Band Luminosity Research Articles

Investigation of the stellar content in the western part of the Carina nebula

We obtained deep $UBVRI$ H$\alpha$ photometric data of the field situated to the west of the main Carina nebula and centered on WR~22. Medium-resolution optical spectroscopy of a subsample of X-ray selected objects along with archival data sets from $Chandra$, $XMM-Newton$ and 2MASS surveys were used for the present study. Different sets of color-color and color-magnitude diagrams are used to determine reddening for the region and to identify young stellar objects (YSOs) and estimate their age and mass. Our spectroscopic results indicate that the majority of the X-ray sources are late spectral type stars. The region shows a large amount of differential reddening with minimum and maximum values of $E(B-V)$ as 0.25 and 1.1 mag, respectively. Our analysis reveals that the total-to-selective absorption ratio $R \rm_{V}$ is $\sim$3.7 $\pm$ 0.1, suggesting an abnormal grain size in the observed region. We identified 467 YSOs and studied their characteristics. The ages and masses of the 241 optically identified YSOs range from $\sim$0.1 to 10 Myr and $\sim$0.3 to 4.8M$_\odot$, respectively. However, the majority of them are younger than 1 Myr and have masses below 2 M$_\odot$. The high mass star WR 22 does not seem to have contributed to the formation of YSOs in the CrW region. The initial mass function slope, $\Gamma$, in this region is found to be -1.13 $\pm$ 0.20 in the mass range of 0.5 < M/M$_\odot$ < 4.8. The $K$-band luminosity function slope ($\alpha$) is also estimated as 0.31 $\pm$ 0.01. We also performed minimum spanning tree analysis of the YSOs in this region, which reveals that there are at least ten YSO cores associated with the molecular cloud, and that leads to an average core radius of 0.43 pc and a median branch length of 0.28 pc.

Singlet oxygen O2(b1Σ g + ) production at altitudes of the polar ionosphere

The relative rate constants of O2(b1Σ + , v= 1–4) production at an inelastic interaction between electronically excited N(2D) atoms and O2(X3Σ − , v = 0) oxygen molecules have been calculated. It was shown that an increase in equilibrium distances between oxygen atoms in NO2 quasi-molecule, produced during the interaction, substantially increases the calculated relative production rates of O2(b1Σ + , v > 1). The obtained coefficients are used to calculate the O2(b1Σ + , v= 1–4) relative populations at 110 km (T = 250 K) and 150 km (T = 500 K) altitudes of the polar ionosphere. The calculated populations have been compared with the results of the published measurements of the Atmospheric system band luminosity intensities, and satisfactory agreement has been obtained for low altitudes.

The ALHAMBRA survey: evolution of galaxy clustering since z ∼ 1

We study the clustering of galaxies as function of luminosity and redshift in the range $0.35 < z < 1.25$ using data from the Advanced Large Homogeneous Area Medium Band Redshift Astronomical (ALHAMBRA) survey. The ALHAMBRA data used in this work cover $2.38 \mathrm{deg}^2$ in 7 independent fields, after applying a detailed angular selection mask, with accurate photometric redshifts, $\sigma_z \lesssim 0.014 (1+z)$, down to $I_{\rm AB} < 24$. Given the depth of the survey, we select samples in $B$-band luminosity down to $L^{\rm th} \simeq 0.16 L^{*}$ at $z = 0.9$. We measure the real-space clustering using the projected correlation function, accounting for photometric redshifts uncertainties. We infer the galaxy bias, and study its evolution with luminosity. We study the effect of sample variance, and confirm earlier results that the COSMOS and ELAIS-N1 fields are dominated by the presence of large structures. For the intermediate and bright samples, $L^{\rm med} \gtrsim 0.6L^{*}$, we obtain a strong dependence of bias on luminosity, in agreement with previous results at similar redshift. We are able to extend this study to fainter luminosities, where we obtain an almost flat relation, similar to that observed at low redshift. Regarding the evolution of bias with redshift, our results suggest that the different galaxy populations studied reside in haloes covering a range in mass between $\log_{10}[M_{\rm h}/(h^{-1}\mathrm{M}_{\odot})] \gtrsim 11.5$ for samples with $L^{\rm med} \simeq 0.3 L^{*}$ and $\log_{10}[M_{\rm h}/(h^{-1}\mathrm{M}_{\odot})] \gtrsim 13.0$ for samples with $L^{\rm med} \simeq 2 L^{*}$, with typical occupation numbers in the range of $\sim 1 - 3$ galaxies per halo.

