arXiv subjects

Mubdi Rahman

Publications and source records attributed to Mubdi Rahman.

50 records · Page 3Linked to original sources

arXiv2016

Galaxy Redshifts from Discrete Optimization of Correlation Functions

We propose a new method of constraining the redshifts of individual extragalactic sources based on celestial coordinates and their ensemble statistics. Techniques from integer linear programming are utilized to optimize simultaneously for the angular two-point cross- and autocorrelation functions. Our novel formalism introduced here not only transforms the otherwise hopelessly expensive, brute-force combinatorial search into a linear system with integer constraints but also is readily implementable in off-the-shelf solvers. We adopt Gurobi, a commercial optimization solver, and use Python to build the cost function dynamically. The preliminary results on simulated data show potential for future applications to sky surveys by complementing and enhancing photometric redshift estimators. Our approach is the first application of integer linear programming to astronomical analysis.

astro-ph.IM↗

arXiv2016

AGN Outflow Shocks on Bonnor-Ebert Spheres

Feedback from Active Galactic Nuclei (AGN) and subsequent jet cocoons and outflow bubbles can have a significant impact on star formation in the host galaxy. To investigate feedback physics on small scales, we perform hydrodynamic simulations of realistically fast AGN winds striking Bonnor-Ebert (BE) spheres and examine gravitational collapse and ablation. We test AGN wind velocities ranging from 300--3,000 km s$^{-1}$ and wind densities ranging from 0.5--10 $m_\mathrm{p}\,\mathrm{cm}^{-3}$. We include heating and cooling of low- and high-temperature gas, self-gravity, and spatially correlated perturbations in the shock, with a maximum resolution of 0.01 pc. We find that the ram pressure is the most important factor that determines the fate of the cloud. High ram pressure winds increase fragmentation and decrease the star formation rate, but also cause star formation to occur on a much shorter time scale and with increased velocities of the newly formed stars. We find a threshold ram pressure of $\sim 2\times10^{-8}$ dyne cm$^{-2}$ above which stars are not formed because the resulting clumps have internal velocities large enough to prevent collapse. Our results indicate that simultaneous positive and negative feedback will be possible in a single galaxy as AGN wind parameters will vary with location within a galaxy.

astro-ph.GA↗

arXiv2016

Cosmological Constraints with Clustering-Based Redshifts

We demonstrate that observations lacking reliable redshift information, such as photometric and radio continuum surveys, can produce robust measurements of cosmological parameters when empowered by clustering-based redshift estimation. This method infers the redshift distribution based on the spatial clustering of sources, using cross-correlation with a reference dataset with known redshifts. Applying this method to the existing SDSS photometric galaxies, and projecting to future radio continuum surveys, we show that sources can be efficiently divided into several redshift bins, increasing their ability to constrain cosmological parameters. We forecast constraints on the dark-energy equation-of-state and on local non-gaussianity parameters. We explore several pertinent issues, including the tradeoff between including more sources versus minimizing the overlap between bins, the shot-noise limitations on binning, and the predicted performance of the method at high redshifts. Remarkably, we find that, once this technique is implemented, constraints on dynamical dark energy from the SDSS imaging catalog can be competitive with, or better than, those from the spectroscopic BOSS survey and even future planned experiments. Further, constraints on primordial non-Gaussianity from future large-sky radio-continuum surveys can outperform those from the Planck CMB experiment, and rival those from future spectroscopic galaxy surveys. The application of this method thus holds tremendous promise for cosmology.

astro-ph.CO↗

arXiv2016

Exploring the SDSS Photometric Galaxies with Clustering Redshifts

We apply clustering-based redshift inference to all extended sources from the Sloan Digital Sky Survey photometric catalogue, down to magnitude r = 22. We map the relationships between colours and redshift, without assumption of the sources' spectral energy distributions (SED). We identify and locate star-forming, quiescent galaxies, and AGN, as well as colour changes due to spectral features, such as the 4000 Å break, redshifting through specific filters. Our mapping is globally in good agreement with colour-redshift tracks computed with SED templates, but reveals informative differences, such as the need for a lower fraction of M-type stars in certain templates. We compare our clustering-redshift estimates to photometric redshifts and find these two independent estimators to be in good agreement at each limiting magnitude considered. Finally, we present the global clustering-redshift distribution of all Sloan extended sources, showing objects up to z ~ 0.8. While the overall shape agrees with that inferred from photometric redshifts, the clustering redshift technique results in a smoother distribution, with no indication of structure in redshift space suggested by the photometric redshift estimates (likely artifacts imprinted by their spectroscopic training set). We also infer a higher fraction of high redshift objects. The mapping between the four observed colours and redshift can be used to estimate the redshift probability distribution function of individual galaxies. This work is an initial step towards producing a general mapping between redshift and all available observables in the photometric space, including brightness, size, concentration, and ellipticity.

