arXiv subjects

Anna Pasquali

Publications and source records attributed to Anna Pasquali.

At least 55 records · Page 3Linked to original sources

arXiv2017

FIGS -- Faint Infrared Grism Survey: Description and Data Reduction

The Faint Infrared Grism Survey (FIGS) is a deep Hubble Space Telescope (HST) WFC3/IR (Wide Field Camera 3 Infrared) slitless spectroscopic survey of four deep fields. Two fields are located in the Great Observatories Origins Deep Survey-North (GOODS-N) area and two fields are located in the Great Observatories Origins Deep Survey-South (GOODS-S) area. One of the southern fields selected is the Hubble Ultra Deep Field. Each of these four fields were observed using the WFC3/G102 grism (0.8$μm$-1.15$μm$ continuous coverage) with a total exposure time of 40 orbits (~ 100 kilo-seconds) per field. This reaches a 3 sigma continuum depth of ~26 AB magnitudes and probes emission lines to $\approx 10^{-17}\ erg\ s^{-1} \ cm^{-2}$. This paper details the four FIGS fields and the overall observational strategy of the project. A detailed description of the Simulation Based Extraction (SBE) method used to extract and combine over 10000 spectra of over 2000 distinct sources brighter than m_F105W=26.5 mag is provided. High fidelity simulations of the observations is shown to significantly improve the background subtraction process, the spectral contamination estimates, and the final flux calibration. This allows for the combination of multiple spectra to produce a final high quality, deep, 1D-spectra for each object in the survey.

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arXiv2017

A High-Resolution Multiband Survey of Westerlund 2 With the Hubble Space Telescope. III. The present-day stellar mass function

We present a detailed analysis of the spatial distribution of the stellar population and the present-day mass function (PDMF) of the Westerlund 2 (Wd2) region using the data from our high resolution multi-band survey with the Hubble Space Telescope. We used state-of-the-art artificial star tests to determine spatially resolved completeness maps for each of the broad-band filters. We reach a level of completeness of 50 % down to F555W=24.8 mag (0.7 $M_\odot$) and F814W=23.3 mag (0.2 $M_\odot$) in the optical and F125W=20.2 mag and F160W=19.4 mag (both 0.12 $M_\odot$) in the infrared throughout the field of view. We had previously reported that the core of Wd2 consists of two clumps: namely the main cluster (MC) and the northern clump (NC). From the spatial distribution of the completeness corrected population, we find that their stellar surface densities are 1114 stars pc$^{-2}$ and 555 stars pc$^{-2}$, respectively, down to F814W=21.8 mag. We find that the present-day mass function (PDMF) of Wd2 has a slope of $Γ=-1.46 \pm 0.06$, which translates to a total stellar cluster mass of $(3.6 \pm 0.3) \cdot 10^4 M_\odot$. The spatial analysis of the PDMF reveals that the cluster population is mass-segregated, most likely primordial. In addition, we report the detection of a stellar population of spatially uniformly distributed low-mass (<0.15 $M_\odot$) stars, extending into the gas ridges of the surrounding gas and dust cloud, as well as a confined region of reddened stars, likely caused by a foreground CO cloud. We find hints that a cloud-cloud collision might be the origin of the formation of Wd2.

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arXiv2017

Observing the products of stellar evolution in the old open cluster M67 with APOGEE

Recent works have shown how the [C/N] ratio in stars after the first dredge-up (FDU) can be used as an age estimator in virtue of its dependence on stellar mass. For this purpose, precise predictions of the surface chemical composition before and after the mixing takes place in the convective envelope of subgiant stars are necessary. Stellar evolution models can provide us with such predictions, although a comparision with objects of known age is needed for calibration. Open clusters are excellent test cases, as they represent a single stellar population for which the age can be derived through, e.g., isochrone fitting. In this study, we present a detailed analysis of stars belonging to the well-known open cluster M67 observed by the APOGEE survey in the twelfth data release of the Sloan Digital Sky Survey and whose chemical properties were derived with the ASPCAP pipeline. We find that the [C/N] abundance of subgiant branch stars is overestimated by $\sim0.2$ dex due to an offset in the determination of the [N/Fe] abundance. Stars on the red giant branch and red clump are shown not to be affected by this offset. We derive $\text{[C/N]}_{FDU}=-0.46\pm 0.03$ dex, which poses a strong constraint on calibrations of $\text{[C/N]}_{FDU}$ as age indicator. We also do not find any clear signature of additional chemical mixing processes that set in after the red giant branch bump. The results obtained for M67 indicate the importance of conducting high-resolution spectroscopic studies of open clusters of different ages in order to establish an accurate age-dating method for field stars.

