arm spurs.Comment: 6 pages, 3 figures, emulate-apj format, accepted in Ap
A Far-infrared Characterization of 24 μm Selected Galaxies at 0 < z < 2.5 using Stacking at 70 μm and 160 μm in the COSMOS Field
We present a study of the average properties of luminous infrared galaxies detected directly at 24 μm in the COSMOS field using a median stacking analysis at 70 μm and 160 μm. Over 35,000 sources spanning 0 ≤ z ≤ 3 and 0.06 mJy ≤ S_(24) ≤ 3.0 mJy are stacked, divided into bins of both photometric redshift and 24 μm flux. We find no correlation of S_(70)/S_(24) flux density ratio with S_(24), but find that galaxies with higher S_(24) have a lower S_(160)/S_(24) flux density ratio. These observed ratios suggest that 24 μm selected galaxies have warmer spectral energy distributions (SEDs) at higher mid-IR fluxes, and therefore have a possible higher fraction of active galactic nuclei. Comparisons of the average S_(70)/S_(24) and S_(160)/S_(24) colors with various empirical templates and theoretical models show that the galaxies detected at 24 μm are consistent with "normal" star-forming galaxies and warm mid-IR galaxies such as Mrk 231, but inconsistent with heavily obscured galaxies such as Arp 220. We perform a χ^2 analysis to determine best-fit galactic model SEDs and total IR luminosities for each of our bins. We compare our results to previous methods of estimating L IR and find that previous methods show considerable agreement over the full redshift range, except for the brightest S_(24) sources, where they overpredict the bolometric IR luminosity at high redshift, most likely due to their warmer dust SED. We present a table that can be used as a more accurate and robust method for estimating bolometric infrared luminosity from 24 μm flux densities
ALMA Imaging of Gas and Dust in a Galaxy Protocluster at Redshift 5.3: [CII] Emission in "Typical" Galaxies and Dusty Starbursts ~1 Billion Years after the Big Bang
We report interferometric imaging of [CII] and OH emission toward the center
of the galaxy protocluster associated with the z=5.3 submillimeter galaxy (SMG)
AzTEC-3, using the Atacama Large (sub)Millimeter Array (ALMA). We detect strong
[CII], OH, and rest-frame 157.7 um continuum emission toward the SMG. The [CII]
emission is distributed over a scale of 3.9 kpc, implying a dynamical mass of
9.7 x 10^10 Msun, and a star formation rate (SFR) surface density of Sigma_SFR
= 530 Msun/yr/kpc2. This suggests that AzTEC-3 forms stars at Sigma_SFR
approaching the Eddington limit for radiation pressure supported disks. We find
that the OH emission is slightly blueshifted relative to the [CII] line, which
may indicate a molecular outflow associated with the peak phase of the
starburst. We also detect and dynamically resolve [CII] emission over a scale
of 7.5 kpc toward a triplet of Lyman-break galaxies with moderate UV-based SFRs
in the protocluster at ~95kpc projected distance from the SMG. These galaxies
are not detected in the continuum, suggesting far-infrared SFRs of <18-54
Msun/yr, consistent with a UV-based estimate of 22 Msun/yr. The spectral energy
distribution of these galaxies is inconsistent with nearby spiral and starburst
galaxies, but resembles those of dwarf galaxies. This is consistent with
expectations for young starbursts without significant older stellar
populations. This suggests that these galaxies are significantly
metal-enriched, but not heavily dust-obscured, "normal" star-forming galaxies
at z>5, showing that ALMA can detect the interstellar medium in "typical"
galaxies in the very early universe.Comment: 15 pages, 12 figures, 4 tables, to appear in ApJ (accepted October
15, 2014
The Faint End Slopes Of Galaxy Luminosity Functions In The COSMOS 2-Square Degree Field
We examine the faint-end slope of the rest-frame V-band luminosity function
(LF), with respect to galaxy spectral type, of field galaxies with redshift
z<0.5, using a sample of 80,820 galaxies with photometric redshifts in the
Cosmic Evolution Survey (COSMOS) field. For all galaxy spectral types combined,
the LF slope, alpha, ranges from -1.24 to -1.12, from the lowest redshift bin
to the highest. In the lowest redshift bin (0.02<z<0.1), where the magnitude
limit is M(V) ~ -13, the slope ranges from ~ -1.1 for galaxies with early-type
spectral energy distributions (SEDs), to ~ -1.9 for galaxies with
low-extinction starburst SEDs. In each galaxy SED category (Ell, Sbc, Scd/Irr,
and starburst), the faint-end slopes grow shallower with increasing redshift;
in the highest redshift bin (0.4<z<0.5), the slope is ~ -0.5 and ~ -1.3 for
early-types and starbursts respectively. The steepness of alpha at lower
redshift could be qualitatively explained by large numbers of faint dwarf
galaxies, perhaps of low surface brightness, which are not detected at higher
redshifts.Comment: 24 pages including 5 figures, accepted to ApJ
The Paα Luminosity Function of H II Regions in Nearby Galaxies from HST/NICMOS
