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Recent papers

Orion as seen by Herschel and Planck

posted Mar 31, 2014, 1:40 PM by Marco Lombardi   [ updated Mar 31, 2014, 2:11 PM ]

A&A in press, by Lombardi M., Hervé B., Alves J., and C.J. Lada   
We present high-resolution, high dynamic range column-density and color-temperature maps of the Orion complex using a combination of Planck dust-emission maps, Herschel dust-emission maps, and 2MASS NIR dust-extinction maps. The column-density maps combine the robustness of the 2MASS NIR extinction maps with the resolution and coverage of the Herschel and Planck dust-emission maps and constitute the highest dynamic range column-density maps ever constructed for the entire Orion complex, covering 0.01 mag < AK < 30 mag, or 2×1020 cm−2 < N < 5×1023 cm−2. We determined the ratio of the 2.2 microns extinction coefficient to the 850 microns opacity and found that the values obtained for both Orion A and B are significantly lower than the predictions of standard dust models, but agree with newer models that incorporate icy silicate-graphite conglomerates for the grain population. We show that the cloud projected pdf, over a large range of column densities, can be well fitted by a simple power law. Moreover, we considered the local Schmidt-law for star formation, and confirm earlier results, showing that the protostar surface density Σ∗ follows a simple law Σ∗Σgasβ, with β~2.
Combined optical-depth and temperature map of Orion A and B.  The image is based on high-resolution Herschel data, where available, and on Planck data, elsewhere.

Pipe nebula extinction map released

posted Mar 24, 2014, 7:50 AM by Marco Lombardi   [ updated Mar 24, 2014, 9:16 AM ]

A&A 454, 781 (2006), by Lombardi et al.   
We just released the complete reduced extinction map of the Pipe nebula, described in Lombardi et al. (2006).  The maps are based on the NICER and NICEST algorithms, and reach a resolution of 1 arcmin (30 arcsec pixel scale).  The maps and ancillary data are available at the Harvard Dataverse repository:

An optical image of the Pipe Nebula (credit: Yury Beletsky) with overimposed in green the 2MASS/NICER extinction map released today.  The lower-right corner of the image is not included in the analysis.

Fitting density models to observational data. The local Schmidt law in molecular clouds

posted Mar 24, 2014, 1:07 AM by Marco Lombardi   [ updated Mar 24, 2014, 2:55 AM ]

A&A 559, id. A90 (2013), by Lombardi et al.   
Abstract. We consider the general problem of fitting a parametric density model to discrete observations, taken to follow a non-homogeneous Poisson point process. This class of models is very common, and can be used to describe many astrophysical processes, including the distribution of protostars in molecular clouds. We give the expression for the likelihood of a given spatial density distribution of protostars and apply it to infer the most probable dependence of the protostellar surface density on the gas surface density. Finally, we apply this general technique to model the distribution of protostars in the Orion molecular cloud and robustly derive the local star formation scaling (Schmidt) law for a molecular cloud. We find that in this cloud the protostellar surface density, ΣYSO, is directly proportional to the square gas column density, here expressed as infrared extinction in the K-band, AK: more precisely,
ΣYSO = (1.65 ± 0.19) (AK/mag)2.03 ± 0.15 stars pc-2.

The maximum-likelihood results for a simulation of the local Schmidt law in a molecular cloud.  The dotted lines show the true, original parameters, while the ellipse the 95% confidence region as derived from the Fisher matrix for this particular problem.

CLASH: New Multiple Images Constraining the Inner Mass Profile of MACS J1206.2-0847

posted Mar 5, 2013, 12:37 AM by Marco Lombardi

ApJ 749, id. 97 (2012), by Adi Zitrin et al.
Abstract. We present a strong-lensing analysis of the galaxy cluster MACS J1206.2-0847 (z = 0.44) using UV, Optical, and IR, HST/ACS/WFC3 data taken as part of the CLASH multi-cycle treasury program, with VLT/VIMOS spectroscopy for some of the multiply lensed arcs. The CLASH observations, combined with our mass model, allow us to identify 47 new multiply lensed images of 12 distant sources. These images, along with the previously known arc, span the redshift range 1 <~ z <~ 5.5, and thus enable us to derive a detailed mass distribution and to accurately constrain, for the first time, the inner mass profile of this cluster. We find an inner profile slope of dlog Σ/dlog θ ~= -0.55 ± 0.1 (in the range [1'', 53''], or 5 kpc <~ r <~ 300 kpc), as commonly found for relaxed and well-concentrated clusters. Using the many systems uncovered here we derive credible critical curves and Einstein radii for different source redshifts. For a source at zs ~= 2.5, the critical curve encloses a large area with an effective Einstein radius of θ E = 28'' ± 3'', and a projected mass of (1.34 ± 0.15) × 1014 M sun. From the current understanding of structure formation in concordance cosmology, these values are relatively high for clusters at z ~ 0.5, so that detailed studies of the inner mass distribution of clusters such as MACS J1206.2-0847 can provide stringent tests of the ΛCDM paradigm.