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arXiv2016

The IMF as a function of supersonic turbulence

Recent studies seem to suggest that the stellar initial mass function (IMF) in early-type galaxies might be different from a classical Kroupa or Chabrier IMF, i.e. contain a larger fraction of the total mass in low-mass stars. From a theoretical point of view, supersonic turbulence has been the subject of interest in many analytical theories proposing a strong correlation with the characteristic mass of the core mass function (CMF) in star forming regions, and as a consequence with the stellar IMF. Performing two suites of smoothed particles hydrodynamics (SPH) simulations with different mass resolutions, we aim at testing the effects of variations in the turbulent properties of a dense, star forming molecular cloud on the shape of the system mass function in different density regimes. While analytical theories predict a shift of the peak of the CMF towards lower masses with increasing velocity dispersion of the cloud, we observe in the low-density regime the opposite trend, with high Mach numbers giving rise to a top-heavy mass distribution. For the high-density regime we do not find any trend correlating the Mach number with the characteristic mass of the resulting IMF, implying that the dynamics of protostellar accretion discs and fragmentation on small scales is not strongly affected by turbulence driven at the scale of the cloud. Furthermore, we suggest that a significant fraction of dense cores are disrupted by turbulence before stars can be formed in their interior through gravitational collapse. Although this particular study has limitations in its numerical resolution, we suggest that our results, along with those from other studies, cast doubt on the turbulent fragmentation models on the IMF that simply map the CMF to the IMF.

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arXiv2016

A High-Resolution Multiband Survey of Westerlund 2 With the Hubble Space Telescope. II. Mass accretion in the Pre-Main Sequence Population

We present a detailed analysis of the pre-main-sequence (PMS) population of the young star cluster Westerlund~2 (Wd2), the central ionizing cluster of the HII region RCW 49, using data from a high resolution multi-band survey with the Hubble Space Telescope. The data were acquired with the Advanced Camera for Surveys in the F555W, F814W, and F658N filters and with the Wide Field Camera 3 in the F125W, F160W, and F128N filters. We find a mean age of the region of 1.04+-0.72 Myr. The combination of dereddened F555W and F814W photometry in combination with F658N photometry allows us to study and identify stars with H_alpha excess emission. With a careful selection of 240 bona-fide PMS H_alpha excess emitters we were able to determine their H_alpha luminosity, which has a mean value L(H_alpha)=1.67 x 10^{-31} erg s^{-1}. Using the PARSEC 1.2S isochrones to obtain the stellar parameters of the PMS stars we determined a mean mass accretion rate \dot M_acc=4.43 x 10^{-8} M_sun yr^{-1} per star. A careful analysis of the spatial dependence of the mass-accretion rate suggests that this rate is ~25% lower in center of the two density peaks of Wd2 in close proximity to the luminous OB stars, compared to the Wd2 average. This rate is higher with increasing distance from the OB stars, indicating that the PMS accretion disks are being rapidly destroyed by the far-ultra-violet radiation emitted by the OB population.

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arXiv2016

Peculiar compact stellar systems in the Fornax cluster

We search for hints to the origin and nature of compact stellar systems in the magnitude range of ultracompact dwarf galaxies in deep wide-field imaging data of the Fornax cluster core. We visually investigate a large sample of 355 spectroscopically confirmed cluster members with V-band equivalent magnitudes brighter than -10 mag for faint extended structures. Our data reveal peculiar compact stellar systems, which appear asymmetric or elongated from their outer light distribution. We characterize the structure of our objects by quantifying their core concentration, as well as their outer asymmetry and ellipticity. For the brighter objects of our sample we also investigate their spatial and phase-space distribution within the cluster. We argue that the distorted outer structure alone that is seen for some of our objects, is not sufficient to decide whether these systems have a star cluster or a galaxy origin. However, we find that objects with low core concentration and high asymmetry (or high ellipticity) are primarily located at larger cluster-centric distances as compared to the entire sample. This supports the hypothesis that at least some of these objects may originate from tidally stripped galaxies.

