Exploring star formation using the filaments in the Sloan Digital Sky Survey Data Release Five

We have quantified the average filamentarity of the galaxy distribution in seven nearly two-dimensional strips from the Sloan Digital Sky Survey Data Release Five (SDSS DR5) using a volume-limited sample in the absolute magnitude range  −21 ≤ M _r ≤−20  . The average filamentarity of star-forming (SF) galaxies, which are predominantly blue, is found to be more than that of other galaxies which are predominantly red. This difference is possibly an outcome of the fact that blue galaxies have a more filamentary distribution. Comparing the SF galaxies with only the other blue galaxies, we find that the two show nearly equal filamentarity. Separately analyzing the galaxies with high star formation rates (SFR) and low SFR, we find that the latter has a more filamentary distribution. We interpret this in terms of two effects. (i) A correlation between the SFR and individual galaxy properties like luminosity with the high-SFR galaxies being more luminous. (ii) A relation between the SFR and environmental effects like the density with the high-SFR galaxies preferentially occurring in high-density regions. These two effects are possibly not independent and are operating simultaneously. We do not find any difference in the filamentarity of SF galaxies and active galactic nuclei.     

The Evolution of Star-Forming Galaxies: The SFR Density of the Universe

The evolution of the Star Formation Rate (SFR) density of the Universe as a function of look-back time is a fundamental parameter in order to understand the formation and evolution of galaxies. The current picture, only outlined in the last years, is that the global SFR density has dropped by about an order of magnitude from a redshift of z∼1.5 to the current value at z=0. Because these SFR density studies are now extended to the whole range in redshift, it becomes mandatory to combine data from different SFR tracers. At low redshifts, optical emission lines are the most widely used. Using Hα as current-SFR tracer, the Universidad Complutense de Madrid (UCM) Survey provided the first estimation of the global SFR density in the Local Universe. The Hα flux in emission is directly related to the number of ionizing photons and, modulo IMF, to the total mass of stars formed. Metallic lines like [OII]λ3727 and [OIII]λ5007 are affected by metallicity and excitation. Beyond redshifts z∼0.4, Hα is not observable in the optical and [OII]λ3727 or UV luminosities have to be used. The UCM galaxy sample has been used to obtain a calibration between [OII]λ3727 luminosity and SFR specially suitable for the different types of star-forming galaxies found by deep spectroscopic surveys in redshifts up to z∼1.5. These calibrations, when applied to recent deep redshift surveys confirm the drop of the SFR density of the Universe since z∼1 previously infered in the UV. However, the fundamental parameter that determines galactic evolution is mass, not luminosity. The mass function for local star-forming galaxies is critical for any future comparison with other galaxy populations of different evolutionary status. Hα velocity-widths for UCM galaxies indicate that besides a small fraction of 10¹⁰-10¹¹ M_⊙ starburst nuclei spirals, the majority have dynamical masses in the ∼10⁹ M_⊙ range. A comparison with published data for faint blue galaxies suggests that star-forming galaxies at z∼1 would have SFR per unit mass and burst strengths similar to those at z=0, but being intrinsically more massive. This revised version was published online in July 2006 with corrections to the Cover Date.     

Emission-line galaxies from SDSS DR4: Statistical studies of the current star formation

For a sample of 8156 emission-line galaxies from the Sloan Digital Sky Survey Data Release 4 (SDSS DR4), we have determined the current star formation rates (SFR) from three parameters: the H_α luminosity of H II regions corrected for the aperture and interstellar extinction (N = 7006 galaxies), the far-infrared luminosity (IRAS data, N = 350), and the monochromatic luminosity in the radio continuum at ν = 1.4 GHz (NVSS data, N = 475). A Salpeter initial mass function with the range of stellar masses 0.1–100 M _⊙ was assumed in the SFR _FIR calculations. In calculating SFR_1.4, we assumed that the fraction of the thermal emission in the total radio continuum emission of the galaxy at 1.4 GHz was 10%. An upper limit for the starburst age has been determined for galaxies with known abundances of heavy elements. We compare our results with those of similar studies for isolated and Markarian H II galaxies.     

