Star Formation Rate Estimates Research Articles

ESTIMATING THE STAR FORMATION RATE AT 1 kpc SCALES IN NEARBY GALAXIES

Using combinations of H\alpha, ultraviolet (UV), and infrared (IR) emission, we estimate the star formation rate (SFR) surface density, \Sigma_SFR, at 1 kpc resolution for 30 disk galaxies that are targets of the IRAM HERACLES CO survey. We present a new physically-motivated IR spectral energy distribution-based approach to account for possible contributions to 24\mum emission not associated with recent star formation. Considering a variety of "reference" SFRs from the literature, we revisit the calibration of the 24\mum term in hybrid (UV+IR or H\alpha+IR) tracers. We show that the overall calibration of this term remains uncertain at the factor of two level because of the lack of wide-field, robust reference SFR estimates. Within this uncertainty, published calibrations represent a reasonable starting point for 1 kpc-wide areas of star-forming disk galaxies but we re-derive and refine the calibration of the IR term in these tracers to match our resolution and approach to 24\mum emission. We compare a large suite of \Sigma_SFR estimates and find that above \Sigma_SFR \sim 10^-3 M_\odot yr^-1 kpc^-2 the systematic differences among tracers are less than a factor of two across two orders of magnitude dynamic range. We caution that methodology and data both become serious issues below this level. We note from simple model considerations that focusing on a part of a galaxy dominated by a single stellar population the intrinsic uncertainty in H\alpha and FUV-based SFRs are \sim 0.3 and \sim 0.5 dex.

PANCHROMATIC ESTIMATION OF STAR FORMATION RATES INBzKGALAXIES AT 1 <z< 3

We determine star formation rates (SFRs) in a sample of color-selected, star-forming (sBzK) galaxies (K(AB)<21.8) in the Extended Chandra Deep Field - South. To identify and avoid active galactic nuclei, we use X-ray, IRAC color, and IR/radio flux ratio selection methods. Photometric redshift-binned, average flux densities are measured with stacking analyses in Spitzer-MIPS IR, BLAST and APEX/LABOCA submillimeter, VLA and GMRT radio and Chandra X-ray data. We include averages of aperture fluxes in MUSYC UBVRIz'JHK images to determine UV-through-radio spectral energy distributions. We determine the total IR luminosities and compare SFR calibrations from FIR, 24 micron, UV, radio and X-ray wavebands. We find consistency with our best estimator, SFR(IR+UV), to within errors for the preferred radio SFR calibration. Our results imply that 24 micron only and X-ray SFR estimates should be applied to high redshift galaxies with caution. Average IR luminosities are consistent with luminous infrared galaxies. We find SFR(IR+UV) for our stacked sBzKs at median redshifts 1.4, 1.8, and 2.2 to be 55+/-6 (random error), 74+/-8 and 154+/-17 Msun yr^-1 respectively, with additional systematic uncertainty of a factor of ~2.

A remarkably high fraction of strong Lyα emitters amongst luminous redshift 6.0 &lt; z &lt; 6.5 Lyman-break galaxies in the UKIDSS Ultra-Deep Survey

We present spectroscopic confirmation of ten highly luminous (L >= 2L*) Lyman alpha emitters in the redshift range 6.01<z<6.49 (nine galaxies and one AGN), initially drawn from a sample of fourteen z_phot >= 6 Lyman break galaxies (LBGs) selected from an area of 0.25 square degrees within the UKIDSS Ultra-deep Survey (UDS). Overall, our high rate of spectroscopic confirmation (>= 71%) and low rate of contamination provides a strong vindication of the photometric redshift analysis used to define the original sample. By considering star-formation rate estimates based on the Ly_alpha and UV continuum luminosity we conclude that our sample is consistent with a Ly_alpha escape fraction of ~25%. Moreover, after careful consideration of the potential uncertainties and biases, we find that 40%-50% of our sample of L >= 2L* galaxies at 6.0<z<6.5 display strong Ly_alpha emission (rest-frame equivalent width >= 25 Angs), a fraction which is a factor of ~2 higher than previously reported for L <= L* galaxies at z~6. Our results suggest that, as the epoch of reionization is approached, it is plausible that the Ly_alpha emitter fraction amongst luminous (L >=2 L*) LBGs shows a similarly sharp increase to that observed in their lower-luminosity (L <= L*) counterparts.