The rise and fall of the Type Ib supernova iPTF13bvn

We investigate iPTF13bvn, a core-collapse (CC) supernova (SN) in the nearby spiral galaxy NGC 5806. This object was discovered by the intermediate Palomar Transient Factory very soon after the explosion and was classified as a stripped-envelope CC SN, likely of Type Ib. A possible progenitor detection in pre-explosion Hubble Space Telescope (HST) images was reported, making this the only SN Ib with such an identification. Based on photometry of the progenitor candidate and on early-time SN data, it was argued that the progenitor candidate is consistent with a single, massive Wolf-Rayet (WR) star. In this work we present follow-up multi-band light-curves and optical spectra of iPTF13bvn. We perform spectral line analysis to track the evolution of the SN ejecta, construct a bolometric light curve and perform hydrodynamical calculations to model this light curve to constrain the synthesized radioactive nickel mass and the total ejecta mass of the SN. Late-time photometry is analyzed to constrain the amount of oxygen. Furthermore, image registration of pre- and post-explosion HST images is performed. Our HST astrometry confirms the location of the progenitor candidate, and follow-up spectra securely classify iPTF13bvn as a SN Ib. Our hydrodynamical model indicates an ejecta mass of 1.9 solar masses and radioactive nickel mass of 0.05 solar masses. The model fit requires the nickel to be highly mixed out in the ejecta. The late-time nebular r'-band luminosity is not consistent with predictions based on the expected oxygen nucleosynthesis in very massive stars. Our bolometric light curve of iPTF13bvn is not consistent with the previously proposed single massive WR-star progenitor scenario. The ejecta mass and the late-time oxygen emission are both significantly lower than what would be expected from a single WR progenitor with a main-sequence mass of at least 30 solar masses.

The local radio-galaxy population at 20 GHz

We have made the first detailed study of the high-frequency radio-source population in the local universe, using a sample of 202 radio sources from the Australia Telescope 20 GHz (AT20G) survey identified with galaxies from the 6dF Galaxy Survey (6dFGS). The AT20G-6dFGS galaxies have a median redshift of z=0.058 and span a wide range in radio luminosity, allowing us to make the first measurement of the local radio luminosity function at 20 GHz. Our sample includes some classical FR-1 and FR-2 radio galaxies, but most of the AT20G-6dFGS galaxies host compact (FR-0) radio AGN which appear lack extended radio emission even at lower frequencies. Most of these FR-0 sources show no evidence for relativistic beaming, and the FR-0 class appears to be a mixed population which includes young Compact Steep-Spectrum (CSS) and Gigahertz-Peaked Spectrum (GPS) radio galaxies. We see a strong dichotomy in the Wide-field Infrared Survey Explorer (WISE) mid-infrared colours of the host galaxies of FR-1 and FR-2 radio sources, with the FR-1 systems found almost exclusively in WISE `early-type' galaxies and the FR-2 radio sources in WISE `late-type' galaxies. The host galaxies of the flat- and steep-spectrum radio sources have a similar distribution in both K--band luminosity and WISE colours, though galaxies with flat-spectrum sources are more likely to show weak emission lines in their optical spectra. We conclude that these flat-spectrum and steep-spectrum radio sources mainly represent different stages in radio-galaxy evolution, rather than beamed and unbeamed radio-source populations.