astro-ph.GA↗

arXiv2016

Exploring the 2MASS Extended and Point Source Catalogs with Clustering Redshifts

The Two-Micron All-Sky Survey (2MASS) has mapped out the low-redshift Universe down to $K_S\sim14$ mag. As its near-infrared photometry primarily probes the featureless Rayleigh-Jeans tail of galaxy spectral energy distributions, colour-based redshift estimation is rather uninformative. Until now, redshift estimates for this dataset have relied on optical follow-up suffering from selection biases. Here we use the newly-developed technique of clustering-based redshift estimation to infer the redshift distribution of the 2MASS sources regardless of their optical properties. We characterise redshift distributions of objects from the Extended Source Catalogue as a function of near-infrared colours and brightness and report some observed trends. We also apply the clustering redshift technique to dropout populations, sources with non-detections in one or more near-infrared bands, and present their redshift distributions. Combining all extended sources, we confirm with clustering redshifts that the distribution of this sample extends up to $z\sim0.35$. % We perform a similar analysis with the Point Source Catalogue and show that it can be separated into stellar and extragalactic contributions with galaxies reaching $z\sim0.7$. We estimate that the Point Source Catalogue contains 1.6 million extragalactic objects: as many as in the Extended Source Catalogue but probing a cosmic volume ten times larger.

astro-ph.GA↗

arXiv2015

On the Metallicity and Origin of the Smith High-Velocity Cloud

The Smith Cloud is a gaseous high-velocity cloud (HVC) in an advanced state of accretion, only 2.9 kpc below the Galactic plane and due to impact the disk in 27 Myr. It is unique among HVCs in having a known distance (12.4+/-1.3 kpc) and a well-constrained 3D velocity (296 km/s), but its origin has long remained a mystery. Here we present the first absorption-line measurements of its metallicity, using HST/COS UV spectra of three AGN lying behind the Cloud together with Green Bank Telescope 21 cm spectra of the same directions. Using Voigt-profile fitting of the S II 1250, 1253, 1259 triplet together with ionization corrections derived from photoionization modeling, we derive the sulfur abundance in each direction; a weighted average of the three measurements gives [S/H]=-0.28+/-0.14, or 0.53+0.21-0.15 solar metallicity. The finding that the Smith Cloud is metal-enriched lends support to scenarios where it represents recycled Galactic material rather than the remnant of a dwarf galaxy or accreting intergalactic gas. The metallicity and trajectory of the Cloud are both indicative of an origin in the outer disk. However, its large mass and prograde kinematics remain to be fully explained. If the cloud has accreted cooling gas from the corona during its fountain trajectory, as predicted in recent theoretical work, its current mass would be higher than its launch mass, alleviating the mass concern.

astro-ph.GA↗

arXiv2014

Inferring the Redshift Distribution of the Cosmic Infrared Background

Cross-correlating the Planck High Frequency Instrument (HFI) maps against quasars from the Sloan Digital Sky Survey (SDSS) DR7, we estimate the intensity distribution of the Cosmic Infrared Background (CIB) over the redshift range 0 < z < 5.We detect redshift-dependent spatial cross-correlations between the two datasets using the 857, 545 and 353 GHz channels and we obtain upper limits at 217 GHz consistent with expectations. At all frequencies with detectable signal we infer a redshift distribution peaking around z ~ 1.2 and find the recovered spectrum to be consistent with emission arising from star forming galaxies. By assuming simple modified blackbody and Kennicutt relations, we estimate dust and star formation rate density as a function of redshift, finding results consistent with earlier multiwavelength measurements over a large portion of cosmic history. However, we note that, lacking mid-infrared coverage, we are not able to make an accurate determination of the mean temperature for the dust responsible for the CIB. Our results demonstrate that clustering-based redshift inference is a valuable tool for measuring the entire evolution history of the cosmic star formation rate from a single and homogeneous dataset.