Spectroscopy of the spatially extended Lyα emission around a quasar at z = 6.4

ABSTRACT We have taken deep, moderate-resolution Keck/DEIMOS spectra of the quasar (QSO) CFHQS J232908.28−030158.8 at z= 6.4. At the wavelength of Lyα, the spectrum shows a spatially extended component, which is significantly more extended than a stellar spectrum, and also a continuum part of the spectrum. The rest-frame linewidth of the extended component is 21 ± 7 Å and thus smaller than that of the QSO (52 ± 4 Å), where they should be identical if the light is incomplete subtraction of the QSO component. Therefore, these comparisons argue for the detection of spatially extended Lyα nebulae around this QSO. This is the first z &amp;gt; 6 QSO where an extended Lyα halo has been observed. Careful subtraction of the central QSO spectrum reveals a lower limit to the Lyα luminosity of (1.7 ± 0.1) × 1043 erg s−1. This emission may be from the theoretically predicted infalling gas in the process of forming a primordial galaxy that is ionized by a central QSO. On the other hand, if it is photoionized by the host galaxy, an estimated star formation rate of &amp;gt;3.0 M⊙ yr−1 is required. If we assume the gas is virialized, we obtain a dynamical mass estimate of Mdyn= 1.2 × 1012 M⊙. The derived MBH/Mhost ratio is 2.1 × 10−4, which is two orders of magnitude smaller than those from more massive z∼ 6 QSOs, and places this galaxy in accordance with the local MBH–σ relation, in contrast to a previous claim on the evolution of the MBH–σ relation at z∼ 6. We do not claim evolution or non-evolution of the MBH–σ relation based on a single object, but our result highlights the importance of investigating fainter QSOs at z∼ 6.

Continuum removal in Hα extragalactic measurements

We point out an important source of error in measurements of extragalactic H-alpha emission and suggest ways to reduce it. The H-alpha line, used for estimating star formation rates, is commonly measured by imaging in a narrow band and a wide band, both which include the line. The image analysis relies on the accurate removal of the underlying continuum. We discuss in detail the derivation of the emission line's equivalent width and flux for extragalactic extended sources, and the required photometric calibrations. We describe commonly used continuum-subtraction procedures, and discuss the uncertainties that they introduce. Specifically, we analyse errors introduced by colour effects. We show that the errors in the measured H-alpha equivalent width induced by colour effects can lead to underestimates as large as 40% and overestimates as large as 10%, depending on the underlying galaxy's stellar population and the continuum-subtraction procedure used. We also show that these errors may lead to biases in results of surveys, and to the underestimation of the cosmic star formation rate at low redshifts (the low z points in the Madau plot). We suggest a method to significantly reduce these errors using a single colour measurement.

On the inconsistency between the estimates of cosmic star formation rate and stellar mass density of high-redshift galaxies

There are mainly two different approaches to measure the cosmic star formation history: direct star formation rate density (SFRD) and stellar mass density rhostar as functions of redshift. Compilations of current observations seem to show a disparity in the two quantities, in the sense that the integral of SFRD is higher than the observed rhostar (after considering gas recycling). Using cosmological smoothed particle hydrodynamics simulations based on the concordance Lambda cold dark matter model, we show that the two quantities become more consistent with each other when we consider the observed galaxy mass limit. The comparison between simulations and (dust corrected) observed cosmic SFRD shows a good agreement, while the observed rhostar is significantly lower than the simulation results. This can be reconciled if the current high-$z$ galaxy surveys are missing faint low-mass galaxies due to their flux limit. Our simulated GSMFs have steep low-mass end slopes of alpha < -2 at z>3, and when these numerous low-mass galaxies are included, the total rhostar matches with the integral of SFRD.