Star formation rates of star-forming galaxies from the WISE All-Sky Survey

We explore correlations between extinction-corrected Hα, Hβ and |${\rm [O\,\small {II}]_{double}}$| luminosities versus 12- and 22-μm band luminosities, based on matching samples from the Sloan Digital Sky Survey (SDSS) and the Wide-field Infrared Survey Explorer (WISE). All the coefficients show strong correlations between Balmer lines and mid-infrared (MIR) luminosities, while the extinction-corrected |${\rm [O\,\small {II}]_{double}}$| shows a weaker correlation with MIR luminosities. The extinction-corrected emission-line (EL) luminosities are more tightly correlated with linear combinations of EL(obs) and MIR luminosities than with pure MIR luminosities. Linear combinations include both direct and dust-obscured star formation activity in galaxies and offer an improved star formation rate (SFR) indicator for star-forming galaxies. The factor that causes log10LMIR/log10LHα to vary is the metallicity. We find that log10(LHα(obs) + α × LMIR) and |${\log _{10}(L[{\rm O}\,\small {II}]_{\rm double}(\mathrm{obs})+\alpha \times L_{\rm MIR})}$|⁠, rather than pure MIR luminosities or |${\rm [O\,\small {II}]_{double}}$| alone, are good proxies for extinction-corrected log10LHα, with a residual that is independent of metallicity. The morphologies have weak correlations with log10LMIR/log10LEL ratios. E(B − V) can be estimated by log10[LMIR/LHα(obs)].

Ks-BAND LUMINOSITY EVOLUTION OF THE ASYMPTOTIC GIANT BRANCH POPULATION BASED ON STAR CLUSTERS IN THE LARGE MAGELLANIC CLOUD

We present a study of Ks band luminosity evolution of the asymptotic giant branch (AGB) population in simple stellar systems using star clusters in the Large Magellanic Cloud (LMC). We determine physical parameters of LMC star clusters including center coordinates, radii, and foreground reddenings. Ages of 83 star clusters are derived from isochrone fitting with the Padova models, and those of 19 star clusters are taken from the literature. The AGB stars in 102 star clusters with log(age) = 7.3 - 9.5 are selected using near-infrared color magnitude diagrams based on 2MASS photometry. Then we obtain the Ks band luminosity fraction of AGB stars in these star clusters as a function of ages. The Ks band luminosity fraction of AGB stars increases, on average, as age increases from log(age) ~ 8.0, reaching a maximum at log(age) ~ 8.5, and it decreases thereafter. There is a large scatter in the AGB luminosity fraction for given ages, which is mainly due to stochastic effects. We discuss this result in comparison with five simple stellar population models. The maximum Ks band AGB luminosity fraction for bright clusters is reproduced by the models that expect the value of 0.7 - 0.8 at log(age) = 8.5 - 8.7. We discuss the implication of our results with regard to the study of size and mass evolution of galaxies.

RR Lyrae variables: visual and infrared luminosities, intrinsic colours and kinematics

We use UCAC4 proper motions and WISE W1-band apparent magnitudes intensity-mean for almost 400 field RR Lyrae variables to determine the parameters of the velocity distribution of Galactic RR Lyrae population and constrain the zero points of the metallicity-<MV> relation and those of the period-metallicity-<MKs>-band and period-metallicity-<MW1>-band luminosity relations via statistical parallax. We find the mean velocities of the halo- and thick-disc RR Lyrae populations in the solar neighbourhood to be (U0(Halo), V0(Halo), W0(Halo)) = (-7 +/- 9, -214 +/- 10, -10 +/- 6) km/s and (U0(Disc), V0(Disc), W0(Disc)) =(-13 +/- 7, -37 +/- 6, -17 +/- 4) km/s, respectively, and the corresponding components of the velocity-dispersion ellipsoids, (sigma VR(Halo), sigma Vphi(Halo), sigma Vtheta(Halo)) = (153 +/- 9, 101 +/- 6, 96 +/- 5) km/s and (sigma VR(Disc), sigma Vphi(Disc), sigma Vtheta(Disc)) = (46 +/- 7, 37 +/- 5, 27 +/- 4) km/s, respectively. The fraction of thick-disc stars is estimated at 0.22 +/- 0.03. The corrected IR period-metallicity-luminosity relations are <MKs> = -0.769 +0.088 [Fe/H]- 2.33 mathoprm log PF and <MW1> = -0.825 + 0.088 [Fe/H] -2.33 mathoprm log PF, and the optical metallicity-luminosity relation, [Fe/H]-<MV>, is <MV> = +1.094 + 0.232 [Fe/H], with a standard error of +/- 0.089, implying an LMC distance modulus of 18.32 +/- 0.09, a solar Galactocentric distance of 7.73 +/- 0.36 kpc, and the M31 and M33 distance moduli of DM(M31) = 24.24 +/- 0.09 (D = 705 +/- 30 kpc) and DM(M33) = 24.36 +/- 0.09 (D = 745 +/- 31 kpc), respectively. Extragalactic distances calibrated with our RR Lyrae star luminosity scale imply a Hubble constant of ~80 km/s/Mpc. Our results suggest marginal prograde rotation for the population of halo RR Lyraes in the Milky Way.