Evolution of oxygen and nitrogen abundances in SDSS galaxies

New expressions for ON and NS calibrations (we call them ON11 and NS11 calibrations) for the determination of oxygen and nitrogen abundances in HII regions are proposed. New criteria for the division of HII regions into three classes (hot, warm, and cold) are also proposed. ON11 and NS11 calibrations provide more accurate determination of oxygen and nitrogen abundances from integrated emission spectra, in comparison with ON and NS calibrations. Oxygen and nitrogen abundances are estimated for a sample of 57 470 SDSS galaxies using ON11 and NS11 calibrations. Strongline flux measurements for SDSS spectra of these galaxies are taken from the MPA/JHU catalog. A ??mass-metallicity?? diagram for several redshift ranges is constructed. It is found that the interstellar medium of galaxies with high masses log(M/M _??) > 11.3 was hardly enriched with either oxygen or nitrogen over the last 4 Gyrs (corresponding to the redshift interval z = 0?C0.4). This indicates that an active stage of star formation in massive galaxies was completed more than 4 Gyrs ago. For galaxies of lower masses, changes in oxygen and nitrogen abundances over the time period under consideration increase as galaxy mass decreases.     

Star-formation rate in compact star-forming galaxies

We use the data for the \(\text{H}\beta\) emission-line, far-ultraviolet (FUV) and mid-infrared 22 μm continuum luminosities to estimate star formation rates \(\langle \mbox{SFR} \rangle \) averaged over the galaxy lifetime for a sample of about 14000 bursting compact star-forming galaxies (CSFGs) selected from the Data Release 12 (DR12) of the Sloan Digital Sky Survey (SDSS). The average coefficient linking \(\langle \mbox{SFR} \rangle \) and the star formation rate \(\mbox{SFR}_{0}\) derived from the \(\text{H}\beta\) luminosity at zero starburst age is found to be 0.04. We compare \(\langle \mbox{SFR} \rangle \mbox{s}\) with some commonly used SFRs which are derived adopting a continuous star formation during a period of \({\sim}\,100~\mbox{Myr}\), and find that the latter ones are 2–3 times higher. It is shown that the relations between SFRs derived using a geometric mean of two star-formation indicators in the UV and IR ranges and reduced to zero starburst age have considerably lower dispersion compared to those with single star-formation indicators. We suggest that our relations for \(\langle \mbox{SFR} \rangle \) determination are more appropriate for CSFGs because they take into account a proper temporal evolution of their luminosities. On the other hand, we show that commonly used SFR relations can be applied for approximate estimation within a factor of \({\sim}\,2\) of the \(\langle \mbox{SFR} \rangle \) averaged over the lifetime of the bursting compact galaxy.     

Interaction-induced star formation in a complete sample of 105 nearby star-forming galaxies

We investigate the clustering properties of a complete sample of 10⁵ star-forming galaxies drawn from the data release 4 (DR4) of the Sloan Digital Sky Survey. On scales less than 100 kpc, the amplitude of the correlation function exhibits a strong dependence on the specific star formation rate (SSFR) of the galaxy. We interpret this as the signature of enhanced star formation induced by tidal interactions. We then explore how the average star formation rate (SFR) in a galaxy is enhanced as the projected separation r _p between the galaxy and its companions decreases. We find that the enhancement strongly depends on r _p, but very weakly on the relative luminosity of the companions. The enhancement is also stronger in low-mass galaxies than in high-mass galaxies. In order to explore whether a tidal interaction is not only sufficient, but also necessary to trigger enhanced star formation in a galaxy, we compute background subtracted neighbour counts for the galaxies in our sample. The average number of close neighbours around galaxies with low to average values of SFR/ M _* is close to zero. At the highest SSFRs, however, more than 40 per cent of the galaxies in our sample have a companion within a projected radius of 100 kpc. Visual inspection of the highest SFR/ M _* galaxies without companions reveals that more than 50 per cent of these are clear interacting or merging systems. We conclude that tidal interactions are the dominant trigger of enhanced star formation in the most strongly star-forming systems. Finally, we find clear evidence that tidal interactions not only lead to enhanced star formation in galaxies, but also cause structural changes such as an increase in concentration.     

Investigation of the correlation between morphology and luminosity for two classes of main galaxies

Using an apparent-magnitude limited Main galaxy sample of the Sloan Digital Sky Survey Data Release 7(SDSS DR7), we investigate the correlation between morphologies and luminosity for the Main galaxy sample. Our Main galaxy sample is divided into two classes: Main galaxies only with TARGET_GALAXY flag (bestPrimtarget = 64), and ones also with other flags. It is found that for the second class Main galaxies, the early-type proportion monotonously increases with increasing luminosity nearly in the whole luminosity region. But for the first class Main galaxies, the early-type proportion increases with increasing luminosity only within a certain luminosity region (−22.2 < M _r  < −19.8). In the high luminosity region (M _r  < −22.2), the early-type proportion of the first class Main galaxies even decreases dramatically with increasing luminosity. We also analyze the correlation between morphologies and luminosity of galaxies around the peak of the redshift distribution ( 0.07 ≤ z ≤ 0.08 ). In such a narrow redshift region, we still observe strong correlation between morphologies and luminosity, which shows that this correlation is fundamental.     