RADIO STACKING REVEALS EVIDENCE FOR STAR FORMATION IN THE HOST GALAXIES OF X-RAY-SELECTED ACTIVE GALACTIC NUCLEI ATz&lt; 1

Nuclear starbursts may contribute to the obscuration of active galactic nuclei (AGNs). The predicted star formation rates are modest, and, for the obscured AGNs that form the X-ray background at z < 1, the associated faint radio emission lies just beyond the sensitivity limits of the deepest surveys. Here, we search for this level of star formation by studying a sample of 359 X-ray selected AGNs at z < 1 from the COSMOS field that are not detected by current radio surveys. The AGNs are separated into bins based on redshift, X-ray luminosity, obscuration, and mid-infrared characteristics. An estimate of the AGN contribution to the radio flux density is subtracted from each radio image, and the images are then stacked to uncover any residual faint radio flux density. All of the bins containing 24 micron-detected AGNs are detected with a signal-to-noise >3sigma in the stacked radio images. In contrast, AGNs not detected at 24 microns are not detected in the resulting stacked radio images. This result provides strong evidence that the stacked radio signals are likely associated with star formation. The estimated star formation rates derived from the radio stacks range from 3 solar masses per year to 29 solar masses per year. Although it is not possible to associate the radio emission with a specific region of the host galaxies, these results are consistent with the predictions of nuclear starburst disks in AGN host galaxies.

Probing star formation across cosmic time with absorption-line systems

We present an empirical connection between cold gas in galactic halos and star formation. Using a sample of more than 8,500 MgII absorbers from SDSS quasar spectra, we report the detection of a 15 sigma correlation between the rest equivalent width W0 of MgII absorbers and the associated OII luminosity, an estimator of star formation rate. This correlation has interesting implications: using only observable quantities we show that MgII absorbers trace a substantial fraction of the global OII luminosity density and recover the overall star formation history of the Universe derived from classical emission estimators up to z~2. We then show that the distribution function of MgII rest equivalent widths, dN/dW0 inherits both its shape and amplitude from the OII luminosity function Phi(L). These distributions can be naturally connected, without any free parameter. Our results imply a high covering factor of cold gas around star forming galaxies: C>0.5, favoring outflows as the mechanism responsible for MgII absorption. We then argue that intervening MgII absorbers and blue-shifted MgII absorption seen in the spectra of star forming galaxies are essentially the same systems. These results not only shed light on the nature of MgII absorbers but also provide us with a new probe of star formation, in absorption, i.e. in a way which does not suffer from dust extinction and with a redshift-independent sensitivity. As shown in this analysis, such a tool can be applied in a noise-dominated regime, i.e. using a dataset for which emission lines are not detected in individual objects. This is of particular interest for high redshift studies.

The Star Formation Reference Survey. I. Survey Description and Basic Data

ABSTRACTStar formation is arguably the most important physical process in the cosmos. It is a fundamental driver of galaxy evolution and the ultimate source of most of the energy emitted by galaxies in the local universe. A correct interpretation of star formation rate (SFR) measures is therefore essential to our understanding of galaxy formation and evolution. Unfortunately, however, no single SFR estimator is universally available or even applicable in all circumstances: the numerous galaxies found in deep surveys are often too faint (or too distant) to yield significant detections with most standard SFR measures, and until now there have been no global multiband observations of nearby galaxies that span all the conditions under which star formation is taking place. To address this need in a systematic way, we have undertaken a multiband survey of all types of star-forming galaxies in the local universe. This project, the Star Formation Reference Survey (SFRS), is based on a statistically valid sample of 369 nearby galaxies that span all existing combinations of dust temperature, SFR, and specific SFR. Furthermore, because the SFRS is blind with respect to AGN fraction and environment, it serves as a means to assess the influence of these factors on SFR. Our panchromatic global flux measurements (including GALEX FUV + NUV, SDSS ugriz, 2MASS JHKs, Spitzer 3–8 μm, and others) furnish uniform SFR measures and the context in which their reliability can be assessed. This article describes the SFRS survey strategy, defines the sample, and presents the multiband photometry collected to date.