Deep luminosity functions and colour–magnitude relations for cluster galaxies at 0.2 &lt; z &lt; 0.6

We derive deep $I$ band luminosity functions and colour-magnitude diagrams from HST imaging for eleven $0.2<z<0.6$ clusters observed at various stages of merging, and a comparison sample of five more relaxed clusters at similar redshifts. The characteristic magnitude $M^*$ evolves passively out to $z=0.6$, while the faint end slope of the luminosity function is $\alpha \sim -1$ at all redshifts. Cluster galaxies must have been completely assembled down to $M_I \sim -18$ out to $z=0.6$. We observe tight colour-magnitude relations over a luminosity range of up to 8 magnitudes, consistent with the passive evolution of ancient stellar populations. This is found in all clusters, irrespective of their dynamical status (involved in a collision or not, or even within subclusters for the same object) and suggests that environment does not have a strong influence on galaxy properties. A red sequence luminosity function can be followed to the limits of our photometry: we see no evidence of a weakening of the red sequence to $z=0.6$. The blue galaxy fraction rises with redshift, especially at fainter absolute magnitudes. We observe bright blue galaxies in clusters at $z > 0.4$ that are not encountered locally. Surface brightness selection effects preferentially influence the detectability of faint red galaxies, accounting for claims of evolution at the faint end.

PHOTOMETRIC PROPERTIES AND LUMINOSITY FUNCTION OF NEARBY MASSIVE EARLY-TYPE GALAXIES

We perform photometric analyses for a bright early-type galaxy (ETG) sample with 2949 galaxies ($M_{\rm r}<-22.5$ mag) in the redshift range of 0.05 to 0.15, drawn from the SDSS DR7 with morphological classification from Galaxy Zoo 1. We measure the Petrosian and isophotal magnitudes, as well as the corresponding half-light radius for each galaxy. We find that for brightest galaxies ($M_{\rm r}<-23$ mag), our Petrosian magnitudes, and isophotal magnitudes to 25 ${\rm mag/arcsec^2}$ and 1\% of the sky brightness are on average 0.16 mag, 0.20 mag, and 0.26 mag brighter than the SDSS Petrosian values, respectively. In the first case the underestimations are caused by overestimations in the sky background by the SDSS PHOTO algorithm, while the latter two are also due to deeper photometry. Similarly, the typical half-light radii ($r_{50}$) measured by the SDSS algorithm are smaller than our measurements. As a result, the bright-end of the $r$-band luminosity function is found to decline more slowly than previous works. Our measured luminosity densities at the bright end are more than one order of magnitude higher than those of Blanton et al. (2003), and the stellar mass densities at $M_{\ast}\sim 5\times10^{11} M_{\odot}$ and $M_{\ast}\sim 10^{12} M_{\odot}$ are a few tenths and a factor of few higher than those of Bernardi et al. (2010). These results may significantly alleviate the tension in the assembly of massive galaxies between observations and predictions of the hierarchical structure formation model.