Galaxy cluster MACS1206 (z = 0.4385) imaged with HST/ACS/WFC3. North is up and east is left. We number the multiply lensed images used and uncovered in this work. The numbers indicate the 50 lensed images, 47 of which are uncovered here and correspond to (at least) 12 newly identified sources and candidates, and the different colors are used to distinguish between them. Note that candidate systems are marked in “c.” Details on the each system are given in Table 1. The overlaid white critical curve corresponds to system 4 at zs = 2.54, enclosing a critical area with an effective Einstein radius of ≃160 kpc at the redshift of this cluster (28′′). The blue critical curve corresponds to the lower redshift of system 1, the giant arc system at zs = 1.033. The composition of this color image is red = F105W+F110W+F125W+F140W+F160W, green = F606W+F625W+F775W+F814W+F850LP, and blue = F435W+F475W. The bottom left inset shows an enlargement of the central core.

Star Formation Rates in Molecular Clouds and the Nature of the Extragalactic Scaling Relations

posted Feb 1, 2012, 4:19 AM by Marco Lombardi   [ updated Feb 1, 2012, 4:20 AM ]

ApJ 745, id. 190 (2012), by Charles J. Lada, Jan Forbrich, Marco Lombardi, and Joao Alves  
Abstract. In this paper, we investigate scaling relations between star formation rates and molecular gas masses for both local Galactic clouds and a sample of external galaxies. We specifically consider relations between the star formation rates and measurements of dense, as well as total, molecular gas masses. We argue that there is a fundamental empirical scaling relation that directly connects the local star formation process with that operating globally within galaxies. Specifically, the total star formation rate in a molecular cloud or galaxy is linearly proportional to the mass of dense gas within the cloud or galaxy. This simple relation, first documented in previous studies, holds over a span of mass covering nearly nine orders of magnitude and indicates that the rate of star formation is directly controlled by the amount of dense molecular gas that can be assembled within a star formation complex. We further show that the star formation rates and total molecular masses, characterizing both local clouds and galaxies, are correlated over similarly large scales of mass and can be described by a family of linear star formation scaling laws, parameterized by f_DG, the fraction of dense gas contained within the clouds or galaxies. That is, the underlying star formation scaling law is always linear for clouds and galaxies with the same dense gas fraction. These considerations provide a single unified framework for understanding the relation between the standard (nonlinear) extragalactic Schmidt-Kennicutt scaling law, that is typically derived from CO observations of the gas, and the linear star formation scaling law derived from HCN observations of the dense gas.

SFR-molecular-mass diagram for local molecular clouds and galaxies from the Gao & Solomon (2004) sample. The solid symbols correspond to measurements of dense cloud masses either from extinction observations of the galactic clouds or HCN observations of the galaxies. The open symbols correspond to measurements of total cloud masses of the same clouds and galaxies, either from extinction measurements for the galactic clouds or CO observations for the galaxies. For the galaxies, pentagons represent the locations of normal spirals, while the positions of starburst galaxies are represented by squares (LIRGs) and inverted triangles (ULIRGs). Triangles represent high-z BzK galaxies.