FAR-INFRARED AND MOLECULAR CO EMISSION FROM THE HOST GALAXIES OF FAINT QUASARS ATz∼ 6

We present new millimeter and radio observations of nine z~6 quasars discovered in deep optical and near-infrared surveys. We observed the 250 GHz continuum in eight of the nine objects and detected three of them. New 1.4 GHz radio continuum data have been obtained for four sources, and one has been detected. We searched for molecular CO (6-5) line emission in the three 250 GHz detections and detected two of them. We study the FIR and radio emission and quasar-host galaxy evolution with a sample of 18 z~6 quasars that are faint at UV/optical wavelengths (rest-frame 1450A magnitudes of m_1450\ge20.2). The average FIR-to-AGN UV luminosity ratio of this faint quasar sample is about two times higher than that of the bright quasars at z~6 (m_1450<20.2). A fit to the average FIR and AGN bolometric luminosities of both the UV/optically faint and bright z~6 quasars, and the average luminosities of samples of submillimeter /millimeter-observed quasars at z~2 to 5, yields a relationship of L_{FIR} {L_{bol}}^{0.62}. Five of the 18 faint z~6 quasars have been detected at 250 GHz. These 250 GHz detections, as well as most of the millimeter-detected optically bright z~6 quasars, follow a shallower trend of L_{FIR} {L_{bol}}^{0.45} defined by the starburst-AGN systems in local and high-z universe. The millimeter continuum detections in the five objects and molecular CO detections in three of them reveal a few x10^8 M_sun of FIR-emitting warm dust and 10^10 M_sun of molecular gas in the quasar host galaxies. All these results argue for massive star formation in the quasar host galaxies, with estimated star formation rates of a few hundred M_sun yr^{-1}. Additionally, the higher FIR-to-AGN luminosity ratio found in these 250 GHz-detected faint quasars also suggests a higher ratio between star formation rate and supermassive black hole accretion rate than the UV/optically most luminous quasars at z~6.

COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies - II. The non-universality of the molecular gas depletion time-scale

We study the relation between molecular gas and star formation in a volume-limited sample of 222 galaxies from the COLD GASS survey, with measurements of the CO(1-0) line from the IRAM 30m telescope. The galaxies are at redshifts 0.025<z<0.05 and have stellar masses in the range 10.0<log(M*/Msun)<11.5. The IRAM measurements are complemented by deep Arecibo HI observations and homogeneous SDSS and GALEX photometry. A reference sample that includes both UV and far-IR data is used to calibrate our estimates of star formation rates from the seven optical/UV bands. The mean molecular gas depletion timescale, tdep(H2), for all the galaxies in our sample is 1 Gyr, however tdep(H2) increases by a factor of 6 from a value of ~0.5 Gyr for galaxies with stellar masses of 10^10 Msun to ~3 Gyr for galaxies with masses of a few times 10^11 Msun. In contrast, the atomic gas depletion timescale remains contant at a value of around 3 Gyr. This implies that in high mass galaxies, molecular and atomic gas depletion timescales are comparable, but in low mass galaxies, molecular gas is being consumed much more quickly than atomic gas. The strongest dependences of tdep(H2) are on the stellar mass of the galaxy (parameterized as log tdep(H2)= (0.36+/-0.07)(log M* - 10.70)+(9.03+/-0.99)), and on the specific star formation rate. A single tdep(H2) versus sSFR relation is able to fit both "normal" star-forming galaxies in our COLD GASS sample, as well as more extreme starburst galaxies (LIRGs and ULIRGs), which have tdep(H2) < 10^8 yr. Normal galaxies at z=1-2 are displaced with respect to the local galaxy population in the tdep(H2) versus sSFR plane and have molecular gas depletion times that are a factor of 3-5 times longer at a given value of sSFR due to their significantly larger gas fractions.

THE ESTIMATION OF STAR FORMATION RATES AND STELLAR POPULATION AGES OF HIGH-REDSHIFT GALAXIES FROM BROADBAND PHOTOMETRY

We explore methods to improve the estimates of star formation rates and mean stellar population ages from broadband photometry of high redshift star-forming galaxies. We use synthetic spectral templates with a variety of simple parametric star formation histories to fit broadband spectral energy distributions. These parametric models are used to infer ages, star formation rates and stellar masses for a mock data set drawn from a hierarchical semi-analytic model of galaxy evolution. Traditional parametric models generally assume an exponentially declining rate of star-formation after an initial instantaneous rise. Our results show that star formation histories with a much more gradual rise in the star formation rate are likely to be better templates, and are likely to give better overall estimates of the age distribution and star formation rate distribution of Lyman break galaxies. For B- and V-dropouts, we find the best simple parametric model to be one where the star formation rate increases linearly with time. The exponentially-declining model overpredicts the age by 100 % and 120 % for B- and V-dropouts, on average, while for a linearly-increasing model, the age is overpredicted by 9 % and 16 %, respectively. Similarly, the exponential model underpredicts star-formation rates by 56 % and 60 %, while the linearly-increasing model underpredicts by 15 % 22 %, respectively. For U-dropouts, the models where the star-formation rate has a peak (near z ~ 3) provide the best match for age -- overprediction is reduced from 110 % to 26 % -- and star-formation rate -- underprediction is reduced from 58 % to 22 %. We classify different types of star-formation histories in the semi-analytic models and show how the biases behave for the different classes. We also provide two-band calibration formulae for stellar mass and star formation rate estimations.