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arXiv2016

The Massive Stellar Population of W49: A Spectroscopic Survey

Massive stars form on different scales ranging from large, dispersed OB associations to compact, dense starburst clusters. The complex structure of regions of massive star formation, and the involved short timescales provide a challenge for our understanding of their birth and early evolution. As one of the most massive and luminous star-forming region in our Galaxy, W49 is the ideal place to study the formation of the most massive stars. By classifying the massive young stars deeply embedded into the molecular cloud of W49, we aim to investigate and trace the star formation history of this region. We analyse near-infrared $K$-band spectroscopic observations of W49 from LBT/LUCI combined with $JHK$ images obtained with NTT/SOFI and LBT/LUCI. Based on $JHK$-band photometry and K-band spectroscopy the massive stars are placed in a Hertzsprung Russell diagram. By comparison with evolutionary models, their age and hence the star formation history of W49 can be investigated. Fourteen O type stars as well as two young stellar objects (YSOs) are identified by our spectroscopic survey. Eleven O-stars are main sequence stars with subtypes ranging from O3 to O9.5, with masses ranging from $\sim 20$ $M_{\odot}$ to $\sim 120$ $M_{\odot}$. Three of the O-stars show strong wind features, and are considered to be Of-type supergiants with masses beyond 100 $M_{\odot}$ . The two YSOs show CO emission, indicative for the presence of circumstellar disks in the central region of the massive cluster. The age of the cluster is estimated as $\sim1.5$ Myr, with star formation still ongoing in different parts of the region. The ionising photons from the central massive stars have not yet cleared the molecular cocoon surrounding the cluster. W49 is comparable to extragalactic star-forming regions and provides us with an unique possibility to study a starburst in detail.

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arXiv2015

Major mergers with small galaxies - the discovery of a Magellanic-type galaxy at z=0.12

We report on the serendipitous discovery of a star-forming galaxy at redshift z=0.116 with morphological features that indicate an ongoing merger. This object exhibits two clearly separated components with significantly different colors, plus a possible tidal stream. Follow-up spectroscopy of the bluer component revealed a low star-forming activity of 0.09 M$_{\odot}$/year and a high metallicity of 12+log(O/H)=8.6. Based on comparison with mass-star-formation-rate and mass-metallicity relations, and on fitting of spectral energy distributions, we obtain a stellar mass of 3x10$^9$ M$_{\odot}$, which renders this object comparable to the Large Magellanic Cloud (LMC). Thus our finding provides a further piece of evidence of a major merger already acting on small, dwarf galaxy-like scales.

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arXiv2015

The grey extinction of the ionizing cluster in NGC 3603 from ultraviolet to optical wavelengths

We use photometry in the F220W, F250W, F330W, F435W filters from the High Resolution Channel of the Advanced Camera for Surveys and photometry in the F555W, F675W, and F814W filters from the Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope to derive individual stellar reddenings and extinctions for stars in the HD 97950 cluster in the giant HII region NGC 3603. The mean line-of-sight reddening for about a hundred main-sequence member stars inside the cluster is $E(F435W-F555W)=1.33\pm0.12$ mag. After correcting for foreground reddening, the total to selective extinction ratio is $R_{F555W}=3.75\pm0.87$ in the cluster. Within the standard deviation associated with $E(\rm λ-F555W)/E(F435W-F555W)$ in each filter, the cluster extinction curve at ultraviolet wavelengths tends to be greyer than the average Galactic extinction laws from Cardelli et al. (1989) and Fitzpatrick et al. (1999). It is closer to the extinction law derived by Calzetti et al. (2000) for starburst galaxies, where the 0.2175 $\rm μm$ bump is absent. This indicates an anomalous extinction in the HD 97950 cluster, which may due to the clumpy dust distribution within the cluster, and the size of dust grains being larger than the average Galactic ISM.

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arXiv2015

A High-Resolution Multiband Survey of Westerlund 2 With the Hubble Space Telescope I: Is the Massive Star Cluster Double?