2MASS wide field extinction maps. IV. The Orion, Monoceros R2, Rosette, and Canis Major star forming regions

posted Nov 14, 2011, 1:55 PM by Marco Lombardi   [ updated Feb 1, 2012, 4:58 AM ]

A&A 535, A16 (2011), by Marco Lombardi, Joao Alves, and Charles Lada  
Abstract. We present a near-infrared extinction map of a large region (approximately 2200 sq. deg) covering the Orion, the Monoceros R2, the Rosette, and the Canis Major molecular clouds. We used robust and optimal methods to map the dust column density in the near-infrared (Nicer and Nicest) towards ~19 million stars of the Two Micron All Sky Survey (2MASS) point source catalog. Over the relevant regions of the field, we reached a 1-σ error of 0.03 mag in the K-band extinction with a resolution of 3 arcmin. We measured the cloud distances by comparing the observed density of foreground stars with the prediction of galactic models, thus obtaining dOrionA = (371 ± 10) pc, dOrionB = (398 ± 12) pc, dMonR2 = (905 ± 37) pc, dRosette = (1330 ± 48) pc, and dCMa = (1150 ± 64) pc, values that compare very well with independent estimates.

The figure to the right is a composite image of Orion, showing the molecular clouds as green areas superimposed to an optical image of the area.  The complementarity between the red HII regions and the green H2 ones is evident.

Direct measurement of the magnification produced by galaxy clusters as gravitational lenses

posted Jul 25, 2011, 12:13 PM by Marco Lombardi   [ updated Feb 1, 2012, 4:58 AM ]

A&A 532, A37, by A. Sonnenfeld, G. Bertin, and M. Lombardi  

Context: Weak lensing is one of the most readily available diagnostic tools to measure the total density profiles of distant clusters of galaxies. Unfortunately, it suffers from the well-known mass-sheet degeneracy, so that weak lensing analyses cannot lead to fully reliable determinations of the total mass of the clusters. One possible way to set the relevant scale of the density profile would be to make a direct measurement of the magnification produced by the clusters as gravitational lenses; in the past, this objective has been addressed in a number of ways, but with no significant success. 

Aims: We revisit a suggestion made a few years ago for this general purpose, based on the use of the fundamental plane as a standard rod for early-type galaxies. Here we move one step further, beyond the simple outline of the idea given earlier, and quantify some statistical properties of this innovative diagnostic tool, with the final goal of identifying clear guidelines for a future observational test of concrete cases, which turns out to be well within the current instrument capabilities. 

Methods: The study is carried out by discussing the statistical properties of fundamental plane measurements for a sample of early-type source galaxies behind a massive cluster, for which a weak lensing analysis is assumed to be available. Some general results are first obtained analytically and then tested and extended by means of dedicated simulations. 

Results: We proceed with determining the optimal way of using fundamental plane measurements to determine the mass scale of a given cluster, which we find to be the study of a sample of early-type galaxies behind the cluster distributed approximately uniformly on the sky. We discuss the role of the redshift distribution of the source galaxies, in relation to the redshift of the lensing cluster and to the limitations of fundamental plane measurements. Simple simulations are carried out for clusters with intrinsic properties similar to those of the Coma cluster. We also show that, within a realistic cosmological scenario, substructures do not contribute much to the magnification signal that we are looking for, but add only a modest amount of scatter. 

Conclusions: We find that for a massive cluster (M200 > 1015 Mȯ) located at redshift 0.3 ± 0.1, a set of about 20 fundamental plane measurements, combined with a robust weak lensing analysis, should be able to lead to a mass determination with a precision of 20% or better.
Simulated Fundamental Plane measurements.  The straight line is the Fundamental Plane expected in the
absence of lensing, viewed edge-on.

Contour plot of the difference between the true and the reconstructed dimensionless mass density from a weak-lensing analysis. In the central region the reconstructed profile underestimates the surface mass density, while in a significant region of the image plane the opposite case occurs.