astro-ph.CO↗

arXiv2014

Clustering-based redshift estimation: method and application to data

We present a data-driven method to infer the redshift distribution of an arbitrary dataset based on spatial cross-correlation with a reference population and we apply it to various datasets across the electromagnetic spectrum to show its potential and limitations. Our approach advocates the use of clustering measurements on all available scales, in contrast to previous works focusing only on linear scales. We also show how its accuracy can be enhanced by optimally sampling a dataset within its photometric space rather than applying the estimator globally. We show that the ultimate goal of this technique is to characterize the mapping between the space of photometric observables and redshift space as this characterization then allows us to infer the clustering-redshift p.d.f. of a single galaxy. We apply this technique to estimate the redshift distributions of luminous red galaxies and emission line galaxies from the SDSS, infrared sources from WISE and radio sources from FIRST. We show that consistent redshift distributions are found using both quasars and absorber systems as reference populations. This technique brings valuable information on the third dimension of astronomical datasets. It is widely applicable to a large range of extra-galactic surveys.

astro-ph.CO↗

arXiv2014

Clustering-based Redshift Estimation: Comparison to Spectroscopic Redshifts

We investigate the potential and accuracy of clustering-based redshift estimation using the method proposed by Ménard et al. (2013). This technique enables the inference of redshift distributions from measurements of the spatial clustering of arbitrary sources, using a set of reference objects for which redshifts are known. We apply it to a sample of spectroscopic galaxies from the Sloan Digital Sky Survey and show that, after carefully controlling the sampling efficiency over the sky, we can estimate redshift distributions with high accuracy. Probing the full colour space of the SDSS galaxies, we show that we can recover the corresponding mean redshifts with an accuracy ranging from $δ$z=0.001 to 0.01. We indicate that this mapping can be used to infer the redshift probability distribution of a single galaxy. We show how the lack of information on the galaxy bias limits the accuracy of the inference and show comparisons between clustering redshifts and photometric redshifts for this dataset. This analysis demonstrates, using real data, that clustering-based redshift inference provides a powerful data-driven technique to explore the redshift distribution of arbitrary datasets, without any prior knowledge on the spectral energy distribution of the sources.

astro-ph.GA↗

arXiv2013

Wide Integral-Field Infrared Spectroscopy of the Bright [Fe II] Shell in the Young Supernova Remnant G11.2-0.3

We present the results of wide integral-field near-infrared (1.0-1.8 um) spectroscopic observations of the southeastern shell of the young core-collapse supernova remnant (SNR) G11.2-0.3. We first construct [Fe II] 1.644 um line images of three bright clumps from the obtained spectral image cubes and compare them with those of other transitions such as [Fe II] 1.257, [Fe II] 1.534 and He I 1.083 um line images. This allows us to estimate the electron density (~ 4,700-9,400 cm^-3) and extinction (Av ~ 16-20 mag) of the shell, including detailed two-dimensional distribution of the properties in the brightest clump, as well as the discovery of a faint high-velocity (~ -440 km/s) component in the clump. Our SNR shock model calculations estimate the preshock number density of ~ 250-500 cm^-3 and shock speed of ~ 80-250 km/s in the [Fe II]-emitting region of the SNR. The comparison between the observed and modelled radial profiles of the line intensities and their ratios reveals that the shell is composed of multiple thin filaments which have been likely formed in episodic mass loss processes of a progenitor star. The discovery of the faint high-velocity component supports the interpretation that the southeastern shell of G11.2-0.3 is mainly composed of circumstellar material with contamination by supernova ejecta and also that its ejected material was expelled primarily in the southeast-northwest direction.