Molecular gas and star formation in early-type galaxies

We present new mm interferometric and optical integral-field unit (IFU) observations and construct a sample of 12 elliptical (E) and lenticular (S0) galaxies with molecular gas which have both CO and optical maps. The galaxies contain 2 × 107 to 5 × 109 M⊙ of molecular gas distributed primarily in central discs or rings (radii 0.5–4 kpc). The molecular gas distributions are always coincident with distributions of optically obscuring dust that reveal tightly wound spiral structures in many cases. The ionized gas always approximately corotates with the molecular gas, evidencing a link between these two gas components, yet star formation is not always the dominant ionization source. The galaxies with less molecular gas tend to have [O iii]/Hβ emission-line ratios at high values not expected for star formation. Most E/S0s with molecular gas have young or intermediate-age stellar populations based on optical colours, ultraviolet colours and absorption linestrengths. The few that appear purely old lie close to the limit where such populations would be undetectable based on the mass fractions of expected young to observed old stars. The 8 μm polycyclic aromatic hydrocarbon (PAH) and 24 μm emission yield similar star formation rate (SFR) estimates of E/S0s, but the total infrared overpredicts the rate due to a contribution to dust heating from older stars. The radio–far-infrared relation also has much more scatter than for other star-forming galaxies. However, despite these biases and additional scatter, the derived star formation rates locate the E/S0 galaxies within the large range of the Schmidt–Kennicutt and constant efficiency star formation laws. Thus, the star formation process in E/S0s is not overwhelmingly different than in other star-forming galaxies, although one of the more reliable tracers (24 μm) points to a possible lower star formation efficiency at a given gas surface density.

MOIRCS DEEP SURVEY. VIII. EVOLUTION OF STAR FORMATION ACTIVITY AS A FUNCTION OF STELLAR MASS IN GALAXIES SINCEz∼ 3

We study the evolution of star formation activity of galaxies at 0.5<z<3.5 as a function of stellar mass, using very deep NIR data taken with Multi-Object Infrared Camera and Spectrograph (MOIRCS) on the Subaru telescope in the GOODS-North region. The NIR imaging data reach K ~ 23-24 Vega magnitude and they allow us to construct a nearly stellar mass-limited sample down to ~ 10^{9.5-10} Msun even at z~3. We estimated star formation rates (SFRs) of the sample with two indicators, namely, the Spitzer/MIPS 24um flux and the rest-frame 2800A luminosity. The SFR distribution at a fixed Mstar shifts to higher values with increasing redshift at 0.5<z<3.5. More massive galaxies show stronger evolution of SFR at z>~1. We found galaxies at 2.5<z<3.5 show a bimodality in their SSFR distribution, which can be divided into two populations by a constant SSFR of ~2 Gyr^{-1}. Galaxies in the low-SSFR group have SSFRs of ~ 0.5-1.0 Gyr^{-1}, while the high-SSFR population shows ~10 Gyr^{-1}. The cosmic SFRD is dominated by galaxies with Mstar = 10^{10-11} Msun at 0.5<z<3.5, while the contribution of massive galaxies with Mstar = 10^{11-11.5} Msun shows a strong evolution at z>1 and becomes significant at z~3, especially in the case with the SFR based on MIPS 24um. In galaxies with Mstar = 10^{10-11.5} Msun, those with a relatively narrow range of SSFR (<~1 dex) dominates the cosmic SFRD at 0.5<z<3.5. The SSFR of galaxies which dominate the SFRD systematically increases with redshift. At 2.5<z<3.5, the high-SSFR population, which is relatively small in number, dominates the SFRD. Major star formation in the universe at higher redshift seems to be associated with a more rapid growth of stellar mass of galaxies.