Effects of galaxy interactions in different environments

We analyse star formation rates (SFRs) derived from photometric and spectroscopic data of galaxies in pairs in different environments using the 2-degree field galaxy redshift survey (2dFGRS) and the Sloan digital sky survey (SDSS). The two samples comprise several thousand pairs, suitable to explore into detail the dependence of star formation activity in pairs on orbital parameters and global environment. We use the projected galaxy density derived from the fifth brightest neighbour of each galaxy, with a convenient luminosity threshold to characterize environment in both surveys in a consistent way. Star formation activity is derived through the η parameter in 2dFGRS and through the SFR normalized to the total mass in stars,  SFR/ M *  , given by Brinchmann et al. in the SDSS-second data release (SDSS-DR2). For both galaxy pair catalogs, the star formation birth rate parameter is a strong function of the global environment and orbital parameters. Our analysis on SDSS pairs confirms previous results found with the 2dFGRS where suitable thresholds for the star formation activity induced by interactions are estimated at a projected distance   r _p= 100  h ⁻¹ kpc  and a relative velocity  Δ V = 350 km s⁻¹  . We observe that galaxy interactions are more effective at triggering important star formation activity in low- and moderate-density environments with respect to the control sample of galaxies without a close companion. Although close pairs have a larger fraction of actively star-forming galaxies, they also exhibit a greater fraction of red galaxies with respect to those systems without a close companion, an effect that may indicate that dust stirred up during encounters could affect colours and, partially, obscure tidally induced star formation.     

Normal galaxies in the all-sky survey by the eROSITA X-ray telescope of the Spectrum-X-Gamma observatory

We analyze the statistical properties of normal galaxies to be detected in the all-sky survey by the eROSITA X-ray telescope of the Spectrum-X-Gamma observatory. With the current configuration and parameters of the eROSITA telescope, the sensitivity of a 4-year-long all-sky survey will be ≈10^?14 erg s^?1 in the 0.5–2 keV band. This will allow ～(1.5–2) × 10⁴ normal galaxies with approximately the same contribution of star-forming and elliptical galaxies to be detected. All galaxies of the X-ray survey are expected to enter into the existing far-infrared (IRAS) or near-infrared (2MASS) catalogs; the sample of star-forming galaxies will be approximately equivalent in sensitivity to the sample of star-forming galaxies in the IRAS catalog of infrared sources. Thus, a large homogeneous sample of normal galaxies with measured X-ray, near-infrared, and far-infrared fluxes will be formed. About 90% of the galaxies in the survey are located within ～200–400 Mpc. A typical (most probable) galaxy will have a luminosity log L _X ～ 40.5–41.0, will be located at a distance of ～70–90 Mpc, and will be either a star-forming galaxy with a star formation rate of ～20M _⊙ yr^?1 whose X-ray emission is produced by ultraluminous X-ray sources (ULXs) or an elliptical galaxy with amass log M _* ～ 11.3 emitting through to a hot interstellar gas. The galaxies within 35 Mpc will collectively contain ～10² ULXs with luminosities log L _X > 40, ～80% of whichwill be the only luminous source in the galaxy. Thus, although the angular resolution of the eROSITA telescope is too low for the luminosity function of compact sources in galaxies to be studied in detail, the survey data will allow one to investigate its bright end and, possibly, to impose constraints on the maximum luminosity of ULXs.     