X-RAY SPECTRAL CUTOFF AND THE LACK OF HARD X-RAY EMISSION FROM TWO ULTRALUMINOUS X-RAY SOURCES M81 X-6 AND HOLMBERG IX X-1

We present broadband X-ray spectral study of two ultraluminous X-ray sources (ULXs), M81 X-6 and Holmberg IX X-1 based on Suzaku & XMM-Newton observations. We perform joint broadband spectral analysis of the brightest sources in the field, i.e. the two ULXs and the active galactic nucleus (AGN) in M81, and demonstrate that the X-ray spectra of the ULXs cut off at energies more than about 3keV with negligible contribution at high energies in the Suzaku HXD/PIN band. The 90% upper limit on the 10-30keV band luminosity of an underlying broadband power-law component is 3.5e38 ergs/s for M81 X-6 and 1.2e39ergs/s for Holmberg IX X-1. These limits are more than an order of magnitude lower than the bolometric (0.1-30 keV) luminosity of 6.8e39ergs/s for M81 X-6 and 1.9e40 ergs/s for Holmberg IX X-1. Our results confirm earlier indications of spectral cut-offs inferred from the XMM-Newton observations of bright ULXs and show that there isn't an additional high energy power-law component contributing significantly to the X-ray emission. The spectral form of the two ULXs are very different from those of Galactic black hole X-ray binaries (BHBs) or AGNs. This implies that the ULXs are neither simply scaled-up versions of stellar mass BHBs or scaled-down versions of AGNs.

MULTI-WAVELENGTH STUDY OF A COMPLETE IRAC 3.6 μm SELECTED GALAXY SAMPLE: A FAIR CENSUS OF RED AND BLUE POPULATIONS AT REDSHIFTS 0.4-1.2

We present a multi-wavelength study of a 3.6 $\mu$m-selected galaxy sample in the Extended Groth strip. The sample is complete for galaxies with stellar mass $>10^{9.5}$ \Msun and redshift $0.4<z<1.2$. In this redshift range, the IRAC 3.6 $\mu$m band measures the rest-frame near-infrared band, permitting nearly unbiased selection with respect to both quiescent and star-forming galaxies. The numerous spectroscopic redshifts available in the EGS are used to train an Artificial Neural Network to estimate photometric redshifts. The distribution of photometric redshift errors is Gaussian with standard deviation ${\sim}0.025(1+z)$, and the fraction of redshift failures (${>}3\sigma$ errors) is about 3.5%. A new method of validation based on pair statistics confirms the estimate of standard deviation even for galaxies lacking spectroscopic redshifts. Basic galaxy properties measured include rest-frame $U-B$ colors, $B$- and $K$-band absolute magnitudes, and stellar masses. We divide the sample into quiescent and star-forming galaxies according to their rest-frame $U-B$ colors and 24 to 3.6 \micron\ flux density ratios and derive rest $K$-band luminosity functions and stellar mass functions for quiescent, star forming, and all galaxies. The results show that massive, quiescent galaxies were in place by $z\approx1$, but lower mass galaxies generally ceased their star formation at later epochs.

On the role of feedback in shaping the cosmic abundance and clustering of neutral atomic hydrogen in galaxies

We investigate the impact of feedback - from supernovae (SNe), active galactic nuclei (AGN) and a photo-ionizing background at high redshifts - on the neutral atomic hydrogen (HI) mass function, the $b_{\rm J}$ band luminosity function, and the spatial clustering of these galaxies at $z$=0. We use a version of the semi-analytical galaxy formation model GALFORM that calculates self-consistently the amount of HI in a galaxy as a function of cosmic time and links its star formation rate to its mass of molecular hydrogen (H$_2$). We find that a systematic increase or decrease in the strength of SNe feedback leads to a systematic decrease or increase in the amplitudes of the luminosity and HI mass functions, but has little influence on their overall shapes. Varying the strength of AGN feedback influences only the numbers of the brightest or most HI massive galaxies, while the impact of varying the strength of photo-ionization feedback is restricted to changing the numbers of the faintest or least HI massive galaxies.Our results suggest that the HI mass function is a more sensitive probe of the consequences of cosmological reionization for galaxy formation than the luminosity function. We find that increasing the strength of any of the modes of feedback acts to weaken the clustering strength of galaxies, regardless of their HI-richness. In contrast, weaker AGN feedback has little effect on the clustering strength whereas weaker SNe feedback increases the clustering strength of HI-poor galaxies more strongly than HI-rich galaxies. These results indicate that forthcoming HI surveys on next generation radio telescopes such as the Square Kilometre Array and its pathfinders will be exploited most fruitfully as part of multiwavelength survey campaigns.