100 μm and 160 μm emission as resolved star-formation rate estimators in M 33 (HERM33ES)

<i>Context. <i/>Over the past few years several studies have provided estimates of the SFR (star-formation rate) or the total infrared luminosity from just one infrared band. However these relations are generally derived for entire galaxies, which are known to contain a large scale diffuse emission that is not necessarily related to the latest star-formation episode.<i>Aims. <i/>We provide new relations to estimate the SFR from resolved star-forming regions at 100 <i>μ<i/>m and 160 <i>μ<i/>m.<i>Methods. <i/>We select individual star-forming regions in the nearby (840 kpc) galaxy M 33. We estimate the SFR combining the emission in H<i>α<i/> and at 24 <i>μ<i/>m to calibrate the emission at 100 <i>μ<i/>m and 160 <i>μ<i/>m as SFR estimators, as mapped with PACS/<i>Herschel<i/>. The data are obtained in the framework of the HERM33ES open time key program.<i>Results. <i/>There is less emission in the HII regions at 160 <i>μ<i/>m than at 100 <i>μ<i/>m. Over a dynamic range of almost 2 dex in <i>Σ<i/><sub><i>SFR<i/><sub/> we find that the 100 <i>μ<i/>m emission is a nearly linear estimator of the SFR, whereas that at 160 <i>μ<i/>m is slightly superlinear.<i>Conclusions. <i/>The behaviour of individual star-forming regions is surprisingly similar to that of entire galaxies. At high <i>Σ<i/><sub><i>SFR<i/><sub/>, star formation drives the dust temperature, whereas uncertainties and variations in radiation-transfer and dust-heated processes dominate at low <i>Σ<i/><sub><i>SFR<i/><sub/>. Detailed modelling of both galaxies and individual star forming regions will be needed to interpret similarities and differences between the two and assess the fraction of diffuse emission in galaxies.

THE PRESENT-DAY STAR FORMATION RATE OF THE MILKY WAY DETERMINED FROM SPITZER -DETECTED YOUNG STELLAR OBJECTS

We present initial results from a population synthesis model aimed at determining the star formation rate of the Milky-Way. We find that a total star formation rate of 0.68 to 1.45 Msun/yr is able to reproduce the observed number of young stellar objects in the Spitzer/IRAC GLIMPSE survey of the Galactic plane, assuming simple prescriptions for the 3D Galactic distributions of YSOs and interstellar dust, and using model SEDs to predict the brightness and color of the synthetic YSOs at different wavelengths. This is the first Galaxy-wide measurement derived from pre-main-sequence objects themselves, rather than global observables such as the total radio continuum, Halpha, or FIR flux. The value obtained is slightly lower than, but generally consistent with previously determined values. We will extend this method in the future to fit the brightness, color, and angular distribution of YSOs, and simultaneously make use of multiple surveys, to place constraints on the input assumptions, and reduce uncertainties in the star formation rate estimate. Ultimately, this will be one of the most accurate methods for determining the Galactic star formation rate, as it makes use of stars of all masses (limited only by sensitivity) rather than solely massive stars or indirect tracers of massive stars.

Near-infrared imaging and spectroscopy of the nuclear region of the disturbed Virgo cluster spiral NGC 4438â˜

We present near-infrared VLT ISAAC imaging and spectroscopy of the peculiar Virgo galaxy NGC 4438, whose nucleus has been classified as a LINER. The data are supplemented by mid-infrared imaging, and compared to previous WFPC2 HST broadband images. Images and position-velocity maps of the [Fe II] and H2 line emissions are presented and compared with the distribution of the optical narrow-line region and radio features. Our results show that shocks (possibly driven by a radio jet) contribute to an important fraction of the excitation of [Fe II], while X-ray heating from a central AGN may be responsible for the H2 excitation. We address the question whether the outflow has an AGN or a starburst origin by providing new estimates of the central star formation rate and the kinetic energy associated with the gas. By fitting a Sersic bulge, an exponential disc and a compact nuclear source to the light distribution, we decomposed NGC 4438's light distribution and found an unresolved nuclear source at 0.8 arcsec resolution with M_K = -18.7 and J-H = 0.69. Our measured bulge velocity dispersion, 142 km/s, together with the standard M_bh-sigma relation, suggests a central black hole mass of log(M_bh/Msun) ~ 7.0. The stellar kinematics measured from the near-infrared CO lines shows a strong peak in the velocity dispersion of 178 km/s in the central 0.5 arcsec, which is possible kinematic evidence of a central black hole. We calculated a general expression for the integrated Sersic profile flux density in elliptical geometry, including the case of 'disky' isophotes.