A sample of radio-loud active galactic nuclei in the Sloan Digital Sky Survey

A sample of 2712 radio-luminous galaxies is defined from the second data release of the Sloan Digital Sky Survey (SDSS) by cross-comparing the main spectroscopic galaxy sample with two radio surveys: the National Radio Astronomy Observatories (NRAO) Very Large Array (VLA) Sky Survey (NVSS) and the Faint Images of the Radio Sky at Twenty centimeters (FIRST) survey. The comparison is carried out in a multistage process and makes optimal use of both radio surveys by exploiting the sensitivity of the NVSS to extended and multicomponent radio sources in addition to the high angular resolution of the FIRST images. A radio source sample with 95 per cent completeness and 98.9 per cent reliability is achieved, far better than would be possible for this sample if only one of the surveys was used. The radio source sample is then divided into two classes: radio-loud active galactic nuclei (AGN) and galaxies in which the radio emission is dominated by star formation. The division is based on the location of a galaxy in the plane of 4000-Å break strength versus radio luminosity per unit stellar mass and provides a sample of 2215 radio-loud AGN and 497 star-forming galaxies brighter than 5 mJy at 1.4 GHz. A full catalogue of positions and radio properties is provided for these sources. The local radio luminosity function is then derived both for radio-loud AGN and for star-forming galaxies and is found to be in agreement with previous studies. By using the radio to far-infrared (FIR) correlation, the radio luminosity function of star-forming galaxies is also compared to the luminosity function derived in the FIR. It is found to agree well at high luminosities but less so at lower luminosities, confirming that the linearity of the radio to FIR correlation breaks down below about 10²² W Hz⁻¹ at 1.4 GHz.     

A Comparison of Star Formation Rate Indicators for Galaxies

With the multi-wavelength data from UV to sub-millimeter in the region of H-ATLAS (Herschel Astrophysical Terahertz Large Area Survey) Science Demonstration Phase (SDP), in combination with the population synthesis model and dust model, the total infrared luminosities of the galaxies were calculated. On this basis, for respectively the strong and weak star-forming galaxies, we studied the differences in the star formation rates calculated by the UV luminosity, infrared luminosity and H_α line, as well as the intrinsic physical origin of such differences. It was found that for the galaxies of strong star-formation activity, the 3 kinds of star formation rate indicators give the basically consistent results with a small dispersion. But at the end of high star formation rate, the star formation rate calculated by the UV luminosity is slightly smaller than that calculated by the H_α-line flux; at the end of low star formation rate, the UV indicator tends to be greater than the H_α indicator; and at both ends, the infrared indicator and H_α indicator have no significant difference. For the weak star-forming galaxies, significant differences exist among the 3 kinds of indicators, and there is a rather large dispersion. The dispersions and systematic difference of the star formation rates calculated by the UV luminosity and H_α line increase with the galactic age and mass. The main cause for the increased systematic difference is that when the extinction of an weak star-forming galaxy is calibrated by its UV continuum spectral slope β, the UV extinction of the galaxy is overestimated, it makes the UV luminosity tends to be large after the extinction correction. In addition, the star formation rates (Hα) of weak starforming galaxies in the MPA/JHU (Max Planck Institute for Astrophysics/Johns Hopkins University) database are generally less than the real values.     

Correlations between environment and other properties of galaxies from the Sloan Digital Sky Survey at fixed color

From the volume-limited Main galaxy sample of the Sloan Digital Sky Survey Data Release 6 (SDSS DR6), we construct three samples with g–r color bins , labeled S1–S3, to investigate how other properties of galaxies depend on environment at fixed color. For each sample, we measure the local three-dimensional galaxy density in a comoving sphere with radius equal to the distance to the 5th nearest galaxy for each galaxy, select about 5% galaxies and construct the two subsamples at both extremes of density. Our study suggests that the environmental dependence of luminosity is mainly due to the environmental dependence of galaxy color and the correlation between color and luminosity. In addition, we preferentially conclude that concentration index and morphologies are not strongly correlated with local density at fixed color, and that galaxy color is a galaxy property very predictive of the local environment. Because SDSS spectroscopy is incomplete for bright galaxies at very low redshifts, we also use a volume-limited Main galaxy sample with a lower redshift limit z = 0.05, which contains 94,954 galaxies at 0.05 < z < 0.089 with −22.40 < M_r < −20.16, and reach the same conclusions.Due to the bimodality of the u–r color distribution, we classify galaxies as ‘red’ and ‘blue’, respectively, and further subdivide the samples into star-forming galaxies and passive ones using Hα equivalent width, W₀(Hα). Results show that color and star formation activity of galaxies are galaxy properties very predictive of the local environment.     

Large primordial gravitational wave background in a class of BSI ΛCDM models

We study the luminosity function and clustering properties of subsamples of local galaxies selected from the Stromlo–APM Survey by the rest-frame equivalent widths of their H α and [O  ii ] emission lines. The b _J luminosity function of star-forming galaxies has a significantly steeper faint-end slope than that for quiescent galaxies: the majority of sub- L * galaxies are currently undergoing significant star formation. Emission-line galaxies are less strongly clustered, both amongst themselves and with the general galaxy population, than are quiescent galaxies. Thus as well as being less luminous, star-forming galaxies also inhabit lower density regions of the Universe than quiescent galaxies.     