Spectral properties of XRBs in dusty early-type galaxies

We present spectral properties of a total of 996 discrete X-ray sources resolved in a sample of 23 dusty early-type galaxies selected from different environments. The combined X-ray luminosity function of all the 996 sources within the optical D25 of the sample galaxies is well described by a broken power law with a break at 2.71×1038ergs−1 and is close to the Eddington limit for a 1.4M⊙ neutron star. Out of the 996, about 63% of the sources have their X-ray luminosities in the range between few ×1037 to 2.0×1039ergs−1 and are like normal LMXBs; about 15–20% with luminosities <few×1037ergs−1 are either super-soft or very-soft sources; while the remainder represents ULXs, HMXBs or unrelated heavily absorbed harder sources. More XRBs have been detected in the galaxies from isolated regions while those from rich groups and clusters host very few sources. The X-ray color-color plot for these sources has enabled us to classify them as SNRs, LMXBs, HMXBs and heavily absorbed AGNs. The composite X-ray spectra of the resolved sources within D25 region of each of the galaxies are best represented by a power law with the average photon spectral index close to 1.65. The contribution of the resolved sources to the total X-ray luminosity of their host is found to vary greatly, in the sense that, in galaxies like NGC 3379 the XRB contribution is about 81% while for NGC 5846 it is only 2%. A correlation has been evidenced between the cumulative X-ray luminosity of the resolved sources against the star formation rate and the Ks band luminosity of the target galaxies indicating their primordial origin.

Star formation in high-redshift quasars: excess [O ii] emission in the radio-loud population

We investigate the [O II] emission line properties of 18,508 quasars at z<1.6 drawn from the Sloan Digital Sky Survey (SDSS) quasar sample. The quasar sample has been separated into 1,692 radio-loud and 16,816 radio-quiet quasars (RLQs and RQQs hereafter) matched in both redshift and i'-band absolute magnitude. We use the [O II]\lambda3726+3729 line as an indicator of star formation. Based on these measurements we find evidence that star-formation activity is higher in the RLQ population. The mean equivalent widths (EW) for [O II] are EW([O II])_RL=7.80\pm0.30 \AA, and EW([O II])_RQ=4.77\pm0.06 \AA, for the RLQ and RQQ samples respectively. The mean [O II] luminosities are \log[L([O II])_RL/W]=34.31\pm0.01 and \log[L([O II])_RQ/W]=34.192\pm0.004 for the samples of RLQs and RQQs respectively. Finally, to overcome possible biases in the EW measurements due to the continuum emission below the [O II] line being contaminated by young stars in the host galaxy, we use the ratio of the [O II] luminosity to rest-frame i'-band luminosity, in this case, we find for the RLQs \log[L([O II])_RL/L_opt]=-3.89\pm0.01 and \log[L([O II])_RQ/L_opt]=-4.011\pm0.004 for RQQs. However the results depend upon the optical luminosity of the quasar. RLQs and RQQs with the same high optical luminosity \log(L_opt/W)>38.6, tend to have the same level of [O II] emission. On the other hand, at lower optical luminosities \log(L_opt/W)<38.6, there is a clear [O II] emission excess for the RLQs. As an additional check of our results we use the [O III] emission line as a tracer of the bolometric accretion luminosity, instead of the i'-band absolute magnitude, and we obtain similar results. Radio jets appear to be the main reason for the [O II] emission excess in the case of RLQs. In contrast, we suggest AGN feedback ensures that the two populations acquire the same [O II] emission at higher optical luminosities.