The Hubble Legacy Archive ACS grism data

posted May 17, 2011, 2:41 PM by Marco Lombardi

A&A 530, A86, by M. Kümmel, P. Rosati, R. Fosbury, J. Haase, R.N. Hook, H. Kuntschner, M. Lombardi, A. Micol, K.K. Nilsson, F. Stoehr, and J.R. Walsh
Abstract. A public release of slitless spectra, obtained with ACS/WFC and the G800L grism, is presented. Spectra were automatically extracted in a uniform way from 153 archival fields (or "associations") distributed across the two Galactic caps, covering all observations to 2008. The ACS G800L grism provides a wavelength range of 0.55-1.00 μm, with a dispersion of 40 Å/pixel and a resolution of ~80 Å for point-like sources. The ACS G800L images and matched direct images were reduced with an automatic pipeline that handles all steps from archive retrieval, alignment and astrometric calibration, direct image combination, catalogue generation, spectral extraction and collection of metadata. The large number of extracted spectra (73,581) demanded automatic methods for quality control and an automated classification algorithm was trained on the visual inspection of several thousand spectra. The final sample of quality controlled spectra includes 47 919 datasets (65% of the total number of extracted spectra) for 32 149 unique objects, with a median iAB-band magnitude of 23.7, reaching 26.5 AB for the faintest objects. Each released dataset contains science-ready 1D and 2D spectra, as well as multi-band image cutouts of corresponding sources and a useful preview page summarising the direct and slitless data, astrometric and photometric parameters. This release is part of the continuing effort to enhance the content of the Hubble Legacy Archive (HLA) with highly processed data products which significantly facilitate the scientific exploitation of the Hubble data. In order to characterize the slitless spectra, emission-line flux and equivalent width sensitivity of the ACS data were compared with public ground-based spectra in the GOODS-South field. An example list of emission line galaxies with two or more identified lines is also included, covering the redshift range 0.2 - 4.6. Almost all redshift determinations outside of the GOODS fields are new. The scope of science projects possible with the ACS slitless release data is large, from studies of Galactic stars to searches for high redshift galaxies.

Two examples of spectra produced in this ACS grism data release.

2MASS wide field extinction maps. III. The Taurus, Perseus, and California cloud complexes

posted Feb 9, 2011, 12:17 AM by Marco Lombardi   [ updated Feb 9, 2011, 3:48 AM ]

A&A 512, A67 (2010), by Marco Lombardi, Charles Lada, and Joao Alves  
Abstract. We present a near-infrared extinction map of a large region in the sky (~ 3500 deg2) in the general directions of Taurus, Perseus, and Aries. The map has been obtained using robust and optimal methods to map dust column density at near-infrared wavelengths (Nicer, described in Lombardi & Alves 2001, A&A, 377, 1023 and Nicest, described in Lombardi 2009, A&A, 493, 735) toward ~23 million stars from the Two Micron All Sky Survey (2MASS) point source catalog. We measure extinction as low as AK = 0.04 mag with a 1-σ significance, and a resolution of 2.5 arcmin in our map. A 250 deg2 section of our map encompasses the Taurus, Perseus, and California molecular cloud complexes. We determine the distances of the clouds by comparing the observed density of foreground stars with the prediction of galactic models, and we obtain results that are in excellent agreement with recent VLBI parallax measurements. We characterize the large-scale structure of the map and find a ~ 25° × 15° region close to the galactic plane (l ~ 135°, b ~ -14°) with small extinction (AK < 0.04 mag); we name this region the Perseus-Andromeda hole. We find that over the region that encompasses the Taurus, Perseus, and California clouds the column density measurements below AK < 0.2 mag are perfectly described by a log-normal distribution, and that a significant deviation is observed at larger extinction values. If turbulence models are invoked to justify the log-normal distribution, the observed departure could be interpreted as the result of the effect of gravity that acts on the cores of the clouds. Finally, we investigate the cloud structure function, and show that significant deviations from the results predicted by turbulent models are observed in at least one cloud.
A NICER extinction map of the Taurus, Perseus, and California dark nebulae.

The probability density function for the extinction measurements obtained in the region above.  The fit with a log-normal distribution is almost perfect, with an excess at high-column densities where star formation occurs.

Stellar properties of z ~ 1 Lyman-break galaxies from ACS slitless grism spectra

posted Feb 6, 2011, 7:32 AM by Marco Lombardi   [ updated Feb 9, 2011, 3:42 AM ]

A&A 526, A10 (2011), by K.K. Nilsson, O. Möller-Nilsson, P. Rosati, M. Lombardi, M. Kümmel, H. Kuntschner, J.R. Walsh, R.A.E. Fosbury  
Abstract.  Lyman-break galaxies are now regularly found in the high redshift Universe by searching for the break in the galaxy spectrum caused by the Lyman-limit redshifted into the optical or even near-IR. At lower redshift, this break is covered by the GALEX UV channels and small samples of z ~ 1 LBGs have been presented in the literature. Here we give results from fitting the spectral energy distributions of a small sub-set of low redshift LBGs and demonstrate the advantage of including photometric points derived from HST ACS slitless grism observations. The results show these galaxies to have very young, star forming populations, while still being massive and dusty. LBGs at low and high redshift show remarkable similarities in their properties, indicating that the LBG selection method picks similar galaxies throughout the Universe.

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