We present first results from a high resolution multi-band survey of the Westerlund 2 region with the Hubble Space Telescope. Specifically, we imaged Westerlund 2 with the Advanced Camera for Surveys through the $F555W$, $F814W$, and $F658N$ filters and with the Wide Field Camera 3 in the $F125W$, $F160W$, and $F128N$ filters. We derive the first high resolution pixel-to-pixel map of the color excess $E(B-V)_g$ of the gas associated with the cluster, combining the H$α$ ($F658N$) and Pa$β$ ($F128N$) line observations. We demonstrate that, as expected, the region is affected by significant differential reddening with a median of $E(B-V)_g=1.87$~mag. After separating the populations of cluster members and foreground contaminants using a $(F814W-F160W)$ vs. $F814W$ color-magnitude diagram, we identify a pronounced pre-main-sequence population in Westerlund 2 showing a distinct turn-on. After dereddening each star of Westerlund 2 individually in the color-magnitude diagram we find via over-plotting PARSEC isochrones that the distance is in good agreement with the literature value of $\sim4.16 \pm 0.33$~kpc. With zero-age-main-sequence fitting to two-color-diagrams, we derive a value of total to selective extinction of $R_V=3.95 \pm 0.135$. A spatial density map of the stellar content reveals that the cluster might be composed of two clumps. We estimate the same age of 0.5-2.0 Myr for both clumps. While the two clumps appear to be coeval, the northern clump shows a $\sim 20 \%$ lower stellar surface density.

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arXiv2014

The Panchromatic High-Resolution Spectroscopic Survey of Local Group Star Clusters - I. General Data Reduction Procedures for the VLT/X-shooter UVB and VIS arm

Our dataset contains spectroscopic observations of 29 globular clusters in the Magellanic Clouds and the Milky Way performed with VLT/X-shooter. Here we present detailed data reduction procedures for the VLT/X-shooter UVB and VIS arm. These are not restricted to our particular dataset, but are generally applicable to different kinds of X-shooter data without major limitation on the astronomical object of interest. ESO's X-shooter pipeline (v1.5.0) performs well and reliably for the wavelength calibration and the associated rectification procedure, yet we find several weaknesses in the reduction cascade that are addressed with additional calibration steps, such as bad pixel interpolation, flat fielding, and slit illumination corrections. Furthermore, the instrumental PSF is analytically modeled and used to reconstruct flux losses at slit transit and for optimally extracting point sources. Regular observations of spectrophotometric standard stars allow us to detect instrumental variability, which needs to be understood if a reliable absolute flux calibration is desired. A cascade of additional custom calibration steps is presented that allows for an absolute flux calibration uncertainty of less than ten percent under virtually every observational setup provided that the signal-to-noise ratio is sufficiently high. The optimal extraction increases the signal-to-noise ratio typically by a factor of 1.5, while simultaneously correcting for resulting flux losses. The wavelength calibration is found to be accurate to an uncertainty level of approximately 0.02 Angstrom. We find that most of the X-shooter systematics can be reliably modeled and corrected for. This offers the possibility of comparing observations on different nights and with different telescope pointings and instrumental setups, thereby facilitating a robust statistical analysis of large datasets.

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arXiv2014

The Discovery of a Very Massive Star in W49

Very massive stars (M>100 M$_{\odot}$) are very rare objects, but have a strong influence on their environment. The formation of this kind of objects is of prime importance in star formation, but observationally still poorly constrained. We report on the identification of a very massive star in the central cluster of the star-forming region W49. We investigate near-infrared K-band spectroscopic observations of W49 from VLT/ISAAC together with JHK images obtained with NTT/SOFI and LBT/LUCI. We derive a spectral type of W49nr1, the brightest star in the dense core of the central cluster of W49. On the basis of its K-band spectrum, W49nr1 is classified as an O2-3.5If* star with a K-band absolute magnitude of -6.27$\pm$0.10 mag. The effective temperature and bolometric correction are estimated from stars of similar spectral type. After comparison to the Geneva evolutionary models, we find an initial mass between 100 M$_{\odot}$ and 180 M$_{\odot}$. Varying the extinction law results in a larger initial mass range of 90 - 250 M$_{\odot}$.