The high-redshift (z &gt; 3) active galactic nucleus population in the 4-Ms Chandra Deep Field-South

We present results from a spectral analysis of a sample of high-redshift (z > 3) X-ray-selected active galactic nucleus (AGN) in the 4-Ms Chandra Deep Field-South (CDF-S), the deepest X-ray survey to date. The sample is selected using the most recent spectroscopic and photometric information available in this field. It consists of 34 sources with median redshift z = 3.7, 80 median net counts in the 0.5–7 keV band and median rest-frame absorption-corrected luminosity L2-10 keV ≈ 1.5 × 1044 erg s−1. Spectral analysis for the full sample is presented and the intrinsic column density distribution, corrected for observational biases using spectral simulations, is compared with the expectations of X-ray background (XRB) synthesis models. We find that ≈57 per cent of the sources are highly obscured (NH > 1023 cm−2). Source number counts in the 0.5-2 keV band down to flux F0.5-2 keV ≈ 4 × 10−17 erg s−1 cm−2 are also presented. Our results are consistent with a decline of the AGN space density at z > 3 and suggest that, at those redshifts, the AGN obscured fraction is in agreement with the expectations of XRB synthesis models.

Multiwavelength Study of the NGC 281 Region

Abstract We present a multiwavelength study of the NGC 281 complex, which contains the young cluster IC 1590 at the center, using deep wide-field optical $ UBVI_{\rm c}$ photometry, slitless spectroscopy along with archival data sets in the near-infrared (NIR) and X-ray regions. The extent of IC 1590 is estimated to be $ \sim$ 6.5 pc. The cluster region shows a relatively small amount of differential reddening. The majority of the identified young stellar objects (YSOs) are low-mass PMS stars having age $ \lt$ 1–2 Myr and mass 0.5–3.5 $ M_{\odot}$ . The slope ($ \Gamma$ ) of the mass function for IC 1590, in the mass range 2 $ \lt$$ M/M_\odot$$ \le$ 54, is found to be $-$ 1.11 $ \pm$ 0.15. The slope of the $ K$-band luminosity function (0.37 $ \pm$ 0.07) is similar to the average value ($ \sim$ 0.4) reported for young clusters. The distribution of gas and dust obtained from the IRAS, CO, and radio maps indicates clumpy structures around the central cluster. The radial distribution of the young stellar objects, their ages, $ \Delta$ ($ H$$-$$ K$ ) NIR-excess, and the fraction of classical T Tauri stars suggest triggered star formation at the periphery of the cluster region. However, deeper optical, NIR, and MIR observations are needed to have a conclusive view of the star-formation scenario in the region. The properties of the Class 0/I and Class II sources detected by using the Spitzer mid-infrared observations indicate that a majority of the Class II sources are X-ray emitting stars, whereas X-ray emission is absent from the Class 0/I sources. The spatial distribution of Class 0/I and Class II sources reveals the presence of three sub-clusters in the NGC 281 West region.

The S-star cluster at the center of the Milky Way

Sagittarius A*, the super-massive black hole at the center of the Milky Way, is surrounded by a small cluster of high velocity stars, known as the S-stars. We aim to constrain the amount and nature of stellar and dark mass associated with the cluster in the immediate vicinity of Sagittarius A*. We use near-infrared imaging to determine the $K_\mathrm{s}$-band luminosity function of the S-star cluster members, and the distribution of the diffuse background emission and the stellar number density counts around the central black hole. This allows us to determine the stellar light and mass contribution expected from the faint members of the cluster. We then use post-Newtonian N-body techniques to investigate the effect of stellar perturbations on the motion of S2, as a means of detecting the number and masses of the perturbers. We find that the stellar mass derived from the $K_\mathrm{s}$-band luminosity extrapolation is much smaller than the amount of mass that might be present considering the uncertainties in the orbital motion of the star S2. Also the amount of light from the fainter S-cluster members is below the amount of residual light at the position of the S-star cluster after removing the bright cluster members. If the distribution of stars and stellar remnants is strongly enough peaked near Sagittarius A*, observed changes in the orbital elements of S2 can be used to constrain both their masses and numbers. Based on simulations of the cluster of high velocity stars we find that at a wavelength of 2.2 $\mu$m close to the confusion level for 8 m class telescopes blend stars will occur (preferentially near the position of Sagittarius A*) that last for typically 3 years before they dissolve due to proper motions.