astro-ph.GA↗

arXiv2012

OB Associations at the Upper End of the Milky Way Luminosity Function

The Milky Way's most luminous, young and massive (M > 10^4 Msun) star clusters and OB associations have largely evaded detection despite knowledge of their surrounding H II regions. We search for these clusters and associations within the 40 star forming complexes from Rahman & Murray in the 13 most luminous WMAP free-free emission sources of the Galaxy. Selecting for objects with the dust-reddened colors of OB stars, we identify new candidate associations using the 2MASS point source catalog. In 40 star forming complexes searched, 22 contain cluster/association candidates with sizes and masses in the range of 3' - 26' and 10^{2.3} - 10^{5} Msun. Of the 22 candidates, at least 7 have estimated masses > 10^4 Msun, doubling the number of such massive clusters known in the Galaxy. Applying our method to a statistically similar set of test locations, we estimate that 3.0 +/- 0.6 of our 22 candidate associations are unrelated to the star forming complexes. In addition, the apparent extinctions of our candidate associations correlate well with the predictions from a Galactic model. These facts, along with the clear detection of a known OB association and the previous spectral verification of one cluster found by this method, validate our method. Only one of the searched WMAP sources remains without a candidate. In 8 of the most luminous WMAP sources, the candidate associations can account for the observed free-free flux. With our new compilation, the Galactic census of young, massive stellar associations may now be about half complete.

astro-ph.GA↗

arXiv2011

Spectroscopic Confirmation of the Dragonfish Association: The Galaxy's Most Luminosity OB Association

Young OB associations with masses greater than 10$^4$ M$_{\sun}$ have been inferred to exist in the Galaxy but have largely evaded detection. Recently, a candidate OB association has been identified within the most luminous star forming complex in the Galaxy, the Dragonfish Nebula. We identify 18 young, massive stars with near-infrared spectroscopy from a sample of 50 members within the candidate OB association, including 15 O-type, and three Luminous Blue Variables or Wolf-Rayet stars. This number matches the expected yield of massive stars from the candidate association, confirming its existence and ability to power the parent star forming complex. These results demonstrate the existence of a 10$^5$ M$_{\sun}$ OB association, more powerful than any previously known in the Galaxy, comparable in mass only to Westerlund 1. Further, the results also validate the color selection method used to identify the association, adding credence to others discovered in the same way.

astro-ph.GA↗

arXiv2011

A Candidate for the Most Luminous OB Association in the Galaxy

The Milky Way harbors giant H II regions which may be powered by star complexes more luminous than any Galactic OB association known. Being across the disk of the Galaxy, however, these brightest associations are severely extinguished and confused. We present a search for one such association toward the most luminous H II region in the recent catalog by Murray and Rahman, which, at ~9.7 kpc, has recombination rate of ~7x10^{51} sec$^{-1}$. Prior searches have identified only small scale clustering around the rim of this shell-like region, but the primary association has not previously been identified. We apply a near-infrared color selection and find an overdensity of point sources toward its southern central part. The colors and magnitudes of these excess sources are consistent with O- and early B-type stars at extinctions $0.96 < A_{K} < 1.2$, and they are sufficiently numerous (406 +/- 102 after subtraction of field sources) to ionize the surrounding H II region, making this a candidate for the most luminous OB association in the Galaxy. We reject an alternate theory, in which the apparent excess is caused by localized extinction, as inconsistent with source demographics.

astro-ph.GA↗

arXiv2010

Massive Star Forming Regions in the Galaxy using the Spitzer GLIMPSE Survey

We examine the thirteen most luminous sources in the WMAP free-free map using the Spitzer GLIMPSE and MSX surveys to identify massive star formation regions, emitting one-third of the Galactic free-free luminosity. We identify star forming regions by a combination of bubble morphology in 8 $\micronm$ (PAH) emission and radio recombination line radial velocities. We find 40 star forming regions associated with our WMAP sources, and determine unique distances to 31. We interpret the bubbles as evidence for radial expansion. The radial velocity distribution for each source allows us to measure the intrinsic speed of a region's expansion. This speed is consistent with the size and age of the bubbles. The high free-free luminosities, combined with negligible synchrotron emission, demonstrate that the bubbles are not driven by supernovae. The kinetic energy of the largest bubbles is a substantial fraction of that measured in the older superbubbles found by Heiles. We find that the energy injected into the ISM by our bubbles is similar to that required to maintain the turbulent motion in the gas disk inside 8 kpc. We report a number of new star forming regions powered by massive ($\textrm{M}_{*} > 10^4 \textrm{M}_\sun$) star clusters. We measure the scale height of the Galactic O stars to be $h_{\textrm{*}} = 35 \pm 5 \pc$. We determine an empirical relationship between the PAH and free-free emission of the form $F_{\textrm{PAH}} \propto F^2_{\textrm{ff}}$. Finally, we find that the bubble geometry is more consistent with a spherical shell rather than a flattened disk.

astro-ph.GA↗