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arXiv2014

The Mice at play in the CALIFA survey: A case study of a gas-rich major merger between first passage and coalescence

We present optical integral field spectroscopy (IFS) observations of the Mice, a major merger between two massive (>10^11Msol) gas-rich spirals NGC4676A and B, observed between first passage and final coalescence. The spectra provide stellar and gas kinematics, ionised gas properties and stellar population diagnostics, over the full optical extent of both galaxies. The Mice provide a perfect case study highlighting the importance of IFS data for improving our understanding of local galaxies. The impact of first passage on the kinematics of the stars and gas has been significant, with strong bars likely induced in both galaxies. The barred spiral NGC4676B exhibits a strong twist in both its stellar and ionised gas disk. On the other hand, the impact of the merger on the stellar populations has been minimal thus far: star formation induced by the recent close passage has not contributed significantly to the global star formation rate or stellar mass of the galaxies. Both galaxies show bicones of high ionisation gas extending along their minor axes. In NGC4676A the high gas velocity dispersion and Seyfert-like line ratios at large scaleheight indicate a powerful outflow. Fast shocks extend to ~6.6kpc above the disk plane. The measured ram pressure and mass outflow rate (~8-20Msol/yr) are similar to superwinds from local ULIRGs, although NGC4676A has only a moderate infrared luminosity of 3x10^10Lsol. Energy beyond that provided by the mechanical energy of the starburst appears to be required to drive the outflow. We compare the observations to mock kinematic and stellar population maps from a merger simulation. The models show little enhancement in star formation during and following first passage, in agreement with the observations. We highlight areas where IFS data could help further constrain the models.

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arXiv2014

A new population of recently quenched elliptical galaxies in the SDSS

We use the Sloan Digital Sky Survey to investigate the properties of massive elliptical galaxies in the local Universe (z\leq0.08) that have unusually blue optical colors. Through careful inspection, we distinguish elliptical from non-elliptical morphologies among a large sample of similarly blue galaxies with high central light concentrations (c_r\geq2.6). These blue ellipticals comprise 3.7 per cent of all c_r\geq2.6 galaxies with stellar masses between 10^10 and 10^11 h^{-2} {\rm M}_{\sun}. Using published fiber spectra diagnostics, we identify a unique subset of 172 non-star-forming ellipticals with distinctly blue urz colors and young (< 3 Gyr) light-weighted stellar ages. These recently quenched ellipticals (RQEs) have a number density of 2.7-4.7\times 10^{-5}\,h^3\,{\rm Mpc}^{-3} and sufficient numbers above 2.5\times10^{10} h^{-2} {\rm M}_{\sun} to account for more than half of the expected quiescent growth at late cosmic time assuming this phase lasts 0.5 Gyr. RQEs have properties that are consistent with a recent merger origin (i.e., they are strong `first-generation' elliptical candidates), yet few involved a starburst strong enough to produce an E+A signature. The preferred environment of RQEs (90 per cent reside at the centers of < 3\times 10^{12}\,h^{-1}{\rm M}_{\sun} groups) agrees well with the `small group scale' predicted for maximally efficient spiral merging onto their halo center and rules out satellite-specific quenching processes. The high incidence of Seyfert and LINER activity in RQEs and their plausible descendents may heat the atmospheres of small host halos sufficiently to maintain quenching.

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arXiv2013

A Lyman Break Galaxy in the Epoch of Reionization from HST Grism Spectroscopy

We present observations of a luminous galaxy at redshift z=6.573 --- the end of the reioinization epoch --- which has been spectroscopically confirmed twice. The first spectroscopic confirmation comes from slitless HST ACS grism spectra from the PEARS survey (Probing Evolution And Reionization Spectroscopically), which show a dramatic continuum break in the spectrum at restframe 1216 A wavelength. The second confirmation is done with Keck + DEIMOS. The continuum is not clearly detected with ground-based spectra, but high wavelength resolution enables the Lyman alpha emission line profile to be determined. We compare the line profile to composite line profiles at redshift z=4.5. The Lyman alpha line profile shows no signature of a damping wing attenuation, confirming that the intergalactic gas is ionized at redshift z=6.57. Spectra of Lyman breaks at yet higher redshifts will be possible using comparably deep observations with IR-sensitive grisms, even at redshifts where Lyman alpha is too attenuated by the neutral IGM to be detectable using traditional spectroscopy from the ground.