THE CHANDRA COSMOS SURVEY. III. OPTICAL AND INFRARED IDENTIFICATION OF X-RAY POINT SOURCES

The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8 Ms, Chandra program that has imaged the central 0.9 deg^2 of the COSMOS field down to limiting depths of 1.9 10^-16 erg cm^-2 s-1 in the 0.5-2 keV band, 7.3 10^-16 erg cm^-2 s^-1 in the 2-10 keV band, and 5.7 10^-16 erg cm^-2 s-1 in the 0.5-10 keV band. In this paper we report the i, K and 3.6micron identifications of the 1761 X-ray point sources. We use the likelihood ratio technique to derive the association of optical/infrared counterparts for 97% of the X-ray sources. For most of the remaining 3%, the presence of multiple counterparts or the faintness of the possible counterpart prevented a unique association. For only 10 X-ray sources we were not able to associate a counterpart, mostly due to the presence of a very bright field source close by. Only 2 sources are truly empty fields. Making use of the large number of X-ray sources, we update the "classic locus" of AGN and define a new locus containing 90% of the AGN in the survey with full band luminosity >10^42 erg/s. We present the linear fit between the total i band magnitude and the X-ray flux in the soft and hard band, drawn over 2 orders of magnitude in X-ray flux, obtained using the combined C-COSMOS and XMM-COSMOS samples. We focus on the X-ray to optical flux ratio (X/O) and we test its known correlation with redshift and luminosity, and a recently introduced anti-correlation with the concentration index (C). We find a strong anti-correlation (though the dispersion is of the order of 0.5 dex) between C and X/O, computed in the hard band, and that 90% of the obscured AGN in the sample with morphological information live in galaxies with regular morphology (bulgy and disky/spiral), suggesting that secular processes govern a significant fraction of the BH growth at X-ray luminosities of 10^43- 10^44.5 erg/s.

CHANDRAOBSERVATIONS OF NGC 4342, AN OPTICALLY FAINT, X-RAY GAS-RICH EARLY-TYPE GALAXY

Chandra X-ray observations of NGC4342, a low stellar mass (M_K=-22.79 mag) early-type galaxy, show luminous, diffuse X-ray emission originating from hot gas with temperature of kT~0.6 keV. The observed 0.5-2 keV band luminosity of the diffuse X-ray emission within the D_25 ellipse is L_0.5-2keV = 2.7 x 10^39 erg/s. The hot gas has a significantly broader distribution than the stellar light, and shows strong hydrodynamic disturbances with a sharp surface brightness edge to the northeast and a trailing tail. We identify the edge as a cold front and conclude that the distorted morphology of the hot gas is produced by ram pressure as NGC4342 moves through external gas. From the thermal pressure ratios inside and outside the cold front, we estimate the velocity of NGC4342 and find that it moves supersonically (M~2.6) towards the northeast. Outside the optical extent of the galaxy we detect ~17 bright (L_0.5-8keV > 3 x 10^37 erg/s) excess X-ray point sources. The excess sources are presumably low-mass X-ray binaries (LMXBs) located in metal-poor globular clusters (GCs) in the extended dark matter halo of NGC4342. Based on the number of excess sources and the average frequency of bright LMXBs in GCs, we estimate that NGC4342 may host roughly 850-1700 GCs. In good agreement with this, optical observations hint that NGC4342 may harbor 1200 +/- 500 GCs. This number corresponds to a GC specific frequency of S_N = 19.9 +/- 8.3, which is among the largest values observed in full-size galaxies.