STAR FORMATION IN THE CENTRAL 400 PC OF THE MILKY WAY: EVIDENCE FOR A POPULATION OF MASSIVE YOUNG STELLAR OBJECTS

The central kpc of the Milky Way might be expected to differ significantly from the rest of the Galaxy with regard to gasdynamics and the formation of young stellar objects (YSOs). We probe this possibility with mid-infrared observations obtained with Infrared Array Camera and Multiband Imaging Photometer on Spitzer and with Midcourse Space Experiment. We use color-color diagrams and spectral energy distribution (SED) fits to explore the nature of YSO candidates (including objects with 4.5 {mu}m excesses possibly due to molecular emission). There is an asymmetry in the distribution of the candidate YSOs, which tend to be found at negative Galactic longitudes; this behavior contrasts with that of the molecular gas, approximately 2/3 of which is at positive longitudes. The small-scale height of these objects suggests that they are within the Galactic center region and are dynamically young. They lie between two layers of infrared dark clouds and may have originated from these clouds. We identify new sites for this recent star formation by comparing the mid-IR, radio, submillimeter, and methanol maser data. The methanol masers appear to be associated with young, embedded YSOs characterized by 4.5 {mu}m excesses. We use the SEDs of these sources to estimate their physicalmore » characteristics; their masses appear to range from {approx}10 to {approx}20 M{sub sun}. Within the central 400 x 50 pc (|l| < 1.{sup 0}3 and |b| < 10') the star formation rate (SFR) based on the identification of Stage I evolutionary phase of YSO candidates is about 0.14 M{sub sun} yr{sup -1}. Given that the majority of the sources in the population of YSOs are classified as Stage I objects, we suggest that a recent burst of star formation took place within the last 10{sup 5} yr. This suggestion is also consistent with estimates of SFRs within the last {approx}10{sup 7} yr showing a peak around 10{sup 5} yr ago. Lastly, we find that the Schmidt-Kennicutt Law applies well in the central 400 pc of the Galaxy. This implies that star formation does not appear to be dramatically affected by the extreme physical conditions in the Galactic center region.« less

X-Ray Selected Type 2 QSOs and Their Host Galaxies

AbstractWe present a large sample of X-ray selected type 2 QSOs from the XMM–COSMOS survey. Type 2 QSOs are luminous AGN whose central engines are obscured by large amounts of gas and dust. The selection criteria we have used are based on high X-ray luminosity (LX &gt; 1044 erg s−1) and heavy obscuration (NH &gt; 1022 cm−2). We derived stellar masses and star-formation rate estimates for the host galaxies from the best fit of the observed photometry. Type 2 QSOs are generally hosted in massive galaxies with on-going star formation.

MID-INFRARED GALAXY LUMINOSITY FUNCTIONS FROM THE AGN AND GALAXY EVOLUTION SURVEY

We present galaxy luminosity functions at 3.6, 4.5, 5.8, and 8.0 micron measured by combining photometry from the IRAC Shallow Survey with redshifts from the AGN and Galaxy Evolution Survey of the NOAO Deep Wide-Field Survey Bootes field. The well-defined IRAC samples contain 3800-5800 galaxies for the 3.6-8.0 micron bands with spectroscopic redshifts and z < 0.6. We obtained relatively complete luminosity functions in the local redshift bin of z < 0.2 for all four IRAC channels that are well fit by Schechter functions. We found significant evolution in the luminosity functions for all four IRAC channels that can be fit as an evolution in M* with redshift, \Delta M* = Qz. While we measured Q=1.2\pm0.4 and 1.1\pm0.4 in the 3.6 and 4.5 micron bands consistent with the predictions from a passively evolving population, we obtained Q=1.8\pm1.1 in the 8.0 micron band consistent with other evolving star formation rate estimates. We compared our LFs with the predictions of semi-analytical galaxy formation and found the best agreement at 3.6 and 4.5 micron, rough agreement at 8.0 micron, and a large mismatch at 5.8 micron. These models also predicted a comparable Q value to our luminosity functions at 8.0 micron, but predicted smaller values at 3.6 and 4.5 micron. We also measured the luminosity functions separately for early and late-type galaxies. While the luminosity functions of late-type galaxies resemble those for the total population, the luminosity functions of early-type galaxies in the 3.6 and 4.5 micron bands indicate deviations from the passive evolution model, especially from the measured flat luminosity density evolution. Combining our estimates with other measurements in the literature, we found (53\pm18)% of the present stellar mass of early-type galaxies has been assembled at z=0.7.