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arXiv2012

Giant Galaxies, Dwarfs, and Debris Survey. I. Dwarf Galaxies and Tidal Features Around NGC 7331

The Giant GAlaxies, Dwarfs, and Debris Survey concentrates on the nearby universe to study how galaxies have interacted in groups of different morphology, density, and richness. In these groups we select the dominant spiral galaxy and search its surroundings for dwarf galaxies and tidal interactions. This paper presents the first results from deep wide-field imaging of NGC 7331, where we detect only four low luminosity candidate dwarf companions and a stellar stream that may be evidence of a past tidal interaction. The dwarf galaxy candidates have surface brightnesses of mu_{r} ~ 23-25 mag/arcsec^{2} with (g-r) colors of 0.57-0.75mag in the Sloan Digital Sky Survey filter system, consistent with their being dwarf spheroidal galaxies (dSph). A faint stellar stream structure on the western edge of NGC 7331 has mu_{g} ~27 mag/arcsec^{2} and a relatively blue color of (g-r)=0.15mag. If it is tidal debris, then this stream could have probably formed from a rare type of interaction between NGC 7331 and a dwarf irregular or transition-type dwarf galaxy. We compare the structure and local environments of NGC 7331 to those of other nearby giant spirals in small galaxy groups. NGC 7331 has a much lower (2%) stellar mass in the form of early-type satellites than found for M31 and lacks the presence of nearby companions like luminous dwarf elliptical galaxies or the Magellanic Clouds. However, our detection of a few dSph candidates suggests that it is not deficient in low-luminosity satellites.

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arXiv2012

Near Infrared spectroscopy of post-starburst galaxies: a limited impact of TP-AGB stars on galaxy SEDs

We present VLT-ISAAC NIR spectro-photometric observations of 16 post-starburst (PSB) galaxies aimed at constraining the debated influence of TP-AGB stars on the SED of galaxies with stellar ages of 0.5-2 Gyr, critical for high-redshift studies. PSB galaxies have negligible on-going star formation and a SED dominated by the stars formed in a recent (<2 Gyr) burst. By spectroscopically selecting PSB galaxies with mean luminosity-weighted ages between 0.5 and 1.5 Gyr and a broad range of metallicities, we explore the parameter space around the peak of the relative energy output of TP-AGB stars. Crucially, we target galaxies at z~0.2, so that two main spectral features of TP-AGB stars (C-molecule band-head drops at 1.41 and 1.77mum), which are heavily absorbed by our atmosphere for targets at z~0, move inside the H and K atmospheric windows and can be constrained for the first time to high accuracy. Our findings provide key constraints to stellar population synthesis models: i) The NIR regions around 1.41 and 1.77mum (rest-frame) are featureless for all galaxies in our sample over the whole range of relevant ages and metallicities at variance with the Maraston (2005) TP-AGB heavy models, which exhibit marked drops there; ii) No flux boosting is observed in the NIR. The optical-NIR SEDs of most of our PSB galaxies can be consistently reproduced with Bruzual & Charlot (2003) models, using either simple stellar populations of corresponding light-weighted ages and metallicities, or a more realistic burst plus an old population containing <~60% of the total stellar mass. In contrast, all combinations of this kind based on the Maraston (2005) models are unable to simultaneously reproduce the smoothness of the NIR spectra and the relatively blue optical-NIR colours in the observations. The data collected in this study appear to disfavour TP-AGB heavy models with respect to TP-AGB light ones.

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arXiv2012

A fundamental problem in our understanding of low mass galaxy evolution

Recent studies have found a dramatic difference between the observed number density evolution of low mass galaxies and that predicted by semi-analytic models. While models accurately reproduce the z=0 number density, they require that the evolution occurs rapidly at early times, which is incompatible with the strong late evolution found observationally. We report here the same discrepancy in two state-of-the-art cosmological hydrodynamical simulations, which is evidence that the problem is fundamental. We search for the underlying cause of this problem using two complementary methods. Firstly, we look for evidence of a different history of today's low mass galaxies in models and observations and we find that the models yield too few young, strongly star-forming galaxies. Secondly, we construct a toy model to link the observed evolution of specific star formation rates (sSFR) with the evolution of the galaxy stellar mass function. We infer from this model that a key problem in both semi-analytic and hydrodynamical models is the presence of a positive instead of a negative correlation between sSFR and stellar mass. A similar positive correlation is found between the specific dark matter halo accretion rate and the halo mass, indicating that model galaxies are growing in a way that follows the growth of their host haloes too closely. It therefore appears necessary to find a mechanism that decouples the growth of low mass galaxies, which occurs primarily at late times, from the growth of their host haloes, which occurs primarily at early times. We argue that the current form of star-formation driven feedback implemented in most galaxy formation models is unlikely to achieve this goal, owing to its fundamental dependence on host halo mass and time. [Abridged]

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