The Nature of the Faint Radio Source Population and Star Formation History derived from Sub-MJY Surveys

We present new results on the nature of the faint radio galaxy population obtained from a deep identification study in the Marano Field. This work represents the highest identification fractions attained to date in literature for sub-mJy radio samples. Star-forming galaxies constitute a significant fraction of the optical counterparts of sub-mJy radio sources, although they are not the dominant identification class (as formerly believed). We also present a new determination of the co-moving star formation density from z=0 to z?0.35 using the radio luminosity function of star-forming galaxies. For the local determination, we used a sample of 231 spiral galaxies taken from the B≤12 Revised Shapley-Ames Catalogue, while for the non-local determination, we used a sample of87 star-forming galaxies taken from the Benn et al. (1993) sample. The inferred local star formation rate is about twice as great as the H_α estimate, while the intermediate 0.05<z<0.35 star formation rate is consistent with coeval ultraviolet and H_α estimates. This is due to large-scale structure, rarely accounted for in estimates of the cosmic star formation history; the situation could improve significantly with deep moderate-area radio surveys.     

Optical and infrared diagnostics of SDSS galaxies in the SWIRE survey

We present the rest-frame optical and infrared colours of a complete sample of  1114 z < 0.3  galaxies from the Spitzer Wide-Area Infrared Extragalactic (SWIRE) Legacy Survey and the Sloan Digital Sky Survey (SDSS). We discuss the optical and infrared colours of our sample and analyse in detail the contribution of dusty star-forming galaxies and active galactic nuclei (AGN) to optically selected red sequence galaxies.
We propose that the optical  ( g − r )  colour and infrared  log( L ₂₄/ L _3.6)  colour of galaxies in our sample are determined primarily by a bulge-to-disc ratio. The  ( g − r )  colour is found to be sensitive to the bulge-to-disc ratio for disc-dominated galaxies, whereas the  log( L ₂₄/ L _3.6)  colour is more sensitive for bulge-dominated systems.
We identify ∼18 per cent (195 sources) of our sample as having red optical colours and infrared excess. Typically, the infrared luminosities of these galaxies are found to be at the high end of star-forming galaxies with blue optical colours. Using emission-line diagnostic diagrams, 78 are found to have an AGN contribution and 117 are identified as star-forming systems. The red  ( g − r )  colour of the star-forming galaxies could be explained by extinction. However, their high optical luminosities cannot. We conclude that they have a significant bulge component.
The number densities of optically red star-forming galaxies are found to correspond to ∼13 per cent of the total number density of our sample. In addition, these systems contribute ∼13 per cent of the total optical luminosity density, and 28 per cent of the total infrared luminosity density of our SWIRE/SDSS sample. These objects may reduce the need for 'dry mergers'.     

The morphology,kinematics and metallicity of blue-core galaxies

We select 107 blue-core galaxies from the MaNGA survey, studying their morphology, kinematics as well as the gas-phase metallicity. Our results are as follows:(i) In our sample, 26% of blue-core galaxies have decoupled gas-star kinematics, indicating external gas accretion;15% have bar-like structure and 8% show post-merger features, such as tidal tails and irregular gas/star velocity field. All these processes/features, such as accreting external misaligned gas, interaction and bar, can trigger gas inflow. Thus the central star-forming activities lead to bluer colors in their centers(blue-core galaxies).(ii) By comparing with the SDSS DR7 star-forming galaxy sample, we find that the blue-core galaxies have higher central gas-phase metallicity than what is predicted by the local mass-metallicity relation. We explore the origin of the higher metallicity, finding that not only the blue-core galaxies, but also the flat-gradient and red-core galaxies all have higher metallicity. This can be explained by the combined effect of redshift and galaxy color.     