The H II region luminosity function (LF) is an important tool for deriving the birthrates and mass distribution of OB associations and is an excellent tracer of the newly formed massive stars and associations. To date, extensive work (predominantly in Hα) has been done from the ground, which is hindered by dust extinction and the severe blending of adjacent (spatially or in projection) H II regions. Reliably measuring the properties of H II regions requires a linear resolution <40 pc, but analyses satisfying this requirement have been done only in a handful of galaxies, so far. As the first space-based work using a galaxy sample, we have selected 12 galaxies from our HST/NICMOS Paα survey and studied the LF and size distribution of H II regions both in individual galaxies and cumulatively, using a virtually extinction-free tracer of the ionizing photon rate. The high angular resolution and low sensitivity to diffuse emission of NICMOS also offer an advantage over ground-based imaging by enabling a higher degree of de-blending of the H II regions. We do not confirm the broken power-law LFs found in ground-based studies. Instead, we find that the LFs, both individual and co-added, follow a single power law dN(L)/dln L ∝ L^–1, are consistent with the mass function of star clusters in nearby galaxies, and are in agreement with the results of the existing analyses with Hubble Space Telescope (HST) data. The individual and co-added size distributions of H II regions are both roughly consistent with dN(D)/dln D ∝ D^–3, but the power-law scaling is probably contaminated by blended regions or complexes
Barred Galaxies at z > 0.7: NICMOS HDFN Observations
Previous optical studies found an unexpected deficit of bars at z > 0.7. To
investigate the effects of bandshifting, we have studied the fraction of barred
spirals in the NICMOS Deep Field North. At z > 0.7 we find at least four barred
spirals, doubling the number previously detected. The number of barred galaxies
is small because these (and previous) data lack adequate spatial resolution. A
typical 5 kpc bar at z > 0.7 is only marginally detectable for WFPC2 at
0.8microns; the NICMOS data have even lower resolution and can only find the
largest bars. The average size of the four bars seen at z > 0.7 is 12 kpc. The
fraction of such large bars (4/95) is higher than that seen in nearby spirals
(1/44); all known selection effects suggest that the observed fraction is a
lower limit. However, important caveats such as small numbers and difficulties
in defining comparable samples at high and low redshifts should be noted. We
conclude that there is no significant evidence for a decrease in the fraction
of barred spirals beyond z ~ 0.7.Comment: Accepted for publication in ApJ Letters, 4 pages in emulate-apj
style, includes 3 figure
A Turnover in the Galaxy Main Sequence of Star Formation at for Redshifts
The relationship between galaxy star formation rates (SFR) and stellar masses
() is re-examined using a mass-selected sample of 62,000
star-forming galaxies at in the COSMOS 2-deg field. Using new
far-infrared photometry from -PACS and SPIRE and -MIPS 24
m, along with derived infrared luminosities from the NRK method based on
galaxies' locations in the restframe color-color diagram vs. , we are able to more accurately determine total SFRs for our complete
sample. At all redshifts, the relationship between median and
follows a power-law at low stellar masses, and flattens to nearly constant SFR
at high stellar masses. We describe a new parameterization that provides the
best fit to the main sequence and characterizes the low mass power-law slope,
turnover mass, and overall scaling. The turnover in the main sequence occurs at
a characteristic mass of about at all redshifts.
The low mass power-law slope ranges from 0.9-1.3 and the overall scaling rises
in SFR as a function of . A broken power-law fit below
and above the turnover mass gives relationships of below the turnover mass and above
the turnover mass. Galaxies more massive than have on average, a much lower specific star formation rate (sSFR) than
would be expected by simply extrapolating the traditional linear fit to the
main sequence found for less massive galaxies.Comment: 16 pages, 7 figures. Accepted for publication in Ap
A Far-IR Characterization of 24 micron Selected Galaxies at 0<z<2.5 using Stacking at 70 microns and 160 microns in the COSMOS Field
We present a study of the average properties of luminous infrared galaxies
detected directly at 24 m in the COSMOS field using a median stacking
analysis at 70m and 160 m. Over 35000 sources spanning 0<z<3 and 0.06
mJy<S_{24}<3.0 mJy are stacked, divided into bins of both photometric redshift
and 24 m flux. We find no correlation of flux density
ratio with , but find that galaxies with higher have a lower
flux density ratio. These observed ratios suggest that 24
m selected galaxies have warmer SEDs at higher mid-IR fluxes, and
therefore have a possible higher fraction of AGN. Comparisons of the average
and colors with various empirical templates
and theoretical models show that the galaxies detected at 24 m are
consistent with "normal" star-forming galaxies and warm mid-IR galaxies such as
Mrk 231, but inconsistent with heavily obscured galaxies such as Arp 220. We
perform a analysis to determine best fit galactic model SEDs and
total IR luminosities for each of our bins. We compare our results to previous
methods of estimating and find that previous methods show
considerable agreement over the full redshift range, except for the brightest
sources, where previous methods overpredict the bolometric IR
luminosity at high redshift, most likely due to their warmer dust SED. We
present a table that can be used as a more accurate and robust method for
estimating bolometric infrared luminosity from 24 m flux densities.Comment: 21 pages, 6 figures, accepted for publication in the Astrophysical
Journa