Evolution of galaxy groups

We study the variations of the properties of groups of galaxies with dynamical masses of 10¹³ M _⊙<M ₂₀₀<10¹⁴ M _⊙, represented by two samples: one has redshifts of z < 0.027 and is located in the vicinity of the Coma cluster, the other has z > 0.027, and is located in the regions of the following superclusters of galaxies: Hercules, Leo, Bootes, Ursa Major, and Corona Borealis. Using the archived data of the SDSS and 2MASX catalogs, we determined the concentration of galaxies in the systems by measuring it as the inner density of the group within the distance of the fifth closest galaxy from the center brighter than M _K = ?23_. ^m 3. We also measured the magnitude gap between the first and the fourth brightest galaxies ΔM ₁₄ located within one half of the selected radius R ₂₀₀, the fraction of early-type galaxies, and the ratio of bright dwarf galaxies (Mr = [?18_. ^m 5,?16_. ^m 5]) to giant galaxies (M _r < ?18_. ^m 5) (DGR) within the radius R ₂₀₀. The main aim of the investigation is to find among these characteristics the ones that reflect the evolution of groups of galaxies.We determined that the ratio of bright dwarf galaxies to early-type giant galaxies on the red sequence depends only on the x-ray luminosity: the DGR increases with luminosity. The fraction of early-type galaxies in the considered systems is equal, on average, to 0.65 ± 0.01, and varies significantly for galaxies with σ₂₀₀ < 300 kms^?1. Based on the luminosity of the brightest galaxy, the magnitude gap between the first and the fourth brightest galaxies in the groups, and on model computations of these parameters, we selected four fossil group candidates: AWM4, NGC0533, NGC0741, and NGC6098 (where the brightest galaxy is a double).We observe no increase in the number of faint galaxies (the α parameter of the Schechter function is less than 1) in our composite luminosity function (LF) for galaxy systems with z < 0.027 in the M _K = [?26m,?21_. ^m 5] range, whereas earlier we obtained α > 1 for the LF of the Hercules and Leo superclusters of galaxies.     

Paired galaxies with different activity levels and their supernovae

We investigate the influence of close neighbor galaxies on the properties of supernovae (SNe) and their host galaxies using 56 SNe located in pairs of galaxies with different levels of star formation (SF) and nuclear activity. The statistical study of SN hosts shows that there is no significant difference between morphologies of hosts in our sample and the larger general sample of SN hosts in the Sloan Digital Sky Survey (SDSS) Data Release 8 (DR8). The mean distance of type II SNe from nuclei of hosts is greater by about a factor of 2 than that of type Ibc SNe. The distributions and mean distances of SNe are consistent with previous results compiled with the larger sample. For the first time it is shown that SNe Ibc are located in pairs with significantly smaller difference of radial velocities between components than pairs containing SNe Ia and II. We consider this as a result of higher star formation rate (SFR) of these closer systems of galaxies. SN types are not correlated with the luminosity ratio of host and neighbor galaxies in pairs. The orientation of SNe with respect to the preferred direction toward neighbor galaxy is found to be isotropic and independent of kinematical properties of the galaxy pair.     

Alignment between galaxies and large-scale structure

Based on the Sloan Digital Sky Survey DR6 (SDSS) and the Millennium Simulation (MS), we investigate the alignment between galaxies and large-scale struc-ture. For this purpose, we develop two new statistical tools, namely the alignment cor-relation function and the cos(20)-statistic. The former is a two-dimensional extension of the traditional two-point correlation function and the latter is related to the ellipticity correlation function used for cosmic shear measurements. Both are based on the cross correlation between a sample of galaxies with orientations and a reference sample which represents the large-scale structure. We apply the new statistics to the SDSS galaxy cat-alog. The alignment correlation function reveals an overabundance of reference galaxies along the major axes of red, luminous (L L*) galaxies out to projected separations of 60 h-1Mpc. The signal increases with central galaxy luminosity. No alignment signal is detected for blue galaxies. The cos(2θ)-statistic yields very similar results. Starting from a MS semi-analytic galaxy catalog, we assign an orientation to each red, luminous and central galaxy, based on that of the central region of the host halo (with size similar to that of the stellar galaxy). As an alternative, we use the orientation of the host halo itself. We find a mean projected misalignment between a halo and its central region of ~ 25°. The misalignment decreases slightly with increasing luminosity of the central galaxy. Using the orientations and luminosities of the semi-analytic galaxies, we repeat our alignment analysis on mock surveys of the MS. Agreement with the SDSS results is good if the central orientations are used. Predictions using the halo orientations as proxies for cen-tral galaxy orientations overestimate the observed alignment by more than a factor of 2. Finally, the large volume of the MS allows us to generate a two-dimensional map of the alignment correlation function, which shows the reference galaxy distribution to be flat-tened parallel to the orientations of red luminous galaxies with axis ratios of ~ 0.5 and ~ 0.75 for halo and central orientations, respectively. These ratios are almost independent of scale out to 60h-1Mpc.